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An X mark (also known as an ex mark or a cross mark or simply an X or ex or a cross ) is used to indicate the concept of negation (for example "no, this has not been verified", "no, that is not the correct answer" or "no, I do not agree") as well as an indicator (for example, in election ballot papers or in maps as an x-marks-the-spot ). Its opposite is often considered to be the O mark used in Japan and Korea or the check mark used in the West. In Japanese, the X mark (❌) is called "batsu" (ばつ) and can be expressed by someone by crossing their arms. [ 1 ]
It is also used as a replacement for a signature for a person who is blind or illiterate and thus cannot write their name. [ 2 ] Typically, the writing of an X used for this purpose must be witnessed to be valid.
Contrary to the negation or negative perception delegated to the letter X, there is a significant resilience in the usage displayed by the letter's placement. This unique letter is also recognized as the symbol of multiplicity, the Roman numerical symbol for 10, and also the mark of a forgotten treasure. As a verb, to X (or ex ) [ 3 ] off / out or to cross off / out means to add such a mark. It is quite common, especially on printed forms and document, for there to be squares in which to place x marks, or interchangeably checks.
It is traditionally used on maps to indicate locations, most famously on treasure maps . [ citation needed ] It is also used as a set of three to mark jugs of moonshine for having completed all distillation steps, while additionally signifying its potency (as high as 150 proof ) relative to legal spirits, which rarely exceed 80 proof (40% ABV).
Among Native Americans in the 18th and 19th centuries, the X mark was used as a signature to denote presence or approval, particularly regarding agreements and treaties. [ 4 ]
In the 21st century, the X mark started to be used to indicate collaborations between fashion brands. [ 5 ]
Unicode provides various related symbols, including:
The mark is generally rendered with a less symmetrical form than the following cross-shaped symbols:
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Xanadu Quantum Technologies is a Canadian quantum computing hardware and software company headquartered in Toronto, Ontario. [ 1 ] [ 2 ] [ 3 ] The company develops cloud accessible photonic quantum computers [ 4 ] [ 5 ] [ 6 ] [ 7 ] and develops open-source software for quantum machine learning and simulating quantum photonic devices. [ 8 ] [ 9 ] [ 10 ]
Xanadu was founded in 2016 by Christian Weedbrook and was a participant in the Creative Destruction Lab's accelerator program. Since then, Xanadu has raised a total of US$245M in funding with venture capital financing from Bessemer Venture Partners , Capricorn Investment Group, Tiger Global Management , In-Q-Tel , Business Development Bank of Canada , OMERS Ventures , Georgian, Real Ventures, Golden Ventures and Radical Ventures [ 11 ] [ 12 ] [ 13 ] [ 14 ] [ 15 ] [ 16 ] and innovation grants from Sustainable Development Technology Canada [ 17 ] [ 18 ] [ 19 ] [ 20 ] and DARPA . [ 21 ]
Xanadu's hardware efforts have been focused on developing programmable Gaussian boson sampling (GBS) devices. GBS is a generalization of boson sampling , which traditionally uses single photons as an input; GBS uses squeezed states of light . [ 22 ] [ 23 ] [ 24 ] [ 25 ] [ 26 ] [ 27 ] In 2020, Xanadu published a blueprint for building a fault-tolerant quantum computer using photonic technology. [ 28 ]
In June 2022 Xanadu reported on a boson sampling experiment summing up to those of Google and University of Science and Technology of China (USTC) . Their setup used loops of optical fiber and multiplexing to replace the network of beam splitters by a single one which made it also more easily reconfigurable. They detected a mean of 125 to 219 photons from 216 squeezed modes (squeezed light follows a photon number distribution so they can contain more than one photon per mode) and claimed to have obtained a speedup 50 million times bigger than previous experiments. [ 29 ] [ 30 ]
In January 2025, Xanadu further advanced photonic quantum computing by demonstrating a scalable modular approach to networking photonic quantum computers. This work, published in Nature , introduced key architectural improvements for integrating multiple photonic quantum processors, significantly enhancing error correction and scalability. [ 31 ]
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https://en.wikipedia.org/wiki/Xanadu_Quantum_Technologies
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A xanthate is a salt or ester of a xanthic acid. The formula of the salt of xanthic acid is [R−O−CS 2 ] − M + (where R is organyl group and M is usually Na or K ). [ 1 ] Xanthate also refers to the anion [R−O−CS 2 ] − . The formula of a xanthic acid is R−O−C(=S)−S−H , such as ethyl xanthic acid , while the formula of a xanthate ester is R−O−C(=S)−S−R' , where R and R' are organyl groups. The salts of xanthates are sometimes called O -organyl dithioates. The esters of xanthic acid are sometimes called O , S -diorganyl esters of dithiocarbonic acid . The name xanthate is derived from Ancient Greek ξανθός ( xanthos ) meaning 'yellowish' or 'golden', and indeed most xanthate salts are yellow. They were discovered and named in 1823 by Danish chemist William Christopher Zeise . These organosulfur compounds are important in two areas: the production of cellophane and related polymers from cellulose and (in mining) for extraction of certain sulphide bearing ores. [ 2 ] They are also versatile intermediates in organic synthesis .
Xanthate salts of alkali metals are produced by the treatment of an alcohol , alkali , and carbon disulfide . The process is called xanthation . [ 2 ] In chemical terminology, the alkali reacts with the alcohol to produce an alkoxide , which is the nucleophile that adds to the electrophilic carbon atom in CS 2 . [ 3 ] Often the alkoxide is generated in situ by treating the alcohol with sodium hydroxide or potassium hydroxide :
For example, sodium ethoxide gives sodium ethyl xanthate . Many alcohols can be used in this reaction. Technical grade xanthate salts are usually of 90–95% purity. Impurities include alkali metal sulfides , sulfates , trithiocarbonates , thiosulfates , sulfites , or carbonates as well as residual raw material such as alcohol and alkali hydroxide . These salts are available commercially as powder, granules, flakes, sticks, and solutions are available.
Some commercially or otherwise useful xanthate salts include:
The OCS 2 core of xanthate salts and esters, like that of the carbonates and the esters has trigonal planar molecular geometry . The central carbon atom is sp 2 -hybridized . The potassium salt of the amyl xanthate ( KS 2 COC 5 H 11 ) has been characterized by X-ray crystallography . The COCS 2 portion of the anion is planar. The C-S bond lengths are both 1.65 Å, and the C-O distance is 1.38 Å. [ 5 ]
Xanthic acids , with the formula ROC(S)SH, can be prepared by treating alkali metal xanthates, e.g. potassium ethyl xanthate , with hydrochloric acid at low temperatures. The methyl and ethyl xanthic acids are oils that are soluble in organic solvents. Benzyl xanthic acid is a solid. They have pKas near 2. [ 6 ] These compounds thermally decompose in the presence of base to the alcohol and carbon disulfide. [ 7 ]
Xanthic acids characteristically decompose:
This reaction is the reverse of the method for the preparation of the xanthate salts. The intermediate in the decomposition is the xanthic acid, ROC(S)SH, which can be isolated in certain cases.
The C-O bond in xanthate esters can be cleaved in various ways, providing a means for deoxygenation of alcohols. In Barton–McCombie deoxygenation , tributyltin hydride is the source of H atom. Several variations of this deoxygenation are known, for example using AIBN and hydrosilanes. [ 8 ]
Xanthates are intermediates in the Chugaev elimination process. They can be used to control radical polymerisation under the RAFT process, also termed MADIX (macromolecular design via interchange of xanthates).
Xanthate anions undergo alkylation to give xanthate esters, which are generally stable: [ 9 ]
They can be oxidized to dixanthogen disulfides :
Acylation of xanthates gives alkyl xanthogen esters (ROC(S)SC(O)R') and related anhydrides. [ 2 ]
Xanthates bind to transition metal cations as bidentate ligands . The charge-neutral complexes are soluble in organic solvents. [ 10 ]
Cellulose reacts with carbon disulfide (CS 2 ) in presence of sodium hydroxide (NaOH) to produces sodium cellulose xanthate, which upon neutralization with sulfuric acid (H 2 SO 4 ) gives viscose rayon or cellophane paper ( Sellotape or Scotch Tape ).
Xanthate salts (e.g. sodium alkyl xanthates, dixanthogen ) are widely used as flotation agents in mineral processing.
Rarely encountered, thioxanthates arise by the reaction of CS 2 with thiolate salts. For example, sodium ethylthioxanthate has the formula C 2 H 5 SCS 2 Na. Dithiocarbamates are also related compounds. They arise from the reaction of a secondary amine with CS 2 . For example, sodium diethyldithiocarbamate has the formula (C 2 H 5 ) 2 NCS 2 Na.
While biodegradable, this class of chemicals may be toxic to life in water at concentrations of less than 1 mg/L. [ 13 ] Water downstream of mining operations is often contaminated with xanthates. [ 14 ]
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https://en.wikipedia.org/wiki/Xanthate
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Xanthine oxidase ( XO or XAO ) is a form of xanthine oxidoreductase, a type of enzyme that generates reactive oxygen species . [ 2 ] These enzymes catalyze the oxidation of hypoxanthine to xanthine and can further catalyze the oxidation of xanthine to uric acid . These enzymes play an important role in the catabolism of purines in some species, including humans. [ 3 ]
Xanthine oxidase is defined as an enzyme activity (EC 1.17.3.2). [ 4 ] The same protein, which in humans has the HGNC approved gene symbol XDH , can also have xanthine dehydrogenase activity (EC 1.17.1.4). [ 5 ] Most of the protein in the liver exists in a form with xanthine dehydrogenase activity, but it can be converted to xanthine oxidase by reversible sulfhydryl oxidation or by irreversible proteolytic modification. [ 6 ] [ 7 ]
The following chemical reactions are catalyzed by xanthine oxidase:
Because XO is a superoxide-producing enzyme, with general low specificity, [ 9 ] it can be combined with other compounds and enzymes and create reactive oxidants, as well as oxidize other substrates.
Bovine xanthine oxidase (from milk) was originally thought to have a binding site to reduce cytochrome c with, but it has been found that the mechanism to reduce this protein is through XO's superoxide anion byproduct, with competitive inhibition by carbonic anhydrase . [ 10 ]
Another reaction catalyzed by xanthine oxidase is the decomposition of S -nitrosothiols (RSNO), a class of reactive nitrogen species, to nitric oxide (NO), which reacts with a superoxide anion to form peroxynitrite under aerobic conditions. [ 11 ]
XO has also been found to produce the strong one-electron oxidant carbonate radical anion from oxidation with acetaldehyde in the presence of catalase and bicarbonate. It was suggested that the carbonate radical was likely produced in one of the enzyme's redox centers with a peroxymonocarbonate intermediate. [ 9 ]
Here is a diagram highlighting the pathways catalyzed by xanthine oxidase.
It is suggested that xanthine oxidoreductase, along with other enzymes, participates in the conversion of nitrate to nitrite in mammalian tissues. [ 12 ]
The protein is large, having a molecular weight of 270 kDa, and has two flavin molecules (bound as FAD), 2 molybdenum atoms, and 8 iron atoms bound per enzymatic unit. The molybdenum atoms are contained as molybdopterin cofactors and are the active sites of the enzyme. The iron atoms are part of [2Fe-2S] ferredoxin iron-sulfur clusters and participate in electron transfer reactions. [ citation needed ]
The active site of XO is composed of a molybdopterin unit with the molybdenum atom also coordinated by terminal oxygen ( oxo ), sulfur atoms and a terminal hydroxide . In the reaction with xanthine to form uric acid, the S=Mo VI O-H group ionizes and the resulting MoVI-O − attacks carbon concomitant with transfer of H − to Mo=S. The resulting HS-Mo IV -O-C center then undergoes 2e oxidation with hydrolysis of the MoVI-O-C group, giving back S=Mo VI -OH, together with xanthine. [ 3 ] Like other known molybdenum-containing oxidoreductases, the oxygen atom introduced to the substrate by XO originates from water rather than from dioxygen (O 2 ). [ citation needed ]
Xanthine oxidase is a superoxide -producing enzyme found normally in serum and the lungs, and its activity is increased during influenza A infection. [ 13 ]
During severe liver damage, xanthine oxidase is released into the blood, so a blood assay for XO is a way to determine if liver damage has happened. [ 14 ]
Because xanthine oxidase is a metabolic pathway for uric acid formation, the xanthine oxidase inhibitor allopurinol is used in the treatment of gout . Since xanthine oxidase is involved in the metabolism of 6-mercaptopurine , caution should be taken before administering allopurinol and 6-mercaptopurine, or its prodrug azathioprine , in conjunction.
Xanthinuria is a rare genetic disorder where the lack of xanthine oxidase leads to high concentration of xanthine in blood and can cause health problems such as renal failure . There is no specific treatment, affected people are advised by doctors to avoid foods high in purine and to maintain a high fluid intake. Type I xanthinuria has been traced directly to mutations of the XDH gene which mediates xanthine oxidase activity. Type II xanthinuria may result from a failure of the mechanism which inserts sulfur into the active sites of xanthine oxidase and aldehyde oxidase , a related enzyme with some overlapping activities (such as conversion of allopurinol to oxypurinol ). [ 15 ]
Inhibition of xanthine oxidase has been proposed as a mechanism for improving cardiovascular health. [ 16 ] A study found that patients with chronic obstructive pulmonary disease ( COPD ) had a decrease in oxidative stress, including glutathione oxidation and lipid peroxidation, when xanthine oxidase was inhibited using allopurinol. [ 17 ] Oxidative stress can be caused by hydroxyl free radicals and hydrogen peroxide, both of which are byproducts of XO activity. [ 18 ]
Increased concentration of serum uric acid has been under research as an indicator for cardiovascular health factors, and has been used to strongly predict mortality, heart transplant, and more in patients. [ 16 ] But it is not clear whether this could be a direct or casual association or link between serum uric acid concentration (and by proxy, xanthine oxidase activity) and cardiovascular health. [ 19 ] States of high cell turnover and alcohol ingestion are some of the most prominent cases of high serum uric acid concentrations. [ 18 ]
Reactive nitrogen species, such as peroxynitrite that xanthine oxidase can form, have been found to react with DNA, proteins, and cells, causing cellular damage or even toxicity. Reactive nitrogen signaling, coupled with reactive oxygen species, have been found to be a central part of myocardial and vascular function, explaining why xanthine oxidase is being researched for links to cardiovascular health. [ 20 ]
Both xanthine oxidase and xanthine oxidoreductase are also present in corneal epithelium and endothelium and may be involved in oxidative eye injury. [ 21 ]
Inhibitors of XO include allopurinol , [ 22 ] oxypurinol , [ 23 ] and phytic acid . [ 24 ] It has also been found to be inhibited by flavonoids , [ 25 ] including those found in Bougainvillea spectabilis ( Nyctaginaceae ) leaves (with an IC 50 of 7.23 μM), typically used in folk medicine . [ 26 ]
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Xanthosine monophosphate ( xanthylate ) is an intermediate in purine metabolism . [ 1 ] It is a ribonucleoside monophosphate . It is formed from IMP via the action of IMP dehydrogenase , and it forms GMP via the action of GMP synthase . Also, XMP can be released from XTP by enzyme deoxyribonucleoside triphosphate pyrophosphohydrolase containing (d)XTPase activity. [ 2 ]
It is abbreviated XMP. [ 3 ]
This biochemistry article is a stub . You can help Wikipedia by expanding it .
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https://en.wikipedia.org/wiki/Xanthosine_monophosphate
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The Xbox 360 video game console was subject to a number of technical problems and failures, some as a result of design flaws. Some issues could be identified by a pattern of red lights on the front face of the console; these colloquially became known as the " Red Ring of Death " or the " RRoD ". [ 1 ] [ 2 ] There were also other issues, such as discs becoming scratched in the drive and " bricking " of consoles due to dashboard updates.
There were many conflicting estimates of the console's unusually high failure rate . [ 3 ] [ 4 ] [ 5 ] The warranty provider SquareTrade estimated it at 23.7% in 2009, [ 6 ] while a Game Informer survey reported 54.2%. [ 7 ] Among the consoles owned by employees of Joystiq , which saw heavy use for games journalism purposes, the failure rate had reached 90% by the end of 2007. [ 8 ] The crisis was ultimately abated from 2009 by design revisions to the later-produced Xbox models; the S model in particular was far more resilient. By 2012 the failure rate for the Xbox 360 family was comparable to the PS3 failure rate. [ 9 ]
The issues proved extremely damaging for Microsoft . Repairs and shipping of replacement hardware cost the company $1.15bn. The issues triggered multiple lawsuits , [ 10 ] cost the Xbox ground in the console wars and threatened the long term viability of the Xbox brand. [ 11 ]
The design of the Xbox 360 was a hurried process subject to a number of late changes. This included the addition of a hard disk drive, which compromised airflow in the machine. The holes in the case were added to try and ameliorate this airflow issue. Time pressures also resulted in insufficient testing. Microsoft were aware of a myriad of technical challenges as early as August 2005, including "overheating graphics chips, cracking heat sinks, cosmetic issues with the hard disk and the front of the box, underperforming graphics memory chips from Infineon , a problem with the DVD drive - and more". Thermal issues with the GPU were ultimately what caused the infamous "Red Ring" issues, while the DVD drive issue was later responsible for scratching discs. An engineer requested a shut down of the production line that month, but this did not occur out of fear of a delay to console delivery in some regions. [ 12 ]
The console launched in November 2005 in North America, swiftly followed by other regions. However, consoles began failing "almost immediately". Microsoft initially dismissed these concerns as "isolated reports", that were within the normal range of failure (around 2%). [ 12 ] [ 13 ] [ 14 ] [ 15 ] [ 16 ] In late 2005, Microsoft's internal data was reporting a failure rate during manufacturing of around 6-7%. These consoles were not shipped to consumers but remained in warehouses. By March 2006, around 30% of consoles manufactured were either returned or had failed checks at the factory. At one point Microsoft's yield was as low as 32% (meaning a failure rate of 68%) [ 17 ]
Peter Moore , the Vice President of Microsoft's Interactive Entertainment Business division in 2015 detailed a conversation he had with Microsoft CEO Steve Ballmer on his planned response to the incident in the mid 2000s. He stated:
"...here's what we have to do: we need to FedEx an empty box to a customer who had a problem - they would call us up - with a FedEx return label to send your box, and then we would FedEx it back to them and fix it. ... I always remember $240m of that was FedEx. ... It was sickening. I was doing a lot of interviews. ... We couldn't figure it out. ... There was a theory. We had changed our solder, which is the way you put the GPU and the fans, to lead-free. ... We think it was somehow the heat coming off the GPU was drying out some of the solder, and it wasn't the normal stuff we'd used, because we had to meet European Standards and take the lead out. ... He said, 'what's it going to cost?' I remember taking a deep breath, looking at Robbie, and saying, 'we think it's $1.15bn, Steve.' He said, 'do it.' There was no hesitation. ... If we hadn't made that decision there and then, and tried to fudge over this problem, then the Xbox brand and Xbox One wouldn't exist today." [ 11 ]
In July 2007 Moore published an open letter recognizing the console's problems, as well as announcing a three-year warranty from the original date of purchase for every Xbox 360 console that experienced the "general hardware failure" (Red Ring) issue. [ 18 ] That October a class action lawsuit was brought against Microsoft due to the problems the console had with disc scratching, which could render games unplayable. [ 19 ] The case was lengthy and worked through the court system over the following decade, with litigation focusing on the validity of class certification. In 2017 the matter was decided by the United States Supreme Court in Microsoft Corp. v. Baker , which settled in favour of Microsoft.
During the Game Developers Conference in February 2008, Microsoft announced that the failure rate had "dropped", but did not mention any specifics. [ 20 ] The same month, electronics warranty provider SquareTrade published an examination of 1040 Xbox 360s and said that they suffered from a failure rate of 16.4%. Of the 171 failures, 60% were due to a general hardware failure (and thus fell under the 3 year extended warranty). And of the remaining 40% which were not covered by the extended warranty, 18% were disc read errors, 13% were video card failures, 13% were hard drive freezes, 10% were power issues and 7% were disc tray malfunctions. [ 21 ] [ 22 ] SquareTrade also stated that its estimates are likely significantly lower than reality due to the time span of the sample (six to ten months), the eventual failure of many consoles that did not occur within this time span and the fact that most owners did not deal with SquareTrade and had their consoles repaired directly through Microsoft via the extended RROD warranty.
From 2009 the crisis began to abate due to design revisions. The Jasper models sold that year had a failure rate of under 4%, with the overall product family rate at around 12% in the first quarter. [ 23 ] The Xbox 360 S launched in 2010 and had a far lower failure rate. The S models did not include segmented outer ring lights like the launch model, and were not included in the extended warranty. [ 24 ] The 360 family as a whole was discontinued in 2016, but Microsoft continued to offer repairs for a time after that. [ 25 ]
Microsoft did not reveal the full technical details of the problem until a 2021 documentary on the history of Xbox, though earlier independant investigations had correctly indentified the issues with the GPU and soldering. [ 26 ] In a nod to the incident, Microsoft sold Red Ring holiday sweaters in December 2024. The item was popular among Microsoft employees. [ 27 ]
The launch model of the Xbox 360 includes four lights in a ring around the power button, on the front face of the console. Green indicated normal operation, while red lights were used for error codes. Most famously, three red lights indicated a "general hardware failure". [ 28 ] The error was coined the "Red Ring of Death" after Windows ' Blue Screen of Death error. The error was sometimes preceded by freeze-ups, graphical problems in the middle of gameplay, such as checkerboard or pinstripe patterns on the screen, and sound errors; mostly consisting of extremely loud noises that couldn't be affected by the volume control, and the console only responding when the power button was pressed to turn it off. [ 29 ] The problem was most prevalent in early models.
This error code was usually caused by the failure of one or more hardware components, although it could indicate that the console is not receiving enough power from the power supply. This coould be caused by a faulty or improperly connected power supply. The three flashing lights could also be caused by power surges. Unplugging and restarting the console fixed this issue in some cases. [ 30 ] [ 31 ]
On the Xbox 360 S and E models, the power button utilizes a different design that does not incorporate the same style of lighting that past models used. [ 32 ] A flashing red light means that the console is overheating, similar to the two-light error code on the original model Xbox 360; however, an on-screen message also appears, telling the user that the console will automatically power off to protect itself from overheating. A solid red light is similar to the one-light error if an "E XX" error message is displayed and a three-light error code if the error message is absent.
The related E74 error caused only a single of the red ring quadrants to illuminate, and the screen to display an error message in multiple languages: "System Error. Contact Xbox Customer Support", with the code E74 at the bottom. Much like the infamous Red Ring issue, the error was related to connection issues with the GPU, but could also be caused by a more general GPU failure or failing eDRAM. The E74 issue was covered by the three-year extended warranty from 2009 as Microsoft considered it part of the same issue as the Red Ring, and customers who previously paid Microsoft for out-of-warranty service to correct the E74 error received a refund. [ 33 ] [ 34 ] [ 35 ]
The console would illuminate all four lights if it could not detect an AV cable. This was not triggered by later revisions of the console which included an HDMI port. In some cases the four lights indicated a more serious problem with the console, followed by a 2-digit error code. [ 36 ] The four lights would also be illuminated briefly by power issues such as surges or brief outages.
Microsoft did not reveal the cause of the issues publicly until 2021, when a 6-part documentary on the history of Xbox was released. The Red Ring issue was caused by the cracking of solder joints inside the GPU flip chip package, connecting the GPU to the substrate interposer, as a result of thermal stress from heating up and cooling back down when the system is power cycled. [ 37 ] Microsoft had switched to lead-free solder due to regulations in the European Union , but using the incorrect alternative resulted in fracturing. [ 12 ]
While the cause was not confirmed by Microsoft until 2021, many independant investigations came to similar conclusions at the time, identifying thermal stress on the GPU and the solder as the culprit. The German computer magazine c't blamed the problem primarily on the use of the wrong type of lead-free solder, a type that when exposed to elevated temperatures for extended periods of time becomes brittle and can develop hair-line cracks that are almost irreparable. [ 38 ] Microsoft designed the chip in-house to cut out the traditional ASIC vendor with the goal of saving money in ASIC design costs. After multiple product failures, Microsoft went back to an ASIC vendor and had the chip redesigned so it would dissipate more heat. [ 39 ] [ 40 ]
The Guardian also claimed that using Xbox Kinect with an old Xenon generation Xbox would cause the Red Ring, but this was denied by Microsoft. [ 41 ]
The design of the disc drive was flawed, and could cause scratches on discs, particularly if the console was moved while the disc was spinning. Unlike the Red Ring issues, the disc scratching was not resolved by hardware revisions and was present in the S and E models. Those versions shipped with a sticker informing users that moving the console while powered on posed a risk. [ 42 ] Even on static footing however, normal floor vibrations that would occur in a household environment were enough to cause disc scratches. [ 43 ] The issue was particularly prevalent in 2006 models.
The issue was subject to multiple independant investigations, initially by the Dutch television program Kassa and later by the European Commissioner for Consumer Protection and the BBC . The BBC investigation in particular involved laboratory conditions for testing. [ 44 ] The issue ultimately led to a Supreme Court case which was ruled in favour of Microsoft in 2017. [ 45 ] [ 46 ]
Although discs scratched by the Xbox 360 were not covered under its warranty, [ 47 ] Microsoft's Xbox Disc Replacement Program [ 48 ] sold customers a new copy of discs scratched by the Xbox 360, if they were published in countries where the Xbox was originally sold, at a cost of $20. [ 49 ] The published list of games that qualify, however, was limited. [ 50 ] Third party games were only ever replaced at the discretion of the publishers. Electronic Arts for example offered replacements made within 90 days of purchase. [ 51 ]
Independant investigations concluded that the disc drives lacked a mechanism to secure the disc solidly in place. [ 52 ] Tilting or moving the console, when operating with a disc spinning inside, can potentially cause damage to the disc and in some cases render the disc unplayable as a result. [ 53 ] Microsoft engineers were aware of the issue ahead of launch, around September or October of 2005. However, installing "bumpers" to prevent the discs moving out of alignment would have added 50 cents to the production cost of each console, and was not implemented. An alternative would have been to slow the disc rotation speed but this would have led to increased loading times, and magnetic adjustments would not have been possible due to the disc tray locking mechanism. [ 54 ]
Several Xbox 360 system updates caused major issues for users.
An update patch released on November 1, 2006 was reported to " brick " consoles, rendering them useless. [ 55 ] The most obvious issue occurs after the installation of the patch, after which the console immediately reboots and shows an error message. Usually, error code E71 is shown during or directly after the booting animation.
In response to the November 2006 update error that "bricked" his console, a California man filed a class action lawsuit against Microsoft in Washington federal court in early December 2006. [ 56 ] The lawsuit seeks $5 million in damages and the free repair of any console rendered unusable by the update. This was the second such lawsuit filed against Microsoft, the first having been filed in December 2005, shortly after the 360's launch. Following Microsoft's extension of the Xbox 360 warranty to a full year, from the previous 90 days, the California man's attorney confirmed to the Seattle Post Intelligencer that the lawsuit had been resolved under confidential terms. [ 57 ]
On November 19, 2008, Microsoft released the " New Xbox Experience " (NXE). This update provided streaming Netflix capability and avatars; however, some users have reported the update has caused their consoles to not properly read optical media. [ 1 ] Others have reported that the update has disabled audio through HDMI connections. [ 58 ] A Microsoft spokesperson stated the company is "aware that a handful of Xbox LIVE users are experiencing audio issues, and are diligently monitoring this issue and working towards a solution." Microsoft released a patch on February 3, 2009 for the HDMI audio issues. [ 59 ]
A patch released in May 2011 prevented some users from playing games from discs. The update involved "a change in the disc reading algorithms", but would simply inform users that the disc was unreadable and ask them to clean it with a cloth. [ 60 ]
In 2007, the official steering wheel peripheral faced issues with overheating and releasing smoke, prompting the "Hotwheels" nickname. Microsoft encouraged users to only use the steering wheel in battery mode rather than while plugged in. [ 61 ] That August a product recall was issued, with Microsoft retrofitting the existing steering wheels to remedy the problem. [ 62 ]
The Nyko Intercooler was a popular aftermarket cooler, purchased by users who wished to improve air flow in an attempt to avoid the red-ring issue. While the exact cause of red-ring was not yet public in the late 2000s, it was known that temperature was an issue. [ 63 ] [ 64 ] Unfortunately, the Nyko Intercooler itself had issues and its usage could cause the red-ring or damage the power DC input. [ 64 ] The Intercooler could also melt itself onto the 360, melt the powercord, or make itself extremely hard to remove. [ 65 ]
Microsoft stated that the peripheral drained too much power from the console (the Intercooler power cord was installed between the Xbox 360 power supply and the console itself), could cause faults to occur, and stated that consoles fitted with the peripheral would have their warranties voided. Nyko released an updated Intercooler that used its own power source, and claimed the problem no longer occurred, but this did not affect Microsoft's stance on the warranty.
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Xcode is a suite of developer tools for building apps on Apple devices. [ 4 ] It includes an integrated development environment (IDE) of the same name for macOS , used to develop software for macOS, iOS , iPadOS , watchOS , tvOS , and visionOS . It was initially released in late 2003; the latest stable release is version 16, released on September 16, 2024, and is available free of charge via the Mac App Store and the Apple Developer website. [ 5 ] Registered developers can also download preview releases and prior versions of the suite through the Apple Developer website. [ 6 ] Xcode includes command-line tools that enable UNIX -style development via the Terminal app in macOS. [ 7 ] They can also be downloaded and installed without the GUI .
Before Xcode, Apple offered developers Project Builder and Interface Builder to develop Mac OS X applications.
Xcode supports source code for the programming languages : Swift , C++ , Objective-C , Objective-C++ , Java , AppleScript , Python , Ruby , ResEdit (Rez), and C , with a variety of programming models, including but not limited to Cocoa , Carbon , and Java. Third parties have added support for GNU Pascal , [ 8 ] Free Pascal , [ 9 ] Ada , [ 10 ] C# , [ 11 ] Go , [ 12 ] Perl , [ 13 ] and D . [ 14 ]
Xcode can build fat binary ( universal binary ) files containing code for multiple architectures with the Mach-O executable format. These helped ease the transitions from 32-bit PowerPC to 64-bit PowerPC, from PowerPC to Intel x86 , from 32-bit to 64-bit Intel, and most recently from Intel x86 to Apple silicon by allowing developers to distribute a single application to users and letting the operating system automatically choose the appropriate architecture at runtime. Using the iOS SDK , tvOS SDK, and watchOS SDK, Xcode can also be used to compile and debug applications for iOS , iPadOS , tvOS , and watchOS .
Xcode includes the GUI tool Instruments , which runs atop a dynamic tracing framework, DTrace , created by Sun Microsystems and released as part of OpenSolaris .
Xcode also integrates built-in support for source code management using the Git version control system and protocol, allowing the user to create and clone Git repositories (which can be hosted on source code repository hosting sites such as GitHub , Bitbucket , and Perforce , or self-hosted using open-source software such as GitLab ), and to commit , push , and pull changes, all from within Xcode, automating tasks that would traditionally be performed by using Git from the command line .
The main application of the suite is the integrated development environment (IDE), also named Xcode. The Xcode suite includes most of Apple's developer documentation, and built-in Interface Builder , an application used to construct graphical user interfaces .
Up to Xcode 4.1, the Xcode suite included a modified version of the GNU Compiler Collection . In Xcode 3.1 up to Xcode 4.6.3, it included the LLVM-GCC compiler, with front ends from the GNU Compiler Collection and a code generator based on LLVM . [ 15 ] In Xcode 3.2 and later, it included the Clang C/C++/Objective-C compiler, with newly-written front ends and a code generator based on LLVM, and the Clang static analyzer . [ 16 ] Starting with Xcode 4.2, the Clang compiler became the default compiler, [ 17 ] Starting with Xcode 5.0, Clang was the only compiler provided.
Up to Xcode 4.6.3, the Xcode suite used the GNU Debugger (GDB) as the back-end for the IDE's debugger . Starting with Xcode 4.3, the LLDB debugger was also provided; starting with Xcode 4.5 LLDB replaced GDB as the default back-end for the IDE's debugger. [ 18 ] Starting with Xcode 5.0, GDB was no longer supplied. [ 19 ]
The Playgrounds feature of Xcode provides an environment for rapid experimentation and development in the Swift programming language. The original version of the feature was announced and released by Apple Inc on June 2, 2014, during WWDC 2014 . [ 20 ] [ 21 ]
Playgrounds provide a testing ground that renders developer code in real time. They have the capability of evaluating and displaying the results of single expressions as they are coded (in line or on a side bar), providing rapid feedback to the programmer. This type of development environment, known as a read-eval-print loop (or REPL) is useful for learning, experimenting and fast prototyping. [ 22 ] [ 23 ] [ 24 ] Playgrounds was used by Apple to publish Swift tutorials and guided tours where the REPL advantages are noticeable . [ 25 ] [ 26 ]
The Playgrounds feature was developed by the Developer Tools department at Apple. According to Chris Lattner , the inventor of Swift Programming Language and Senior Director and Architect at the Developer Tools Department, Playgrounds was "heavily influenced by Bret Victor's ideas, by Light Table and by many other interactive systems". [ 27 ] Playgrounds was announced by Apple Inc. on June 2, 2014, during WWDC 2014 as part of Xcode 6 and released in September.
In September 2016, the Swift Playgrounds application for iPad (also available on macOS starting in February 2020) was released, incorporating these ideas into an educational tool. Xcode's Playgrounds feature continued development, with a new step-by-step execution feature introduced in Xcode 10 at WWDC 2018 . [ 28 ]
Formerly, Xcode supported distributing a product build process over multiple systems. One technology involved was named Shared Workgroup Build , which used the Bonjour protocol to automatically discover systems providing compiler services, and a modified version of the free software product distcc to facilitate the distribution of workloads. Earlier versions of Xcode provided a system named Dedicated Network Builds . These features are absent in the supported versions of Xcode.
Xcode also includes Apple's WebObjects tools and frameworks for building Java web applications and web services (formerly sold as a separate product). As of Xcode 3.0, Apple dropped [ 29 ] WebObjects development inside Xcode; WOLips [ 30 ] should be used instead. Xcode 3 still includes the WebObjects frameworks.
Xcode 1.0 was released in fall 2003. Xcode 1.0 was based on Project Builder , but had an updated user interface (UI), ZeroLink, Fix & Continue, distributed build support, and Code Sense indexing.
The next significant release, Xcode 1.5, had better code completion and an improved debugger.
Xcode 2.0 was released with Mac OS X v10.4 "Tiger". It included the Quartz Composer visual programming language, better Code Sense indexing for Java, and Ant support. It also included the Apple Reference Library tool, which allows searching and reading online documentation from Apple's website and documentation installed on a local computer.
Xcode 2.1 could create universal binary files. It supported shared precompiled headers , unit testing targets, conditional breakpoints, and watchpoints. It also had better dependency analysis.
The final version of Xcode for Mac OS X v10.4 was 2.5.
Xcode 3.0 was released with Mac OS X v10.5 "Leopard". Notable changes since 2.1 include [ 31 ] the DTrace debugging tool (now named Instruments ), refactoring support, context-sensitive documentation, and Objective-C 2.0 with garbage collection . It also supports Project Snapshots, which provide a basic form of version control; Message Bubbles, which show build errors debug values alongside code; and building four-architecture fat binaries (32 and 64-bit Intel and PowerPC).
Xcode 3.1 was an update release of the developer tools for Mac OS X, and was the same version included with the iPhone SDK. It could target non-Mac OS X platforms, including iPhone OS 2.0. It included the GCC 4.2 and LLVM GCC 4.2 compilers. Another new feature since Xcode 3.0 is that Xcode's SCM support now includes Subversion 1.5.
Xcode 3.2 was released with Mac OS X v10.6 "Snow Leopard" and installs on no earlier version of OS X. It supports static program analysis , among other features. It also drops official support for targeting versions earlier than iPhone OS 3.0. But it is still possible to target older versions, and the simulator supports iPhone OS 2.0 through 3.1. Also, Java support is "exiled" in 3.2 to the organizer. [ 32 ]
Xcode 3.2.6 is the last version that can be downloaded for free for users of Mac OS X Snow Leopard (though it’s not the last version that supports Snow Leopard; 4.2 is). Downloading Xcode 3.2.6 requires a free registration at Apple's developer site.
In June 2010, at the Apple Worldwide Developers Conference version 4 of Xcode was announced during the Developer Tools State of the Union address. Version 4 of the developer tools consolidates the Xcode editing tools and Interface Builder into one application, among other enhancements. [ 33 ] [ 34 ] Apple released the final version of Xcode 4.0 on March 9, 2011. The software was made available for free to all registered members of the $99 per year Mac Developer program and the $99 per year iOS Developer program. It was also sold for $4.99 to non-members on the Mac App Store (no longer available). Xcode 4.0 drops support for many older systems, including all PowerPC development and software development kits (SDKs) for Mac OS X 10.4 and 10.5, and all iOS SDKs older than 4.3. The deployment target can still be set to produce binaries for those older platforms, but for Mac OS platforms, one is then limited to creating x86 and x86-64 binaries. Later, Xcode was free to the general public. Before version 4.1, Xcode cost $4.99. [ 35 ]
Xcode 4.1 was made available for free on July 20, 2011 (the day of Mac OS X Lion 's release) to all users of Mac OS X Lion on the Mac App Store. On August 29, 2011, Xcode 4.1 was made available for Mac OS X Snow Leopard for members of the paid Mac or iOS developer programs. [ 36 ] Xcode 4.1 was the last version to include GNU Compiler Collection (GCC) instead of only LLVM GCC or Clang .
On October 12, 2011, Xcode 4.2 was released concurrently with the release of iOS 5.0, and it included many more and improved features, such as storyboarding and automatic reference counting (ARC) . [ 17 ] Xcode 4.2 is the last version to support Mac OS X 10.6 "Snow Leopard" , but is available only to registered developers with paid accounts; without a paid account, 3.2.6 is the latest download that appears for Snow Leopard. [ 37 ]
Xcode 4.3, released on February 16, 2012, is distributed as one application bundle, Xcode.app, installed from the Mac App Store. Xcode 4.3 reorganizes the Xcode menu to include development tools. [ 38 ] Xcode 4.3.1 was released on March 7, 2012 to add support for iOS 5.1. [ 39 ] Xcode 4.3.2 was released on March 22, 2012 with enhancements to the iOS Simulator and a suggested move to the LLDB debugger versus the GDB debugger (which appear to be undocumented changes). [ citation needed ] Xcode 4.3.3, released in May 2012, featured an updated SDK for Mac OS X 10.7.4 "Lion" and a few bug fixes. [ 40 ]
Xcode 4.4 was released on July 25, 2012. [ 41 ] It runs on both Mac OS X Lion (10.7) and OS X Mountain Lion (10.8) and is the first version of Xcode to contain the OS X 10.8 "Mountain Lion" SDK. Xcode 4.4 includes support for automatic synthesizing of declared properties, new Objective-C features such as literal syntax and subscripting, improved localization, and more. [ 42 ] On August 7, 2012, Xcode 4.4.1 was released with a few bug fixes.
On September 19, 2012, iOS 6 and Xcode 4.5 were released. Xcode added support for iOS 6 and the 4-inch Retina Display on iPhone 5 and iPod Touch 5th generation. It also brought some new Objective-C features to iOS, simplified localization, and added auto-layout support for iOS. [ 18 ] On October 3, 2012, Xcode 4.5.1 was released with bug fixes and stability improvements. [ 5 ] Less than a month later, Xcode 4.5.2 was released, with support for iPad Mini and iPad with Retina Display, and bug fixes and stability improvements.
On January 28, 2013, iOS 6.1 and Xcode 4.6 were released.
On June 10, 2013, at the Apple Worldwide Developers Conference , version 5 of Xcode was announced. [ 43 ] On September 18, 2013, Xcode 5.0 was released. It shipped with iOS 7 and OS X 10.8 Mountain Lion SDKs. However, support for OS X 10.9 Mavericks was only available in beta versions. Xcode 5.0 also added a version of Clang generating 64-bit ARM code for iOS 7. Apple removed support for building garbage collected Cocoa binaries in Xcode 5.1. [ 44 ]
On June 2, 2014, at the Worldwide Developers Conference, Apple announced version 6 of Xcode. One of the most notable features was support for Swift , an all-new programming language developed by Apple. Xcode 6 also included features like Playgrounds and live debugging tools. [ 45 ] On September 17, 2014, at the same time, iOS 8 and Xcode 6 were released. Xcode could be downloaded on the Mac App Store .
On June 8, 2015, at the Apple Worldwide Developers Conference , Xcode version 7 was announced. It introduced support for Swift 2, and Metal for OS X , and added support for deploying on iOS devices without an Apple Developer account. [ 46 ] Xcode 7 was released on September 16, 2015.
On June 13, 2016, at the Apple Worldwide Developers Conference , Xcode version 8 was announced; a beta version was released the same day. It introduced support for Swift 3. [ 47 ] Xcode 8 was released on September 13, 2016.
On June 5, 2017, at the Apple Worldwide Developers Conference , Xcode version 9 was announced; a beta version was released the same day. It introduced support for Swift 4 and Metal 2. It also introduced remote debugging on iOS and tvOS devices wirelessly, through Wi-Fi. [ 48 ]
Xcode 9 was publicly released on September 19, 2017. [ 49 ]
On June 4, 2018, at the Apple Worldwide Developers Conference , Xcode version 10 was announced; a beta version was released the same day. Xcode 10 introduced support for the Dark Mode announced for macOS Mojave , the collaboration platforms Bitbucket and GitLab (in addition to already supported GitHub ), training machine learning models from playgrounds, and the new features in Swift 4.2 and Metal 2.1, as well as improvements to the editor and the project build system. [ 50 ] Xcode 10 also dropped support for building 32-bit macOS apps [ 51 ] and no longer supports Subversion integration. [ 52 ]
Xcode 10 was publicly released on September 17, 2018. [ 53 ]
On June 3, 2019, at the Apple Worldwide Developers Conference , Xcode version 11 was announced; a beta version was released the same day. Xcode 11 introduced support for the new features in Swift 5.1, as well as the new SwiftUI framework (although the interactive UI tools are available only when running under macOS 10.15 ). [ 54 ] It also supports building iPad applications that run under macOS; includes integrated support for the Swift Package Manager; and contains further improvements to the editor, including a "minimap" that gives an overview of a source code file with quick navigation. [ 55 ] Xcode 11 requires macOS 10.14 or later [ 54 ] and Xcode 11.4 requires 10.15 or later. [ 56 ]
Xcode 11 was publicly released on September 20, 2019.
On June 22, 2020, at the Apple Worldwide Developers Conference , Xcode version 12 was announced; a beta version was released the same day. Xcode 12 introduced support for Swift 5.3 and requires macOS 10.15.4 or later. Xcode 12 dropped building apps for iOS 8 and the lowest version of iOS supported by Xcode 12 built apps is iOS 9. Xcode 12.1 also dropped support for building apps for Mac OS X 10.6 Snow Leopard. The minimum version of macOS supported by Xcode 12.1 built apps is OS X 10.9 Mavericks. [ 57 ]
Xcode 12 was publicly released on September 16, 2020.
On June 7, 2021, at the Apple Worldwide Developers Conference , Xcode version 13 was announced; a beta version was released the same day. The new version introduced support for Swift 5.5 and requires macOS 11.3 or later. Xcode 13 contains SDKs for iOS / iPadOS 15 , macOS 12 , watchOS 8 , and tvOS 15 . Xcode 13’s major features include the new concurrency model in Swift projects, [ 58 ] improved support for version control providers (such as GitHub ), including the ability to browse, view, and comment on pull requests right in the app interface, and support for Xcode Cloud, Apple’s newly-launched mobile CI/CD service (it also has a web version).
Xcode 13 was publicly released on September 20, 2021.
On June 6, 2022, at the Apple Worldwide Developers Conference , Xcode version 14 was announced; a beta version was released the same day. Xcode 14 dropped support for building 32-bit iOS apps. [ 59 ] Xcode 14 dropped support for building apps for iOS 9 and 10 (these versions of iOS supported 32-bit iOS apps) and the minimum version of iOS supported by Xcode 14 built apps is iOS 11. Xcode 14 also dropped building apps for macOS 10.12 Sierra. The minimum version of macOS supported by Xcode 14 built apps is macOS 10.13 High Sierra. [ 57 ]
Xcode 14 was publicly released on September 12, 2022.
On June 5, 2023, at the Apple Worldwide Developers Conference , Xcode version 15 was announced; a beta version was released the same day. Xcode 15 dropped support for building apps for iOS 11 and the minimum version of iOS supported by Xcode 15 built apps is iOS 12. [ 57 ]
Xcode 15 was publicly released on September 18, 2023.
On June 10, 2024, at the Apple Worldwide Developers Conference , Xcode version 16 was announced; a beta version was released the same day. [ 57 ] Xcode 16 introduced predictive code completion on Apple silicon Macs, along with the Swift Testing framework. [ 60 ]
Xcode 16 was publicly released on September 16, 2024.
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The R1 (internally called XCON , for e X pert CON figurer) program was a production-rule -based system written in OPS5 by John P. McDermott of Carnegie Mellon University in 1978 to assist in the ordering of DEC 's VAX computer systems by automatically selecting the computer system components based on the customer's requirements.
In developing the system, McDermott made use of experts from both DEC's PDP/11 and VAX computer systems groups. These experts sometimes even disagreed amongst themselves as to an optimal configuration. The resultant "sorting it out" had an additional benefit in terms of the quality of VAX systems delivered.
XCON first went into use in 1980 in DEC's plant in Salem, New Hampshire . It eventually had about 2500 rules. By 1986, it had processed 80,000 orders, and achieved 95–98% accuracy. It was estimated to be saving DEC $25M a year by reducing the need to give customers free components when technicians made errors, by speeding the assembly process, and by increasing customer satisfaction.
Before XCON, when ordering a VAX from DEC, every cable, connection, and bit of software had to be ordered separately. (Computers and peripherals were not sold complete in boxes as they are today.) The sales people were not always very technically expert, so customers would find that they had hardware without the correct cables, printers without the correct drivers, a processor without the correct language chip, and so on. This meant delays and caused a lot of customer dissatisfaction and resultant legal action. XCON interacted with the sales person, asking critical questions before printing out a coherent and workable system specification/order slip.
XCON's success led DEC to rewrite XCON as XSEL —a version of XCON intended for use by DEC's salesforce to aid a customer in properly configuring their VAX (so they would not, say, choose a computer too large to fit through their doorway or choose too few cabinets for the components to fit in). Location problems and configuration were handled by yet another expert system, XSITE .
McDermott's 1980 paper [ 1 ] on R1 won the AAAI Classic Paper Award in 1999. Footnote 2 gave a humorous explanation for the name "R1" as "Four years ago I couldn't even say "knowledge engineer", now I ... [are one.]".
This business software article is a stub . You can help Wikipedia by expanding it .
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Bis(pentafluorophenyl)xenon is an unstable organic compound of xenon . [ 1 ] [ 2 ] It consists of two fluorinated phenyl rings connected to xenon.
Bis(pentafluorophenyl)xenon is a molecular substance. In the solid form it crystallises in the monoclinic system with space group P 2 1 / n . [ 3 ] The unit cell has four molecules with a = 13.635 Å. b = 8.248 Å. c = 11.511 Å, β = 102.624°. The unit cell volume is 1263.18 Å 3 . [ 4 ]
The molecules have carbon to xenon to carbon bonds in nearly a straight line (the bond angle is at least 175°). The carbon–xenon bond lengths are 2.35 and 2.39 Å. The two pentafluorophenyl rings are twisted by 72° with respect to each other. [ 3 ]
Bis(pentafluorophenyl)xenon decomposes above −20 °C and can explode. [ 1 ]
Xe(C 6 F 5 ) 2 is prepared from the [(CH 3 ) 4 N]F catalyzed reactions of (CH 3 ) 3 SiC 6 F 5 and XeF 2 in propionitrile, propionitrile/acetonitrile, acetonitrile, or CH 2 Cl 2 , at -60 to -40 °C as the first [10-Xe-2] species with two xenon-carbon bonds as a colorless solid that decomposes above −20 °C and spontaneously at 20 °C. [ 1 ] C 6 F 5 XeF [ 1 ] [ 2 ] is formed as an intermediate which has been characterized by NMR spectroscopy.
Xe(C 6 F 5 ) 2 is also formed from the reaction of C 6 F 5 XeF with Cd(C 6 F 5 ) 2 [ 2 ]
However, the direct introduction of the C 6 F 5 group into XeF 2 with Cd(C 6 F 5 ) 2 is not successful. [ 2 ]
Bis(pentafluorophenyl)xenon is crystallized from dichloromethane at −40 °C. [ 3 ]
Bis(pentafluorophenyl)xenon reacts with mercury to make bis(pentafluorophenyl)mercury. [ 1 ]
Bis(pentafluorophenyl)xenon reacts with hydrogen fluoride to form pentafluorophenyl xenon fluoride C 6 F 5 XeF. [ 2 ] In acetonitrile solution bis(pentafluorophenyl)xenon decomposes to form C 6 F 5 -C 6 F 5 (C 12 F 10 ) and xenon. [ 5 ] But in dichloromethane solution the product is mostly pentafluorobenzene . [ 5 ]
It reacts with iodine to make pentafluoroiodobenzene (C 6 F 5 I). [ 2 ]
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Xenon dibromide is an unstable chemical compound with the chemical formula XeBr 2 . It was only produced by the decomposition of iodine-129 : [ 2 ] [ 3 ]
Attempts to prepare this compound by combining elemental xenon and bromine only resulted in the XeBr radical . [ 4 ] This compound is expected to be less stable than xenon difluoride and xenon dichloride . It is also expected to decompose to xenon and bromine. [ 1 ]
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Xenon monochloride (XeCl) is an exciplex which is used in excimer lasers and excimer lamps emitting near ultraviolet light at 308 nm. It is most commonly used in medicine . Xenon monochloride was first synthesized in the 1960s. Its kinetic scheme is very complex and its state changes occur on a nanosecond timescale. In the gaseous state, at least two kinds of xenon monochloride are known: XeCl and Xe 2 Cl , whereas complex aggregates form in the solid state in noble gas matrices . The excited state of xenon resembles halogens and it reacts with them to form excited molecular compounds.
Molecules that are only stable in electronically excited states are called excimer molecules , but may be called exciplex molecules if they are heteronuclear . The exciplex halides constitute an important class of rare gas halides with formula RgX. Rg is the noble gas, and X is the halogen. These molecules are de-excited by emitting a photon whose energy is some Electronvolts . Therefore, the wavelength of the light produced is in the visible or ultraviolet spectra. Gas or gaseous mixtures that may lead to the formation of these molecules is a quasi-ideal laser medium since the population inversion [ citation needed ] is directly obtained when the excimer is formed. The other consequence of the unstable ground state is that the excimer or exciplex species must be generated by an external excitation (either through a discharge, an electron beam, microwave, or radiation). At least two gases must be used to generate exciplexes: a halogen donor and a rare gas. [ 1 ] However, as shown in Table 1, not all rare gas halide molecules lead to the development of lasers; some may not even exist. Multiple molecules and applications have been developed. [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 6 ] [ 7 ] [ 8 ] [ 9 ] [ 10 ]
Several review articles related to xenon chloride laser technology and its applications have been published. [ 11 ] [ 12 ] [ 13 ] [ 14 ]
Some authors [ 11 ] [ 14 ] stress the importance of accurately determining the kinetics of the laser medium when rare-gas halides are involved. Recent results have provided insight into the physical chemistry of the laser medium. [ 15 ] [ 16 ] [ 17 ] Spectroscopic investigations are limited to the visible-near ultraviolet region where exciplex lasers operate. Only binary gas mixtures of xenon and a chlorine donor, or ternary mixtures that also include a buffer gas (a rare gas indicated by Rg) will be considered. The most interesting chlorine donors are CCl 4 and HCl because of their use in laser technology, and Cl 2 (see Figure 1).
XeCl and Xe 2 Cl are most important in laser applications amongst the xenon chlorides. Although discharge lamps based on low-pressure mixtures of xenon and a chlorine donor emit incoherent light, they are reliable and easy to operate. [ 18 ]
The idea that the noble gases can form halides arose in the early 1920s: [ 19 ] A. von Antropoff [ 20 ] and Oddo [ 21 ] suggested that krypton and xenon may form bromides and chlorides . In 1933, Yost and Kaye [ 22 ] unsuccessfully tried to synthesize xenon chloride by illuminating a mixture of xenon (70 torr of pressure) and chlorine (225 torr) with a mercury-vapor lamp .
Xenon monochlorides were first synthesized in 1965. [ 23 ] Later, solid XeCl 2 and XeCl 4 compounds were synthesized at low temperatures. In 1991, Prosperio et al. [ 24 ] demonstrated the existence of XeCl 2 in the gaseous state, which is important for lasing kinetics, although it emits an uninteresting infrared light . [ 25 ]
In 1973 Riveros et al. [ 26 ] synthesized XeCl − ions in the gaseous phase at a pressure of 10 −4 torr . This ionic molecule attracted little interest.
Systematic studies of XeCl were initiated in 1975 by Velazco and Setser, [ 27 ] who demonstrated 304 nm emission from XeCl * . This emission was obtained by mixing xenon atoms ( Xe 3 P 2 ) with chlorine gas Cl 2 or other chlorinated compounds ( NOCl and SOCl 2 ). The excitation was provided by a cold cathode discharge; the total pressure was a few torr. Months later, Ewing and Brau [ 28 ] reported lasing from a XeCl film 2 Σ 1/2 + → 2 Σ 1/2 + at 308 nm, which was most promising for industrial applications. The preferred chlorine donor for XeCl laser is HCl. The reasons given are:
Three years later Lorentz et al. [ 33 ] performed experiments at high pressures (a few atmospheres ) in a mixture containing ( Ar / XeCl 2 ) and found an emission centered at 450 nm which was attributed to XeCl 2 .
The first XeCl 2 laser was developed in 1980. [ 34 ] [ 35 ] This type of laser is likely to be tunable over a wide range of wavelengths (30 nm) in the visible spectrum . This is true even if the absorption phenomena occur on the side of shorter wavelengths and therefore limits the laser action at the red region of the electromagnetic spectrum from light emission . Solid state experiments with Xe 2 Cl * [ 36 ] suggest that the gaseous state is more suitable for the development of this type of laser. The measured amplification was correct in the solid state. [ 37 ] The liquid state [ 38 ] seems like an ideal dye laser although implementation seems complex and costly. Presently, the Xe 2 Cl laser has not been industrially developed. Unlike XeCl, the best chlorine donor is CCl 4 [ 39 ] while no laser action occurs when using HCl. [ 34 ]
Four molecules are a priori synthesized in the mixtures. Note particularly the possibility of synthesizing these under the experimental conditions of lasers and their roles.
XeHCl was observed in a gaseous medium. However, this molecule has only been detected via emission spectra in the microwave, radio and far infrared regions, [ 40 ] but with emission predicted by two theoretical studies at 232 nm [ 41 ] and 129 nm. [ 42 ] Note, however, that when nearly in aggregate, it is more likely to be stable at the solid state . It is the same for Xe 3 Cl which can theoretically emit at 500 nm, [ 43 ] while this activity has never been observed in the gaseous state.
XeH has three known emission lines. They were observed at 190 nm, [ 44 ] 250 nm [ 45 ] and 660 nm. [ 46 ] However, they have never manifested in the laser spectra, which leads to the assumption that XeH is not formed under the experimental conditions. Conversely, the XeH + ion is formed in the mixtures used in lasers. It plays a significant role in kinetics in the synthesis of XeCl * , [ 47 ] through a reaction that competes with the creation of Xe + ions (shown below):
HCl + + Xe → Xe + + HCl (80 ± 10%)
HCl + + Xe → XeH + + HCl (20 ± 10%)
The rate constant of the entire process is 6.4 × 10 −10 cm 3 s −1 (± 20%).
Xe + ion is a pivotal precursor in the formation of the exciplex molecule.
The potential curves presented in Figure 2 are the results of theoretical [ 48 ] [ 49 ] [ 50 ] and experimental [ 51 ] works.
Common characteristics for all halide states of the noble gases includes a group of related excited states B, C and D and a lower group of dissociatively or weakly bound states A and X. States B, D and X have Σ symmetry (Λ = 1/2) while the C state has π symmetry (Λ = 3/2). The state A is itself split into two sub-states, a symmetry Σ, A 1/2 and the other symmetry π, A 3/2 .
The ionization potential of noble gases in their lowest excited state is close to the electron affinity of halogen atoms. Thus, the rare gas halide molecules are formed by an ionic bond since the excited electron of the noble gas is partly transferred to the halogen atom. The molecule so formed is therefore stable as is the case of the states B, C and D.
This electron transfer does not occur with ground state atoms. As the rare gas atoms are not reactive. This is the case for states A and X.
These states are correlated with ground state Xe + ions and Cl − . The spin-orbital splitting of the Xe + ion into two states ( 2 P 3/2 and 2 P 1/2 ) is important; also the states B and D to which they are correlated are significantly far away. For the minimum potential curves corresponding to almost the same value of the internuclear distance (r e #0.3 nm), the energy difference measured experimentally is about 9940 cm −1 . [ 52 ] [ 53 ] [ 54 ] This is in agreement with the energy of separation of Xe + ( 2 P 3/2 ) and Xe + ( 2 P 1/2 ) states valued at 10574 cm −1 .
Potential curves of the states B and C intersect adiabatically with a potential curve correlated to Xe* + Cl at large internuclear distances: 7.1 nm experimentally [ 55 ] and 7.19 nm [ 56 ] and 6.3 nm [ 10 ] theoretically. A more recent theoretical investigation specifies these intersection phenomena. [ 57 ] States B and C merging at long-distance, intersect two successive potential curves correlated to Xe* + Cl. The lowest correlated to Xe ( 3 P 2 ) + Cl ( 2 P 3/2 ) is 7.25 nm and after that, the next correlated to Xe ( 3 P 1 ) + Cl ( 2 P 3/2 ) is intercepted at 18.68 nm. As this intersection occurs at a great distance, the ionic character of the binding of these states near the equilibrium internuclear distance r e is virtually unaffected.
This situation is slightly different for state D that crosses these two potential curves at a much shorter distance. [ 57 ] Indeed, state D intersects Xe ( 3 P 2 ) + Cl
( 2 P 3/2 ) only at 0.89 nm and Xe ( 3 P 1 ) + Cl ( 2 P 3/2 ) at 1.02 nm.
The distinction between states B and C is that they are correlated with Xe + ions whose semi-occupied orbital p is in a plane parallel to the internuclear axis for the state B and perpendicular to this axis for the state C. [ 58 ]
On an examination of the energy position of the potential curve of states B and C, their proximity results in some difficulty. the values of the energy gap (E B – E C ) between the two states is enumerated in Table 2. The data is highly dispersed; computed values, in particular, are far removed from all experimental values. These were determined mostly from the intensity ratios of the two emissions XeCl * centered at 308 nm and 345 nm, either with or without corrections by the participation of transition (B → A). [ 59 ] The most direct measure is given by Jouvet et al. [ 60 ] Excitation spectra of XeCl * directly provide the energy difference between the vibrational levels v′=0 and v″=0 which correspond respectively to the B and C states. This value of 90 cm −1 is close to other measurements from studies in kinetics . [ 16 ] [ 61 ] [ 62 ]
I: measurement derived from the value of the intensity ratios of the XeCl emissions centered at 308 and 345 nm (see § 3-1-1)
C: measurement derived from a kinetic study providing the coupling constants between these two states.
* : Emission at 345 nm is not corrected for the contribution XeCl (B → A)
** : XeCl is in the solid state.
Positioning of state B in relation to state C is theoretically justified by considering the configuration interaction between the ionic and covalent character states of similar symmetry. [ 65 ] [ 73 ] [ 74 ] In a state 2 Σ (as states B and X), a simply occupied orbital is located closer to an orbital of another atom such that the interaction or exchange of charges between two atoms are larger and easier than in a state 2 π (like states C and A 3/2 ), where a simply occupied orbital is perpendicular to the molecular axis and far away from another atom. The correction introduced by this phenomenon in terms of energy values is much more important for Σ states than for the π states. [ 73 ] This interaction greatly increases the energy of state B relative to that of state C. Hence, the positioning on the observed potential curves from Figure 2.
The lowest states are correlated with ground state xenon and chlorine atoms.
Due to spin-orbital splitting of the chlorine atom level at 881 cm −1 [ 75 ] into two states, ( 2 P 3/2 ) and ( 2 P 1/2 ), state A is divided into two sub-states. However, the effect of the spin-orbital coupling here is significantly weaker than in the case of Xe + ion. At large internuclear distances, an energy gap of 882 cm −1 between A 1/2 and A 3/2 was experimentally measured in the solid state in a neon matrix. [ 76 ] Thus, this value is very close to the energy separation of states Cl ( 2 P 3/2 ) and Cl ( 2 P 1/2 ). This confirms the theoretical assumptions of state correlations between XeCl state A and Cl. At large distances state A 3/2 is similar to state X. Becker et al., [ 77 ] who laid out the interaction potentials of 35 Cl ( 2 P 3/2 and 2 P 1/2 ) and Xe ( 1 S 0 ) from the analysis of quasi–elastic scattering in collisions produced from crossed beams, has experimentally confirmed this result.
Unlike some other noble gas halides, XeCl has a non-dissociative ground state. This bonding character was demonstrated experimentally well before theoretical studies of XeCl in solid state argon matrices at 20K [ 54 ] and later in the gaseous state. [ 53 ] [ 55 ]
The Van der Waals force between atoms [ 78 ] is not strong enough in state X to explain the presence of a potential well that when low (the depth is in the order of kilotorr) [ clarification needed ] can contain between 12 and 20 vibrational levels (see Table 3). The relative increase in the binding energy of state X compared to state A can also be explained by taking into account the configuration interaction. [ 79 ] State A is also very lightly bound with binding energy half that of state X.
The energy E v'j' M of a known state M with a vibrational level v' with the rotational quantum number j is:
E v'j' M = T e (M) + E Vib (M) + E Rot (M) where T e (M), E Vib (M) and E Rot (M) respectively denote the molecule's vibrational and rotational electronic energies.
The main features of the electronic states of a known state M are usually the dissociation energy D e , the inter-atomic distance r e and energy of the bottom of the potential well E M . For XeCl, different reported values of these quantities are summarized in Tables 4, 5 and 6. They were determined theoretically or experimentally for isotope 35 Cl in the solid or gaseous state.
Dissociation energies have been calculated or measured for different states of the excimer. Some states have more measurements than others. States A, C and D have too few measurements for a statistical analysis. For state B, the four values are not consistent with each other
For state X, there are six values, two of which are outliers. That of Flannery [ 10 ] is an old, imprecise theoretical estimate. That of Tellinghuisen et al. [ 55 ] is the first experimental determination made in 1976. Seven years later [ 83 ] the same team corrected this value and closed the gap on the most recent estimates. The remaining four values seem to be the only reliable ones. D e is (with a probability of 95%) between 278.3 cm −1 and 285.3 cm −1 . The interval corresponds to a 1.3% fluctuation around 281.5 cm -1 . Indeed, among the selected determinations are two measures with high uncertainty, [ 77 ] [ 81 ] and a third which the author does not indicate. [ 79 ] The value of D e of state X, depends on the number of vibrational levels contained in the well and sets the number of transitions bound → bound that can be achieved. This result is fundamental to a better understanding of XeCl laser spectroscopy.
The interatomic distance for states A, C and D has few measurements, but they are close. On average, state A is 0.408 nm, state D, 0.307 nm and state C, 0.311 nm.
For the state X, the theoretical determination of Adrian and Jette [ 86 ] is statistically far from the others. Omitting it, using confidence level of 95% state X r e , will be in the range: 0.318 nm < r e < 0.326 nm.
The value of Tellinghuisen et al. [ 55 ] is at the limit of the interval. If ignored, the other three authors announce an identical value of 0.323 nm.
Tellinghuisen's value for state B is far from others for r e . It is the same for Ewing et Brau, [ 56 ] the oldest study of the noble gas halides which is based on the analogy of the excited rare gas with alkali metals . These are only estimates. These two values will be discarded to give a confidence interval at 95% for the interatomic distance of state B: 0.2993 nm < r e < 0.3319 nm.
Table 6 shows that there is very little information for states X, A and D. For state X, Sur et al. [ 81 ] arbitrarily took bottom of the well X as the origin of their energy scale. It is therefore not a direct measurement. Therefore, the state X as well as state A have been the subject of only one study; that of Aquilanti et al. . [ 79 ] For state D, two quite different determinations exist.
As was seen in an earlier section, the location of states B and C is problematic.
State B has the most attention from researchers. Two measures are statistically far from the others. Besides the previously mentioned study by Ewing and Brau, [ 56 ] the old theoretical work of Hay and Dunning are among the doubtful determinations [ 49 ] which will be broached soon. Without considering these values, the experimental work provides a confidence interval at a very narrow 95% threshold: from 32380.1 cm −1 to 32415.3 cm −1 .
In contrast, no conclusion can be drawn statistically given the small number of measurements of state C. However, further analysis will illuminate despite the non-matching character values in Table 6. Indeed, the positioning of the C states with respect to state B has resulted in many publications.
A statistical analysis of the values of Table 2 provides a step by step approach to the confidence interval at 95% which is the following: 76.8 cm −1 < (E B - E C ) < 100.2 cm −1 . Only four measures belong to this interval. This is the direct determination of Jouvet et al. [ 60 ] and three values deduced from kinetic studies. [ 50 ] [ 61 ] [ 62 ] On the other hand, a point estimate gives 88.5 cm −1 and the only measure to be consistent with it (given the absolute error indicated) is from Jouvet et al. . [ 60 ] at (90 ± 2 cm −1 ). Statistical study then confirms the conclusions reached in paragraph 1.1.
The confidence intervals listed above for state B and the energy difference (E B – E C ) produce an interval for E C : 32279.9 cm −1 < E C < 32338.4 cm −1 .
Under these conditions, only the value of Jouvet et al. [ 60 ] in Table 6 is consistent with this range. The three doubtful determinations include that of Hay and Dunning [ 49 ] with a defective value for E B . Another early theoretical study by Clugston and Gordon [ 87 ] also resulted in this interval. The same is true for solid state experimental work carried out by Fajardo and Apkarian. [ 72 ]
Calculating the mean of the two values in Table 6 yields 43838.45 cm -1 . The energy gap of state B is then of the order of 11400 cm −1 . Shostak and Strong [ 52 ] experimentally determined the energy difference between states A and B. They found 9900 cm −1 . The difference between these values (E B – E D ) is very sharp. Considering only work by Sur et al. , [ 81 ] the energy difference between states B and D becomes of the order of 9950 cm −1 which is close to that of Shostak and Strong. [ 52 ] This observation casts fresh doubts on the theoretical work of Hay and Dunning [ 49 ] for which (E B – E D ) is 10888 cm −1 .
With respect to the electronic structure, it appears that older studies pose a problem regarding some of their results. [ 10 ] [ 49 ] [ 55 ] [ 56 ] [ 87 ] On the other hand, work carried out by Fajardo and Apkarian [ 72 ] is not always consistent with observations of the gaseous state. Moreover, recent theoretical studies do not eliminate significant differences with experimental results. [ 42 ] [ 43 ]
The removal of the values of Hay and Dunning, [ 49 ] reduces to a determination the values of D e for states C and D, and makes the three other values relating to state B homogenous. Among these Tellinghuisen et al. [ 55 ] poses a problem for other values. The energy D e for state B then has an average value of 36184 cm −1 .
The vibrational energy of level v’ of any state M can be calculated as:
E Vib (M) = ω e (v’+1/2) – ω e x e (v’+1/2) 2
where ω e and (ω e x e ) indicates respectively, the basic vibrational frequency and the anharmonicity constant . Their respective determinations are collected in Tables 7 and table 8.
The values of ω e are grouped together in Table 7.
States X, C and D have only four determinations. No measure can be considered statistically distant from others, despite disparities.
State B offers nine determinations. A statistical analysis leads to a confidence interval of 95%: 194.7 cm −1 < ω e < 195.4 cm −1 .
Six values in Table 7 are strange. Three of them markedly so. They are old publications of which two (Hay and Dunning [ 49 ] and Brau and Ewing [ 89 ] ) were central to the previous section. Golde's [ 92 ] results were based on the same method as that used by Brau and Ewing. [ 89 ]
The other three measures that are outside the range are more recent. Kvaran et al. [ 90 ] researched the solid state. Like Fajardo and Apkarian, [ 72 ] they observed significant differences in the gaseous state. In contrast, most surprising are disagreements between Jouvet et al. [ 60 ] and Tamagake et al. [ 74 ] which were studies with good results. Finally, among the values that agreed with these ranges are many studies that were more theoretical [ 42 ] [ 88 ] than experimental. [ 55 ] [ 81 ]
In conclusion, Tellinghuisen et al. [ 55 ] gives very good results on both state B and state X.
The reported results on state C are rather questionable. [ 49 ] [ 72 ] [ 87 ] The work by Jouvet et al. [ 60 ] is at the extreme compared to other State B measures.
As for state D, excluding results by Hay and Dunning [ 49 ] makes it more cohesive than the other three values.
Finally it is necessary to specify the values of ω e for states X, C and D. The main interest of this clarification would be a better resolution of the vibrational structure of the transition used in the laser, which requires a better knowledge of state X. On the other hand, the structure of state C is important because it plays a fundamental role in laser kinetics .
Table 8 shows anharmonicity constant measurements for the various states. The measurements for constants of anharmonicity for states X, C and D are very inconsistent.
Six measurements for state B produce the confidence interval at 95%:
0.532 cm −1 < ω e x e < 0.669 cm −1 .
The work by Jouvet et al. [ 60 ] is statistically far from the others and the authors cannot explain this difference. Hay and Dunning [ 49 ] give correct forecasts, as does the vibrational structure study by Tellinghuisen et al. . [ 55 ]
The following expression denotes rotational energy: E rot (M) = B’.K’ ef – D’.(K’ ef ) 2 , where K’ ef = j’(j’+1) ± (1/2).δ(j’+1/2) ;
B’ and D’ respectively are the rotational constant and the first centrifugal distortion constant. Their values are indicated in table 9 and table 10. δ is a parameter that is equal to 2.0 for state B [ 62 ] and 0.4 for state X. [ 93 ]
Therefore, the rotational structures is very poorly known. Despite that, one should notice the consistency of some measurements made on B’.
When they are in a configuration belonging to metastable states np 5 (n+1)s 1 , (n = 5 for xenon), rare gases possess properties of polarizability and elastic scattering similar to those of alkali metals . [ 95 ] The valence electron, s, of the excited rare gas has a bond energy close to that of the alkali metal that follows it in the periodic table . In older publications, [ 56 ] [ 92 ] [ 96 ] [ 97 ] this analogy that is applicable only for the heavier rare gases, is used to study the behavior of these gases with halogen donors. The alkali metals have good chemical affinity for halogens and should have affinity for excited rare gases. Experimentally the collision cross section of metastable states of rare gases with the halogens is similar to that of alkali metals with halogens. [ 96 ] [ 97 ] [ 98 ] Thus, an excited xenon has an electronic structure close to that of caesium , so that it can react with a chlorine donor in order to form XeCl * .
Significant differences between alkali metals and excited rare gases exist in their molecular symmetry. The number of states of rare gas halides is greater than that of alkali metal salts. This is due to spin-orbital splitting of atoms and ions of rare gases.
The first condition to produce XeCl is to make xenon reactive. To do this, it must be either excited, ionized or both. Several methods of external excitation have been used. The most common are electric shocks, [ 27 ] electron beams, [ 39 ] laser excitation, [ 99 ] microwaves [ 100 ] and α particles. [ 15 ]
The excitation is not selective and formation of XeCl * may follow many paths. Their relative importance varies with the conditions, especially with pressure, mode of excitation and the halogen donor. When ternary mixtures are involved, the creation process of XeCl is more complicated. Nevertheless, the addition of a buffer gas offers many advantages. Other rare gases are cheaper than xenon, but they (along with their excited species and their ions) absorb less than xenon at 308 nm. Thus, the buffer gas can be used in very high proportions without much alteration of the laser's output power. Under these conditions, the proportions of xenon and HCl must match those required to produce the desired amount of exciplex molecule. The essential role of the buffer gas is to transfer to xenon atoms the necessary excitation energy. This transfer can be regarded as instantaneous. It can result in excitation or ionization of xenon or the formation of a Rg Xe + ion. [ 4 ] Each of these species may then react with the chlorine donor to form XeCl * . On the other hand, the formation of neutral species of RgXe does not seem to be important. [ 5 ]
The two main ways of exciplex synthesis are collision (between chlorine molecules and xenon, where at least one species is excited) and ion recombination. The buffer gas is sometimes a partner in the former and almost always involved in the latter.
The formation of XeCl * is extremely effective since Konovalov et al. [ 101 ] observed emission of XeCl in krypton while xenon was present only in trace amounts (0.2%).
XeCl * synthesis occurs when a mixture containing xenon and chlorine ( Cl 2 ) is excited by means of a laser emitting between 304 nm and 312 nm. [ 99 ] Two reactions are then induced: [ 102 ]
In the latter case, a transient complex is formed [ 103 ] (Xe- Cl 2 ) * in the state ( 1 Π u ). [ 104 ] Therefore, two dissociation pathways are possible from the time a photon is absorbed by the Cl-Cl pair or the pair Xe-Cl from ( Xe-Cl 2 ) * in the state ( 1 Π u ). [ 104 ] [ 105 ]
Xe-Cl 2 ( 1 Π u ) + hν → Xe-Cl 2 ( 1 Π g ) → Xe + Cl 2 − → XeCl(B,C) + Cl
Xe- Cl 2 ( 1 Π u ) + hν → Xe-Cl(X)-Cl + hν → Xe-Cl(B)-Cl → XeCl(B) + Cl
The rate constant of the reaction was measured by considering the photon as a third partner. It is 6 × 10 −29 cm 6 s −1 . [ 106 ]
Similar results were obtained with other chlorine donors including HCl and CCl 4 .
In all cases, the molecules of XeCl(B, C) are always produced in states with a strong vibrational excitation.
Numerous processes' importance rests on the type and excitation of the species in collision. The principal leftover in all cases is the emissions that follow from binary collisions.
These reactions involve the chlorine donor at the ground state and an excited atom of xenon, both in the first 6s, Xe * and at higher levels Xe ** such as level 6p.
Generally, these reactions can describe the result of collisions of the noble gas atoms (Rg) and halogen donors (RX), where X is a halogen atom and R a radical molecule. [ 107 ] The products of the reactions depend strongly on the type of rare gas and halogen donor. In our case where Rg = Xe and X = Cl, the nature of the products follows this rule. [ 50 ] [ 108 ] In some cases, this collision may not provide any halide rare gas. [ 50 ]
The atom Rg and the molecule RX follow when they approach the lowest adiabatic potential and the reaction proceeds by the orbital mechanism controlled at the crossover of the ionic-covalent. The reagents (Rg and RX) approach on a covalent diabatic surface. They then form a complex Rg * ...RX at a fairly large internuclear distance. Its potential is V(Rg, RX). When the distance becomes sufficiently small, it may be that V(Rg, RX) intersects an ionic potential surface ( Rg + ...RX − ). The crossover can occur through the transfer of an electron from Rg to RX. This is known as a harpoon mechanism. In this case, the atoms continue on the new surface. This leads to a diffusion reaction and the formation of RgX * .
Figure 3 shows the process of creating XeCl * which involves Rg=Xe and X=Cl. After its transfer, the electron occupies an antibonding orbital of RCl. In the presence of Xe + , R Cl − splits into R and Cl − . Xe * ions and Cl − then recombine to form XeCl in states B, C and D because there is no new force between Cl − and R. The vibrational excitation of XeCl * is always important. In total, everything takes place according to the reaction equation:
Xe* + RCl → XeCl * (B,C,D) + R with rate constant of k MX
However, the competitive formation of XeCl * reactions occur before or after the intersection. They correspond to the interactions of the V potential ( Rg * , RX * ) and V (Rg + RX * ).
In general, this situation occurs when the ionic surface is intersected by covalent surfaces where RX is in its lowest excited state. The distribution of output depends on the number and nature of output channels that are possible following collisions. [ 107 ] [ 109 ] The most common occurs at the intersection of the potential surfaces by a transfer of electronic energy that can cause a dissociation of the excited acceptor:
Rg * + RX → (Rg + ...RX − ) → Rg(B,C,D) + RX * with rate constant k ET
Rg * + RX → (Rg + ...RX − ) → Rg + R + X with rate constant k D
This pathway tends to become less important as the complexity of RX increases
It is also possible that the transfer took place in a state that is not correlated to the RX * ion but at very high Rydberg states in the neutral molecule and lying just below the limits of ionization. Critical factors regulating the branching ratios are the potential energies interrelated with the molecular ion (V I ), the Rydberg group close to the ionization (V II ) or an initial excited atom (V III ). The importance of these pathways increases with the depth of hole V ( Rg * , RX * ).
When the highly separated asymptotic energy levels are in the order V I > V II > V III and the potential energies (V II ) are attractive, the first failed intersection is encountered when the approach of reacting atoms favors output of (V II ) rather than the anionic (V I ). Since (V II ) has a cationic center that remains strongly linked, it preferentially leads to a transfer of excitation. This is the dissociative excitation reaction:
Rg * + RX → Rg + R * + X or Rg + R + X * with rate constant k DE
If V III > V II at long distance, the Penning ionization pathway or associative ionization are possible: [ 107 ]
Penning ionization : Rg * + RX → Rg + RX + + e − with rate constant k PI
Associative ionization: Rg * + RX → (RgRX) + + e − with rate constant k AI
In (V I ) bonding with an halogen atom is in principle, weak and atomic transfer is enhanced between Rg and R. This potential thus leads to the formation of the exciplex.
There are therefore a priori five competitive ways of synthesizing RGX. For XeCl * an excited xenon atom collides with a chlorine donor. These five reactions were all observed for various chlorine donors. [ 109 ] To quantify the proportion of produced exciplex, it is customary to define the branching ratio. It shows the rate of formation of XeCl, as denoted by Γ XeCl :
Γ XeCl = k MX / (k MX + k AI + k PI + k ET + k DE + k D )
Γ XeCl measurements were effectuated for several chlorine donors and principally for the 6s and 6p states of xenon.
Xe(6s or 6p) + RCl → products with rate constant k Q
k Q is the total rate constant and is calculated as: k Q = k MX + k AI + k PI + k ET + k DE + k D
The results for Cl 2 , CCl 4 and HCl (v = 0) are summarized in Tables 11–13. Γ XeCl is set equal to 1 by Setser Ku [ 102 ] where the chlorine donor is Cl 2 . This decision is justified by the fact that for Xe* + Cl 2 we have V II > V I > V III , which according to Simons [ 107 ] fixes an unlikely channel for the excitation transfer.
A first analysis of Tables 11-13 shows that the results are in good agreement when several measurements were made for the same reaction. We find that most collisions had their rate constants measured only once. Moreover, with rare exceptions, these determinations for K Q and Γ XeCl are limited to the lowest excited states of atomic xenon. This shows the need for new measures to confirm the available experimental results and estimate the role of other states that do not fail to form if one makes use of, as for the lasers, non-selective modes of excitation.
An important result for XeCl lasers is evident in an initial analysis. Xe(6s) + HCl (v = 0) does not produce XeCl. However, according to the estimates of Kannari et al. [ 118 ] 5% of exciplex synthesis occurs through the harpoon reaction. In addition, Xe(6p) states produce 2.5% of this amount.
Molecular chlorine reacts efficiently with these xenon states. Since Cl 2 is formed in gaseous mixtures (Figure 1), this reaction is important in the kinetics of XeCl lasers.
Reaction with CCl 4 is faster than Cl 2 by an order of magnitude, but it is still effective. This reaction is important in the kinetics of Xe 2 lasers.
If the chlorine donor is HCl, the situation is more complex. Two situations are apparent:
According to other authors, the set of vibrational levels are taken into account. And for V ≥ 1, Kannari et al. [ 121 ] proposed a synthesis rate constant of 5.6 × 10 −10 cm 3 s −1 and Γ XeCl = 26%. Experiments are necessary to clarify this aspect of laser kinetics . [ 118 ]
The synthetic reactions of XeCl are generally more effective than the 6s state. This applies for the three chlorine donors indicated graphically in tables 11, 12, and 13.
The rate constants are twice faster for chlorine than for HCl and CCl 4 .
For HCl, the situation is different from the previous case. If the total rate constants are of the same order of magnitude as those of the 6s states, the branching ratios Γ XeCl are high. The result explains the forecast by Kannari et al. [ 118 ] regarding the effectiveness of the rate of synthesis of XeCl * from Xe(6p).
With reference to the potential curves of Figure 3, the potential curves of V( Xe** + RX) and V( Xe + + RX − ) intersect at a greater internuclear distance than 6s states in a region of strong interactions. [ 115 ] This explains why the production of XeCl is more effective after the intersection than in the 6s states [ 102 ] [ 115 ] irrespective of the chlorine donor, as seen for Cl 2 , HCl, CCl 4 , and also for chlorofluorocarbons [ 122 ] in the states 6p[1/2] 0 and 6p[3/2] 2 .
Competitive reactions occur. One of them has been experimentally observed and quantified – the collisional relaxation induced by HCl: [ 123 ]
Xe(6p[3/2] 2 ) + HCl → Xe(6s[5/2] 2 0 ) + HCl with rate constant k a or k a = 4.3 × 10 −11 cm 3 s −1 .
This represents only 6% of the value of k Q from table 12 for the (6p[3/2] 2 ) state. As the proportions of exciplex synthesis is placed at 60%, one should conclude that there are other important competitive processes at play.
The summarized results in Table 12 relate to HCl (v=0). For 6p states, the role of vibrational excitation of HCl in the kinetics of XeCl formation is poorly understood. Some authors argue for rate constants neighboring state v=0 if HCl is vibrationally excited, but this results are based on analogies. An experimental clarification is therefore needed. The rate constant for v=1 is placed at 5.6 × 10 −10 cm 3 s −1 . [ 116 ] The same value is used for v=2. [ 120 ] Kannari et al. [ 121 ] is still not likely to reduce the different vibrational levels of HCl and for v≥1, 8.2 × 10 −10 cm 3 s −1 is proposed.
Experiments conducted with Cl 2 show that the effectiveness of XeCl formation increases with the excitation energy of the xenon atom; the rate constant of synthesis is multiplied by three when one goes beyond the 6s states to the 7p states (table 11).
The rate of XeCl * synthesis increases by an order of magnitude when one goes beyond the 6s states to the 6p states when CCl 4 (table 13) is utilized.
HCL is ambiguous. An examination of Table 12 shows that the increase in k Q does not appear to increase significantly with the xenon excitation. So far, no measurements go beyond the 5d[3/2] state that is roughly of the same energy as the 6p state. The rate of synthesis also seems very effective from the 7s[3/2] states [ 70 ] without there being any known numerical value. The available information does not support assuming a more efficient rate of synthesis of the exciplex as the excitation of xenon gradually increases. Indeed, for the state 5d[5/2] 3 0 , there is only an excitation with a reaction rate constant of 3.2 × 10 −12 cm 3 s −1 : [ 123 ]
Xe(5d[5/2] 2 0 ) + HCl → Xe(6p[3/2] 2 ) + HCl
Also, the Rydberg states do not appear to have produced XeCl. The observed reactions for Xe(31f) [ 124 ] are the following:
Xe(31f) + HCl(J) → Xe(31l) + HCl(J) (α)
Xe(31f) + HCl(J) → Xe(nl) + HCl(J-1) if J≤5 (β)
Xe(31f) + HCl(J) → Xe + + e − + HCl(J-1) if J>5 (γ)
The total rate constant is k T = (11.3 ± 3.0) × 10 –7 cm 3 s −1 , divided into the following:
k α = (5.5 ± 2.5) × 10 –7 cm 3 s −1 (l-changing)
k β = (4.8 ± 2.4) × 10 –7 cm 3 s −1 (n-changing)
k γ = (0.9 ± 0.4) × 10 –7 cm 3 s −1 (ionisation)
Note that the reaction (γ) produces an important XeCl precursor, namely Xe + .
Harpoon reactions play an important role in laser kinetics.
For Xe 2 Cl lasers, the situation is simple when reacted with CCl 4 . For the XeCl laser, the harpooning kinetics is more complex. Despite its weak proportion in a gaseous mixture, Cl 2 is produced much effectively from the exciplex through harpooning. The 6s states do not come into play in the production of XeCl * to the extent that they give rise to collisions with molecules of vibrationally excited HCl.
The kinetics of the vibrational excitation of HCl is therefore fundamental. At least the first six levels of vibration should be taken into consideration in order to build a satisfactory model. [ 125 ] [ 126 ] [ 127 ] [ 128 ] This vibrational excitation is produced by the following electrons:
HCl(v) + e − → HCl(v’) + e − (EV) with rate constant K .
The rate constants of (EV) were measured for the following transitions: v=0→v’=1, v=0→v’=2, v=1→ v’=2 et v=2→v’=3. An empirical law can then be proposed: [ 127 ]
K v→v+1 = v K 0→1
K v→v+2 = v K 0→2
The values for K are dependent on the electron energy distribution as shown in Figure 4.
In the harpoon reactions, the rate of synthesis of the B state with respect to that of the C state is included between 1 and 2 whatever the nature of the rare gas halide. [ 58 ] Nevertheless, one notices a clear increase in the proportion of state B with respect to state C when pressure increases. [ 96 ] This relation is also strongly influenced by the nature of the chlorine donor. It is 1.2 for CCl 4 [ 96 ] and 1.3 for Cl 2 . [ 58 ] The excitation state of xenon is important. For the case of Cl 2 , it was observed [ 112 ] that the rate of synthesis of the B state could be five times higher than the C state if Xe(6p[1/2] 0 ) takes part in the reaction than if they in strongly excited states.
Other reactions are involved in the reactive collisions between neutral species but they play a negligible role.
It is difficult to find reactions involving the molecules of xenon and HCL in published literature.
Lorents [ 70 ] only measured the rate constant of decomposition of Xe 2 * by HCl as (8.2 ± 0.8) × 10 –10 cm 3 s −1 without stating the resulting products.
In contrast, Bibinov et Vinogradov [ 108 ] observed the following reaction with Cl 2 :
Xe 2 * + Cl 2 → XeCl * + Cl + Xe
Exciplex synthesis was by harpooning. The rate constant was estimated at 7.1 × 10 −10 cm 3 s −1 . [ 121 ]
Castillejo et al. [ 129 ] observed an HCl emission between 200 and 240 nm due to the B transition B( 1 Σ + ) → X ( 1 Σ + ) (see figure 5). This emission disappears with increase in the pressure of xenon and XeCl(B) appears. In other words, XeCl(B) could be synthesized by the reaction:
HCl (B 1 Σ + ) + Xe ( 1 S O ) → XeCl(B) + H
The rate constant is estimated at 5 × 10 −10 cm 3 s −1 . [ 130 ]
Another output pathway seems competitive to exciplex synthesis within the same collision which product should be:
Xe + + H + Cl + e − and the associated rate constant associated is 1 × 10 −10 cm 3 s −1 . [ 121 ]
Cl 2 is synthesized in the laser through the following reaction:
Cl * + HCl → Cl 2 * + Cl
The rate constant is 1 × 10 −10 cm 3 s −1 . [ 121 ] Exciplex synthesis occurs through the following reaction:
Xe + Cl 2 * ( 1 Σ u + ) → XeCl * + Cl with rate constant k u
The values of k u are given in table 14. The results from Zuev et al. [ 131 ] is statistically distant from the others although recent. Ignoring it, the average value should be k u = 2.6 × 10 −10 cm 3 s −1 .
A corresponding reaction could be found for the Cl 2 * (D’ 3 π 2g ) [ 108 ] state.
They are essentially produced in ternary mixtures and are of the type:
Xe** + Cl 2 + M → XeCl * + Cl + M with rate constant k c
The rate constant k c is given in table 15. Notice only the processes where M=Ar are negligible.
As for helium , there are two reactions:
Xe* + Cl + He → XeCl * + He
Xe** + Cl + He → XeCl * + He
The rate constants are respectively, 10 −27 cm 6 s −1 and 3 × 10 −27 cm 6 s −1 . [ 134 ]
There also exist data where the xenon atoms are at the ground state:
Xe + Cl + M → XeCl (X) + M where M = Ne or Xe
In both cases, the rate constant is: 1.2 × 10 −33 cm 6 s −1 . [ 135 ]
Chlorine, Cl 2 , synthesized in a gaseous mixture could induce the following reactions:
Xe + Cl 2 → XeCl 2
Xe* + Cl 2 + Xe → Xe + + Cl 2 − + Xe → ( XeCl 2 ) * + Xe [ 136 ]
As the sublimation temperature of XrCl 2 is t s = 80 °C, this molecule is synthesized at room temperature, in the solid state within the gaseous mixture. This causes a parasitic lasing phenomenon called "laser snow". [ 137 ]
Some authors have proposed increasing the temperature to make XeCl 2 sublime. It then becomes reactive and actively participates in the synthesis of XeCl * :
XeCl 2 * → XeCl * + Cl
Xe* + XeCl 2 → 2 XeCl *
The temperature increase procures two advantages: to eliminate the parasitic laser phenomenon and increase XrCl production. However, the increase should not be of much importance so that XeCl 2 does not dissociate which would destroy the preceding reaction.
In ternary mixtures, RgCl exciplexes could be synthesized, possibly leading to the formation of XeCl * through so-called displacement reactions . They have been observed when the Rg is Ar or Kr: [ 135 ] [ 138 ]
RgCl * + Xe → XeCl * + Rg with rate constant k d or k d =1.5 × 10 −10 cm 3 s −1 for Rg = Ar
Inversely, RgCl synthesis consumes the available chlorine reducing the rate of XeCl production. The laser quality may be negatively affected as was the case with krypton. [ 139 ]
This review will be limited to synthetic reactions of XeCl * , excluding ionic recombination. A second pathway exists and will be considered.
According to several authors [ 115 ] [ 140 ] [ 141 ] bimolecular reactions ( Xe + + Cl − , Xe2+ + Cl − and Rg Xe + + Cl − ) are not involved.
Ternary reactions are typically:
Xe + + Cl − + Rg → XeCl * + Rg (3)
Xe + 2 + Cl − + Rg → XeCl * + Rg + Xe (4)
RgXe + + Cl − + Rg → XeCl * + 2 Rg (5)
Xenon ions are synthesized directly in the discharge or through successive reactions that involve Rg + , Rg 2+ as well as other ionic or excited species. Figure 1 gives an example where Rg=Ne and figure 6 where Rg=He. [ 116 ] [ 119 ] [ 142 ] [ 130 ] [ 143 ] [ 144 ]
The Cl − ions are basically formed by dissociative attachment from an HCl electron: [ 32 ]
HCl(v) + e − → H + Cl − (AD)
In that same case, the rate constant (AD) depends on the energy distribution of the electrons as illustrated in Figure 4.
The third element Rg is passive chemically. It only serve to stabilize the reaction. [ 145 ] Therefore, the authors only took the recombination rates of the positive and negative ions into consideration. These vary in a significant way with the total pressure of the gaseous mixture, the buffer gas and temperature.
Reactions (3) and (4) were experimentally demonstrated for all the rare gases. Figure 7 and Figure 8 show the influence of the buffer gas and pressure on the rate of recombination of these reactions when helium and then neon are utilized as buffer gases. This rate of recombination is of the same order of magnitude in both cases, of about some 10 −6 cm 3 s −1 . Apparently the influence of temperature has only been studied for neon. (See Figure 9.) The rate of recombination α 3 in reaction (3) is at maximum at 180K for an absolute pressure of 294.2 kPa. [ 146 ] α 3 is therefore 4.2 × 10 −6 cm 3 s −1 .
The more refined analysis of reaction (4) was carried out by Bates et Morgan. [ 147 ] who found that the Monte-Carlo method , Flannery's equation and Langevin's theory can give good results only when the pressure is above 1 atm . This is the norm for lasers. The proposed "tidal" theory agrees with the experimental measurements of Mezyk et al. [ 140 ] which is evident in Figure 10. The rate of recombination α 4 for reaction (4) is of the same order of magnitude as α 3 .
Reaction (5) is only observed when Rg is neon or argon. For this reaction, the evolution of the rate of recombination α 5 in the presence of pressurized neon is shown in figure 6. Imada et al. [ 148 ] studied the influence of temperature for a fixed total pressure of 294 kPa. The maximum value of α 5 is obtained at 120K and α 5 = 7.5 × 10 −6 cm 3 s −1 .
For argon only two estimations are available at room temperature. At a pressure of 2 atm, α 5 = 2.10 −6 cm 3 s −1 [ 149 ] and at a pressure of 1 atm, α 5 is 1 × 10 −6 cm 3 s −1 . [ 65 ]
Reaction (5) does not favor a transitory complex Rg XeCl * as an intermediate stage. [ 57 ] The following reaction, therefore, plays a minor role:
Rg Xe + + Cl − + Rg → Rg XeCl * + Rg → XeCl * + 2 Rg
On the contrary, the principal synthetic pathway is given by:
Rg Xe + + Cl − + Rg → 2 Rg + Xe + + Cl − → XeCl * + 2Rg
Kannari et al. . [ 142 ] estimated the contribution of each of the three recombination and harpooning reactions for three types of mixtures. The results are shown in Table 16. Reaction (3) provides the bulk of the exciplex molecules and generally the harpooning reactions play a secondary role. When helium is used, in contrast, the harpooning reactions contributes about 10–15% of XeCl * synthesis. [ 144 ] [ 150 ] Other authors only estimate this contribution at 1% when the ionic pathway is involved. [ 125 ] These theoretical conclusions are confirmed by experimental methods for the generality of the buffer gases and for other chlorine donors. [ 144 ] [ 151 ] The "harpoon" reactions, notwithstanding, are important despite their low contributions. These harpoon reactions are the reactions which are set in motion after the first excitation. Ionic recombinations, which then provide the bulk of the exciplex molecules, kick off 20 ns later. [ 144 ]
In table 16, the column named "others" shows 5.8% for neon, meaning that other recombination pathways are possible.
Xe3+ ions are synthesized in the gaseous mixtures used in lasers. These ions react with Cl -10− in order to produce XeCl. Nevertheless, this reaction is only a little contribution to the kinetics of the laser. [ 152 ]
Xe+* ions react with Cl − in order to produce XeCl * . [ 15 ] [ 153 ] Alekhin et al. [ 153 ] have also synthesized XeCl * using NaCl vapors. XeCl * is the product of the lowest vibrational states (v≤20) using highly excited Xe* ions in a bimolecular reaction. The rate of synthesis is estimated to be between 2 × 10 −10 and 1 × 10 −9 cm 3 s −1 . A corresponding reaction is proposed using HCl. [ 15 ] This conclusion is based on the presence of the states which are responsible for the third continuum of xenon – only Xe2+ ions, since XeCl * is not produced. [ 146 ] [ 148 ] On the contrary, Xe* ion participation in the reaction is compatible with the observations of other authors. Several authors [ 144 ] [ 150 ] [ 154 ] have confirmed the presence of Xe* ions (6s 4 P 3/2 ) in the laser mixtures. Their concentration is a thousand times greater than that of Xe* ions in the harpoon reaction. [ 125 ] On the other hand, the concentration of these ions and that of XeCl * and Cl − as a factor of time is not incompatible with the synthesis of exciplex molecules using Xe + . The beginning of the decline in Xe+* and Cl − is related to an increasing acceleration of the rate of synthesis of XeCl * . The distribution during harpoon reactions between states B and C occurs in random proportions in experimental conditions.
The first estimate of the ionic pathways was made by Tysone and Hoffman [ 155 ] who suggested 76% for states B and 24% for states C. Successively, the buffer gases are neon, argon and krypton. Ohwa and Kushner [ 156 ] published similar values: 77% for states B and 23% for states C. They used a quaternary mixture containing a buffer gas (using neon) from hydrogen, H 2 .
A recent and more detailed study was conducted by Tsuji et al. [ 141 ] in a mixture of helium as buffer gas. They found that:
– States D are especially formed from Xe + ion, ( 2 P 1/2 ) ;
– States B and C are exclusively produced from Xe + ion ( 2 P 3/2 ) in the following proportions: States B – 62.6% and States C – 38.4%. The rate of production of XeCl * is 98%. [ 157 ] There is then few competing reactions.
In laboratory experiments, the number of the Xe + ( 2 P 1/2 ) and Xe + ( 2 P 3/2 ) states are the same. In addition, the rate constants of reaction (3) relative to these two states of xenon are similar. However, under these conditions, the number of states D formed is very low with respect to the number of states B and C. The rate of XeCl(D) formation with respect to XeCl(B, C) is estimated at 0.033±0.006. The faster dissociation of [ Xe + ( 2 P 1/2 ) Cl − ] * with respect to that of [ Xe + ( 2 P 3/2 ) Cl − ] * is responsible for this situation.
The corresponding spectra demonstrated in Figure 11 was observed by virtually all authors who studied mixtures that were based on xenon and a chlorine donor.
Two theoretical studies have enabled identification of the emission spectra. [ 42 ] [ 49 ] Five transitions have heightened intensities that correspond to ΔΩ = 0 i.e., a parallel polarization to the internuclear axis. The starting states are always ionic and the product states are covalent. The characteristics of these emissions are as shown in Table 17.
The most probable UV transitions are the B→X and D→X. They have the Σ→Σ type. The other transitions, B→A, C→A and D→A, have the nature Π→Π and are much less probable. [ 73 ]
Other theoretically weaker transitions have not yet resulted in an observation with the exception of Hay and Dunning, [ 49 ] who made provisions for four transitions that are perpendicularly polarized at the internuclear axis; in other words, with ΔΩ = ±1. Only Ewing and Brau [ 89 ] noted an emission centered at 425 nm attributed to a 2 Σ→ 2 Π transition. Finally, Krauss [ 73 ] suggested the possibility of an emission of the D→B type whose transition period is itself very weak. Table 6 places this at 931 nm.
The principal emissions were observed and reported as in Table 17.
The B→X line is observed at 308 nm (Figure 11) while the theoretical prediction of its existence was clearly weak. This is the narrowest emission and the final state shows a somewhat shallow potential well. Just like the rare gas halides, this emission has the strongest transition period. That is why it is the preferred emission in XeCl lasers. [ 4 ]
Experimentally, the (C→A) and (B→A) lines overlap, [ 59 ] producing a continuum centered at 345 nm, often of low amplitude as can be observed in Figure 11. The width of the emission depends on the transition tending to a strongly repulsive state. Koltz et al. placed this continuum at between 312 and 460 nm. [ 50 ] The weak observed intensities are attributed to the weakness of the probabilities of the transition of the two emissions opposite that of the B→X and by the small amounts of states C formed with respect to states B as was previously seen. Other authors have called attention to the absorption phenomena of molecule Xe 2 Cl at this wavelength. [ 158 ] According to Kannari et al. , reaction (3) is the principal pathway for synthesis of states B and C. [ 142 ] Tsuji et al. estimated the proportions of states B and C formed: 38% for state C and 62% state B. [ 141 ] The value of the transition probabilities (theoretical value of I B→A /I B→X = 0.07; experimental value of 0.05), [ 50 ] so the contribution of (B→A) emission is about 10%. Several authors [ 6 ] [ 59 ] [ 159 ] claimed that a laser based on the 345 nm emission could be developed, especially at pressures of about 10 atmospheres when states B and C are thermalized. Meanwhile, no concrete result had been discovered as of 2014.
The (D→X) transition centered at 235.5 nm has not been systematically observed. The corresponding line appears weak as in the case in Figure 12. Its optical width is similar to that of (B→X) emission because it leads to the same weakly bound state of X. [ 53 ] In contrast, the relative intensity of the (B→X) and (D→X) emissions considerably vary from one author to the other: I D→X /I B→X = 1/3 by Shuker, [ 53 ] 1/25 to 1/50 by Sur et al. [ 81 ] and 0.14 by Taylor et al. . [ 160 ] The latter authors noted that the relation is pressure-independent. It remains unlikely that a laser could be developed using this transition as Shuker had predicted. [ 53 ]
The spectra did not show any D→A emission. Nevertheless, Hassal et Ballik [ 100 ] saw a line at 246 nm with very weak intensity (figure 12) without attributing it to the transition under consideration.
State D emissions are negligible for XeCl spectroscopy. Attributing the absence of D→A as for D→B to the weakly associated transition probability, [ 42 ] [ 49 ] [ 73 ] the same cannot be said for D→X. From Table 17, the D→X emission should be of lesser intensity than B→X. In this case, the possible explanation could be due to the weak production of state D, either by the ionic pathway [ 141 ] or by the harpoon reaction using states Xe( 3 P). [ 97 ] The principal path of XeCl * synthesis is reaction (3) and the relation of the number of states B to that of state D is 0.053. From Table 17, it is likely that state D will de-excite exclusively towards state X. Table 17's transition probabilities show I D→X /I B→X ≈6.2%, with results of the order of magnitude of Sur et al. [ 81 ] and not far from that of Taylor et al. . [ 160 ]
These emissions are more or less degraded for short wavelengths as the emission spectrum of the (B→X) line shows in figure 13. A corresponding oscillation phenomenon with the same wavelength was observed in the absorption spectra. [ 52 ] Besides, the (D→X) emission has the same line structure as (B→X). [ 81 ]
The width and oscillatory nature of these lines are bound to the existence of transitions arising from high vibrational levels of excited radiative states. [ 50 ] [ 74 ] [ 92 ] The vibrational excitation is a result of the energy left after exciplex molecule formation. This energy depends on both the state of the xenon atom/ion involved in the reaction and the halogen donor. [ 58 ] [ 74 ] [ 115 ] For the 345 nm emission, the transitions at a high vibrational level are more dispersed towards the red region for C→A 3/2 than for B→A 1/2 because the repulsive barrier of A 3/2 is steeper and closer to the higher state of the emission than is A 1/2 . [ 74 ]
The oscillatory nature of these spectra tends to disappear with an increase of pressure, showing only the peaks arising from the v≤2 level when the pressure is above 1 atm. This shows that the vibrational relaxation effectively depopulates the highest vibrational levels. [ 10 ] [ 92 ] On the other hand, the disappearance of the elevated levels is faster for state B than for state C because state C has a much longer lifetime. [ 74 ] The vibrational relaxation of states B and C play an important role in the chemical kinetics of XeCl lasers.
Beyond 5 atm, these lines increase in width, possibly due to collisional enlargement induced by rays or due to the entire rotational structure. [ 161 ]
The isotopic effects are negligible for xenon but marked for chlorine. The vibrational lines associated with the heaviest isotope 37 Cl are lightly displaced towards the greatest wavelengths. For example, the gap reads 1.51Å for the 4-0 line of B→X. [ 55 ]
Values for states B, C and D are shown in Table 18 for the vibrational level v=0. These are states B and C which have resulted in more determinations.
In state B, two values are statistically distant from the others. [ 67 ] [ 162 ] They correspond to the oldest measurements. Without taking them into account, the confidence interval obtained in ns is: 8<τ B <12.3.
For state C, the dispersion is more important. Grieneisen et al.' s determination [ 67 ] is still statistically distant from the others as well as the two theoretical values [ 42 ] [ 49 ] along with a measurement obtained at the solid state. [ 76 ] When the above is disregarded, the confidence interval, in ns, then becomes: 129.1<τ C <135.9.
Using average values, the relation τ B /τ C is 0.0764. It is adequately comparable with a direct measure which is 0.087 ± 0.009. [ 64 ] This relation is important because it plays an important role in the vibrational relaxation of states B and C.
A systematic study of the lifetimes of several vibrational levels (v≤136) of states B and C was conducted as reported in Table 19. [ 163 ]
Lifetimes increase by a factor of 4 when v goes from 0 to 100. A graphical extrapolation of the data relative to state B is shown in Figure 14.
For state D, only three determinations are relatively close to one another. At the gaseous state, Shuker [ 53 ] noted that D→X emission has a time-based dependence similar to B→X emission, which is in line with the previous magnitudes as the lifetime of the B state is of the order of 10 ns. However, other measures are necessary to precisely value τ D .
The influences of xenon and HCl will be discussed first, followed by the role of the diverse buffer gases and of the chlorine donors.
The only process of destruction of states B and C of XeCl, other than the radiative process, which has been proved is:
XeCl * + HCl → Other products and not XeCl (6) with rate constant of k H
XeCl * + Xe → Other products and not XeCl (7) with rate constant of k X
XeCl * + 2 Xe → Other products and not XeCl and Xe 2 Cl or → Xe 2 Cl * + Xe (8) with rate constant of k DX
XeCl * + Xe + HCl → Other products and not XeCl (9) with rate constant of k M
XeCl * + e − → Xe + Cl + e − (10) with rate constant of k e
As of 2014 no result had been found for state D.
The values obtained for states B and C are collected in Table 20. The authors assume that the reaction rates are identical for the two states.
Reaction (9) has been observed only once, recently. [ 16 ] Comparison data are therefore not available. In contrast, the other reactions have been repeatedly observed and quantified.
For k H , three measures are statistically distant from the others. [ 16 ] [ 155 ] [ 164 ] The last (older) two are superior to the others. The first, a recent measure, is the only experiment which proved process (9) which had been neglected. Measurements made by Rives et al. , [ 16 ] k H must be multiplied by 2 which puts them at the same level as the other values. Taking reaction (9) into account, the set of values of k H must be revised downward except for Rives et al. . [ 16 ] A confidence interval is difficult to obtain in these conditions.
For k X , a statistical analysis is very difficult because of the high dispersion of significant absolute values of doubled uncertainties. Lorents [ 70 ] provided only an upper limit. Rives et al. [ 16 ] results leave open to question whether this process is computable, considering its weak rate constant. Statistically, k X , should not surpass 6.12 × 10 −12 cm 3 s −1 . [ 61 ] One other (old) measure, [ 164 ] had already provided an erroneous value for k H . Another measure [ 61 ] was strongly revised downwards six years later. [ 62 ]
Reaction (8) which does not lead to the production of Xe 2 Cl * is of negligible importance. [ 62 ] [ 111 ] The measurements given for k DX are well dispersed and the confidence interval contains only three values. [ 16 ] [ 162 ] [ 165 ] Two of the excluded measurements are of doubtful estimations, [ 135 ] [ 168 ] while the others are correspondingly direct measures [ 62 ] [ 70 ] [ 71 ] [ 131 ] [ 155 ] that provided good results. Hanging over k DX is a great uncertainty, but the average value is representative of the overall results, that is, 9.1 × 10 −31 cm 6 s −1 .
The measured values of k e display a strong dispersion. Only four values are statistically close [ 119 ] [ 130 ] [ 155 ] [ 166 ] The average value of 9.6 × 10 −8 cm 3 s −1 is relatively close to the only direct measure. [ 166 ]
Lou [ 171 ] also suggested other products for reaction (10):
XeCl * + e − → Xe + + Cl − (k e1 = 1.8 × 10 −7 cm 3 s −1 ) or → Xe* + Cl + e − (k e2 = 1.2 × 10 −7 cm 3 s −1 )
Some differences were noticed for reactions of type (6) accounting for the vibrational levels of the collision partners:
XeCl * (v=0) + HCl(v=1) → Xe + HCl + Cl + Cl (6a) with rate constant of k Ha
XeCl * (v=0) + HCl(v=2) → Xe + HCl + Cl + Cl (6b) with rate constant of k Hb
XeCl(B,C;v≠0) + HCl(v=0) → Other products and not XeCl (6c) with rate constant of k Hc
The values of the rate constants are summarized in Table 21. They are well dispersed and do not correspond to any direct measurement. These values are specifically based on analogous estimations.
Reactions that correspond to reactions (6) and (7) are evident when XeCl is in the ground state of X(v=0). These phenomena affect laser performance and are therefore important. The rate constants are assembled in Table 22. These rates do not vary with the vibrational level of the colliding molecules. Only one direct measurement exists; [ 30 ] the others are estimates.
The addition of a third gas in significant quantities also affects the kinetics of disappearance of XeCl(B,C). It induces reactions which are similar to those produced by xenon:
Double collision (11) : XeCl(B,C) + Rg → Xe + Cl + Rg rate constant of k 11
Triple collision (12) : XeCl(B,C) + 2 Rg → Xe + Cl + 2 Rg rate constant of k 12
Mixed triple collision (13) : XeCl(B,C) + Xe + Rg → 2 Xe + Cl + Rg rate constant of k 13
The rate constants of the three processes are grouped in tables 23–25.
Reactions (11) and (13) are always important while reaction (12) has a negligible contribution. The results are greatly dispersed. Differences may reach orders of magnitude. Four references [ 62 ] [ 70 ] [ 164 ] [ 175 ] have resulted in direct measurements of reaction rates. Others are estimates. These are based on correspondences and only indicative. No information is available for krypton.
Competitive reactions are evident for the totality of these reactions.
The reactions of (11) are competitive for displacement reactions. In this case, the products are RgCl(B). They have only been observed in the case where Rg = Kr: [ 138 ]
XeCl * + Kr → KrCl + Xe
The rate constant is 0.7 × 10 −9 cm 3 s −1 . [ 139 ] Therefore, this reaction is more effective than quenching. It plays an important role in laser kinetics. It is also as fast as the process of creation of XeCl * by harpoon reaction. Table 20 concerns one of the principal pathways of destruction of the exciplex molecule.
For Brashears et al. , [ 177 ] it is possible to obtain the triatomic complex, Rg XeCl * , as product. This is a competitive reaction when collisions that produce dissociated atoms occur. Emissions of KrXeCl at 370 nm have been observed, [ 177 ] along with ArXeCl at 326 nm [ 178 ] and NeXeCl at 434 nm. [ 91 ] The rate constants have not been measured, except for Rg=Kr, which is 9 × 10 −33 cm 6 s −1 . [ 62 ]
However, the creation of ArXeCl seems to be preferential by a competitive reaction (13):
Xe* + Ar + Xe → Ar XeCl *
The rate constant is 4 × 10 −30 cm 6 s −1 . [ 9 ] It is then of the same order of magnitude as (13).
However, the synthesis of the Xe 2 Cl * trimer is the most frequent competitive reaction of (13).
For helium, Baginskii et al. [ 174 ] provided a solution using Xe * 2 + Cl + He of which the rate constant is 1.5 × 10 −31 cm 6 s −1 .
A corresponding reaction for (11) was demonstrated for XeCl at the ground state. The rate constants are summarized in Table 26. The measurements are greatly dispersed (only one is direct) and data on krypton are absent. [ 30 ] The others are based, more or less, on estimates. Amongst these, one [ 179 ] is statistically distant from the others. On using neon, the rate constant for XeCl(X, v=1) has been estimated as 1 × 10 −11 cm 3 s −1 . [ 156 ]
The main reactions are those corresponding to reaction (6):
XeCl * + RCl → Other products and not XeCl (14) rate constant of k R
The values of the rate constants through RCl = Cl 2 or CCl 4 are summarized in table 27. The three chlorine donors studied (HCl, Cl 2 and CCl 4 ) have rates of quenching of the same order of magnitude.
All the measurements in Table 27 were experimental. For chlorine, only one (recent) value is statistically distant from the others. [ 61 ] The absolute difference is not very great versus the other determinations. An average value for k R for chlorine is 5 × 10 −10 cm 3 s −1 , which is very close to a measure relative to CCl 4 .
For chlorine, Grieneisen et al. [ 67 ] pointed to two different values for the rate constant for states B and C. They were respectively estimated as (8.8 ± 1.5) × 10 −10 cm 3 s −1 and (3.3 ± 0.3) × 10 −10 cm 3 s −1 . This is a direct measure of the process of destruction through binary collision with Cl 2 that includes all the phenomena and not just quenching. As states B and C are energetically close, collisional coupling is acting on the two states. A similar result for xenon seems to reinforce this hypothesis.
Some atoms of free chlorine exist in the conditions which matter for lasers. The following quenching reactions is provided for:
XeCl * + Cl → Xe + 2Cl
Two authors have estimated the rate constant: 1.4 × 10 −9 cm 3 s −1 [ 119 ] and 8 × 10 −10 cm 3 s −1 . [ 135 ]
The presence of impurities, I m , such as the chlorocarbons (the consequence of corrosion [ 181 ] ), NO, CO 2 , O 2 , CO, N 2 O, H 2 O could have an effect on the chemical kinetics of disappearance of XeCl * since binary collisions I m – XeCl * possess rate constants of the order of 3 × 10 −10 cm 3 s −1 , [ 167 ] thus making them comparable to the XeCl * + RCl reaction. However, given the usual impurity levels, the reaction frequencies are negligible. A practical solution has been proposed to eliminate them that involves introducing 1 torr of H 2 . [ 181 ]
The weak energetic gap (about 100 cm −1 ) between these two states (Table 2), suggests that a coupling was produced. However, this result was not exactly quantified nor was it later confirmed. No collisional coupling phenomenon induced by chlorine has been detected recently.
The role of electrons is also not well known in the coupling process. According to Finn et al. , [ 164 ] its role is negligible, although Johnson et al. [ 135 ] gave an elevated rate constant. This rate is the same, according to them, for B to C and C to B transfers. The energy difference between B and C is not zero (see Table 2). The reaction rate was estimated at 2 × 10 −8 cm 3 s −1 .
These couplings are demonstrated through binary collisions using an atom of xenon:
XeCl(B ; v’ = 0) + Xe → XeCl(C ; v’ = 0,1) + Xe (15) rate constant of k BC
XeCl(C ; v’ = 0, 1) + Xe → XeCl(B ; v’ = 0) + Xe (16) rate constant of k CB
The measurements of rate constants are not very consistent as can be seen in Table 28.
r
In experiments by Inoue et al. , [ 61 ] the vibrational levels v’=0.1 were directly excited. This is not the case in other experiments. [ 16 ] [ 71 ] The last value [ 121 ] is only a theoretical estimation based on similarities with other reactions. The energetic gap ΔE = E B – E C deduced from k CB and k BC , suggests that further information could follow. Assuming that the states E B and E C were thermalized:
k BC /k CB = exp(ΔE/kT) since the statistical weights of the two states are the same. [ 49 ]
ΔE, was also inferred by Inoue et al. [ 61 ] as 85 cm −1 , and as 119 cm −1 by Rives et al. , [ 16 ] while 22 cm −1 was the measurement given by Le Calvé et al. [ 71 ] (see Table 2). Only the first two values are values of ΔE which are compatible with 100 cm −1 , the accepted order of magnitude. A clear difference exists between these two; an order of magnitude separates the values of k BC and k CB in the two experiments. [ 16 ] [ 61 ] Grieneisen et al. [ 67 ] provided only the global rate of destruction of states B and C, in other words, quenching and coupling. For the destruction of state C, they found (15.5 ± 0.9) × 10 −12 cm 3 s −1 and for state B (10.3 ± 0.9) × 10 −12 cm 3 s −1 , which are intermediate values between those of Inoue et al. [ 61 ] and Rives et al. . [ 16 ] Recall that quenching by xenon only has a weak influence (Table 20). Inoue et al. [ 61 ] notably did not take account of reaction (9). If the same approach is taken for the results by Rives et al. , [ 16 ] the values of k BC and k CB are close to those of Inoue et al. . [ 61 ] As was explained for k x and k H , taking account of the process (9) modifies the values of the reaction rate. On this point, Rives et al. [ 16 ] is more precise than Inoue et al. . [ 61 ]
The advantage of Inoue et al.' s [ 61 ] result was in vibrational resolution, as k BC and k CB vary with the vibrational level v. For level v=70 to 130, rate constants between 15 and 20 × 10 −11 cm 3 s −1 were observed. [ 163 ] k BC and k CB seems to then grow with v.
Since most of the time XeCl(B, C) is formed with a strong vibrational excitation, knowledge of the exact estimate of the variation of k BC and k CB with v; and the chemical kinetics of the vibrational relaxation and its importance relative vis-à-vis to the coupling process are important.
Collisional coupling is produced by binary collisions with an atom of a rare gas, Rg:
XeCl(B) + Rg → XeCl(C) + Rg (17) rate constant of k BC Rg
XeCl(C) + Rg → XeCl(B) + Rg (18) rate constant of k CB Rg
Dreiling and Setser [ 163 ] provide order of magnitude values for k BC Rg and k CB Rg for a given vibrational level. The results are shown in Table 29. This shows that the rate constants increase regularly when the vibrational level, v, of XeCl * is higher and the rare gas, Rg, is heavier.
Using helium, experiments have been made at low and high pressures. [ 66 ] At high pressures, the transfer constants are of the order of (1.5 ± 0.7) × 10 −12 cm 3 s −1 and at low pressures 3.0 × 10 −11 cm 3 s −1 . A strong pressure induces a vibrational relaxation such that the values of v involved in the transfer are weak and vice versa for weak pressures. The only available direct determination for k BC He gives a value less than 3 × 10 −13 cm 3 s −1 . [ 69 ]
For neon, the values of the rate of transfer at low and high pressure are respectively, 3.0 × 10 −11 cm 3 s −1 and (0.8 ± 0.4) × 10 −12 cm 3 s −1 . [ 66 ] They are inferior to those of Table 29. The direct measurement of the rate constant k BC Ne gives a value less than 3.10 −13 cm 3 s −1 . [ 69 ] Finally, according to Ohwa, [ 156 ] the order of magnitude of the two rate of coupling constants would be 4.8 × 10 −12 cm 3 s −1 for v=4.
For argon, the results increase. At low pressures, the order of magnitude would only be 6.0 × 10 −11 cm 3 s −1 . [ 66 ] Other authors [ 65 ] published rates of transfer of 1.2 ± 0.4 × 10 −4 cm 3 s −1 for a pressure interval starting from 10 to 1000 torr. Direct measurements of k BC Ar and k CB Ar are available without specifying the vibrational levels involved: [ 50 ]
k BC Ar = 36 × 10 −4 cm 3 s −1 and k CB Ar = 21 × 10 −11 cm 3 s −1
Meanwhile, Yu et al. [ 69 ] noted a variation with temperature of k BC Ar :
k BC Ar = (4 ± 2) × 10 −12 cm 3 s −1 at 300K and k BC Ar = (2 ± 1) × 10 −12 cm 3 s −1 at 230K.
For krypton, we can only make an estimation:
k BC Kr = (4) × 10 −12 cm 3 s −1 . [ 69 ]
It is clear that the collisional coupling process induced by the rare gases are not well established. Different authors give different order of magnitudes. The uncertainty on the rate constants is therefore as important as for that of xenon. The vibrational excitation seems to play a role that is still not well defined. Direct measurements for k BC Rg and k CB Rg are not available. From the first estimations, the phenomena seem important in the kinetics of gaseous mixtures.
XeCl * is more often synthesized with strong vibrational excitation and can reach vibration quantum numbers as high as v=100. [ 182 ] This induces some vibrational relaxation that is formed by binary collision with an atom of a rare gas. [ 183 ]
Only a single measurement for xenon and level v=2 has been published.
XeCl(B; v = 2) + Xe → XeCl(B; v’ = 0.1) + Xe rate constant of k v
where k v = (2 ± 1) × 10 −10 cm 3 s −1 . [ 61 ]
Most of the known results are related to buffer gases. Yet, only Dreiling and Sester [ 163 ] completed measurements. The vibrational relaxation can be written as:
XeCl * (v) + Rg → XeCl * (v’) + Rg (19)
The orders of magnitude of k v Rg are summarized in Table 30. k v Rg increases with the vibrational level of XeCl * and heavier rare gases, Rg. Values of k v Rg are assumed to be the same for states B and C.
For helium and krypton, no comparison is available.
For neon, only the reaction with first two vibrational levels of B have been documented:
XeCl(B; v = 1) + Ne → XeCl(B ; v = 0) + Ne with rate constant of k v Ne =(0.3 to 0.5) × 10 −11 cm 3 s −1 . [ 184 ]
For argon, the values of k v Ar has been determined for v=33, 60 and 75. [ 90 ] Their values, respectively, are (17 ± 5) × 10 −11 ; (31 ± 9) × 10 −11 and (43 ± 10) × 10 −11 cm −11 . Other authors placed the figure for k v Ar between (10 and 15) × 10 −11 [ 155 ] agreeing on the order of magnitude.
The chemical kinetics due to collisional coupling of states B and C and vibrational relaxation are not well known. The few available results often disagree, although a general idea of the situation is possible. For high vibrational levels, coupling overrides the vibrational relaxation while the contrary is true for the lowest levels, [ 58 ] even if a rare gas is involved.
The various destructive processes of XeCl(B), differ in importance. A mixture optimized for lasers is used. Neon is favored over argon because the latter strongly absorbs via the Ar + 2 ion at 308 nm. [ 135 ] Therefore, a ternary mixture (Ne/Xe/HCl) is used. The total pressure is fixed at 3 atm, the respective partial pressures is 2268.6 torr, 10 torr and 1.4 torr. The rate constants are the average values of the most reliable estimates.
The results are summarized in Table 31. For reaction (19), only the lowest vibrational levels are accounted. The lower frequency of disappearance limit is 0.40 ns −1 . This process induces the highest destruction, indicating that XeCl(B) synthesized with high vibrational excitation is quickly relaxed by binary collision with neon and (probably) also by xenon. This suggests that other processes are really noticeable only after XeCl(B) is on the v=0 level, which is why reaction (17) uses the value of k BC Do relative to a low v. Once the relaxation is complete other processes take over. Depopulation by spontaneous emission is very important as well as reactions (11) and (17). These two processes lack refined measurements and determinations overall. The role of the xenon coupling is not better known but has less influence than the destruction by binary collision with HCl. Other better known processes are negligible. In particular all termolecular reactions are negligible.
Generally, Rg 2 X molecules are less stable than RgX. [ 7 ] Xe 2 Cl is of double interest. It can cause perturbations in laser XeCl performance because it absorbs well at 308 nm and enables the development of another type of laser based on an Xe 2 Cl emission.
Initial studies on the Xe 2 Cl molecule [ 33 ] [ 185 ] found:
The potential curves calculated by Huestis et al. [ 187 ] from the DIM (Diatomics In Molecules) method are presented in Figure 15.
The three lowest states are covalent and repulsive. They are correlated to XeCl(X or A) and to an atom of xenon at the ground state. The experimental value of the energy at state 1 2 Γ is 0.273 eV. [ 33 ] It is compatible with these potential curves. The following three states are ionic. The bound state 4 2 Γ is correlated to XeCl(B) + Xe; the following, 5 2 Γ, a repulsive state, is correlated to XeCl(C) + Xe.
Last and George [ 43 ] made a determination of the potential curves using another method, the DIIS (Diatomics In Ionic Systems) method without considering spin-orbital coupling. They found, like Huestis et al. [ 187 ] that the 4 2 Γ state is the lowest ionic state. At the bottom of the well, this state has the configuration of an isosceles triangle, such that the distance between the equilibrium positions of Xe and Cl is 3.23 Å. According to Adams and Chabalowski [ 42 ] the Xe–Cl distance is 3.39 Å.
Initially, the potential curves of the different states were plotted by maintaining a constant and equal Xe-Xe distance at 3.25 Å (figure 16). Last and George discovered nine states (three covalent and six ionic). The potential curves of the antisymmetric states 4 2 Γ π and 6 2 Γ π are almost coincident with the potential curves of the symmetric states 5 2 Γ and 6 2 Γ. The 3 2 Γ and 7 2 Γ states highlighted by Huestin et al. are absent since the spin-orbital coupling were not taken into account. Inversely, three states, (2 2 Γ π , 4 2 Γ π and 6 2 Γ π ) with the π symmetry, were not included in their diagrams. [ 187 ]
A second study kept the separation of Xe-Cl at 3.23 Å (figure 17).
* In 4 2 Γ π state, the molecule with isosceles triangle configuration such as the Xe-Cl and Xe-Xe distances are respectively 3.13 and 4.23 Å. The state is 0.8 eV above the 4 2 Γ state. [ 43 ] * At the ground state, 1 2 Γ forms a Van der Waals complex. It has a bond-dissociation energy of 0.075eV and a dissymmetric triangular configuration. The Xe–Cl distances are 3.23 and 4.06 Å and the Xe–Cl–Xe angle is 74.4°. [ 43 ] * The second excited state 2 2 Γ is also a Van der Waals complex. It has a symmetrical geometry and an Xe–Cl distance of 3.99 Å with an Xe–Cl–Xe angle of 68.4°. Its dissociation energy is 0.055 eV. [ 43 ]
Another way of describing Xe–Cl–Xe finds the stable state to be linear and symmetric. At the ground state, the Xe-Cl distance should be 3.24 Å and the dissociation energy 0.076 eV. An excited state could exist with a geometric distance of Xe-Cl of 3.06 Å. [ 43 ] This state, which is not shown in Figures 16 and 17, would possess an energy higher than 0.72 eV to that of the 4 2 Γ state. The bonding would be ionic.
Only an experiment conducted at the solid state [ 72 ] can be compared to these theoretical results. The special state studied was the 4 2 Γ state. The isosceles triangle structure of this state was confirmed. Three quantities can be compared with theoretical predictions. The Xe-Xe distance is measured at 3.17 Å and that of Xe-Cl at 3 Å. The agreement in values is best for the energy at the bottom of the well that was evaluated at 3.15 eV. The fundamental vibrational frequencies for Xe–Xe, is ω x = 123 cm −1 and for Xe–Cl, ω c = 180 cm −1 .
Three principal pathways of Xe 2 Cl * synthesis are energetically possible through collisions and two others through photodissociation :
Xe * 2 (A 1 Σ) + Cl 2 → Xe 2 Cl * + Cl (20)
Xe * + Xe + Rg → Xe 2 Cl * + Rg (21)
Xe 2 + + Cl − + Rg → Xe 2 Cl * + Rg (22)
XeCl * (X) + Xe + hν → Xe 2 Cl * (23)
Xe + Cl + Xe + hν → Xe 2 Cl * (24)
where Rg is a rare gas, probably xenon or a buffer gas.
The authors disagree on the relative importance of these synthetic processes. The processes depend on experimental conditions.
Reaction (20) is a very energetic harpoon reaction. It involves Xe * 2 excited state. According to Bruce et al. , [ 112 ] this is the dominant synthetic pathway. Other authors though do not share this view since they believe that this reaction is weak, [ 187 ] or indeed negligible. [ 188 ] Its rate constant has not yet been measured.
Reactions (23) and (24) were only recently discovered. [ 106 ]
According to a theoretical computation, [ 147 ] the rate of recombination α’ of the Xe + 2 and Cl − ions when Rg = Xe (reaction (22)) was, at the first instance, estimated as 1 × 10 –7 cm 3 s −1 . The same authors later revised this value downward as: α’ = 5 × 10 –8 cm 3 s −1 . [ 189 ] This result was confirmed experimentally. [ 187 ] [ 190 ] According to computations, this reaction could become important at high pressures at which Xe 2 Cl * becomes the principal reaction product, to the detriment of XeCl * (reaction (4)).
The synthesis of Xe 2 Cl * is principally through pathway (21). According to a recent study, [ 62 ] the reaction can be interpreted as the result of two successive reactions, the second reaction corresponding to a vibrational relaxation through collision using Rg:
XeCl(B,C) + Xe ↔ Xe 2 Cl *(v)
Xe 2 Cl *(v) + Rg → Xe 2 Cl * + Rg
The starting vibrational levels of Xe 2 Cl *(v) are above the limit of dissociation of the state in XeCl * + Xe.
In contrast, Yu et al. [ 69 ] believe that the formation of Xe 2 Cl * is through a triatomic complex, RgXeCl * , mainly :
XeCl * + Rg → RgXeCl * where Rg≠Xe
RgXeCl * + Xe → Xe 2 Cl * Rg
These reactions have been observed in only argon and krypton.
The second reaction is one of displacement. Another reaction is competitive to it when xenon is replaced by krypton. This quenching process should have a rate constant higher than 1 × 10 −13 cm 3 s −1 . [ 69 ] [ 177 ]
The lifetime of the RgXeCl * complex is not well known. It is estimated at 200 ns for KrXeCl [ 69 ] [ 177 ] and 40 ns for NeXeCl. [ 91 ] This interval in time is sufficient for the second collision to have a chance of being produced.
The rate constants have been measured as summarized in table 32. If Rg≠Xe, only two direct measurements have been carried out. [ 39 ] [ 62 ] The last [ 191 ] is only an evaluation.
As for xenon, notice that the totality of the k DX constants of table 20 could be taken as those of the fifth column of table 32 since k DX could be merged with reaction (21). [ 62 ]
Theoretical studies [ 158 ] [ 185 ] show that the allowed transitions are (figure 15) :
4 2 Γ → 1 2 Γ (A)
4 2 Γ → 2 2 Γ (B)
4 2 Γ → 3 2 Γ (C)
The starting states are always the same and the corresponding wavelengths, λ Th , are indicated in Table 33. They can be compared to experimental values, λ Obs .
Experimentally, Fajardo and Apkarian [ 72 ] observed two transitions (A) and (B) in the spectral domain, even while there was a significant wavelength shift. In most cases, a very large continuum (approximately 80 nm) was observed covering the three emissions. The maximum positioning oscillated between 450 and 500 nm. An example of this sort of spectrum is given in Figure 11. On computation, the limits of short wavelength emissions were evaluated at 443 nm. [ 101 ]
According to Last and George, [ 43 ] the Xe–Cl–Xe linear molecule ought to have produced an emission approaching the ground state at 321 nm and the transition moment should be elevated to 3.9 D. As of 2014, however, no experiment confirms this prediction.
At the solid state, the Xe 2 Cl * emission shifts towards the red range and is centered around 570 nm. [ 192 ] [ 193 ] A corresponding result is observed in the liquid state. [ 194 ] This phenomenon should be owed to a distortion of the potential curves arising from molecular interactions which are closest to themselves than at the gaseous state. [ citation needed ] A theoretical study [ 195 ] attributes this to the polarization of the xenon matrix by Xe 2 + Cl − and by Van der Waals forces.
Emission of Xe 2 Cl * trimer is only observed at high pressures of the rare gas (xenon or buffer gas) and fluorescence increases with the pressure of xenon. [ 33 ] These results follow because the synthetic pathway of Xe 2 Cl * is similar to that of reaction (21). Considering the values of the rate constant of reactions of type (21), the reaction frequency does not deviate in a significant way even when the rare gas pressure is close to 200 torr. Reaction (22) only takes place under pressure of several atmospheres. [ 189 ]
The only state where Xe 2 Cl is the original parent of a luminous emission is 4 2 Γ). Several determinations of its lifetime obtained at the gaseous state are summarized in Table 34. The results vary and the uncertainties involved are important. The confidence interval obtained within a threshold of 5% lies between 240 and 253 ns. Of these, four values are not included. [ 62 ] [ 80 ] [ 167 ] [ 190 ] Given the strong absolute uncertainty, another measure [ 111 ] has a common interval within the confidence interval.
Measurements realized at the solid state provide values that are yet more dispersed such as is shown in Table 35.
Beyond the radiative disexcitation, the Xe 2 Cl (4 2 Γ) state is destroyed by a double collision with RCl. In practical terms, every author agrees that double collision is the dominant destruction pathway of Xe 2 Cl when collision is involved, whatever the chlorine donor. Therefore, Xe 2 Cl * emissions are only observed at weak concentrations of RCl. [ 15 ] [ 112 ] [ 167 ] The values of the rate constants for reactions (24) are given in Table 36.
Xe 2 Cl * + RCl → Other products except Xe 2 Cl (24)
There are only two determinations for CCl 4 and these are coincident. For HCl, two values are statistically distant from others. [ 152 ] [ 198 ] Giving an explanation for this distance remains difficult. The confidence interval at a threshold of 5% is from 4 to 7 × 10 −10 cm 3 s −1 .
In the case of chlorine, Cl 2 , only one half of measurements are statistically close. [ 80 ] [ 111 ] [ 165 ] [ 167 ] Even so, this closeness is difficult to explain. Its confidence interval at the threshold of 5% varies from 3.7 to 4.5 × 10 −10 cm 3 s −1 . The three chlorine donors appear to have a corresponding influence on the collisional destruction of Xe 2 Cl * .
To estimate the rate constant of the reaction:
Xe 2 Cl * + Cl → 2 Xe + 2 Cl
The value is 1 × 10 −9 cm 3 s −1 . [ 200 ]
These are uniquely binary reactions:
Xe 2 Cl * + Rg → Other products except Xe 2 Cl (25)
The disappearance of Xe 2 Cl * by collision on a xenon atom was observed by Grieneisen et al., [ 67 ] the reaction constant was estimated at 6 × 10 −15 cm 3 s −1 . However, this reaction has not been demonstrated by other authors. [ 39 ] [ 70 ] [ 165 ] [ 197 ] [ 199 ] The upper bound of the rate constant of reaction (25) is 1 × 10 −17 cm 3 s −1 , [ 197 ] although other authors placed this limit at 4 to 7 × 10 −14 cm 3 s −1 [ 165 ] [ 199 ] or 5 × 10 −13 cm 3 s −1 . [ 39 ] The value used by Kannari et al. , [ 121 ] 8 × 10 −12 cm 3 s −1 , has no basis.
For ternary mixtures, the role of the buffer gas is not well known.
For argon, (3 ± 1) × 10 −14 cm 3 s −1 [ 39 ] and (1.5 ± 0.4) × 10 −14 cm 3 s −1 are available. [ 196 ]
For helium, 5 × 10 −13 cm 3 s −1 [ 152 ] and 3 × 10 −14 cm 3 s −1 are available. [ 119 ]
The rate of reactions of Xe 2 Cl * + e − → 2 Xe + Cl + e − (26) does not have consistent estimates. They are summarized in Table 37.
The impurities have a lesser influence in the chemical decay of Xe 2 Cl than XeCl * . [ 167 ] The bimolecular rate constants of disappearance of Im – Xe 2 Cl * are an order of magnitude lower than the relative rate constants for binary collisions ImXeCl * . Yet, for CO 2 and nitric oxide , NO, the rate constants are of the same order of magnitude, about some 10 −10 cm 3 s −1 . Impurity levels, most often low, may influence the measurements. The reaction frequencies are negligible.
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Xenon dichloride (XeCl 2 ) is a xenon compound and the only known stable chloride of xenon . The compound can be prepared by using microwave discharges towards the mixture of xenon and chlorine, and it can be isolated from a condensate trap . One experiment [ which? ] tried to use xenon, chlorine and boron trichloride to produce XeCl 2 ·BCl 3 , but only generated xenon dichloride. [ 1 ]
However, it is still doubtful whether xenon dichloride is a true compound or a Van der Waals molecule composed of a xenon atom and a chlorine molecule connected by a secondary bond . [ 2 ]
This inorganic compound –related article is a stub . You can help Wikipedia by expanding it .
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Xenon tetrachloride is an unstable [ 1 ] inorganic compound with the chemical formula XeCl 4 . Unlike other noble gas / halide compounds, it cannot be synthesized by simply combining the elements , by using a more-active halogenating agent, or by substitution of other halides on tetrahaloxenon compounds. Instead, a decay technique can be used, starting with K 129 ICl 4 . The iodine-129 atom of the 129 ICl – 4 covalent cluster is radioactive and undergoes beta decay to become xenon-129 . [ 2 ] [ 3 ] The resulting XeCl 4 molecule has a square planar molecular geometry analogous to xenon tetrafluoride . [ 4 ]
Alternately, the product can be obtained by subjecting the elements to an electric discharge. [ 1 ]
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Xenon difluoride is a powerful fluorinating agent with the chemical formula XeF 2 , and one of the most stable xenon compounds . Like most covalent inorganic fluorides it is moisture-sensitive. It gradually decomposes on contact with water vapor , but is otherwise stable in storage. Xenon difluoride is a dense, colourless crystalline solid.
It has a nauseating odour and low vapor pressure . [ 6 ]
Xenon difluoride is a linear molecule with an Xe–F bond length of 197.73 ± 0.15 pm in the vapor stage, and 200 pm in the solid phase. The packing arrangement in solid XeF 2 shows that the fluorine atoms of neighbouring molecules avoid the equatorial region of each XeF 2 molecule. This agrees with the prediction of VSEPR theory, which predicts that there are 3 pairs of non-bonding electrons around the equatorial region of the xenon atom. [ 1 ]
At high pressures, novel, non-molecular forms of xenon difluoride can be obtained. Under a pressure of ~50 GPa , XeF 2 transforms into a semiconductor consisting of XeF 4 units linked in a two-dimensional structure, like graphite . At even higher pressures, above 70 GPa, it becomes metallic, forming a three-dimensional structure containing XeF 8 units. [ 7 ] However, a recent theoretical study has cast doubt on these experimental results. [ 8 ]
The Xe–F bonds are weak. XeF 2 has a total bond energy of 267.8 kJ/mol (64.0 kcal/mol), with first and second bond energies of 184.1 kJ/mol (44.0 kcal/mol) and 83.68 kJ/mol (20.00 kcal/mol), respectively. However, XeF 2 is much more robust than KrF 2 , which has a total bond energy of only 92.05 kJ/mol (22.00 kcal/mol). [ 9 ]
Synthesis proceeds by the simple reaction:
The reaction needs heat, irradiation, or an electrical discharge. The product is a solid. It is purified by fractional distillation or selective condensation using a vacuum line. [ 10 ]
The first published report of XeF 2 was in October 1962 by Chernick, et al. [ 11 ] However, though published later, [ 12 ] XeF 2 was probably first created by Rudolf Hoppe at the University of Münster , Germany, in early 1962, by reacting fluorine and xenon gas mixtures in an electrical discharge. [ 13 ] Shortly after these reports, Weeks, Chernick, and Matheson of Argonne National Laboratory reported the synthesis of XeF 2 using an all-nickel system with transparent alumina windows, in which equal parts xenon and fluorine gases react at low pressure upon irradiation by an ultraviolet source to give XeF 2 . [ 14 ] Williamson reported that the reaction works equally well at atmospheric pressure in a dry Pyrex glass bulb using sunlight as a source. It was noted that the synthesis worked even on cloudy days. [ 15 ]
In the previous syntheses the fluorine gas reactant had been purified to remove hydrogen fluoride . Šmalc and Lutar found that if this step is skipped the reaction rate proceeds at four times the original rate. [ 16 ]
In 1965, it was also synthesized by reacting xenon gas with dioxygen difluoride . [ 17 ]
XeF 2 is soluble in solvents such as BrF 5 , BrF 3 , IF 5 , anhydrous hydrogen fluoride , and acetonitrile , without reduction or oxidation. Solubility in hydrogen fluoride is high, at 167 g per 100 g HF at 29.95 °C. [ 1 ]
Other xenon compounds may be derived from xenon difluoride. The unstable organoxenon compound Xe(CF 3 ) 2 can be made by irradiating hexafluoroethane to generate CF • 3 radicals and passing the gas over XeF 2 . The resulting waxy white solid decomposes completely within 4 hours at room temperature. [ 18 ]
The XeF + cation is formed by combining xenon difluoride with a strong fluoride acceptor, such as an excess of liquid antimony pentafluoride ( SbF 5 ):
Adding xenon gas to this pale yellow solution at a pressure of 2–3 atmospheres produces a green solution containing the paramagnetic Xe + 2 ion, [ 19 ] which contains a Xe−Xe bond: ("apf" denotes solution in liquid SbF 5 )
This reaction is reversible; removing xenon gas from the solution causes the Xe + 2 ion to revert to xenon gas and XeF + , and the color of the solution returns to a pale yellow. [ 20 ]
In the presence of liquid HF , dark green crystals can be precipitated from the green solution at −30 °C:
X-ray crystallography indicates that the Xe–Xe bond length in this compound is 309 pm , indicating a very weak bond. [ 18 ] The Xe + 2 ion is isoelectronic with the I − 2 ion, which is also dark green. [ 21 ] [ 22 ]
Bonding in the XeF 2 molecule is adequately described by the three-center four-electron bond model.
XeF 2 can act as a ligand in coordination complexes of metals. [ 1 ] For example, in HF solution:
Crystallographic analysis shows that the magnesium atom is coordinated to 6 fluorine atoms. Four of the fluorine atoms are attributed to the four xenon difluoride ligands while the other two are a pair of cis - AsF − 6 ligands. [ 23 ]
A similar reaction is:
In the crystal structure of this product the magnesium atom is octahedrally-coordinated and the XeF 2 ligands are axial while the AsF − 6 ligands are equatorial.
Many such reactions with products of the form [M x (XeF 2 ) n ](AF 6 ) x have been observed, where M can be calcium , strontium , barium , lead , silver , lanthanum , or neodymium and A can be arsenic , antimony or phosphorus . Some of these compounds feature extraordinarily high coordination numbers at the metal center. [ 24 ]
In 2004, results of synthesis of a solvate where part of cationic centers were coordinated solely by XeF 2 fluorine atoms were published. [ 25 ] Reaction can be written as:
This reaction requires a large excess of xenon difluoride. The structure of the salt is such that half of the Ca 2+ ions are coordinated by fluorine atoms from xenon difluoride, while the other Ca 2+ ions are coordinated by both XeF 2 and AsF − 6 .
Xenon difluoride is a strong fluorinating and oxidizing agent. [ 26 ] [ 27 ] With fluoride ion acceptors, it forms XeF + and Xe 2 F + 3 species which are even more powerful fluorinators. [ 1 ]
Among the fluorination reactions that xenon difluoride undergoes are:
XeF 2 is selective about which atom it fluorinates, making it a useful reagent for fluorinating heteroatoms without touching other substituents in organic compounds. For example, it fluorinates the arsenic atom in trimethylarsine , but leaves the methyl groups untouched: [ 30 ]
XeF 2 can similarly be used to prepare N -fluoroammonium salts, useful as fluorine transfer reagents in organic synthesis (e.g., Selectfluor ), from the corresponding tertiary amine: [ 31 ]
XeF 2 will also oxidatively decarboxylate carboxylic acids to the corresponding fluoroalkanes : [ 32 ] [ 33 ]
Silicon tetrafluoride has been found to act as a catalyst in fluorination by XeF 2 . [ 34 ]
Xenon difluoride is also used as an isotropic gaseous etchant for silicon , particularly in the production of microelectromechanical systems (MEMS), as first demonstrated in 1995. [ 35 ] Commercial systems use pulse etching with an expansion chamber [ 36 ] Brazzle, Dokmeci, et al. describe this process: [ 37 ]
The mechanism of the etch is as follows. First, the XeF 2 adsorbs and dissociates to xenon and fluorine atoms on the surface of silicon. Fluorine is the main etchant in the silicon etching process. The reaction describing the silicon with XeF 2 is
XeF 2 has a relatively high etch rate and does not require ion bombardment or external energy sources in order to etch silicon.
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Xenon tetrafluoride is a chemical compound with chemical formula XeF 4 . It was the first discovered binary compound of a noble gas . [ 3 ] It is produced by the chemical reaction of xenon with fluorine : [ 4 ] [ 5 ]
This reaction is exothermic , releasing an energy of 251 kJ /mol. [ 3 ]
Xenon tetrafluoride is a colorless crystalline solid that sublimes at 117 °C. Its structure was determined by both NMR spectroscopy and X-ray crystallography in 1963. [ 6 ] [ 7 ] The structure is square planar , as has been confirmed by neutron diffraction studies. [ 8 ] According to VSEPR theory , in addition to four fluoride ligands, the xenon center has two lone pairs of electrons. These lone pairs are mutually trans .
The original synthesis of xenon tetrafluoride occurred through direct 1:5-molar-ratio combination of the elements in a nickel ( Monel ) vessel at 400 °C. [ 9 ] The nickel does not catalyze the reaction, [ citation needed ] but rather protects the container surfaces against fluoride corrosion. Controlling the process against impurities is difficult, as xenon difluoride ( XeF 2 ), tetrafluoride, and hexafluoride ( XeF 6 ) are all in chemical equilibrium, the difluoride favored at low temperatures and little fluorine and the hexafluoride favored at high temperatures and excess fluorine. [ 9 ] [ 10 ] Fractional sublimation (xenon tetrafluoride is particularly involatile) or other equilibria generally allow purification of the product mixture. [ 9 ]
The elements combine more selectively when γ- or UV-irradiated in a nickel container or dissolved in anhydrous hydrogen fluoride with catalytic oxygen . That reaction is believed selective because dioxygen difluoride at standard conditions is too weak an oxidant to generate xenon(VI) species. [ 9 ]
Alternatively, fluoroxenonium perfluorometallate salts pyrolyze to XeF 4 . [ 9 ]
Xenon tetrafluoride hydrolyzes at low temperatures to form elemental xenon , oxygen , hydrofluoric acid , and aqueous xenon trioxide : [ 11 ]
It is used as a precursor for synthesis of all tetravalent Xe compounds. [ 9 ] Reaction with tetramethylammonium fluoride gives tetramethylammonium pentafluoroxenate , which contains the pentagonal XeF − 5 anion. The XeF − 5 anion is also formed by reaction with cesium fluoride : [ 12 ]
Reaction with bismuth pentafluoride ( BiF 5 ) forms the XeF + 3 cation: [ 13 ]
The XeF + 3 cation in the salt XeF 3 Sb 2 F 11 has been characterized by NMR spectroscopy. [ 14 ]
At 400 °C, XeF 4 reacts with xenon to form XeF 2 : [ 10 ]
The reaction of xenon tetrafluoride with platinum yields platinum tetrafluoride and xenon: [ 10 ]
Xenon tetrafluoride has few applications. It has been shown to degrade silicone rubber for analyzing trace metal impurities in the rubber. XeF 4 reacts with the silicone to form simple gaseous products, leaving a residue of metal impurities. [ 15 ]
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Xenon hexafluoride is a noble gas compound with the formula XeF 6 . It is one of the three binary fluorides of xenon that have been studied experimentally, the other two being XeF 2 and XeF 4 . All of them are exergonic and stable at normal temperatures. XeF 6 is the strongest fluorinating agent of the series. It is a colorless solid that readily sublimes into intensely yellow vapors.
Xenon hexafluoride can be prepared by heating of XeF 2 at about 300 °C under 6 MPa (60 atmospheres) of fluorine. With NiF 2 as catalyst, however, this reaction can proceed at 120 °C even in xenon-fluorine molar ratios as low as 1:5. [ 2 ] [ 3 ]
The structure of XeF 6 required several years to establish in contrast to the cases of XeF 2 and XeF 4 . In the gas phase the compound is monomeric . VSEPR theory predicts that due to the presence of six fluoride ligands and one lone pair of electrons the structure lacks perfect octahedral symmetry , and indeed electron diffraction combined with high-level calculations indicate that the compound's point group is C 3v . It is a fluxional molecule . O h is only insignificantly higher, indicating that the minimum on the energy surface is very shallow. [ 4 ]
129 Xe and 19 F NMR spectroscopy indicates that in solution the compound assumes a tetrameric structure: four equivalent xenon atoms are arranged in a tetrahedron surrounded by a fluctuating array of 24 fluorine atoms that interchange positions in a "cogwheel mechanism".
Six polymorphs of XeF 6 are known. [ 5 ] including one that contains XeF + 5 ions with bridging F − ions. [ 6 ]
Xenon hexafluoride hydrolyzes, ultimately affording xenon trioxide : [ 7 ]
XeF 6 is a Lewis acid , binding one and two fluoride anions:
Salts of the octafluoroxenate(VI) anion (XeF 2− 8 ) are very stable, decomposing only above 400 °C. [ 8 ] [ 9 ] [ 10 ] This anion has been shown to have square antiprismatic geometry, based on single-crystal X-ray counter analysis of its nitrosonium salt, (NO) 2 XeF 8 . [ 11 ] The sodium and potassium salts are formed directly from sodium fluoride and potassium fluoride : [ 10 ]
These are thermally less stable than the caesium and rubidium salts, which are synthesized by first forming the heptafluoroxenate salts:
which are then pyrolysed at 50 °C and 20 °C, respectively, to form the yellow [ 12 ] octafluoroxenate salts: [ 8 ] [ 9 ] [ 10 ]
These salts are hydrolysed by water, yielding various products containing xenon and oxygen. [ 10 ]
The two other binary fluorides of xenon do not form such stable adducts with fluoride.
XeF 6 reacts with strong fluoride acceptors such as RuF 5 [ 6 ] and BrF 3 ·AuF 3 [ 13 ] to form the XeF + 5 cation:
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Xenon dioxide , or xenon(IV) oxide , is a compound of xenon and oxygen with formula XeO 2 which was synthesized in 2011. It is synthesized at 0 °C by hydrolysis of xenon tetrafluoride in aqueous sulfuric acid : [ 2 ]
XeO 2 has an extended (chain or network) structure in which xenon and oxygen have coordination numbers of four and two respectively. The geometry at xenon is square planar , consistent with VSEPR theory for four ligands and two lone pairs (or AX 4 E 2 in the notation of VSEPR theory).
The XeO 2 network does not share a crystal structure of SiO2 (which has tetrahedral coordination at Si), but XeO 2 units are believed to intermix with SiO 2 in Earth's mantle . Computational studies suggest that xenon cannot displace silicon directly, but can fill pre-existing silicon vacancies. The stability of the resulting material under standard conditions depends on its allotrope . Patterned off quartz, it likely decomposes; but materials patterned off fibrous silica may be metastable. [ 3 ]
In addition, the existence of an XeO 2 molecule was predicted by an ab initio quantum chemistry method several years earlier by Pyykkö and Tamm, but these authors did not consider an extended structure. [ 4 ]
XeO 2 is a yellow-orange solid. [ 5 ] It is an unstable compound, with a half-life of about two minutes, disproportionating into XeO 3 and xenon gas. Its structure and identity was confirmed by cooling it to −150 °C so that Raman spectroscopy could be performed before it decomposed. [ 2 ] [ 1 ]
At -78 °C, the majority of XeO 2 decomposed over a period of 72 hours, which was identified by the fading of the original yellow product to a pale yellow. Almost all yellow color indicating pure XeO 2 disappeared over the span of 1 week. [ 2 ]
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Xenon dioxydifluoride is an inorganic chemical compound with the formula XeO 2 F 2 . [ 3 ] [ 2 ] At room temperature it exists as a metastable solid, which decomposes slowly into xenon difluoride , but the cause of this decomposition is unknown. [ 2 ]
Xenon dioxydifluoride is prepared by reacting xenon trioxide with xenon oxytetrafluoride . [ 2 ]
This inorganic compound –related article is a stub . You can help Wikipedia by expanding it .
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Xenon trioxide is an unstable compound of xenon in its +6 oxidation state . It is a very powerful oxidizing agent , and liberates oxygen from water slowly, accelerated by exposure to sunlight. It is dangerously explosive upon contact with organic materials. When it detonates, it releases xenon and oxygen gas.
Synthesis of xenon trioxide is by aqueous hydrolysis of XeF 6 : [ 2 ]
The resulting xenon trioxide crystals are a strong oxidising agent and can oxidise most substances that are at all oxidisable. However, it is slow-acting and this reduces its usefulness. [ 3 ]
Above 25 °C, xenon trioxide is very prone to violent explosion:
When it dissolves in water, an acidic solution of xenic acid is formed:
This solution is stable at room temperature and lacks the explosive properties of xenon trioxide. It oxidises carboxylic acids quantitatively to carbon dioxide and water . [ 4 ]
Alternatively, 15-crown-5 coordinates to xenon trioxide to give a complex stable at room-temperature against mechanical shock. [ 5 ]
Alternatively, it dissolves in alkaline solutions to form xenates . The HXeO − 4 anion is the predominant species in xenate solutions. [ 6 ] These are not stable and begin to disproportionate into perxenates (+8 oxidation state) and xenon and oxygen gas. [ 7 ] Solid perxenates containing XeO 4− 6 have been isolated by reacting XeO 3 with an aqueous solution of hydroxides. Xenon trioxide reacts with inorganic fluorides such as KF, RbF, or CsF to form stable solids of the form MXeO 3 F . [ 8 ]
Hydrolysis of xenon hexafluoride or xenon tetrafluoride yields a solution from which colorless XeO 3 crystals can be obtained by evaporation. [ 2 ] The crystals are stable for days in dry air, but readily absorb water from humid air to form a concentrated solution. The crystal structure is orthorhombic with a = 6.163 Å, b = 8.115 Å, c = 5.234 Å, and 4 molecules per unit cell. The density is 4.55 g/cm 3 . [ 9 ]
XeO 3 should be handled with great caution. Samples have detonated when undisturbed at room temperature. Dry crystals react explosively with cellulose. [ 9 ] [ 10 ]
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Xenon tetroxide is a chemical compound of xenon and oxygen with molecular formula XeO 4 , remarkable for being a relatively stable compound of a noble gas . It is a yellow crystalline solid that is stable below −35.9 ° C ; above that temperature it is very prone to exploding and decomposing into elemental xenon and oxygen (O 2 ). [ 4 ] [ 5 ]
All eight valence electrons of xenon are involved in the bonds with the oxygen, and the oxidation state of the xenon atom is +8. Oxygen is the only element that can bring xenon up to its highest oxidation state; even fluorine can only give XeF 6 (+6).
Two other short-lived xenon compounds with an oxidation state of +8, XeO 3 F 2 and XeO 2 F 4 , are accessible by the reaction of xenon tetroxide with xenon hexafluoride . XeO 3 F 2 and XeO 2 F 4 can be detected with mass spectrometry . The perxenates are also compounds where xenon has the +8 oxidation state.
At temperatures above −35.9 °C, xenon tetroxide is very prone to explosion, decomposing into xenon and oxygen gases with Δ H = −643 kJ/mol:
Xenon tetroxide dissolves in water to form perxenic acid and in alkalis to form perxenate salts:
Xenon tetroxide can also react with xenon hexafluoride to give xenon oxyfluorides:
All syntheses start from the perxenates , which are accessible from the xenates through two methods. One is the disproportionation of xenates to perxenates and xenon:
The other is oxidation of the xenates with ozone in basic solution:
Barium perxenate is reacted with sulfuric acid and the unstable perxenic acid is dehydrated to give xenon tetroxide: [ 6 ]
Any excess perxenic acid slowly undergoes a decomposition reaction to xenic acid and oxygen:
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Xenon oxydifluoride is an inorganic compound with the molecular formula XeOF 2 . The first definitive isolation of the compound was published on 3 March 2007, producing it by the previously-examined route of partial hydrolysis of xenon tetrafluoride . [ 1 ]
The compound has a T-shaped geometry. [ 1 ] It is a weak Lewis acid , adducing acetonitrile and forming the trifluoroxenate(IV) ion in hydrogen fluoride . With strong fluoride acceptors, the latter generates the hydroxydifluoroxenonium(IV) ion (HOXeF + 2 ), suggesting a certain Brønsted basicity as well. [ 2 ]
Although stable at low temperatures, it rapidly decomposes upon warming, either by losing the oxygen atom or by disproportionating into xenon difluoride and xenon dioxydifluoride : [ 1 ]
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Xenon oxytetrafluoride ( Xe O F 4 ) is an inorganic chemical compound . It is an unstable colorless liquid [ 2 ] [ 3 ] with a melting point of −46.2 °C (−51.2 °F; 227.0 K) [ 4 ] that can be synthesized by partial hydrolysis of XeF 6 , or the reaction of XeF 6 with silica [ 3 ] or NaNO 3 : [ 5 ]
A high-yield synthesis proceeds by the reaction of XeF 6 with POF 3 at −196 °C (−320.8 °F; 77.1 K). [ 6 ]
Like most xenon oxides, it is extremely reactive, and it hydrolyses in water to give hazardous and corrosive products, including hydrogen fluoride :
In addition, some ozone and fluorine is formed.
XeOF 4 reacts with H 2 O in the following steps:
The XeO 3 formed is a dangerous explosive, decomposing explosively to Xe and O 2 :
In its liquid form, XeOF 4 exhibits amphoteric behaviour, forming complexes with both strong Lewis bases like CsF and strong Lewis acids like SbF 5 . [ 7 ] It forms a 1:1 adduct with XeF 2 , isostructural with XeF 2 · IF 5 , [ 8 ] as well as various heavy alkali metal fluorides. [ 4 ]
The reaction of XeOF 4 with XeO 3 provides a convenient synthesis route for XeO 2 F 2 . [ 9 ]
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Xenon hexafluoroplatinate is the product of the reaction of platinum hexafluoride with xenon , in an experiment that proved the chemical reactivity of the noble gases . This experiment was performed by Neil Bartlett at the University of British Columbia , who formulated the product as "Xe + [PtF 6 ] − ", although subsequent work suggests that Bartlett's product was probably a salt mixture and did not in fact contain this specific salt. [ 1 ]
"Xenon hexafluoroplatinate" is prepared from xenon and platinum hexafluoride (PtF 6 ) as gaseous solutions in SF 6 . The reactants are combined at 77 K and slowly warmed to allow for a controlled reaction.
The material described originally as "xenon hexafluoroplatinate" is probably not Xe + [PtF 6 ] − . The main problem with this formulation is "Xe + ", which would be a radical and would dimerize or abstract a fluorine atom to give XeF + . Thus, Bartlett discovered that Xe undergoes chemical reactions, but the nature and purity of his initial mustard yellow product remains uncertain. [ 2 ] Further work indicates that Bartlett's product probably contained [XeF] + [PtF 5 ] − , [XeF] + [Pt 2 F 11 ] − , and [Xe 2 F 3 ] + [PtF 6 ] − . [ 3 ] The title "compound" is a salt, consisting of an octahedral anionic fluoride complex of platinum and various xenon cations. [ 4 ]
It has been proposed that the platinum fluoride forms a negatively charged polymeric network with xenon or xenon fluoride cations held in its interstices . A preparation of "XePtF 6 " in HF solution results in a solid which has been characterized as a [PtF 5 ] − polymeric network associated with XeF + . This result is evidence for such a polymeric structure of xenon hexafluoroplatinate. [ 2 ]
In 1962, Neil Bartlett discovered that a mixture of platinum hexafluoride gas and oxygen formed a red solid. [ 5 ] [ 6 ] The red solid turned out to be dioxygenyl hexafluoroplatinate , O + 2 [PtF 6 ] − . Bartlett noticed that the ionization energy for O 2 (1175 kJ mol −1 ) was very close to the ionization energy for Xe (1170 kJ mol −1 ). He then asked his colleagues to give him some xenon "so that he could try out some reactions", [ 7 ] whereupon he established that xenon indeed reacts with PtF 6 . Although, as discussed above, the product was probably a mixture of several compounds, Bartlett's work was the first proof that compounds could be prepared from a noble gas . Since Bartlett's observation, many well-defined compounds of xenon have been reported including XeF 2 , XeF 4 , and XeF 6 . [ 3 ]
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Xellia ApS is a Danish multinational pharmaceutical and life sciences company headquartered in Copenhagen [ 1 ] specialized in the production of anti-biotics , including Vancomycin [ 2 ] and Bacitracin . [ 3 ]
The company's US base of operations is in Buffalo Grove, Illinois , [ 4 ] [ 5 ] with additional facilities in Ohio , and North Carolina . [ 6 ] [ 7 ] In 2018, Xellia's sales in the United States accounted for 60% of its total revenue. [ 5 ]
The company's main output is bulk drug chemicals which are wholesaled to other companies for packaging and distribution. Recently some prepackaged goods have been produced.
Active drug substances manufactured are: [ 8 ]
Previously the company also produced:
In 2019, Xellia partnered with Civica Rx to produce generic Vancomycin and Daptomycin . The partnership's stated aim is to remedy chronic drug shortages that have affected the American pharmaceutical market. [ 4 ] [ 19 ] [ 20 ] [ 21 ] [ 22 ]
Xellia has been discussed as a potential alternative to Chinese companies for anti-biotic primary ingredients in the context of the China-United States trade war . [ 23 ]
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Xenbase is a Model Organism Database (MOD) , providing informatics resources, as well as genomic and biological data on Xenopus frogs. [ 1 ] Xenbase has been available since 1999, and covers both X. laevis and X. tropicalis Xenopus varieties. [ 2 ] As of 2013 all of its services are running on virtual machines in a private cloud environment, making it one of the first MODs to do so. [ 3 ] Other than hosting genomics data and tools, Xenbase supports the Xenopus research community though profiles for researchers and laboratories, and job and events postings.
Xenbase runs in a cloud environment. [ 3 ] Its virtual machines are running in a VMware vSphere environment on two servers, with automatic load balancing and fault tolerance . Xenbase software uses Java , JSP , JavaScript , AJAX , XML , and CSS . It also uses Apache Tomcat and the Postgres database. The same hardware and software platforms support Echinobase .
Xenbase offers two levels of support. Full support includes full genome integration in the database, including gene pages, BLAST, JBrowse, and genome downloads. Partial support provides BLAST, JBrowse, and download options, but no gene page integration.
Full support: Xenbase's primary mission is to provide comprehensive support for the following frogs
Partial support:
The Xenopus model organism is responsible for large amounts of new knowledge on embryonic development and cell biology. Xenopus has a number of unique experimental advantages as a vertebrate model. Paramount among these is the robustness of early embryos and their amenability to microinjection and microsurgery. This makes them a particularly attractive system for testing the ectopic activity of gene products and loss-of-function experiments using antagonizing reagents such as morpholinos, dominant-negatives and neomorphic proteins. Morpholinos are synthetic oligonucleotides that can be used to inhibit nuclear RNA splicing or mRNA translation and are the common gene inhibition reagent in Xenopus as neither siRNA or miRNA have yet been shown to reproducibly function in frog embryos. [ 4 ] Xenopus embryos develop very quickly and form a full set of differentiated tissues within days of fertilization, allowing rapid analysis of the effects of manipulating embryonic gene expression . [ 5 ] The large size of embryos and amenability to microinjection also makes them extremely well suited to microarray approaches. Furthermore, these same characteristics make Xenopus, one of the few vertebrate model organisms suited for chemical screens. [ 6 ] Xenbase provides a large database of images illustrating the full genome, movies detailing embryogenesis , and multiple online tools useful for designing and conducting experiments using Xenopus .
Xenopus can be used to model human diseases caused by common genes. [ 7 ] Xenbase supports this by mapping Disease Ontology and OMIM diseases to Xenopus genes and publications. Xenopus phenotype data, as well as links to comparable human and mouse phenotypes and diseases (via the Monarch Initiative ) are also provided.
Xenbase provides many tools useful for both professional research as well as academic learning. Highlighted below are a few of the tools, along with a brief description. For full details on provided tools, users are referred to Xenbase's publications. [ 8 ] A detailed introduction to using Xenabse comes in. [ 9 ]
The Nobel Prize for Medicine or Physiology was awarded to John B. Gurdon and Shinya Yamanaka on October 8, 2012. [ 12 ] for nuclear reprogramming in Xenopus. [ 13 ]
Importance: Gurdon's experiments challenged the dogma of the time which suggested that the nucleus of a differentiated cell is committed to their fate (Example: a liver cell nucleus remains a liver cell nucleus and cannot return to an undifferentiated state).
Specifically, John Gurdon's experiments showed that a mature or differentiated cell nucleus can be returned to its immature undifferentiated form; this is the first instance of cloning of a vertebrate animal.
Experiment : Gurdon used a technique known as nuclear transfer to replace the killed-off nucleus of a frog ( Xenopus ) egg with a nucleus from a mature cell (intestinal epithelial). The tadpoles resulting from these eggs did not survive long (past the gastrulation stage), however, further transformation of the nuclei from these Xenopus eggs to a second set of Xenopus eggs resulted in fully developed tadpoles. This process (transfer of nuclei from cloned cells) is referred to as serial transplantation.
To provide examples of how Xenbase could be used to facilitate academic research, two research articles are briefly described below.
This paper uses Xenbase resources to create and characterize mutations in Xenopus tropicalis . Goda et al., performed a large scale forward genetics screen on X. tropicalis embryos to identify novel mutations (2006). Defects were noted and put into 10 different categories as follows: eye, ear, neural crest/pigment, dwarf, axial, gut, cardiovascular, head, cardiovascular plus motility, and circulation. Further studies were performed on the whitehart mutant "wha" which does not have normal circulating blood. The Xenopus Molecular Marker Resource page was used to design a microarray experiment which compared wild type (normal circulation) and "wha" mutant X. tropicalis . Analysis of microarray data revealed that 216 genes had significant changes in expression, with genes involved in hemoglobin and heme biosynthesis being the most affected, consistent with the observation that "wha" may have a role in hematopoiesis.
The 2013 paper by Suzuki et al. describes the use of a relatively new gene knockdown technique in X. laevis . Traditionally, antisense morpholino oligonucleotides have been the method of choice to study the effects of transient gene knockdown in Xenopus .
In comparison to morpholinos which disrupt gene expression by inhibiting translational machinery TALENs disrupt gene expression by binding to DNA and introducing double stranded breaks. [ 16 ] [ 17 ] Xenbase was utilized to obtain publicly available sequences for tyrosinase (tyr) and Pax6 , needed for TALEN design. Knockdown of both Pax6 and tyr was highly efficient using TALENs, suggesting that gene disruption using TALENs may be an alternative or better method to use in comparison to antisense morpholino's.
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Xenic acid is a proposed noble gas compound with the chemical formula H 2 XeO 4 or XeO 2 (OH) 2 . It has not been isolated, and the published characterization data are ambiguous. [ 2 ]
Salts of xenic acid are called xenates , containing the HXeO − 4 anion, such as monosodium xenate . They tend to disproportionate into xenon gas and perxenates : [ 3 ]
The energy given off is sufficient to form ozone from diatomic oxygen:
Salts containing the deprotonated anion XeO 2− 4 are presently unknown. [ 3 ]
This inorganic compound –related article is a stub . You can help Wikipedia by expanding it .
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Xenoarchaeology , a branch of xenology dealing with extraterrestrial cultures , is a hypothetical form of archaeology that exists mainly in works of science fiction. The field is concerned with the study of the material remains to reconstruct and interpret past life-ways of alien civilizations. Xenoarchaeology is not currently practiced by mainstream archaeologists due to the current lack of any material for the discipline to study.
The name derives from Greek xenos (ξένος) which means 'stranger, alien', and archaeology 'study of ancients'.
Xenoarchaeology is sometimes called astroarchaeology or exoarchaeology , although some would argue that the prefix exo- would be more correctly applied to the study of human activities in a space environment. [ 1 ]
Other names for xenoarchaeology, or specialised fields of interest, include Probe SETI (Search for Extra-Terrestrial Intelligence), extraterrestrial archaeology, space archaeology , SETA (Search for Extra-Terrestrial Artifacts), Dysonian SETI, Planetary SETI, SETT (Search for Extra-Terrestrial Technology), SETV (Search for Extra-Terrestrial Visitation), [ 2 ] extraterrestrial anthropology, areoarchaeology and selenoarchaeology. [ 3 ]
It is arguably the case that, due to the immense distances between stars, any evidence we discover of extraterrestrial intelligence, whether it be an artifact or an electromagnetic signal, may come from a long-vanished civilization. Thus the entire SETI project can be seen as a form of archaeology. [ 4 ] [ 5 ] [ 6 ] Additionally, due to the extreme age of the universe , there may be a reasonable expectation for astrobiology research to produce evidence of extinct alien life prior to the discovery of alien life itself. [ 7 ]
The study of alien cultures might offer us glimpses into our own species' past or future development. [ 8 ] [ 9 ]
Vicky Walsh argued for the existence of "exo-artifacts" using the principle of mediocrity and the Drake equation . She proposed that a theoretical and speculative field of archaeology be established in order to test outlandish claims and to prepare for a time when undeniably extraterrestrial artifacts needed to be analysed. "If it is possible to construct an abstract archaeology that can be tested and refined on earth and then applied to areas beyond our planet, then the claims for ETI remains on the moon and Mars may really be evaluated in light of established archaeological theory and analysis". [ 10 ]
Ben McGee similarly proposed the creation of a set of interdisciplinary, proactive xenoarchaeological guidelines, arguing that identifying suspected artifacts of astrobiology is all that is required to justify establishing a methodology for xenoarchaeology. He emphasized the necessity of proactive xenoarchaeological work in order to avoid future bias, mischaracterization, and information mismanagement, and he cites three scenarios under which such a methodology or set of guidelines would be useful, those being " remote sensing " of a potential xenoarchaeological artifact, encountering an artifact during "human exploration," and "terrestrial interception" of an artifact. [ 7 ]
The origins of the field have been traced [ 11 ] to theories about a hypothetical Martian civilization based on observations of what were perceived as canals on Mars . These theories, of which Percival Lowell was the most famous exponent, were apparently inspired by a mistranslation of a quote by Giovanni Schiaparelli .
The 1997 Theoretical Archaeology Group conference featured a session on "archaeology and science fiction". [ 12 ]
The 2004 annual meeting of the American Anthropological Association featured a session Anthropology, Archaeology and Interstellar Communication . [ 13 ]
Planetary SETI is concerned with the search for extraterrestrial structures on the surface of bodies in the Solar System . Claims for evidence of extraterrestrial artifacts can be divided into three groups, the Moon, Mars, and the other planets and their satellites. [ 3 ]
Examples of sites of interest include the "bridge" sighted in the Mare Crisium in 1953, and the "Blair Cuspids", "an unusual arrangement of seven spirelike objects of varying heights" at the western edge of the Mare Tranquillitatis , photographed by the Lunar Orbiter 2 on 20 November 1966. [ 14 ] In 2006, Ian Crawford proposed that a search for alien artifacts be conducted on the Moon. [ 15 ]
Percival Lowell 's mistaken identification of Martian canals [ 16 ] was an early attempt to detect and study an alien culture from its supposed physical remains. [ citation needed ] More recently, there was interest in the supposed Face on Mars , an example of the psychological phenomenon of pareidolia . [ 17 ]
The Society for Planetary SETI Research is a loose organization of researchers interested in this field. The organization does not endorse any particular conclusions drawn by its members on particular sites. [ 18 ]
A great deal of research and writing has been done, and some searches conducted for extraterrestrial probes in the Solar System. [ 19 ] This followed the work of Ronald N. Bracewell .
Robert Freitas , [ 20 ] [ 21 ] [ 22 ] Christopher Rose and Gregory Wright have argued that interstellar probes can be a more energy-efficient means of communication than electromagnetic broadcasts. [ 23 ]
If so, a solar centric Search for Extraterrestrial Artifacts (SETA) [ 24 ] would seem to be favored over the more traditional radio or optical searches. Robert A. Freitas coined the term SETA in the 1980s. [ 25 ]
On the basis that the Earth-Moon or Sun-Earth libration orbits might constitute convenient parking places for automated extraterrestrial probes, unsuccessful searches were conducted by Freitas and Valdes. [ 26 ] [ 27 ]
In a 1960 paper, Freeman Dyson proposed the idea of a Dyson sphere , a type of extraterrestrial artifact able to be searched for and studied at interstellar distances. Following that paper, several searches have been conducted. [ 28 ]
In a 2005 paper, Luc Arnold proposed a means of detecting smaller, though still mega-scale, artifacts from their distinctive transit light curve signature. [ 29 ] (see Astroengineering ) .
A subculture of enthusiasts studies purported structures on the Moon or Mars . These controversial "structures" (such as the Face on Mars ) are not accepted as more than natural features by most scientists, examples of the pareidolia phenomenon.
Palaeocontact or ancient astronaut theories, espoused by Erich von Däniken and others, are further examples of fringe theories. These claim that the Earth was visited in prehistoric times by extraterrestrial beings.
Xenoarchaeological themes are common in science fiction. Works about the exploration of enigmatic extraterrestrial artifacts have been satirically categorized as Big Dumb Object stories.
Some of the more prominent examples of xenoarchaeological fiction include Arthur C. Clarke's novel Rendezvous with Rama , H. Beam Piper's short story Omnilingual , and Charles Sheffield's Heritage Universe series.
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Xenobiology ( XB ) is a subfield of synthetic biology , the study of synthesizing and manipulating biological devices and systems. [ 1 ] The name "xenobiology" derives from the Greek word xenos , which means "stranger, alien". Xenobiology is a form of biology that is not (yet) familiar to science and is not found in nature. [ 2 ] In practice, it describes novel biological systems and biochemistries that differ from the canonical DNA – RNA -20 amino acid system (see central dogma of molecular biology ). For example, instead of DNA or RNA, XB explores nucleic acid analogues , termed xeno nucleic acid (XNA) as information carriers. [ 3 ] It also focuses on an expanded genetic code [ 4 ] and the incorporation of non- proteinogenic amino acids , or “xeno amino acids” into proteins. [ 5 ] [ 6 ]
"Astro" means "star" and "exo" means "outside". Both exo- and astrobiology deal with the search for naturally evolved life in the Universe, mostly on other planets in the circumstellar habitable zone . (These are also occasionally referred to as xenobiology. [ 2 ] ) Whereas astrobiologists are concerned with the detection and analysis of life elsewhere in the Universe, xenobiology attempts to design forms of life with a different biochemistry or different genetic code than on planet Earth. [ 2 ]
In xenobiology, the aim is to design and construct biological systems that differ from their natural counterparts on one or more fundamental levels. Ideally these new-to-nature organisms would be different in every possible biochemical aspect exhibiting a very different genetic code. [ 13 ] The long-term goal is to construct a cell that would store its genetic information not in DNA but in an alternative informational polymer consisting of xeno nucleic acids (XNA), different base pairs, using non-canonical amino acids and an altered genetic code. So far cells have been constructed that incorporate only one or two of these features.
Originally this research on alternative forms of DNA was driven by the question of how life evolved on earth and why RNA and DNA were selected by (chemical) evolution over other possible nucleic acid structures. [ 14 ] Two hypotheses for the selection of RNA and DNA as life's backbone are either they are favored under life on Earth's conditions, or they were coincidentally present in pre-life chemistry and continue to be used now. [ 15 ] Systematic experimental studies aiming at the diversification of the chemical structure of nucleic acids have resulted in completely novel informational biopolymers. So far a number of XNAs with new chemical backbones or leaving group of the DNA have been synthesized, [ 3 ] [ 16 ] [ 17 ] [ 18 ] e.g.: hexose nucleic acid (HNA); threose nucleic acid (TNA), [ 19 ] glycol nucleic acid (GNA) cyclohexenyl nucleic acid (CeNA). [ 20 ] The incorporation of XNA in a plasmid, involving 3 HNA codons, has been accomplished already in 2003. [ 21 ] This XNA is used in vivo (E coli) as template for DNA synthesis. This study, using a binary (G/T) genetic cassette and two non-DNA bases (Hx/U), was extended to CeNA, while GNA seems to be too alien at this moment for the natural biological system to be used as template for DNA synthesis. [ 22 ] Extended bases using a natural DNA backbone could, likewise, be transliterated into natural DNA, although to a more limited extent. [ 23 ]
Aside being used as extensions to template DNA strands, XNA activity has been tested for use as genetic catalysts . Although proteins are the most common components of cellular enzymatic activity , nucleic acids are also used in the cell to catalyze reactions. A 2015 study found several different kinds of XNA, most notably FANA (2'-fluoroarabino nucleic acids), as well as HNA, CeNA and ANA (arabino nucleic acids) could be used to cleave RNA during post-transcriptional RNA processing acting as XNA enzymes, hence the name XNAzymes. FANA XNAzymes also showed the ability to ligate DNA, RNA and XNA substrates. [ 15 ] Although XNAzyme studies are still preliminary, this study was a step in the direction of searching for synthetic circuit components that are more efficient than those containing DNA and RNA counterparts that can regulate DNA, RNA, and their own, XNA, substrates.
While XNAs have modified backbones, other experiments target the replacement or enlargement of the genetic alphabet of DNA with unnatural base pairs. For example, DNA has been designed that has – instead of the four standard bases A, T, G, and C – six bases A, T, G, C, and the two new ones P and Z (where Z stands for 6-Amino-5-nitro3-(l'-p-D-2'-deoxyribofuranosyl)-2(1H)-pyridone, and P stands for 2-Amino-8-(1-beta-D-2'-deoxyribofuranosyl)imidazo[1,2-a]-1,3,5-triazin-4 (8H)). [ 24 ] [ 25 ] [ 26 ] In a systematic study, Leconte et al. tested the viability of 60 candidate bases (yielding potentially 3600 base pairs) for possible incorporation in the DNA. [ 27 ]
In 2002, Hirao et al. developed an unnatural base pair between 2-amino-8-(2-thienyl)purine (s) and pyridine-2-one (y) that functions in vitro in transcription and translation toward a genetic code for protein synthesis containing a non-standard amino acid. [ 28 ] In 2006, they created 7-(2-thienyl)imidazo[4,5-b]pyridine (Ds) and pyrrole-2-carbaldehyde (Pa) as a third base pair for replication and transcription, [ 29 ] and afterward, Ds and 4-[3-(6-aminohexanamido)-1-propynyl]-2-nitropyrrole (Px) was discovered as a high fidelity pair in PCR amplification. [ 30 ] [ 31 ] In 2013, they applied the Ds-Px pair to DNA aptamer generation by in vitro selection (SELEX) and demonstrated the genetic alphabet expansion significantly augment DNA aptamer affinities to target proteins. [ 32 ]
In May 2014, researchers announced that they had successfully introduced two new artificial nucleotides into bacterial DNA, alongside the four naturally occurring nucleotides, and by including individual artificial nucleotides in the culture media, were able to passage the bacteria 24 times; they did not create mRNA or proteins able to use the artificial nucleotides. [ 33 ] [ 34 ] [ 35 ]
Neither the XNA nor the unnatural bases are recognized by natural polymerases . One of the major challenges is to find or create novel types of polymerases that will be able to replicate these new-to-nature constructs. In one case a modified variant of the HIV - reverse transcriptase was found to be able to PCR-amplify an oligonucleotide containing a third type base pair. [ 36 ] [ 37 ] Pinheiro et al. (2012) demonstrated that the method of polymerase evolution and design successfully led to the storage and recovery of genetic information (of less than 100bp length) from six alternative genetic polymers based on simple nucleic acid architectures not found in nature, xeno nucleic acids . [ 38 ]
One of the goals of xenobiology is to rewrite the genetic code . The most promising approach to change the code is the reassignment of seldom used or even unused codons. [ 39 ] In an ideal scenario, the genetic code is expanded by one codon, thus having been liberated from its old function and fully reassigned to a non-canonical amino acid (ncAA) ("code expansion"). As these methods are laborious to implement, and some short cuts can be applied ("code engineering"), for example in bacteria that are auxotrophic for specific amino acids and at some point in the experiment are fed isostructural analogues instead of the canonical amino acids for which they are auxotrophic. In that situation, the canonical amino acid residues in native proteins are substituted with the ncAAs. Even the insertion of multiple different ncAAs into the same protein is possible. [ 40 ] Finally, the repertoire of 20 canonical amino acids can not only be expanded, but also reduced to 19. [ 41 ] By reassigning transfer RNA (tRNA)/aminoacyl-tRNA synthetase pairs the codon specificity can be changed. Cells endowed with such aminoacyl-[tRNA synthetases] are thus able to read [mRNA] sequences that make no sense to the existing gene expression machinery. [ 42 ] Altering the codon: tRNA synthetases pairs may lead to the in vivo incorporation of the non-canonical amino acids into proteins. [ 43 ] [ 44 ] In the past reassigning codons was mainly done on a limited scale. In 2013, however, Farren Isaacs and George Church at Harvard University reported the replacement of all 321 TAG stop codons present in the genome of E. coli with synonymous TAA codons, thereby demonstrating that massive substitutions can be combined into higher-order strains without lethal effects. [ 45 ] Following the success of this genome wide codon replacement, the authors continued and achieved the reprogramming of 13 codons throughout the genome, directly affecting 42 essential genes. [ 46 ]
An even more radical change in the genetic code is the change of a triplet codon to a quadruplet and even quintuplet codon pioneered by Sisido in cell-free systems [ 47 ] and by Schultz in bacteria. [ 48 ] Finally, non-natural base pairs can be used to introduce novel amino acid in proteins. [ 49 ]
The goal of substituting DNA by XNA may also be reached by another route, namely by engineering the environment instead of the genetic modules. This approach has been successfully demonstrated by Marlière and Mutzel with the production of an E. coli strain whose DNA is composed of standard A, C and G nucleotides but has the synthetic thymine analogue 5-chlorouracil instead of thymine (T) in the corresponding positions of the sequence. These cells are then dependent on externally supplied 5-chlorouracil for growth, but otherwise they look and behave as normal E. coli . These cells, however, are currently not yet fully auxotrophic for the Xeno-base since they are still growing on thymine when this is supplied to the medium. [ 50 ]
Xenobiological systems are designed to convey orthogonality to natural biological systems. A (still hypothetical) organism that uses XNA, [ 51 ] different base pairs and polymerases and has an altered genetic code will hardly be able to interact with natural forms of life on the genetic level. Thus, these xenobiological organisms represent a genetic enclave that cannot exchange information with natural cells. [ 52 ] Altering the genetic machinery of the cell leads to semantic containment. In analogy to information processing in IT, this safety concept is termed a “genetic firewall”. [ 2 ] [ 53 ] The concept of the genetic firewall seems to overcome a number of limitations of previous safety systems. [ 54 ] [ 55 ] A first experimental evidence of the theoretical concept of the genetic firewall was achieved in 2013 with the construction of a genomically recoded organism (GRO). In this GRO all known UAG stop codons in E.coli were replaced by UAA codons, which allowed for the deletion of release factor 1 and reassignment of UAG translation function. The GRO exhibited increased resistance to T7 bacteriophage, thus showing that alternative genetic codes do reduce genetic compatibility. [ 56 ] This GRO, however, is still very similar to its natural “parent” and cannot be regarded to have a genetic firewall. The possibility of reassigning the function of large number of triplets opens the perspective to have strains that combine XNA, novel base pairs, new genetic codes, etc. that cannot exchange any information with the natural biological world.
Regardless of changes leading to a semantic containment mechanism in new organisms, any novel biochemical systems still has to undergo a toxicological screening. XNA, novel proteins, etc. might represent novel toxins, or have an allergic potential that needs to be assessed. [ 57 ] [ 58 ]
Xenobiology might challenge the regulatory framework, as currently laws and directives deal with genetically modified organisms and do not directly mention chemically or genomically modified organisms. Taking into account that real xenobiology organisms are not expected in the next few years, policy makers do have some time at hand to prepare themselves for an upcoming governance challenge. Since 2012, the following groups have picked up the topic as a developing governance issue: policy advisers in the US, [ 59 ] four National Biosafety Boards in Europe, [ 60 ] the European Molecular Biology Organisation, [ 61 ] and the European Commission's Scientific Committee on Emerging and Newly Identified Health Risks (SCENIHR) in three opinions (Definition, [ 62 ] risk assessment methodologies and safety aspects, [ 63 ] and risks to the environment and biodiversity related to synthetic biology and research priorities in the field of synthetic biology. [ 64 ] ).
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A xenobiotic is a chemical substance found within an organism that is not naturally produced or expected to be present within the organism. It can also cover substances that are present in much higher concentrations than are usual. Natural compounds can also become xenobiotics if they are taken up by another organism, such as the uptake of natural human hormones by fish found downstream of sewage treatment plant outfalls, or the chemical defenses produced by some organisms as protection against predators. [ 1 ] The term "xenobiotic" is also used to refer to organs transplanted from one species to another.
The term "xenobiotics", however, is very often used in the context of pollutants such as dioxins and polychlorinated biphenyls and their effect on the biota , because xenobiotics are understood as substances foreign to an entire biological system, i.e. artificial substances, which did not exist in nature before their synthesis by humans. The term xenobiotic is derived from the Greek words ξένος (xenos) = foreigner, stranger and βίος (bios) = life, plus the Greek suffix for adjectives -τικός, -ή, -όν (-tikos, -ē, -on). Xenobiotics may be grouped as carcinogens , drugs, environmental pollutants, food additives , hydrocarbons , and pesticides.
The body removes xenobiotics by xenobiotic metabolism . This consists of the deactivation and the excretion of xenobiotics and happens mostly in the liver. Excretion routes are urine, feces, breath, and sweat. Hepatic enzymes are responsible for the metabolism of xenobiotics by first activating them (oxidation, reduction, hydrolysis, and/or hydration of the xenobiotic), and then conjugating the active secondary metabolite with glucuronic acid , sulfuric acid , or glutathione , followed by excretion in bile or urine. An example of a group of enzymes involved in xenobiotic metabolism is hepatic microsomal cytochrome P450 . These enzymes that metabolize xenobiotics are very important for the pharmaceutical industry because they are responsible for the breakdown of medications. A species with this unique cytochrome P450 system is Drosophila mettleri , which uses xenobiotic resistance to exploit a wider nesting range including both soil moistened with necrotic exudates and necrotic plots themselves.
Although the body is able to remove xenobiotics by reducing it to a less toxic form through xenobiotic metabolism then excreting it, it is also possible for it to be converted into a more toxic form in some cases. This process is referred to as bioactivation and can result in structural and functional changes to the microbiota. [ 2 ] Exposure to xenobiotics can disrupt the microbiome community structure, either by increasing or decreasing the size of certain bacterial populations depending on the substance. Functional changes that result vary depending on the substance and can include increased expression in genes involved in stress response and antibiotic resistance , changes in the levels of metabolites produced, etc. [ 3 ]
Organisms can also evolve to tolerate xenobiotics. An example is the co-evolution of the production of tetrodotoxin in the rough-skinned newt and the evolution of tetrodotoxin resistance in its predator, the Common Garter Snake . In this predator–prey pair, an evolutionary arms race has produced high levels of toxin in the newt and correspondingly high levels of resistance in the snake. [ 4 ] This evolutionary response is based on the snake evolving modified forms of the ion channels that the toxin acts upon, so becoming resistant to its effects. [ 5 ] Another example of a xenobiotic tolerance mechanism is the use of ATP-binding cassette (ABC) transporters , which is largely exhibited in insects. [ 6 ] Such transporters contribute to resistance by enabling the transport of toxins across the cell membrane, thus preventing accumulation of these substances within cells.
Xenobiotic substances are an issue for sewage treatment systems, since they are many in number, and each will present its own problems as to how to remove them (and whether it is worth trying to)
Some xenobiotics substances are resistant to degradation. Xenobiotics such as polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), and trichloroethylene (TCE) accumulate in the environment due to their recalcitrant properties and have become an environmental concern due to their toxicity and accumulation. This occurs particularly in the subsurface environment and water sources, as well as in biological systems, having the potential to impact human health. [ 7 ] Some of the main sources of pollution and the introduction of xenobiotics into the environment come from large industries such as pharmaceuticals, fossil fuels, pulp and paper bleaching and agriculture. [ 8 ] For example, they may be synthetic organochlorides such as plastics and pesticides, or naturally occurring organic chemicals such as polyaromatic hydrocarbons (PAHs) and some fractions of crude oil and coal.
Microorganisms may be a viable solution to this issue of environmental pollution by the degradation of the xenobiotics; a process known as bioremediation . [ 9 ] Microorganisms are able to adapt to xenobiotics introduced into the environment through horizontal gene transfer , in order to make use of such compounds as energy sources. [ 8 ] This process can be further altered to manipulate the metabolic pathways of microorganisms in order to degrade harmful xenobiotics under specific environmental conditions at a more desirable rate. [ 8 ] Mechanisms of bioremediation include both genetically engineering microorganisms and isolating the naturally occurring xenobiotic degrading microbes. [ 9 ] Research has been conducted to identify the genes responsible for the ability of microorganisms to metabolize certain xenobiotics and it has been suggested that this research can be used in order to engineer microorganisms specifically for this purpose. [ 9 ] Not only can current pathways be engineered to be expressed in other organisms, but the creation of novel pathways is a possible approach. [ 8 ]
Xenobiotics may be limited in the environment and difficult to access in areas such as the subsurface environment. [ 8 ] Degradative organisms can be engineered to increase mobility in order to access these compounds, including enhanced chemotaxis . [ 8 ] One limitation of the bioremediation process is that optimal conditions are required for proper metabolic functioning of certain microorganisms, which may be difficult to meet in an environmental setting. [ 7 ] In some cases a single microorganism may not be capable of performing all metabolic processes required for degradation of a xenobiotic compound and so "syntrophic bacterial consortia" may be employed. [ 8 ] In this case, a group of bacteria work in conjunction, resulting in dead end products from one organism being further degraded by another organism. [ 7 ] In other cases, the products of one microorganisms may inhibit the activity another, and thus a balance must be maintained. [ 8 ]
Many xenobiotics produce a variety of biological effects, which is used when they are characterized using bioassay . Before they can be registered for sale in most countries, xenobiotic pesticides must undergo extensive evaluation for risk factors, such as toxicity to humans, ecotoxicity , or persistence in the environment. For example, during the registration process, the herbicide, cloransulam-methyl was found to degrade relatively quickly in soil. [ 10 ]
The term xenobiotic is also used to refer to organs transplanted from one species to another. For example, some researchers hope that hearts and other organs could be transplanted from pigs to humans. Many people die every year whose lives could have been saved if a critical organ had been available for transplant. Kidneys are
currently the most commonly transplanted organ. Xenobiotic organs would need to be developed in such a way that they would not be rejected by the immune system.
Drug metabolism – Xenobiotic metabolism is redirected to the special case: Drug metabolism.
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Xenobiotic metabolism (from the Greek xenos "stranger" and biotic "related to living beings") is the set of metabolic pathways that modify the chemical structure of xenobiotics , which are compounds foreign to an organism's normal biochemistry, such as drugs and poisons. These pathways are a form of biotransformation present in all major groups of organisms, and are considered to be of ancient origin. These reactions often act to detoxify poisonous compounds; however, in cases such as in the metabolism of alcohol , the intermediates in xenobiotic metabolism can themselves be the cause of toxic effects.
Xenobiotic metabolism is divided into three phases. In phase I, enzymes such as cytochrome P450 oxidases introduce reactive or polar groups into xenobiotics. These modified compounds are then conjugated to polar compounds in phase II reactions. These reactions are catalysed by transferase enzymes such as glutathione S-transferases . Finally, in phase III, the conjugated xenobiotics may be further processed, before being recognised by efflux transporters and pumped out of cells.
The reactions in these pathways are of particular interest in medicine as part of drug metabolism and as a factor contributing to multidrug resistance in infectious diseases and cancer chemotherapy . The actions of some drugs as substrates or inhibitors of enzymes involved in xenobiotic metabolism are a common reason for hazardous drug interactions . These pathways are also important in environmental science , with the xenobiotic metabolism of microorganisms determining whether a pollutant will be broken down during bioremediation , or persist in the environment. The enzymes of xenobiotic metabolism, particularly the glutathione S-transferases are also important in agriculture, since they may produce resistance to pesticides and herbicides .
That the exact compounds an organism is exposed to will be largely unpredictable, and may differ widely over time, is a major characteristic of xenobiotic toxic stress. [ 1 ] The major challenge faced by xenobiotic detoxification systems is that they must be able to remove the almost-limitless number of xenobiotic compounds from the complex mixture of chemicals involved in normal metabolism . The solution that has evolved to address this problem is an elegant combination of physical barriers and low-specificity enzymatic systems.
All organisms use cell membranes as hydrophobic permeability barriers to control access to their internal environment. Polar compounds cannot diffuse across these cell membranes , and the uptake of useful molecules is mediated through transport proteins that specifically select substrates from the extracellular mixture. This selective uptake means that most hydrophilic molecules cannot enter cells, since they are not recognised by any specific transporters. [ 2 ] In contrast, the diffusion of hydrophobic compounds across these barriers cannot be controlled, and organisms, therefore, cannot exclude lipid -soluble xenobiotics using membrane barriers.
However, the existence of a permeability barrier means that organisms were able to evolve detoxification systems that exploit the hydrophobicity common to membrane-permeable xenobiotics. These systems therefore solve the specificity problem by possessing such broad substrate specificities that they metabolise almost any non-polar compound. [ 1 ] Useful metabolites are excluded since they are polar, and in general contain one or more charged groups.
The detoxification of the reactive by-products of normal metabolism cannot be achieved by the systems outlined above, because these species are derived from normal cellular constituents and usually share their polar characteristics. However, since these compounds are few in number, specific enzymes can recognize and remove them. Examples of these specific detoxification systems are the glyoxalase system , which removes the reactive aldehyde methylglyoxal, [ 3 ] and the various antioxidant systems that eliminate reactive oxygen species. [ 4 ]
The metabolism of xenobiotics is often divided into three phases: modification, conjugation, and excretion. These reactions act in concert to detoxify xenobiotics and remove them from cells.
In phase I, a variety of enzymes acts to introduce reactive and polar groups into their substrates. One of the most common modifications is hydroxylation catalysed by the cytochrome P-450-dependent mixed-function oxidase system . These enzyme complexes act to incorporate an atom of oxygen into nonactivated hydrocarbons, which can result in either the introduction of hydroxyl groups or N-, O- and S-dealkylation of substrates. [ 5 ] The reaction mechanism of the P-450 oxidases proceeds through the reduction of cytochrome-bound oxygen and the generation of a highly-reactive oxyferryl species, according to the following scheme: [ 6 ]
NADPH + H + + RH → NADP + + H 2 O + ROH {\displaystyle {\mbox{NADPH}}+{\mbox{H}}^{+}+{\mbox{RH}}\rightarrow {\mbox{NADP}}^{+}+{\mbox{H}}_{2}{\mbox{O}}+{\mbox{ROH}}\,}
In subsequent phase II reactions, these activated xenobiotic metabolites are conjugated with charged species such as glutathione (GSH), sulfate , glycine , or glucuronic acid . These reactions are catalysed by a large group of broad-specificity transferases, which in combination can metabolise almost any hydrophobic compound that contains nucleophilic or electrophilic groups. [ 1 ] One of the most important of these groups are the glutathione S-transferases (GSTs). The addition of large anionic groups (such as GSH) detoxifies reactive electrophiles and produces more polar metabolites that cannot diffuse across membranes, and may, therefore, be actively transported.
After phase II reactions, the xenobiotic conjugates may be further metabolised. A common example is the processing of glutathione conjugates to acetylcysteine ( mercapturic acid ) conjugates. [ 7 ] Here, the γ-glutamate and glycine residues in the glutathione molecule are removed by Gamma-glutamyl transpeptidase and dipeptidases . In the final step, the cystine residue in the conjugate is acetylated .
Conjugates and their metabolites can be excreted from cells in phase III of their metabolism, with the anionic groups acting as affinity tags for a variety of membrane transporters of the multidrug resistance protein (MRP) family. [ 8 ] These proteins are members of the family of ATP-binding cassette transporters and can catalyse the ATP-dependent transport of a huge variety of hydrophobic anions, [ 9 ] and thus act to remove phase II products to the extracellular medium, where they may be further metabolised or excreted. [ 10 ]
The detoxification of endogenous reactive metabolites such as peroxides and reactive aldehydes often cannot be achieved by the system described above. This is the result of these species' being derived from normal cellular constituents and usually sharing their polar characteristics. However, since these compounds are few in number, it is possible for enzymatic systems to utilize specific molecular recognition to recognize and remove them. The similarity of these molecules to useful metabolites therefore means that different detoxification enzymes are usually required for the metabolism of each group of endogenous toxins. Examples of these specific detoxification systems are the glyoxalase system , which acts to dispose of the reactive aldehyde methylglyoxal , and the various antioxidant systems that remove reactive oxygen species .
Studies on how people transform the substances that they ingest began in the mid-nineteenth century, with chemists discovering that organic chemicals such as benzaldehyde could be oxidized and conjugated to amino acids in the human body. [ 11 ] During the remainder of the nineteenth century, several other basic detoxification reactions were discovered, such as methylation , acetylation , and sulfonation .
In the early twentieth century, work moved on to the investigation of the enzymes and pathways that were responsible for the production of these metabolites. This field became defined as a separate area of study with the publication by Richard Williams of the book Detoxication mechanisms in 1947. [ 12 ] This modern biochemical research resulted in the identification of glutathione S -transferases in 1961, [ 13 ] followed by the discovery of cytochrome P450s in 1962, [ 14 ] and the realization of their central role in xenobiotic metabolism in 1963. [ 15 ] [ 16 ]
Databases
Drug metabolism
Microbial biodegradation
History
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Xenobots , named after the clawed frog ( Xenopus laevis ), [ 1 ] are synthetic lifeforms that are designed by computers to perform some desired function and built by combining together different biological tissues. [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 6 ] There is debate among scientists whether xenobots are robots, organisms, or something else entirely.
The first xenobots were built by Douglas Blackiston according to blueprints generated by an AI program, which was developed by Sam Kriegman . [ 3 ]
Xenobots built to date have been less than 1 millimeter (0.04 inches) wide and composed of just two things: skin cells and heart muscle cells , both of which are derived from stem cells harvested from early ( blastula stage ) frog embryos. [ 7 ] The skin cells provide rigid support and the heart cells act as small motors, contracting and expanding in volume to propel the xenobot forward.
The shape of a xenobot's body, and its distribution of skin and heart cells, are automatically designed in simulation to perform a specific task, using a process of trial and error (an evolutionary algorithm ).
Xenobots have been designed to walk, swim, push pellets, carry payloads, and work together in a swarm to aggregate debris scattered along the surface of their dish into neat piles.
They can survive for weeks without food and heal themselves after lacerations. [ 2 ]
Other kinds of motors and sensors have been incorporated into xenobots.
Instead of heart muscle, xenobots can grow patches of cilia and use them as small oars for swimming. [ 8 ] However, cilia-driven xenobot locomotion is currently less controllable than cardiac-driven xenobot locomotion. [ 9 ] An RNA molecule can also be introduced to xenobots to give them molecular memory: if exposed to specific kind of light during behavior, they will glow a prespecified color when viewed under a fluorescence microscope . [ 9 ]
Xenobots can also self-replicate. Xenobots can gather loose cells in their environment, forming them into new xenobots with the same capability. [ 10 ] [ 11 ] [ 12 ]
Currently, xenobots are primarily used as a scientific tool to understand how cells cooperate to build complex bodies during morphogenesis . [ 1 ] However, the behavior and biocompatibility of current xenobots suggest several potential applications to which they may be put in the future.
Xenobots are composed solely of frog cells, making them biodegradable and environmentally friendly robots.
Unlike traditional technologies, xenobots do not generate pollution or require external energy inputs during their life-cycle. They move using energy from fat and protein naturally stored in their tissue, which lasts about a week, at which point they simply turn into dead skin cells. [ 2 ] Additionally, since swarms of xenobots tend to work together to push microscopic pellets in their dish into central piles, [ 2 ] it has been speculated that future xenobots might be able to find and aggregate tiny bits of ocean-polluting microplastics into a large ball of plastic that a traditional boat or drone could gather and bring to a recycling center.
In future clinical applications, such as targeted drug delivery, xenobots could be made from a human patient’s own cells, which would virtually eliminate the immune response challenges inherent in other kinds of micro-robotic delivery systems.
Such xenobots could potentially be used to scrape plaque from arteries , and with additional cell types and bioengineering, locate and treat disease.
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Aubrey David Nicholas Jasper de Grey ( / d ə ˈ ɡ r eɪ / ; born 20 April 1963) is an English biomedical gerontologist . He is the author of The Mitochondrial Free Radical Theory of Aging (1999) and co-author of Ending Aging (2007). De Grey is known for his view that medical technology may enable human beings alive today not to die from age-related causes. As an amateur mathematician , he has contributed to the study of the Hadwiger–Nelson problem in geometric graph theory , making the first progress on the problem in over 60 years.
De Grey is an international adjunct professor of the Moscow Institute of Physics and Technology . [ 3 ] In August 2021, he was removed as the Chief Science Officer of the SENS Research Foundation after he had allegedly attempted to interfere in a probe investigating sexual harassment allegations against him. In September 2021, an independent investigation concluded that he had made offensive remarks to two women.
De Grey was born on 20 April 1963 [ 4 ] [ 5 ] and brought up in London, England. [ 6 ] He told The Observer that he never knew his father, and that his mother Cordelia, an artist, encouraged him in the areas in which she herself was weakest: science and mathematics. [ 7 ] De Grey was educated at Sussex House School [ 8 ] and Harrow School . [ 9 ] He attended university at Trinity Hall, Cambridge , graduating with a BA in computer science in 1985. [ 10 ]
After graduation in 1985, de Grey joined Sinclair Research as an artificial intelligence (AI) researcher and software engineer. In 1986, along with SRL colleague Aaron Turner, he co-founded Man-Made Minions Ltd. in order to pursue the development of an automated formal program verifier . [ 11 ] At a graduate party in Cambridge, de Grey met fruit fly geneticist Adelaide Carpenter , [ 12 ] whom he would marry in 1991. [ 13 ] Through her, he was introduced to the intersection of biology and programming when her boss needed someone who knew about computers and biology to take over the running of a database on fruit flies. [ 14 ] In the early 1990s, he switched fields from AI research to biomedical gerontology, after realising that "biologists by and large were not terribly interested in doing anything about aging". [ 15 ] He educated himself in biology by reading journals and textbooks, attending conferences, and being tutored by Professor Carpenter. [ 16 ] [ 17 ] From 1992 to 2006, he was in charge of software development at the university's Genetics Department for the FlyBase genetic database. [ 18 ]
Cambridge awarded de Grey a Ph.D. by publication in biology on 9 December 2000. [ 10 ] [ 19 ] The degree was based on his 1999 book The Mitochondrial Free Radical Theory of Aging , in which de Grey wrote that obviating damage to mitochondrial DNA might by itself extend lifespan significantly, though he said it was more likely that cumulative damage to mitochondria is a significant cause of senescence, but not the single dominant cause. [ citation needed ]
In 2005, de Grey argued that most of the fundamental knowledge needed to develop effective anti-aging medicine already exists, and that the science is ahead of the funding. He described his work as identifying and promoting specific technological approaches to the reversal of various aspects of aging, or, as he puts it, "... the set of accumulated side effects from metabolism that eventually kills us." [ 20 ]
As of 2005 [update] , de Grey's work centered on a detailed plan called strategies for engineered negligible senescence (SENS), which is aimed at preventing age-related physical and cognitive decline. In March 2009, he co-founded the SENS Research Foundation (named SENS Foundation until early 2013), a non-profit organisation based in California, United States, where he served until 2021 as Chief Science Officer. The foundation "works to develop, promote and ensure widespread access to regenerative medicine solutions to the disabilities and diseases of aging", [ 21 ] focusing on the strategies for engineered negligible senescence. Before March 2009, the SENS research program was mainly pursued by the Methuselah Foundation , co-founded by de Grey.
A major activity of the Methuselah Foundation is the Methuselah Mouse Prize , [ 22 ] a prize designed to incentivize research into effective life extension interventions by awarding monetary prizes to researchers who stretch the lifespan of mice to unprecedented lengths. De Grey stated in March 2005 "if we are to bring about real regenerative therapies that will benefit not just future generations, but those of us who are alive today, we must encourage scientists to work on the problem of aging." The prize reached US$4.2 million in February 2007.
In 2007, de Grey wrote the book Ending Aging with the assistance of Michael Rae. [ 23 ]
In a 2008 broadcast on Franco-German TV network Arte , de Grey claimed that the first human to live 1,000 years was probably already alive, and might even be between 50 and 60 years old already. [ 24 ]
In 2012, de Grey inherited more than US$16 million , US$13 million of which he donated to the SENS Research Foundation. [ 25 ]
In 2022, de Grey started the Longevity Escape Velocity Foundation, which funded and launched a project focusing on robust mouse rejuvenation . [ 26 ]
On 8 April 2018, de Grey posted a paper to arXiv explicitly constructing a unit-distance graph in the plane that cannot be colored with fewer than five colors, increasing the previously known lower bound by one and making the first progress on the problem in over 60 years. [ 27 ] The previous lower bound of four was due to the problem 's original proposal in 1950 by Hugo Hadwiger and Edward Nelson . [ 28 ] De Grey's graph has 1581 vertices, but it has since been reduced to 510 by independent researchers. [ 29 ] [ 30 ] [ 31 ]
De Grey was formerly Vice President of New Technology Discovery at AgeX Therapeutics , a startup in the longevity space helmed by Michael D. West . He was appointed to the position within the company in July 2017. [ 32 ] [ 33 ] [ 34 ]
Xenocatabolism is a concept in medical bioremediation that relies upon introducing into the body microbial enzymes that break down pathogenic lysosomal, cytosolic and extracellular aggregates. The term, also called xenohydrolysis, was coined by de Grey, building upon the work of others. [ 35 ] [ 36 ] De Grey posited that there are microbes that feed on substances such as amyloid , cholesterol and other related substances in places that are full of human remains , such as graveyards . This was based on the microbial infallibility hypothesis. He states that "the biomedical approach would be to identify the genetic basis for that capacity, and to put one or two genes into ourselves, thereby enhancing our own ability to break things down, and to thereby get rid of things that we cannot naturally break down". [ 37 ] In order to add credibility to the concept, de Grey created an experiment using soil from a graveyard and took the bacteria from it. He used lipofuscin , "one of the major things that accumulates indigestibly in the body" – which some of the bacteria broke down, lending credibility to the hypothesis. De Grey presented this theory on May 29, 2007, at the Googleplex Google TechTalks. [ citation needed ]
De Grey believes that medical technology may enable human beings alive today not to die from age-related causes. [ 38 ]
He coined the term Methuselarity , which he defines as the moment when medical therapies will rejuvenate people enough to continue living healthily until the next improved generation of rejuvenation biotechnology, and so on, indefinitely. According to de Grey, that is synonymous with the point where science achieves longevity escape velocity –the minimum rate at which those therapies need to be improved in order to allow people not to suffer from age-related ill-health at any age. [ 39 ] In 2022, he stated that there is a 50% chance that this breakthrough was only 15 years away. [ 40 ]
However, de Grey views the fatalistic attitude toward aging in society , as he sees it, as a hurdle in the rapid development of anti-aging medicine , which he calls " pro-aging trance ". [ 41 ]
In 2005, MIT Technology Review , in cooperation with the Methuselah Foundation, announced a US$20,000 prize for any molecular biologist who could demonstrate that SENS was "so wrong that it is unworthy of learned debate." The judges of the challenge were Rodney Brooks , Anita Goel , Vikram Sheel Kumar , Nathan Myhrvold , and Craig Venter . Five submissions were made, of which three met the terms of the challenge. De Grey wrote a rebuttal to each submission, and the challengers wrote responses to each rebuttal. The judges concluded that none of the challengers had disproved SENS, but the magazine opined that Preston Estep 's submission was particularly eloquent and well written, and awarded the contestant US$10,000. The judges also noted "the proponents of SENS have not made a compelling case for SENS", and wrote that many of its proposals could not be verified with the current level of scientific knowledge and technology, concluding that "SENS does not compel the assent of many knowledgeable scientists; but neither is it demonstrably wrong." [ 42 ] The critics single out three proposed therapies for criticism: somatic telomerase deletion, somatic mitochondrial genome engineering, and the use of transgenic microbial hydrolase. [ 43 ]
Later in 2005, he was the subject of an associated critical editorial article in the MIT Technology Review , which viewed his theories as oversimplifying anti-aging as a scientific goal, and expressed concern at a lack of ethical and moral considerations towards anti-aging research. [ 44 ]
A 2005 article about SENS published in the viewpoint section of EMBO Reports by 28 scientists concluded that none of de Grey's hypotheses "have ever been shown to extend the lifespan of any organism, let alone humans". [ 45 ] The SENS Research Foundation, of which de Grey was a co-founder, acknowledged this, stating, "If you want to reverse the damage of aging right now I'm afraid the simple answer is, you can't." [ 46 ] Moreover, de Grey argued that this reveals a serious gap in understanding between basic scientists and technologists and between biologists studying aging and those studying regenerative medicine. [ 47 ]
The 31-member Research Advisory Board of de Grey's SENS Research Foundation have signed an endorsement of the plausibility of the SENS approach. [ 48 ] In 2021, the National Institute on Aging (NIA), a division of the U.S. National Institutes of Health (NIH), showcased a SENS research project and provided a grant for the research. [ 49 ] [ 50 ]
According to de Grey, automation will take over most jobs in the future. [ 51 ] He also believes that the introduction of a universal basic income will be necessary to find "some new way to distribute wealth that doesn’t depend on being paid to do things we wouldn’t otherwise do". [ 51 ]
De Grey is a cryonicist , having signed up with Alcor . [ 52 ] When asked in an interview about his views on cryonics, he answered that "[...] it's an absolute tragedy that cryonics is still such a backwater publicly and that a large majority of people still believe that it has no chance of ever working", arguing "If people understood it better, there would be more research done to develop better cryopreservation technologies, and more people would have a chance at life." [ 53 ]
In August 2021, following allegations of sexual harassment by two women, de Grey was put on administrative leave by the SENS Research Foundation (SRF). [ 54 ]
Both women describe situations in which he, {...} explicitly spoke with them about sex. According to Halioua’s account, he told her that it was her responsibility to sleep with SENS donors to encourage financial contributions. Deming was 17 when, she alleges, de Grey told her he wanted to speak with her about his “adventurous love life.” Deming says that when she recently became aware that this was not a one-off incident, she was “angry to realize that Aubrey inappropriately propositioned more than one woman, many in the community knew about it, and no one did anything,” she writes. [ 55 ]
The SRF board of directors decided to remove de Grey from his position as chief science officer, severing all ties with him following the report that he had allegedly attempted to interfere with the investigation. [ 56 ]
The independent investigator determined that de Grey made inappropriate remarks to Deming and Halioua. [ 57 ] The investigator also decided de Grey's attempt to communicate with Halioua via a third party constituted interference, although de Grey stated he believed that phase of the investigation had concluded. The investigator found that various other allegations against de Grey were not substantiated. [ 58 ] [ 59 ]
In March 2022, SRF released a statement regarding de Grey's employment affirming that while his actions "did substantiate instances of poor judgment and boundary-crossing behaviors, Dr. de Grey is not a sexual predator." [ 60 ]
De Grey's removal from SRF led three major donors to sue SRF for fraud, where they alleged that at the time they had donated (July 2021) the SRF board was already engineering de Grey’s removal but had concealed this from the wider world. [ 61 ] [ 62 ] The donors said that had they known that de Grey would not be substantially responsible for disbursing donated funds they would not have donated. The suit was settled within three months and did not proceed to a discovery phase. [ 62 ] The nature of the settlement was not made public, but de Grey has stated that the LEV Foundation, his new organization, was formed "as a result". [ 63 ] Another lawsuit making similar accusations was filed by donor April Okita on November 21st, 2024. [ 64 ]
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Xenoestrogens are a type of xenohormone that imitates estrogen . They can be either synthetic or natural chemical compounds . Synthetic xenoestrogens include some widely used industrial compounds, such as PCBs , BPA , and phthalates , which have estrogenic effects on a living organism even though they differ chemically from the estrogenic substances produced internally by the endocrine system of any organism. Natural xenoestrogens include phytoestrogens which are plant-derived xenoestrogens. Because the primary route of exposure to these compounds is by consumption of phytoestrogenic plants, they are sometimes called "dietary estrogens". Mycoestrogens , estrogenic substances from fungi , are another type of xenoestrogen that are also considered mycotoxins . [ 1 ] [ 2 ]
Xenoestrogens are clinically significant because they can mimic the effects of endogenous estrogen and thus have been implicated in precocious puberty and other disorders of the reproductive system. [ 3 ] [ 4 ]
Xenoestrogens include pharmacological estrogens (in which estrogenic action is an intended effect, as in the drug ethinylestradiol used in contraceptive pills ), but other chemicals may also have estrogenic effects. Xenoestrogens have been introduced into the environment by industrial, agricultural and chemical companies and consumers only in the last 70 years or so, but archiestrogens exist naturally. Some plants (like the cereals and the legumes) are using estrogenic substances possibly as part of their natural defence against herbivore animals by controlling their fertility. [ 5 ] [ 6 ]
The potential ecological and human health impact of xenoestrogens is of growing concern. [ 7 ] The word xenoestrogen is derived from the Greek words ξένο (xeno, meaning foreign), οἶστρος (estrus, meaning sexual desire) and γόνο (gene, meaning "to generate") and literally means "foreign estrogen ". Xenoestrogens are also called "environmental hormones" or "EDC" (Endocrine Disrupting Compounds, or Endocrine disruptor for short). Most scientists that study xenoestrogens, including The Endocrine Society , regard them as serious environmental hazards that have hormone disruptive effects on both wildlife and humans. [ 8 ] [ 9 ] [ 10 ] [ 11 ] [ 12 ]
The onset of puberty is characterized by increased levels of hypothalamic gonadotropin releasing hormone (GnRH). GnRH triggers the secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the anterior pituitary gland , which in turn causes the ovaries to respond and secrete estradiol . Increases in gonadal estrogen promote breast development, female fat distribution and skeletal growth. Adrenal androgen and gonadal androgen result in pubic and axillary hair. [ 13 ] [ 14 ] Peripheral precocious puberty caused by exogenous estrogens is evaluated by assessing decreased levels of gonadotrophins. [ 15 ]
Xenoestrogens in plastics, packaged food, drink trays and containers, (more so, when they've been heated in the Sun, or an oven), may interfere with pubertal development by actions at different levels – hypothalamic-pituitary axis, gonads, peripheral target organs such as the breast, hair follicles and genitals. Exogenous chemicals that mimic estrogen can alter the functions of the endocrine system and cause various health defects by interfering with synthesis, metabolism, binding or cellular responses of natural estrogens. [ 14 ] [ 16 ] [ 17 ] [ 18 ]
Although the physiology of the reproductive system is complex, the action of environmental exogenous estrogens is hypothesized to occur by two possible mechanisms. Xenoestrogens may temporarily or permanently alter the feedback loops in the brain, pituitary, gonads, and thyroid by mimicking the effects of estrogen and triggering their specific receptors or they may bind to hormone receptors and block the action of natural hormones. Thus it is plausible that environmental estrogens can accelerate sexual development if present in a sufficient concentration or with chronic exposure. [ 16 ] [ 18 ] [ 19 ] [ 20 ] The similarity in the structure of exogenous estrogens and the estrogens has changed the hormone balance within the body and resulted in various reproductive problems in females. [ 14 ] The overall mechanism of action is binding of the exogenous compounds that mimic estrogen to the estrogen binding receptors and cause the determined action in the target organs. [ 21 ]
Xenoestrogens have been implicated in a variety of medical problems, and during the last 10 years many scientific studies have found hard evidence of adverse effects on human and animal health. [ 33 ]
There is a concern that xenoestrogens may act as false messengers and disrupt the process of reproduction . Xenoestrogens, like all estrogens, can increase growth of the endometrium , so treatments for endometriosis include avoidance of products which contain them. Likewise, they are avoided in order to prevent the onset or aggravation of adenomyosis . Studies have implicated observations of disturbances in wildlife with estrogenic exposure. For example, discharge from human settlement including runoff and water flowing out of wastewater treatment plants release a large amount of xenoestrogens into streams, which lead to immense alterations in aquatic life. With a bioaccumulation factor of 10 5 –10 6 , fish are extremely susceptible to pollutants. [ 34 ] Streams in more arid conditions are thought to have more effects due to higher concentrations of the chemicals arising from lack of dilution. [ 35 ]
When comparing fish from above a wastewater treatment plant and below a wastewater treatment plant, studies found disrupted ovarian and testicular histopathology, gonadal intersex, reduced gonad size, vitellogenin induction, and altered sex ratios. [ 35 ]
The sex ratios are female biased because xenoestrogens interrupt gonadal configuration causing complete or partial sex reversal. When comparing adjacent populations of white sucker fish, the exposed female fish can have up to five oocyte stages and asynchronously developing ovaries versus the unexposed female fish who usually have two oocyte stages and group-synchronously developing ovaries. Previously, this type of difference has only been found between tropical and temperate species. [ 35 ]
Sperm concentrations and motility perimeters are reduced in male fish exposed to xenoestrogens in addition to disrupt stages of spermatogenesis. [ 24 ] [ 35 ] Moreover, xenoestrogens have been leading to vast amounts of intersex in fish. For example, one study indicates the numbers of intersex in white sucker fish to be equal to the number of males in the population downstream of a wastewater treatment plant. No intersex members were found upstream from the plant. Also, they found differences in the proportion of testicular and ovarian tissue and its degree of organization between the intersex fish. [ 35 ] Furthermore, xenoestrogens expose fish to CYP1A inducers through inhibiting a putative labile protein and enhancing the Ah receptor, which has been linked to epizootics of cancer and the initiation of tumors. [ 34 ]
The induction of CYP1A has been established to be a good bioindicator for xenoestrogen exposure. In addition, xenoestrogens stimulate vitellogenin (Vtg), which acts as a nutrient reserve, and Zona readiata proteins (Zrp), which forms eggshells. Therefore, Vtg and Zrp are biomarkers to exposure for fish. [ 36 ]
Another potential effect of xenoestrogens is on oncogenes , specifically in relation to breast cancer . Some scientists doubt that xenoestrogens have any significant biological effect, in the concentrations found in the environment. [ 37 ] However, there is substantial evidence in a variety of recent studies to indicate that xenoestrogens can increase breast cancer growth in tissue culture . [ 38 ] [ 39 ] [ 40 ] [ 41 ]
It has been suggested that very low levels of a xenoestrogen, Bisphenol A , could affect fetal neural signalling more than higher levels, indicating that classical models where dose equals response may not be applicable in susceptible tissue. [ 42 ] As this study involved intra-cerebellar injections, its relevance to environmental exposures is unclear, as is the role of an estrogenic effect compared to some other toxic effect of bisphenol A.
Other scientists argue that the observed effects are spurious and inconsistent, or that the quantities of the agents are too low to have any effect. [ 43 ] A 1997 survey of scientists in fields pertinent to evaluating estrogens found that 13 percent regarded the health threats from xenoestrogens as "major," 62 percent as "minor" or "none," and 25 percent were unsure. [ 44 ]
There has been speculation that falling sperm counts in males may be due to increased estrogen exposure in utero. [ 45 ] Sharpe in a 2005 review indicated that external estrogenic substances are too weak in their cumulative effects to alter male reproductive functioning, but indicates that the situation appears to be more complex as external chemicals may affect the internal testosterone -estrogen balance. [ 46 ]
The ubiquitous presence of such estrogenic substances is a significant health concern, both individually and for a population. Life relies on the transmission of biochemical information to the next generation, and the presence of xenoestrogens may interfere with this transgenerational information process through "chemical confusion" (Vidaeff and Sever), [ 47 ] who state: "The results do not support with certainty the view that environmental estrogens contribute to an increase in male reproductive disorders, neither do they provide sufficient grounds to reject such a hypothesis."
A 2008 report demonstrates further evidence of widespread effects of feminizing chemicals on male development in each class of vertebrate species as a worldwide phenomenon. [ 48 ] Ninety-nine percent of over 100,000 recently introduced chemicals are underregulated, according to the European Commission. [ 48 ]
Agencies such as the United States Environmental Protection Agency and the World Health Organization International Programme on Chemical Safety are charged to address these issues. [ citation needed ]
Puberty is a complex developmental process defined as the transition from childhood to adolescence and adult reproductive function. [ 13 ] [ 19 ] [ 49 ] [ 50 ] The first sign of female puberty is an acceleration of growth followed by the development of a palpable breast bud ( thelarche ). The median age of thelarche is 9.8 years. Although the sequence may be reversed, androgen dependent changes such as growth of axillary and pubic hair, body odor and acne (adrenarche) usually appears 2 years later. Onset of menstruation (menarche) is a late event (median 12.8 years), occurring after the peak of growth has passed. [ 13 ]
Puberty is considered precocious ( precocious puberty ) if secondary sex characteristics occur before the age of 8 in girls and 9 years in boys. [ 13 ] [ 15 ] Increased growth is often the first change in precocious puberty, followed by breast development and growth of pubic hair. However, thelarche , adrenarche , and accelerated growth can occur simultaneously and although uncommon, menarche can be the first sign. [ 13 ] Precocious puberty can be classified into central (gonadotropin-dependent) precocious puberty or peripheral (gonadotropin-independent) puberty. [ 13 ] [ 19 ] [ 50 ] [ 51 ] Both central and peripheral precocious puberty have been linked to exposure to exogenous estrogenic compounds. [ 50 ] [ 51 ]
Central precocious puberty is due to early maturation of the hypothalamic–pituitary–gonadal (HPG) axis. Majority of central precocious puberty cases are spontaneous or arise from an unknown cause, but some of these cases arise from organic lesions, environmental factors, and endocrine disrupting chemicals. [ 51 ] Central precocious puberty is most commonly caused through idiopathic (unknown) reasons in girls, but there is an increased risk of these organic causes for central precocious puberty in boys. [ 51 ]
Peripheral precocious puberty is independent of gonadotropin and thus does not activate the HPG axis. [ 51 ] Peripheral precocious puberty in females most commonly shows through ovarian follicular cysts, which may cause vaginal bleeding. [ 51 ] LH receptor activating mutations ( familial testotoxicosis ) are autosomal dominate diseases found in male children. [ 51 ] [ 52 ] These diseases are usually characterized by enlarged testis and can be an indication of peripheral precocious puberty in boys. [ 51 ]
Age of onset of puberty is influenced by many factors such as genetics, nutritional status, ethnicity and environmental factors including socio-economic conditions and geographical location. [ 3 ] [ 53 ] A decline of age at onset of puberty from 17 years of age to 13 years of age has occurred over a period of 200 years until the middle of the 20th century. [ 3 ] [ 16 ] [ 49 ] Trends toward earlier puberty have been attributed to improved public health and living conditions. [ 54 ] A leading hypothesis for this change toward early puberty is improved nutrition resulting in rapid body growth, increased weight and fat deposition. [ 55 ] However, recent studies have shown that chemical exposure to environmental estrogen disruptors the HPG axis and result in precocious puberty. [ 56 ] [ 57 ] In 1999, US Food and Drug Administration has recommended to not take estrogen in food of more than 0.43 ng/day for boys and 3.24 ng/day for females. [ 58 ] Two recent epidemiologic studies in the United States (PROS and NMANES III) [ 59 ] highlighted a recent unexpected advance in sexual maturation in girls. [ 3 ] [ 4 ] [ 60 ] American, European and Asian studies suggest breast development in girls occurs at a much younger age than a few decades ago, irrespective of race and socioeconomic conditions. [ 16 ] [ 49 ] [ 55 ] Environmental chemical exposure is one of the factors implicated in the recent downward trend of earlier sexual maturation. [ 16 ] [ 49 ] [ 60 ]
Epidemiology
The prevalence of precocious puberty is difficult to determine as it is highly variable depending on the population from which the data has been collected. The Danish national registry estimated that roughly 20-23 per 10,000 (0.2%) of girls and 5 per 10,000 (0.05%) of boys suffer from a form of precocious puberty. [ 61 ] An additional study conducted in Korea reported a where 55.9 per 100,000 girls and 1.7 per 100,000 boys indicated signs of central precocious puberty. [ 62 ]
Since 1979, pediatric endocrinologists in Puerto Rico recognized an increase in number of patients with premature thelarche . [ 63 ] The presence of phthalates were measured in the blood of 41 girls experiencing early onset breast development and matched set of controls. The average age of girls with premature thelarche was 31 months. They found high phthalate levels in the girls suffering from premature thelarche compared to the controls. [ 64 ] Not all cases of premature thelarche in the study sample contained elevated levels of phthalate esters and there was concern whether artificial contamination from vinyl lab equipment and tubing invalidated the results, hence weakening the link between exposure and causation. [ 63 ] [ 65 ]
Dr. Massart and colleagues from the University of Pisa studied the increased prevalence of precocious puberty in a region of northwest Tuscany. This region of Italy is represented by a high density of navy yards and greenhouses where exposures to pesticides and mycoestrogens (estrogens produced by fungi) are common. Although unable to identify a definitive cause of the high rates of precocious puberty, the authors concluded environmental pesticides and herbicides may be implicated. [ 66 ]
Animal feed was contaminated with several thousand pounds of polybrominated biphenyl in Michigan in 1973 resulting in high exposures of PBB in the population via milk and other products from contaminated cows. Perinatal exposure of children was estimated by measuring PBB in serum of mothers some years after exposure. Girls that had been exposed to high PBB levels through lactation had an earlier age of menarche and pubic hair development than girls who had less perinatal exposure. The study noted there no differences found in the timing of breast development among the cases and controls. [ 16 ] [ 20 ] [ 65 ]
The Great Lakes have been polluted with industrial wastes (mainly PCBs and DDT) since the beginning of the 20th century. These compounds have accumulated in birds and sports fish. A study was designed to assess the impact of consumption of contaminated fish on pregnant women and their children. Concentrations of maternal serum PCB and DDE and their daughters' age at menarche were reviewed. In multivariate analysis, DDE but not PCB was linked with a lowered age of menarche. [ 20 ] [ 63 ] [ 65 ] Limitations of the study included indirect measurement of the exposure and self reporting of menarche. [ 20 ]
Precocious puberty has numerous significant physical, psychological and social implications for young children. It has been associated with metabolic disorders ( insulin resistance and diabetes ), increased cardiometabolic risk (high blood pressure and cholesterol levels), obesity, [ 60 ] [ 67 ] increased cancer risk ( breast [ 60 ] and endometrial for girls and testicular for boys). [ 68 ] Precocious puberty is linked with other gynecologic disorders such as endometriosis, adenomyosis, polycystic ovarian syndrome and infertility. [ 17 ] [ 69 ] [ 70 ] Premature pubertal growth spurt and accelerated bone maturation will result in premature closure of distal epiphysis which causes reduced adult height and short stature. [ 67 ] Precocious puberty can lead to psychosocial distress , a poor self-image, and poor self-esteem. [ 71 ] Girls with secondary sex characteristics at such a young age are more likely to be bullied and suffer from sexual abuse. [ 17 ] [ 69 ] [ 71 ] Studies indicate that girls who become sexually mature at earlier ages are also more likely to engage in risk-taking behaviors such as smoking, alcohol or drug use, and engage in unprotected sex. [ 67 ] [ 71 ]
The current literature is inadequate to provide the information we need to assess the extent to which environmental chemicals contribute to precocious puberty. [ 60 ] Gaps in our knowledge are the result of limitations in the designs of studies, small sample sizes, challenges to conducting exposure assessment and the few number of chemicals studied. [ 60 ] Unfortunately exposure is inferred and not actually measured in available studies. [ 17 ] The ability to detect the possible role of chemicals in altering pubertal development is confounded by many nutritional, genetic and lifestyle factors capable of affecting puberty and the complex nature of the reproductive endocrine system. [ 55 ] [ 72 ] Other research challenges include shifts in exposure levels among populations over time and simultaneous exposures to multiple compounds. [ 72 ] Overall the literature does not with certainty support the contention that environmental chemicals or dietary factors are having widespread effects on human sexual development. However data does not refute such a hypothesis either. Accelerated sexual development is plausible in individuals exposed to high concentration of estrogenic substances. There is a concerning steady increase in exposure to a wide variety of xenoestrogens in the industrial world. Further research is needed to assess the impact of these compounds on pubertal development.
Non-human animal studies have shown that exposure to environmental contaminants with estrogenic activity can accelerate the onset of puberty. A potential mechanism has been described in rats exposed to DDT or beta-estradiol in which GnRH pulsatile secretion was found to be increased. [ 20 ] [ 73 ] Oral exposure of female rats to xenoestrogens has been shown to cause pseudo precocious puberty (early vaginal opening and early first estrus). [ 53 ] [ 74 ] [ 75 ] [ 76 ] A study of dioxin in immature female rats induced early follicular development [ 77 ] and phthalates are known to decrease the anogenital distance in newborn rats. [ 65 ] Although this article focuses on the effects of xenoestrogens and reproductive function in females, numerous animal studies also implicate environmental estrogens' and androgens' adverse effects on the male reproduction system. [ 77 ] Administration of estrogens to developing male animals reduces testicular weight and decreases sperm production. [ 18 ] The small phallus size of male alligators has been linked to contamination of their natural Florida habitat with DDT. [ 67 ] [ 77 ] Data from animal research is abundant demonstrating the adverse effects on reproduction of hormonally active compounds found in the environment. [ 18 ] [ 77 ] [ 78 ] [ 79 ]
Atrazine is widely used as an herbicide to control broad-leaf weed species that grow in crops such as corn, sugarcane, hay and winter wheat. Atrazine is also applied to Christmas trees, residential lawns, golf courses, and other recreational areas. Atrazine is the second largest selling pesticide in the world and estimated to be the most heavily used herbicide in the United States. [ 14 ] Atrazine has been implicated in interfering with the neuroendocrine system, blocking the release of gonadotropin-releasing hormone (GnRH) which in turn reduces luteinizing hormone (LH) and follicle stimulating hormone (FSH) levels. [ 80 ]
BPA ( Bisphenol A ) is the monomer used to manufacture polycarbonate plastic and epoxy resins used as a lining in most food and beverage cans. BPA global capacity is in excess of 6.4 billion pounds (2.9 × 10 9 kg) per year and thus is one of the highest-volume chemicals produced worldwide. [ 81 ] The ester bonds in the BPA-based polycarbonates could be subject to hydrolysis and leaching of BPA. But in the case of epoxypolymers formed from bisphenol A, it is not possible to release bisphenol A by such a reaction. It is also noteworthy that, of the bisphenols, bisphenol A is a weak xenoestrogen. Other compounds, such as bisphenol Z, have been shown to have stronger estrogenic effects in rats. [ 82 ]
It has been suggested that biphenol A and other xenoestrogens might cause disease to humans [ 72 ] and animals. [ 78 ] BPA exposure is linked to dysfunctions in human systems including the immune, neuroendocrine, and excretory systems. The damage that results in these dysfunctions is via the mechanisms of enzyme interference, cellular oxidation, epigenetic changes, and the breaking of DNA strands. [ 83 ]
Bisphenol S (BPS), an analog of BPA, has also been shown to alter estrogenic activity. [ 84 ] [ 85 ] One study demonstrated that when cultured rat pituitary cells were exposed to low levels of BPS, it altered the estrogen-estradiol signaling pathway and led to the inappropriate release of prolactin. [ 85 ]
DDT ( Dichlorodiphenyltrichloroethane ) was widely used in pesticides for agricultural purposes until it was banned in 1972 in the United States. DDT's hazardous effects on the environment include being linked to the production of fragile eggshells in birds and showed a 90% decline in the birth rates of alligators. [ 86 ] Though it is banned in the United States, DDT continues to be used in many parts of the world for agricultural use, insect control, and to fight the spread of malaria. [ 14 ] [ 17 ] [ 65 ] [ 78 ]
DDT and its metabolites DDE and DDD are persistent in the environment and accumulate in fatty tissues. In vertebrates, DDT is unable to be broken down and remains within the organism. There is little risk of DDT causing an increase in health risk upon exposure in adulthood, but in key developmental periods prenatally and in adolescence, there has been evidence to suggest an increased risk of breast cancer. [ 86 ]
Dioxin , a group of highly toxic chemicals are released during combustion processes, pesticide manufacturing and chlorine bleaching of wood pulp . Dioxin is discharged into waterways from pulp and paper mills. Consumption of animals fats is thought to be the primary pathway for human exposure. [ 14 ] [ 17 ] [ 54 ] The connection between dioxin and dioxin-like compound (DLC) exposure and human disease is one not well established. Bioassays performed in animals does not show a strong connection between the two. [ 87 ]
Endosulfan is an insecticide used on numerous vegetables, fruits, cereal grains and trees. Endosulfan can be produced as a liquid concentrate, wettable powder or smoke tablet. Human exposure occurs through food consumption or ground and surface water contamination. [ 14 ] [ 88 ] Endosulfan exposure is known to cause seizures that are the result of hyper-stimulation of the central nervous system ( CNS ). Upon significant exposure and accumulation in the system, toxicity of the major organs such as the heart, liver and kidneys has been reported and can lead to death within hours. [ 89 ]
Both PBBs and PBDEs belong to the same class of chemicals known as brominated flame retardants (BFRs). [ 90 ] PBBs ( Polybrominated biphenyls ) are chemicals added to plastics used in computer monitors, televisions, textiles and plastics foams to make them more difficult to burn. Manufacturing of PBBs in the United States stopped in 1976, however because they do not degrade easily. PBBs continue to be found in soil, water and air. PBDEs ( Polybrominated biphenyl ethers ) behave similarly to PBBs in that they are also a flame retardant. PBDEs are not chemically bound to the items they are attached to, and thus can leech into the environment. [ 91 ] [ 14 ] [ 20 ] [ 78 ]
PCBs ( Polychlorinated biphenyls ) are man made organic chemicals known as chlorinated hydrocarbons . PCBs were manufactured primarily for use as insulating fluids and coolants given their chemical stability, low flammability and electrical insulating properties. PCBs were banned in 1979 but, like DDT, continue to persist in the environment. [ 14 ] [ 17 ] [ 65 ] The effects of PCBs are not limited to the environment. There have been associations revealed between maternal PCB levels and conditions such as asthma, eczema, roseola , and upper respiratory infections. [ 92 ]
Phthalates are plasticizers providing durability and flexibility to plastics such as polyvinyl chloride. High molecular weight phthalates are used in flooring , wall coverings and medical device such as intravenous bags and tubing. Low molecular weight phthalates are found in perfumes, lotions, cosmetics, varnishes, lacquers and coatings including timed releases in pharmaceuticals. [ 14 ] [ 78 ] [ 93 ] Exposure to phthalates can have varying effects in humans depending on maturity. In adults, phthalate exposure has been linked to conditions like asthma, metabolic disorders like type II diabetes and insulin resistance, allergies, and asthma. In children, exposure to phthalates has a marked difference when compared to adults, having been associated with disrupted reproductive hormone levels and thyroid function. [ 94 ]
Zeranol is currently used as an anabolic growth promoter for livestock in the US [ 95 ] and Canada. [ 96 ] It has been banned in the EU since 1985, [ 97 ] but is still present as a contaminant in food through meat products that were exposed to it. [ 14 ]
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Xenogamy (Greek xenos =stranger, gamos =marriage) is the transfer of pollen grains from the anther to the stigma of a different plant. This is the only type of cross pollination which during pollination brings genetically different types of pollen grains to the stigma. [ 1 ]
The term xenogamy (along with geitonogamy and autogamy ) was first suggested by Kerner in 1876. [ 2 ] Cross-pollination involves the transfer of pollen grains from the flower of one plant to the stigma of the flower of another plant.
The main characteristics which facilitate cross-pollination are:
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Xenohormesis is a hypothesis that posits that certain molecules such as plant polyphenols , which indicate stress in the plants, can have benefits for another organism ( heterotrophs ) that consumes it. Or in simpler terms, xenohormesis is interspecies hormesis . The expected benefits include improve lifespan and fitness, by activating the animal's cellular stress response. [ 1 ]
This may be useful to evolve, as it gives possible cues about the state of the environment. If the plants an animal is eating have increased polyphenol content, it means the plant is under stress and may signal famines. Using the chemical cues the heterotophs could preemptively prepare and defend itself before conditions worsen. A possible example may be resveratrol , which is famously found in red wine, which modulates over two dozen receptors and enzymes in mammals. [ 1 ]
Xenohormesis could also explain several phenomena seen in the ethno-pharmaceutical (traditional medicine) side of things. Such as in the case of cinnamon , which in a few studies has been found to have an effect on lipid, glucose, and HbA1c levels in type 2 diabetes , but upon meta analysis was found to have no significant effect. One group of authors suggested this might be caused by the cinnamon used in one study differing from the other in xenohormetic properties. [ 2 ]
Some explanations as to why this works, is first and foremost, it could be a coincidence. Especially for cases which partially venomous products, cause a positive stress in the organism. The second is that it is a shared evolutionary attribute, as both animals and plants share a huge amount of homology between their pathways. The third is that there is evolutionary pressure to evolve to better respond to the molecules. The latter is proposed mainly by Howitz and his team. [ 2 ]
There also might be the problem that our focus on maximizing the crop output, may be losing many of the xenohormetic advantages. Under the xenohormesis hypothesis, although the ideal conditions will cause the plant to increase its crop output, the reduced stress of the ideal conditions might reduce beneficial xenohormetic effects in humans.
The term xenohormesis was first coined by Kondrad T. Howitz and David A. Sinclair , in the 2004 paper "Small molecules that regulate lifespan: evidence for xenohormesis". [ 3 ] Xeno comes from Greek, meaning foreign, and hormesis is the adaptive response of organisms and cells to stress. [ 1 ]
There are many applications for this products, mainly found in the micronutrients. One of the most obvious ones is in the pharmaceutical section. Such as the antimalarial artemisinin which is synthesised from Artemisia annua . The density of this product also increases with stress, which is probably not a coincidence, however not in a case of mutualistic xenohormesis. [ 2 ]
It has also been shown that it affects macro nutrients, cold shock may increase the level of unsaturated fatty acids . [ 2 ]
Some times xenohormesis is targeted to specific organisms. Such as in the case Acacias , which produce food bodies , which the ants consume, in turn protecting the tree. If a herbivore feeds on an Acacia, it will release more of these bodies. In turn attracting more ants and/or wasps. There are also many cases of mutualistic xenohormesis for the dispersal of a seeds and pollen. Though not all xenohormesis is mutualistic. [ 2 ]
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Xenohormones or environmental hormones are compounds produced outside of the human body that exhibit endocrine hormone -like properties. They may be either of natural origin, such as phytoestrogens , which are derived from plants, or of synthetic origin. These compounds can cause endocrine disruption by multiple mechanisms including acting directly on hormone receptors, affecting the levels of natural hormones in the body, and by altering the expression of hormone receptors. [ 1 ] [ 2 ] The most commonly occurring xenohormones are xenoestrogens , which mimic the effects of estrogen . Other xenohormones include xenoandrogens (anabolic-androgenic steroids) and xenoprogesterones. [ 3 ] [ 4 ] Xenohormones are used for a variety of purposes including contraceptive & hormonal therapies, and agriculture. However, exposure to certain xenohormones early in childhood development can lead to a host of developmental issues including infertility, thyroid complications, and early onset of puberty. Exposure to others later in life has been linked to increased risks of testicular, prostate, ovarian, and uterine cancers.
The term is derived from the Greek words ξένος ( xenos ), meaning "stranger". [ 5 ] The prefix "xeno-" is added because xenohormones are foreign to the body, even though they mimic natural hormones. [ citation needed ]
Xenohormones can come from a variety of sources, both natural and man-made. Man-made xenoestrogens are often found in cosmetic products, some foods, certain pharmaceuticals, plastic products, flame retardants, and pesticides. [ 6 ] [ 7 ] Naturally occurring xenoestrogens include phytoestrogens (estrogen-like compounds from plants) and mycoestrogens (estrogen-like compounds from fungi). [ 8 ]
While natural xenohormones exist, there are not as many compounds found in nature which are capable of interacting with human androgen receptors , so humans are most likely to come into contact with man-made xenoandrogens by taking anabolic steroids or through pollutants which contain xenoandrogens. "Organochlorine pesticides, polychlorinated biphenyls (PCBs), and polychlorinated dibenzo- p -dioxins /dibenzofurans (PCDDs/PCDFs)" are several pesticides known to contain xenoandrogens. [ 9 ]
Xenohormones are found in a variety of different consumer products, agricultural products, and chemicals. Common sources of xenohormones include:
There are many alternatives to prevent pregnancy to stop using oral contraceptives that use xenohormones such as Dropspirenone and Ethinyl Estroidal . Non-hormonal birth control includes, but not subject to the use of a: diaphragm, cervical cap, sponge, copper IUD, spermicide, vaginal gel, male condom, and female condom. [ 11 ]
When present in excessive amounts in the human body, xenohormones can cause a host of health issues due to their disruption of the endocrine system. The name given to these exogenous (coming from an external source) hormones is endocrine disruptors , due to their tendency to mimic the behaviors of naturally produced bodily hormones. [ 15 ] Endocrine disruptors have also been found to affect the levels and behaviors of a number of other bodily hormones. Because of this, it is difficult to establish a definitive relationship between xenohormones and health problems, making effects hard to predict.
Xenohormones pose a problem because they are retained by the body in fat tissue for a very long time. As we are exposed more and more to these chemicals, they build up within the body in ever-increasing amounts, a process known as bioaccumulation . With the amount of xenohormones we are exposed to every day, the health effects of xenohormones are becoming more relevant.
The human body's endocrine system functions through hormones that act as messengers within the body. Hormones, upon release, travel through the body to their receptors and trigger a physiological response. Hormones naturally work at very low concentrations in the body. This means that even low concentrations of xenohormones in the body can act as an excess and have a profound effect on the body's endocrine system.
The levels of hormones present in the body at any given time are tightly controlled through feedback mechanisms. When xenohormones are present in the body, they alter the levels of hormones in the body and therefore alter the feedback mechanisms that the endocrine system relies on.
Xenohormones can interact with the human endocrine system because they are structurally similar to natural hormones. This similarity allows for xenohormones to act on hormone receptors, usually either as an agonist or antagonist . [ 15 ] Agonists activate a receptor by binding to the receptor, enhancing the effect of the natural hormone. Antagonists inhibit the activation of a receptor by preventing the binding of the natural hormone to its receptor. In this way, xenohormones act as endocrine disruptors by increasing or decreasing the activation of hormone receptors in the body.
Xenohormones can often act on multiple hormone receptor types and enact multiple different effects. For example, BPA acts as an agonist of estrogen receptors and as an antagonist of androgen receptors. [ 15 ] Methoxychlor is an organochlorine pesticide that can act on both estrogen receptors and androgen receptors. [ 2 ]
Other than having effects by directly acting on the endocrine receptors, xenohormones can also act to decrease the availability of natural hormones. Phthalates inhibit testosterone synthesis and decrease the production of natural androgens in the body. [ 2 ] Dioxins and some organochlorine pesticides (OCPs) can cause increased metabolism of estrogen, decreasing the amount of estrogen in the body. [ 2 ] Xenohormones can also alter the expression of hormone receptors to either increase or decrease the amount of receptors available in tissues. [ 2 ]
When xenohormone exposure occurs during the early developmental stages of life, the effects tend to be permanent. The consequences of excessive xenohormone exposure in adulthood are different, and typically more temporary in nature. This is to say that the health risks can be minimized if the individual is removed from their state of excessive exposure. Xenohormone-related issues in adults frequently take the form of increased cancer risk in reproductive/secondary sexual areas (breast, uterine, ovarian, prostate, and testicular).
Xenoestrogens are xenohormones that mimic the effects of natural estrogen. When present in the body, xenoestrogens can bind with estrogen receptors in the brain, leading to a disruption in the gonadal endocrine system.
Xenoestrogen exposure during different developmental periods can have differing effects on the reproductive system. Prenatal and perinatal exposure results in greater reproductive defects than exposure in adult life. [ 2 ]
The negative effects of excessive xenoestrogen involve a long list of developmental abnormalities, especially when the exposure occurs during a critical postnatal period . When high levels of xenoestrogen are experienced shortly after birth, urogenital tract and nervous system development are hindered, as they are known to be especially sensitive to hormonal disruption.
Known xenoestrogens include bisphenol A (BPA) , the organochlorine pesticide methoxychlor , and the insecticide endosulfan . [ 2 ]
Xenoandrogens are xenohormones that mimic the effects of natural androgen hormones. Androgen hormones are often associated with males and include the major hormone testosterone. [ 16 ] Androgens work on the metabolic system playing roles in muscle growth, bone formation, and endocrine function. [ 16 ]
There are not many compounds found in nature that are capable of interacting with human androgen receptors , so humans are most likely to come into contact with man-made xenoandrogens by taking anabolic steroids or through pollutants that contain xenoandrogens. Organochlorine pesticides, polychlorinated biphenyls (PCBs), and polychlorinated dibenzo- p -dioxins /dibenzofurans (PCDDs/PCDFs) are several pesticides known to contain xenoandrogens.
Research indicates that exposure to certain xenohormones can result in severe health risks, including infertility, early onset puberty, thyroid problems, endometriosis, and certain types of cancers. The effect that xenohormones have on the health of humans is complex with a wide range and can effect multiple systems and process within the human body. It has also been claimed that certain xenoestrogens, most notably phytoestrogens and mycoestrogens can potentially have beneficial health effects, though it is not yet clear to what extent the benefits are present or whether they outweigh the possible health risks of these compounds. [ 17 ] [ 18 ] Xenohormones and other endocrine disrupting compounds (EDCs) can block and disrupt the natural function of hormones and the functionality of endocrine system in the human body, so conditions related to hormone imbalance or an improperly functioning endocrine system are possible after exposure. [ 19 ] The specific risk that a xenohormone has on a person is based on the exposure level and their biological make up. There has been difficult comprehensive research on the specific outcomes on the relationship of xenohormone exposure and its effect on the health of humans. There have also been research on ways to mitigate the risk and its threats to public health like the development of a biomarker to identify and measure the level of xenohormones in blood to approximate the level of exposure. [ 20 ]
Xenohormones can negatively impact the reproductive health of men by disrupting their hormone levels and sperm production which leads to reduced fertility. There is also an increased rate of prostate cancer or prostate related disorders in males due to xenohormone exposure. There are also studies done on how during certain stages of development xenohormone exposure can effect sexual maturation and the development of secondary sexual characteristics in males. [ 21 ]
Xenohormones can have a negative impact on the reproductive health of females by disrupting their hormone levels and the natural flow of their menstrual cycles. This disruption can lead to miscarriages, fertility problems, and irregular ovulation. Hormone imbalances in females caused by xenohormones caused health issues like thyroid cancer and polycystic ovary syndrome. There is also an increased risk on breast cancer or breast related disorders depending onto length of exposure to xenohormones. In older women going through menopause the exposure to xenohormones can cause a dramatic increase in their menopausal symptoms from hot flashed to severe mood swings. [ 22 ]
Researchers suspect that xenohormones promotes the growth of cancer by causing aboral cell growth and playing a role in altering hormone levels. Certain xenohormones have been detected in the breast tissue of humans with breast cancer, which hints at a correlation between xenoestrogen exposure and breast cancer. This can occur in both men and women, although women may be more likely to develop breast cancer from xenohormones due to the popularity of cosmetic products that contain endocrine disruptors among women. It is also important to note that it may also be the case that women simply develop breast cancer in general more often than men, as there is no conclusive evidence that xenoestrogen-related breast cancers are more common among women than men after adjusting for the differing rates of breast cancer. Xenohormones are also linked to increased risks of susceptibility to testicular, prostate, ovarian, and uterine cancers. [ 23 ] [ 24 ]
Several women treated with xenoestrogens while pregnant showed signs an effect to the central nervous system of their offspring leading to them having psychiatric/somatic disorders. There was a significant amount of offspring treated with xenohormones during the prenatal period that showed an alteration of genes will lead to psychosis and other psychiatric disorders during their neurodevelopment. The primary part of the brain affected by the prenatal xenohormone exposure to the offspring and mother were the hippocampus and amygdala. There was also evidence of autism spectrum disorders and learning disabilities in addition to the mood and behavior disorders found in the offspring of mothers treated with xenohormones. The effects found to be transgenerational so that if the offspring grew to have children as an adult the alternation of genes caused by xenohormones in the offspring carry over to future offspring for multiple generations. [ 25 ]
The use of xenohormones in both agriculture and industry raises concerns about their effect on the environment and public health. Xenohormones have been observed to contaminate food and water through the use of pesticides, hormone treatments in livestock, and plastic packing such as water bottles. [ 26 ]
In addition to posing health threats for humans, EDCs in the environment also pose health risks to wildlife. [ 27 ] Common source of EDCs in the environment are agricultural pesticides, which are often effective due to their effects on the endocrine systems of pest species. [ 28 ] A well known example of a xenohormone having detrimental effects on wildlife is the pesticide xenoestrogen DDT, which causes reproductive defects in birds and can persist in the environment, leading to bioaccumulation in the food chain. [ 28 ] [ 29 ] However, it can be difficult to accurately measure the effects of EDCs on the environment, as the extent of the influence a compound can have on an organism can vary across taxa. [ 28 ] Pesticides which affect the endocrine system of a target species can have led to unintended effects on other species. [ 28 ] For example, PCBs can interrupt animal fetal development, cause changes in an animal's response to stress, and cause thyroid and immune function diseases. [ 27 ] Plastics specifically pose a commendable environment threat due to the fact that many of them do not decompose. Xenohormones in plastic litter have the potential to contaminate natural water sources and expose both humans and wildlife to a variety of different EDCs. [ 26 ]
Regulations on the use of xenohormone-containing substances vary by country. In the United States, the Endocrine Disruptor Screening and Testing Advisory Committee was formed in 1996 and developed the Endocrine Disruptor Screening Program (EDSP). [ 30 ] [ 31 ] The EDSP is used by the EPA and other regulatory bodies to screen chemicals such as pesticides and potential environmental pollutants for their effects on the endocrine systems of humans and wildlife. [ 31 ] Because xenohormones such as BPA have demonstrated health concerns for humans and animals, both the Environmental Protection Agency (EPA) and FDA have conducted research and issued statements and regulations to reduce their impact on public health and the environment. [ 32 ] In March 2010, the EPA published its Bisphenol A (BPA) Action Plan, which details measures to reduce the impact of BPA on aquatic species. [ 32 ] Under this action plan, the EPA is considering listing BPA as a substance that may present a significant risk to the environment on the Toxic Substances Control Act Concern List. [ 32 ] The EU banned pesticides from containing EDCs in the passing of the 2009 Plant Protection Products Regulation as well as the 2012 Biocidal Products Regulation. [ 33 ]
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Xenointoxication is a form of pest control in which an ectoparasite 's host animal is dosed with a substance that is poisonous to the parasite. When the parasite feeds on its host, it is poisoned, and eventually dies. [ citation needed ]
An example of this strategy is the experimental use of oral ivermectin in humans to kill bed bugs and parasitic worms . [ 1 ] [ 2 ] This technique has also been used to combat other ectoparasites. [ 3 ] [ 4 ]
This method was unsuccessful in a 1969 study attempting to control Triatoma infestans in chicken houses because even though some bugs that fed on the treated birds did die, so did the birds, and the birds that survived produced fewer eggs. [ 5 ]
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Xenon-135 ( 135 Xe ) is an unstable isotope of xenon with a half-life of about 9.2 hours. 135 Xe is a fission product of uranium and it is the most powerful known neutron -absorbing nuclear poison (2 million barns ; [ 1 ] up to 3 million barns [ 1 ] under reactor conditions [ 2 ] ), with a significant effect on nuclear reactor operation. The ultimate yield of xenon-135 from fission is 6.3%, though most of this is from fission-produced tellurium-135 and iodine-135 .
In a typical nuclear reactor fueled with uranium-235 , the presence of 135 Xe as a fission product presents designers and operators with problems due to its large neutron cross section for absorption. Because absorbing neutrons can impair a nuclear reactor's ability to increase power, reactors are designed to mitigate this effect and operators are trained to anticipate and react to these transients. This practice dates to the first fission piles , constructed by the Manhattan Project during the Second World War . Enrico Fermi suspected that 135 Xe would act as a powerful neutron poison and followed the advice of Emilio Segrè by contacting his student Chien-Shiung Wu . Wu's unpublished paper on 135 Xe verified Fermi's guess that it absorbed neutrons and was the cause of the disruptions to the B Reactor then in use at Hanford , Washington to breed plutonium for the American implosion bomb . [ 3 ] [ 4 ]
During periods of steady state operation at a constant neutron flux level, the 135 Xe concentration builds up to its equilibrium value for that reactor power in about 40 to 50 hours. When the reactor power is increased, 135 Xe concentration initially decreases because the burn up is increased at the new higher power level. Because 95% of the 135 Xe production is from decay of 135 I , which has a 6.57 hour half-life, the production of 135 Xe remains constant; at this point, the 135 Xe concentration reaches a minimum. The concentration then increases to the new equilibrium level (more accurately steady state level) for the new power level in roughly 40 to 50 hours. During the initial 4 to 6 hours following the power change, the magnitude and the rate of change of concentration is dependent upon the initial power level and on the amount of change in power level; the 135 Xe concentration change is greater for a larger change in power level. When reactor power is decreased, the process is reversed. [ 5 ]
Iodine-135 is a fission product of uranium with a yield of about 6% (counting also the 135 I produced almost immediately from decay of fission-produced tellurium-135). [ 6 ] This 135 I decays with a 6.57 hour half-life to 135 Xe. Thus, in an operating nuclear reactor, 135 Xe is being continuously produced. 135 Xe has a very large neutron absorption cross-section, so in the high-neutron-flux environment of a nuclear reactor core, the 135 Xe soon absorbs a neutron and becomes effectively stable 136 Xe . (The half life of 136 Xe is >10 21 years, and it is not treated as a radioisotope.) Thus, in about 50 hours, the 135 Xe concentration reaches equilibrium where its creation by 135 I decay is balanced with its destruction by neutron absorption.
When reactor power is decreased or shut down by inserting neutron-absorbing control rods, the reactor neutron flux is reduced and the equilibrium shifts initially towards higher 135 Xe concentration. The 135 Xe concentration peaks about 11.1 hours after reactor power is decreased. Since 135 Xe has a 9.2 hour half-life, the 135 Xe concentration gradually decays back to low levels over 72 hours.
The temporarily high level of 135 Xe with its high neutron absorption cross-section makes it difficult to restart the reactor for several hours. The neutron-absorbing 135 Xe acts like a control rod, reducing reactivity. The inability of a reactor to be started due to the effects of 135 Xe is sometimes referred to as xenon-precluded start-up, and the reactor is said to be "poisoned out". [ 7 ] The period of time that the reactor is unable to overcome the effects of 135 Xe is called the "xenon dead time".
If sufficient reactivity control authority is available, the reactor can be restarted, but the xenon burn-out transient must be carefully managed. As the control rods are extracted and criticality is reached, neutron flux increases many orders of magnitude and the 135 Xe begins to absorb neutrons and be transmuted to 136 Xe . The reactor burns off the nuclear poison. As this happens, the reactivity and neutron flux increases, and the control rods must be gradually reinserted to counter the loss of neutron absorption by the 135 Xe. Otherwise, the reactor neutron flux will continue to increase, burning off even more xenon poison, on a path to runaway criticality . The time constant for this burn-off transient depends on the reactor design, power level history of the reactor for the past several days, and the new power setting. For a typical step up from 50% power to 100% power, 135 Xe concentration falls for about 3 hours. [ 8 ]
Xenon poisoning was a contributing factor to the Chernobyl disaster ; during a run-down to a lower power, a combination of operator error and xenon poisoning caused the reactor thermal power to fall to near-shutdown levels. The crew's resulting efforts to restore power placed the reactor in a highly unsafe configuration. A flaw in the SCRAM system inserted positive reactivity, causing a thermal transient and a steam explosion that tore the reactor apart.
Reactors using continuous reprocessing like many molten salt reactor designs might be able to extract 135 Xe from the fuel and avoid these effects. Fluid fuel reactors cannot develop xenon inhomogeneity because the fuel is free to mix. Also, the Molten Salt Reactor Experiment demonstrated that spraying the liquid fuel as droplets through a gas space during recirculation can allow xenon and krypton to leave the fuel salts. Removing 135 Xe from neutron exposure improves neutron economy, but causes the reactor to produce more of the long-lived fission product 135 Cs . The long lived (but 76000 times less radioactive) caesium-135 condenses in a separate tank after the decay of 135 Xe, and is physically separate from the 30.05 year half life caesium-137 ( 137 Cs) produced in the fuel, and it is practical to handle them separately (fission yield is approximately 6% for both).
A 135 Xe atom that does not capture a neutron undergoes beta decay to 135 Cs , one of the 7 long-lived fission products , while a 135 Xe that does capture a neutron becomes almost-stable 136 Xe.
The probability of capturing a neutron before decay varies with the neutron flux, which itself depends on the kind of reactor, fuel enrichment and power level; and the 135 Cs / 136 Xe ratio switches its predominant branch very near usual reactor conditions.
Estimates of the proportion of 135 Xe during steady-state reactor operation that captures a neutron include 90%, [ 9 ] 39%–91% [ 10 ] and "essentially all". [ 11 ] For instance, in a (somewhat high) neutron flux of 10 14 n·cm −2 ·s −1 , the xenon cross section of σ = 2.65 × 10 −18 cm 2 ( 2.65 × 10 6 barn) would lead to a capture probability of 2.65 × 10 −4 s −1 , which corresponds to a half-life of about one hour. Compared to the 9.17 hour half-life of 135 Xe, this nearly ten-to-one ratio means that under such conditions, essentially all 135 Xe would capture a neutron before decay. But if the neutron flux is lowered to one-tenth of this value, like in CANDU reactors, the ratio would be 50-50, and half the 135 Xe would decay to 135 Cs before neutron capture.
136 Xe from neutron capture ends up as part of the eventual stable fission xenon which also includes 134 Xe, 132 Xe, and 131 Xe produced by fission and beta decay rather than neutron capture.
Nuclei of 133 Xe, 137 Xe, and 135 Xe that have not captured a neutron all beta decay to isotopes of caesium . Fission produces 133 Xe, 137 Xe, and 135 Xe in roughly equal amounts but, after neutron capture, fission caesium contains more stable 133 Cs (which however can become 134 Cs on further neutron activation ) and highly radioactive 137 Cs than 135 Cs .
Large thermal reactors with low flux coupling between regions may experience spatial power oscillations [ 12 ] because of the non-uniform presence of xenon-135. Xenon-induced spatial power oscillations occur as a result of rapid perturbations to power distribution that cause the xenon and iodine distribution to be out of phase with the perturbed power distribution. This results in a shift in xenon and iodine distributions that causes the power distribution to change in an opposite direction from the initial perturbation.
The instantaneous production rate of xenon-135 is dependent on the iodine-135 concentration and therefore on the local neutron flux history. On the other hand, the destruction rate of xenon-135 is dependent on the instantaneous local neutron flux.
The combination of delayed generation and high neutron-capture cross section produces a diversity of impacts on nuclear reactor operation. The mechanism is described in the following four steps.
With little change in overall power level, these oscillations can significantly change the local power levels. This oscillation may go unnoticed and reach dangerous local flux levels if only the total power of the core is monitored. Therefore, most PWRs use tandem power range excore neutron detectors to monitor upper and lower halves of the core separately.
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Xenon compounds are compounds containing the element xenon (Xe). After Neil Bartlett's discovery in 1962 that xenon can form chemical compounds, a large number of xenon compounds have been discovered and described. Almost all known xenon compounds contain the electronegative atoms fluorine or oxygen. The chemistry of xenon in each oxidation state is analogous to that of the neighboring element iodine in the immediately lower oxidation state. [ 1 ]
Three fluorides are known: XeF 2 , XeF 4 , and XeF 6 . XeF is theorized to be unstable. [ 2 ] These are the starting points for the synthesis of almost all xenon compounds.
The solid, crystalline difluoride XeF 2 is formed when a mixture of fluorine and xenon gases is exposed to ultraviolet light. [ 3 ] The ultraviolet component of ordinary daylight is sufficient. [ 4 ] Long-term heating of XeF 2 at high temperatures under an NiF 2 catalyst yields XeF 6 . [ 5 ] Pyrolysis of XeF 6 in the presence of NaF yields high-purity XeF 4 . [ 6 ]
The xenon fluorides behave as both fluoride acceptors and fluoride donors, forming salts that contain such cations as XeF + and Xe 2 F + 3 , and anions such as XeF − 5 , XeF − 7 , and XeF 2− 8 . The green, paramagnetic Xe + 2 is formed by the reduction of XeF 2 by xenon gas. [ 1 ]
XeF 2 also forms coordination complexes with transition metal ions. More than 30 such complexes have been synthesized and characterized. [ 5 ]
Whereas the xenon fluorides are well characterized, the other halides are not. Xenon dichloride , formed by the high-frequency irradiation of a mixture of xenon, fluorine, and silicon or carbon tetrachloride , [ 7 ] is reported to be an endothermic, colorless, crystalline compound that decomposes into the elements at 80 °C. However, XeCl 2 may be merely a van der Waals molecule of weakly bound Xe atoms and Cl 2 molecules and not a real compound. [ 8 ] Theoretical calculations indicate that the linear molecule XeCl 2 is less stable than the van der Waals complex. [ 9 ] Xenon tetrachloride and xenon dibromide are more unstable that they cannot be synthesized by chemical reactions. They were created by radioactive decay of 129 ICl − 4 and 129 IBr − 2 , respectively. [ 10 ] [ 11 ]
Three oxides of xenon are known: xenon trioxide ( XeO 3 ) and xenon tetroxide ( XeO 4 ), both of which are dangerously explosive and powerful oxidizing agents, and xenon dioxide (XeO 2 ), which was reported in 2011 with a coordination number of four. [ 12 ] XeO 2 forms when xenon tetrafluoride is poured over ice. Its crystal structure may allow it to replace silicon in silicate minerals. [ 13 ] The XeOO + cation has been identified by infrared spectroscopy in solid argon . [ 14 ]
Xenon does not react with oxygen directly; the trioxide is formed by the hydrolysis of XeF 6 : [ 15 ]
XeO 3 is weakly acidic, dissolving in alkali to form unstable xenate salts containing the HXeO − 4 anion. These unstable salts easily disproportionate into xenon gas and perxenate salts, containing the XeO 4− 6 anion. [ 16 ]
Barium perxenate, when treated with concentrated sulfuric acid , yields gaseous xenon tetroxide: [ 7 ]
To prevent decomposition, the xenon tetroxide thus formed is quickly cooled into a pale-yellow solid. It explodes above −35.9 °C into xenon and oxygen gas, but is otherwise stable.
A number of xenon oxyfluorides are known, including XeOF 2 , XeOF 4 , XeO 2 F 2 , and XeO 3 F 2 . XeOF 2 is formed by reacting OF 2 with xenon gas at low temperatures. It may also be obtained by partial hydrolysis of XeF 4 . It disproportionates at −20 °C into XeF 2 and XeO 2 F 2 . [ 17 ] XeOF 4 is formed by the partial hydrolysis of XeF 6 ... [ 18 ]
...or the reaction of XeF 6 with sodium perxenate, Na 4 XeO 6 . The latter reaction also produces a small amount of XeO 3 F 2 .
XeO 2 F 2 is also formed by partial hydrolysis of XeF 6 . [ 19 ]
XeOF 4 reacts with CsF to form the XeOF − 5 anion, [ 17 ] [ 20 ] while XeOF 3 reacts with the alkali metal fluorides KF , RbF and CsF to form the XeOF − 4 anion. [ 21 ]
Xenon can be directly bonded to a less electronegative element than fluorine or oxygen, particularly carbon . [ 22 ] Electron-withdrawing groups, such as groups with fluorine substitution, are necessary to stabilize these compounds. [ 16 ] Numerous such compounds have been characterized, including: [ 17 ] [ 23 ]
Other compounds containing xenon bonded to a less electronegative element include F–Xe–N(SO 2 F) 2 and F–Xe–BF 2 . The latter is synthesized from dioxygenyl tetrafluoroborate, O 2 BF 4 , at −100 °C. [ 17 ] [ 24 ]
An unusual ion containing xenon is the tetraxenonogold(II) cation, AuXe 2+ 4 , which contains Xe–Au bonds. [ 25 ] This ion occurs in the compound AuXe 4 (Sb 2 F 11 ) 2 , and is remarkable in having direct chemical bonds between two notoriously unreactive atoms, xenon and gold , with xenon acting as a transition metal ligand. A similar mercury complex (HgXe)(Sb 3 F 17 ) (formulated as [HgXe 2+ ][Sb 2 F 11 – ][SbF 6 – ]) is also known. [ 26 ] Xenon reversibly complexes gaseous M(CO) 5 , where M=Cr, Mo, or W. p -block metals also bind noble gases: XeBeO has been observed spectroscopically and both XeBeS and FXeBO are predicted stable. [ 27 ]
The compound Xe 2 Sb 2 F 11 contains a Xe–Xe bond, the longest element-element bond known (308.71 pm = 3.0871 Å ). [ 28 ]
In 1995, M. Räsänen and co-workers, scientists at the University of Helsinki in Finland , announced the preparation of xenon dihydride (HXeH), and later xenon hydride-hydroxide (HXeOH), hydroxenoacetylene (HXeCCH), and other Xe-containing molecules. [ 29 ] In 2008, Khriachtchev et al. reported the preparation of HXeOXeH by the photolysis of water within a cryogenic xenon matrix. [ 30 ] Deuterated molecules, HXeOD and DXeOH, have also been produced. [ 31 ]
In addition to compounds where xenon forms a chemical bond , xenon can form clathrates —substances where xenon atoms or pairs are trapped by the crystalline lattice of another compound. One example is xenon hydrate (Xe· 5 + 3 ⁄ 4 H 2 O), where xenon atoms occupy vacancies in a lattice of water molecules. [ 32 ] This clathrate has a melting point of 24 °C. [ 33 ] The deuterated version of this hydrate has also been produced. [ 34 ] Another example is xenon hydride (Xe(H 2 ) 8 ), in which xenon pairs ( dimers ) are trapped inside solid hydrogen . [ 35 ] Such clathrate hydrates can occur naturally under conditions of high pressure, such as in Lake Vostok underneath the Antarctic ice sheet. [ 36 ] Clathrate formation can be used to fractionally distill xenon, argon and krypton. [ 37 ]
Xenon can also form endohedral fullerene compounds, where a xenon atom is trapped inside a fullerene molecule. The xenon atom trapped in the fullerene can be observed by 129 Xe nuclear magnetic resonance (NMR) spectroscopy. Through the sensitive chemical shift of the xenon atom to its environment, chemical reactions on the fullerene molecule can be analyzed. These observations are not without caveat, however, because the xenon atom has an electronic influence on the reactivity of the fullerene. [ 38 ]
When xenon atoms are in the ground energy state , they repel each other and will not form a bond. When xenon atoms becomes energized, however, they can form an excimer (excited dimer) until the electrons return to the ground state . This entity is formed because the xenon atom tends to complete the outermost electronic shell by adding an electron from a neighboring xenon atom. The typical lifetime of a xenon excimer is 1–5 nanoseconds, and the decay releases photons with wavelengths of about 150 and 173 nm . [ 39 ] [ 40 ] Xenon can also form excimers with other elements, such as the halogens bromine , chlorine , and fluorine . [ 41 ]
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Xenon dichloride (XeCl 2 ) is a xenon compound and the only known stable chloride of xenon . The compound can be prepared by using microwave discharges towards the mixture of xenon and chlorine, and it can be isolated from a condensate trap . One experiment [ which? ] tried to use xenon, chlorine and boron trichloride to produce XeCl 2 ·BCl 3 , but only generated xenon dichloride. [ 1 ]
However, it is still doubtful whether xenon dichloride is a true compound or a Van der Waals molecule composed of a xenon atom and a chlorine molecule connected by a secondary bond . [ 2 ]
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Xenon dioxide , or xenon(IV) oxide , is a compound of xenon and oxygen with formula XeO 2 which was synthesized in 2011. It is synthesized at 0 °C by hydrolysis of xenon tetrafluoride in aqueous sulfuric acid : [ 2 ]
XeO 2 has an extended (chain or network) structure in which xenon and oxygen have coordination numbers of four and two respectively. The geometry at xenon is square planar , consistent with VSEPR theory for four ligands and two lone pairs (or AX 4 E 2 in the notation of VSEPR theory).
The XeO 2 network does not share a crystal structure of SiO2 (which has tetrahedral coordination at Si), but XeO 2 units are believed to intermix with SiO 2 in Earth's mantle . Computational studies suggest that xenon cannot displace silicon directly, but can fill pre-existing silicon vacancies. The stability of the resulting material under standard conditions depends on its allotrope . Patterned off quartz, it likely decomposes; but materials patterned off fibrous silica may be metastable. [ 3 ]
In addition, the existence of an XeO 2 molecule was predicted by an ab initio quantum chemistry method several years earlier by Pyykkö and Tamm, but these authors did not consider an extended structure. [ 4 ]
XeO 2 is a yellow-orange solid. [ 5 ] It is an unstable compound, with a half-life of about two minutes, disproportionating into XeO 3 and xenon gas. Its structure and identity was confirmed by cooling it to −150 °C so that Raman spectroscopy could be performed before it decomposed. [ 2 ] [ 1 ]
At -78 °C, the majority of XeO 2 decomposed over a period of 72 hours, which was identified by the fading of the original yellow product to a pale yellow. Almost all yellow color indicating pure XeO 2 disappeared over the span of 1 week. [ 2 ]
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Xenon isotope geochemistry uses the abundance of xenon (Xe) isotopes and total xenon to investigate how Xe has been generated, transported, fractionated, and distributed in planetary systems. Xe has nine stable or very long-lived isotopes. Radiogenic 129 Xe and fissiogenic 131,132,134,136 Xe isotopes are of special interest in geochemical research . [ 1 ] The radiogenic and fissiogenic properties can be used in deciphering the early chronology of Earth. [ 2 ] [ 3 ] Elemental Xe in the atmosphere is depleted and isotopically enriched in heavier isotopes relative to estimated solar abundances. [ 1 ] The depletion and heavy isotopic enrichment can be explained by hydrodynamic escape to space that occurred in Earth's early atmosphere. Differences in the Xe isotope distribution between the deep mantle (from Ocean Island Basalts, or OIBs ), shallower Mid-ocean Ridge Basalts ( MORBs ), and the atmosphere can be used to deduce Earth's history of formation and differentiation of the solid Earth into layers.
Xe is the heaviest noble gas in the Earth's atmosphere . It has seven stable isotopes ( 126 Xe, 128 Xe, 129 Xe, 130 Xe, 131 Xe, 132 Xe, 134 Xe) and two isotopes ( 124 Xe, 136 Xe) with long-lived half-lives . Xe has four synthetic radioisotopes with very short half-lives, usually less than one month.
Xenon-129 can be used to examine the early history of the Earth. 129 Xe was derived from the extinct nuclide of iodine, iodine-129 or 129 I (with a half-life of 15.7 Million years , or Myr), which can be used in iodine-xenon (I-Xe) dating. The production of 129 Xe stopped within about 100 Myr after the start of the Solar System because 129 I became extinct. [ 1 ] In the modern atmosphere, about 6.8% [ 4 ] of atmospheric 129 Xe originated from the decay 129 I in the first ~100 Myr of the Solar System's history, i.e., during and immediately following Earth's accretion .
Fissiogenic Xe isotopes were generated mainly from the extinct nuclide, plutonium-244 or 244 Pu (half-life of 80 Myr), and also the extant nuclide, uranium-238 or 238 U (half-life of 4468 Myr). Spontaneous fission of 238 U has generated ~5% [ 5 ] as much fissiogenic Xe as 244 Pu. Pu and U fission produce the four fissiogenic isotopes, 136 Xe, 134 Xe, 132 Xe, and 131 Xe in distinct proportions. A reservoir that remains an entirely closed system over Earth's history has a ratio of Pu- to U-derived fissiogenic Xe reaching to ~27. [ 6 ] Accordingly, the isotopic composition of the fissiogenic Xe for a closed-system reservoir would largely resemble that produced from pure 244 Pu fission. [ 7 ] Loss of Xe from a reservoir after 244 Pu becomes extinct (500 Myr) would lead to a greater contribution of 238 U fission to the fissiogenic Xe. [ 1 ]
Differences in the abundance of isotopes among natural samples are extremely small (almost always below 0.1% or 1 per mille ). Nevertheless, these very small differences can record meaningful geological processes. [ 8 ] To compare these tiny but meaningful differences, isotope abundances in natural materials are often reported relative to isotope abundances in designated standards, with the delta (δ) notation. The absolute values of Xe isotopes are normalized to atmospheric 130 Xe. [ 9 ] Define δ X e = [ ( i X e / 130 X e ) s a m p l e / ( i X e / 130 X e ) a t m − 1 ] × 1000 {\displaystyle {\rm {\delta _{Xe}=[(^{i}Xe/^{130}Xe)_{sample}/(^{i}Xe/^{130}Xe)_{atm}-1]\times 1000}}} where i = 124, 126, 128, 129, 131, 132, 134, 136.
Iodine-129 decays with a half-life of 15.7 Ma into 129 Xe, resulting in excess 129 Xe in primitive meteorites relative to primordial Xe isotopic compositions. The property of 129 I can be used in radiometric chronology . However, as detailed below, the age of Earth's formation cannot be deduced directly from I-Xe dating. The major problem is the Xe closure time, or the time when the early Earth system stopped gaining substantial new material from space. When the Earth became closed for the I-Xe system, Xe isotope evolution began to obey a simple radioactive decay law as shown below and became predictable.
The principle of radiogenic chronology is , [ 5 ] if at time t 1 the quantity of a radioisotope is P 1 while at some previous time this quantity was P 0 , the interval between t 1 and t 0 is given by the law of radioactive decay as
Δ t = t 1 − t 0 = ( 1 / λ ) l n ( P 0 / P 1 ) {\displaystyle {\rm {\Delta t=t_{1}-t_{0}=(1/\lambda )ln(P_{0}/P_{1})}}}
Here λ {\displaystyle \lambda } is the decay constant of the radioisotope, which is the probability of decay per nucleus per unit time. The decay constant is related to the half life t 1/2 , by t 1/2 = ln(2)/ λ {\displaystyle \lambda }
The I-Xe system was first applied in 1975 to estimate the age of the Earth. [ 10 ] For all Xe isotopes, the initial isotope composition of iodine in the Earth is given by
( 129 I 127 I ) E = ( 129 I 127 I ) 0 e − λ Δ t E {\displaystyle {\rm {({\frac {^{129}I}{^{127}I}})_{E}=({\frac {^{129}I}{^{127}I}})_{0}e^{-\lambda \Delta t_{E}}}}} where ( 129 I 127 I ) E {\displaystyle {\rm {({\frac {^{129}I}{^{127}I}})_{E}}}} is the isotopic ratios of iodine at the time that Earth primarily formed, ( 129 I 127 I ) 0 {\displaystyle {\rm {({\frac {^{129}I}{^{127}I}})_{0}}}} is the isotopic ratio of iodine at the end of stellar nucleosynthesis , and Δ t E {\displaystyle {\rm {\Delta t_{E}}}} is the time interval between the end of stellar nucleosynthesis and the formation of the Earth. The estimated iodine-127 concentration in the Bulk Silicate Earth (BSE) (= crust + mantle average) ranges from 7 to 10 parts per billion (ppb) by mass. [ 11 ] [ 12 ] If the BSE represents Earth's chemical composition, [ 2 ] the total 127 I in the BSE ranges from 2.26×10 17 to 3.23×10 17 moles. The meteorite Bjurböle is 4.56 billion years old [ 13 ] with an initial 129 I/ 127 I ratio of 1.1×10 −4 , [ 14 ] so an equation can be derived as
( 129 I 127 I ) E = ( 129 I 127 I ) B e − λ ( Δ t E − Δ t B ) {\displaystyle {\rm {({\frac {^{129}I}{^{127}I}})_{E}=({\frac {^{129}I}{^{127}I}})_{B}e^{-\lambda (\Delta t_{E}-\Delta t_{B})}}}}
where Δ t E − Δ t B {\displaystyle {\rm {\Delta t_{E}-\Delta t_{B}}}} is the interval between the formation of the Earth and the formation of meteorite Bjurböle. Given the half life of 129 I of 15.7 Myr, and assuming that all the initial 129 I has decayed to 129 Xe, the following equation can be derived:
Δ t E − Δ t B = 1 λ l n ( ( 129 I / 127 I ) B ( 129 I / 127 I ) E ) = 1 λ l n ( ( 129 I / 127 I ) B ( 129 X e / 127 I ) B S E ) {\displaystyle {\rm {\Delta t_{E}-\Delta t_{B}={\frac {1}{\lambda }}ln({\frac {(^{129}I/^{127}I)_{B}}{(^{129}I/^{127}I)_{E}}})={\frac {1}{\lambda }}ln({\frac {(^{129}I/^{127}I)_{B}}{(^{129}Xe/^{127}I)_{BSE}}})}}}
129 Xe in the modern atmosphere is 3.63×10 13 grams. The iodine content for BSE lies between 10 and 12 ppb by mass. Consequently, Δ t E − Δ t B {\displaystyle {\rm {\Delta t_{E}-\Delta t_{B}}}} should be 108 Myr, [ 2 ] [ 3 ] [ 10 ] i.e., the Xe-closure age is 108 Myr younger than the age of meteorite Bjurböle. The estimated Xe closure time was ~4.45 billion years ago when the growing Earth started to retain Xe in its atmosphere, which is coincident with ages derived from other geochronology dating methods. [ 2 ] [ 3 ] [ 15 ]
There are some disputes about using I-Xe dating to estimate the Xe closure time. First, in the early solar system, planetesimals collided and grew into larger bodies that accreted to form the Earth. But there could be a 10 7 to 10 8 years time gap in Xe closure time between the Earth's inner and outer regions. [ 16 ] Some research support 4.45 Ga probably represents the time when the last giant impactor (Martian-size) hit Earth, [ 3 ] but some regard it as the time of core-mantle differentiation . [ 2 ] The second problem is that the total inventory of 129 Xe on Earth may be larger than that of the atmosphere since the lower mantle hadn't been entirely mixed, which may underestimate 129 Xe in the calculation. Last but not least, if Xe gas not been lost from the atmosphere during a long interval of early Earth's history, the chronology based on 129 I- 129 Xe would need revising [ 17 ] since 129 Xe and 127 Xe could be greatly altered.
Compared with solar xenon, Earth's atmospheric Xe is enriched in heavy isotopes by 3 to 4% per atomic mass unit (amu). [ 18 ] However, the total abundance of xenon gas is depleted by one order of magnitude relative to other noble gases. [ 15 ] The elemental depletion while relative enrichment in heavy isotopes is called the " Xenon paradox ". A possible explanation is that some processes can specifically diminish xenon rather than other light noble gases (e.g. Krypton ) and preferentially remove lighter Xe isotopes.
In the last 2 decades, two categories of models have been proposed to solve the xenon paradox. The first assumes that the Earth accreted from porous planetesimals, and isotope fractionation happened due to gravitational separation. [ 19 ] However, this model cannot reproduce the abundance and isotopic composition of light noble gases in the atmosphere. The second category supposes a massive impact resulted in an aerodynamic drag on heavier gases. [ 20 ] Both the aerodynamic drag and the downward gravitational effect lead to a mass-dependent loss of Xe gases. But following research suggested that Xe isotope mass fractionation shouldn't be a rapid, single event. [ 21 ]
Research published since 2018 on noble gases preserved in Archean (3.5–3.0 Ga old) samples may provide a solution to the Xe paradox. [ 21 ] [ 22 ] Isotopically mass fractionated Xe is found in tiny inclusions of ancient seawater in Archean barite [ 23 ] and hydrothermal quartz. [ 24 ] The distribution of Xe isotopes lies between the primordial solar and the modern atmospheric Xe isotope patterns. The isotopic fractionation gradually increases relative to the solar distribution as Earth evolves over its first 2 billion years. [ 21 ] This two billion-year history of evolving Xe fractionation coincides with early solar system conditions including high solar extreme ultraviolet ( EUV ) radiation [ 15 ] [ 25 ] [ 26 ] and large impacts that could energize large rates of hydrogen escape to space that are big enough to drag out xenon. However, models of neutral xenon atoms escaping cannot resolve the problem that other lighter noble gas elements don't show the signal of depletion or mass-dependent fractionation . For example, because Kr is lighter than Xe, Kr should also have escaped in a neutral wind. Yet the isotopic distribution of atmospheric Kr on Earth is significantly less fractionated than atmospheric Xe. [ 16 ]
A current explanation [ 28 ] [ 29 ] is that hydrodynamic escape can preferentially remove lighter atmospheric species and lighter isotopes of Xe in the form of charged ions instead of neutral atoms. Hydrogen is liberated from hydrogen-bearing gases (H 2 or CH 4 ) by photolysis in the early Earth atmosphere. Hydrogen is light and can be abundant at the top of the atmosphere and escape. In the polar regions where there are open magnetic field lines, hydrogen ions can drag ionized Xe out from the atmosphere to space even though neutral Xe cannot escape. [ 27 ]
The mechanism is summarized as below.
Xe can be directly photo-ionized by UV radiation in range of 91.2 n m < λ < 102.3 n m {\displaystyle 91.2\ {\rm {{nm}<\lambda <102.3nm}}} [ 28 ]
X e + h v → X e + + e − {\displaystyle {\rm {Xe+hv\rightarrow Xe^{+}+e^{-}}}}
Or Xe can be ionized by change exchange with H 2 and CO 2 through
C O 2 + + X e → C O 2 + X e + {\displaystyle {\rm {CO_{2}^{+}+Xe\rightarrow CO_{2}+Xe^{+}}}}
H 2 + + X e → H X e + + H {\displaystyle {\rm {H_{2}^{+}+Xe\rightarrow HXe^{+}+H}}}
where H + and CO 2 + can come from EUV dissociation. Xe + is chemically inert in H, H 2 , or CO 2 atmospheres. [ 30 ] As a result, Xe + tends to persist. These ions interact strongly with each other through the Coulomb force and are finally dragged away by strong ancient polar wind . Isotope mass fractionation accumulates as lighter isotopes of Xe + preferentially escape from the Earth. A preliminary model suggests that Xe can escape in the Archean if the atmosphere contains >1% H 2 or >0.5% methane. [ 28 ]
When O 2 levels increased in the atmosphere, Xe + could exchange positive charge with O 2 though [ 30 ]
X e + + O 2 → X e + O 2 + {\displaystyle {\rm {Xe^{+}+O_{2}\rightarrow Xe+O_{2}^{+}}}}
From this reaction, Xe escape stopped when the atmosphere became enriched in O 2 . As a result, Xe isotope fractionation may provide insights into the long history of hydrogen escape that ended with the Great Oxidation Event (GOE) . [ 28 ] Understanding Xe isotopes is promising to reconstruct hydrogen or methane escape history that irreversibly oxidized the Earth and drove biological evolution toward aerobic ecological systems. [ 21 ] [ 31 ] [ 32 ] Other factors, such as the hydrogen (or methane) concentration becoming too low or EUV radiation from the aging Sun becoming too weak, can also cease the hydrodynamic escape of Xe, [ 28 ] but are not mutually exclusive.
Organic hazes on Archean Earth could also scavenge isotopically heavy Xe. [ 25 ] Ionized Xe can be chemically incorporated into organic materials, [ 33 ] going through the terrestrial weathering cycle on the surface. The trapped Xe is mass fractionated by about 1% per amu in heavier isotopes [ 28 ] but they may be released again and recover the original unfractionated composition, making them not sufficient to totally resolve Xe paradox.
Observed atmospheric Xe is depleted relative to Chondritic meteorites by a factor of 4 to 20 when compared to Kr. [ 27 ] In contrast, the stable isotopes of Kr are barely fractionated. This mechanism is unique to Xe since Kr + ions are quickly neutralized via [ 34 ]
K r + + H 2 → K r H + + H {\displaystyle {\rm {Kr^{+}+H_{2}\rightarrow KrH^{+}+H}}}
K r H + + e − → K r + H {\displaystyle {\rm {KrH^{+}+e^{-}\rightarrow Kr+H}}}
Therefore, Kr can be rapidly returned to neutral and wouldn't be dragged away by the charged ion wind in the polar region. Hence Kr is retained in the atmosphere.
The signal of mass-independent fractionation of sulfur isotopes, known as MIF-S, correlates with the end of Xe isotope fractionation. During the Great Oxidation Event (GOE), the ozone layer formed when O 2 rose, accounting for the end of the MIF-S signature. The disappearance of the MIF-S signal has been regarded as changing the redox ratio of Earth's surface reservoirs. [ 35 ] [ 36 ] However, potential memory effects of MIF-S due to oxidative weathering can lead to large uncertainty on the process and chronology of GOE. [ 37 ] Compared to the MIF-S signals, hydrodynamic escape of Xe is not affected by the ozone formation and may be even more sensitive to O 2 availability, [ 32 ] promising to provide more details about the oxidation history of Earth. [ 32 ]
Xe isotopes are also promising in tracing mantle dynamics in Earth's evolution. The first explicit recognition of non-atmospheric Xe in terrestrial samples came from the analysis of CO 2 -well gas in New Mexico, displaying an excess of 129 I-derived or primitive source 129 Xe and high content in 131-136 Xe due to the decay of 238 U. [ 38 ] At present, the excess of 129 Xe and 131-136 Xe has been widely observed in mid-ocean ridge basalt (MORBs) [ 39 ] [ 40 ] [ 41 ] and Oceanic island basalt (OIBs). [ 42 ] Because 136 Xe receives more fissiogenic contribution than other heavy Xe isotopes, 129 Xe (decay of 129 I) and 136 Xe are usually normalized to 130 Xe when discussing Xe isotope trends of different mantle sources. [ 11 ] MORBs' 129 Xe/ 130 Xe and 136 Xe/ 130 Xe ratios lie on a trend from atmospheric ratios to higher values [ 39 ] and seemingly contaminated by the air. Oceanic island basalt (OIBs) data lies lower than those in MORBs, implying different Xe sources for OIBs and MORBs. [ 42 ]
The deviations in 129 Xe/ 130 Xe ratio between air and MORBs show that mantle degassing occurred before 129 I was extinct, otherwise 129 Xe/ 130 Xe in the air would be the same as in the mantle. [ 45 ] [ 46 ] The differences in the 129 Xe/ 130 Xe ratio between MORBs and OIBs may indicate that the mantle reservoirs are still not thoroughly mixed. The chemical differences between OIBs and MORBs still await discovery. [ 11 ]
To obtain mantle Xe isotope ratios, it is necessary to remove contamination by atmospheric Xe, which could start before 2.5 billion years ago. [ 47 ] [ 48 ] Theoretically, the many non-radiogenic isotopic ratios ( 124 Xe/ 130 Xe, 126 Xe/ 130 Xe, and 128 Xe/ 130 Xe) can be used to accurately correct for atmospheric contamination if slight differences between air and mantle can be precisely measured. Still, we cannot reach such precision with current techniques. [ 11 ]
On Mars, Xe isotopes in the present atmosphere are mass fractionated relative to their primordial composition from in situ measurement of the Curiosity Rover at Gale Crater , Mars. [ 49 ] Paleo-atmospheric Xe trapped in the Martian regolith breccia NWA 11220 is mass-dependently fractionated relative to solar Xe by ~16.2‰. [ 29 ] The extent of fractionation is comparable for Mars and Earth, which may be compelling evidence that hydrodynamic escape also occurred in the Mars history. The regolith breccia NWA7084 and the >4 Ga orthopyroxene ALH84001 Martian meteorites trap ancient Martian atmospheric gases with little if any Xe isotopic fractionation relative to modern Martian atmospheric Xe. [ 50 ] Alternative models for Mars consider that the isotopic fractionation and escape of Mars atmospheric Xe occurred very early in the planet's history and ceased around a few hundred million years after planetary formation rather than continuing during its evolutionary history [ 21 ] [ 51 ]
Xe has not been detected in Venus's atmosphere. 132 Xe has an upper limit of 10 parts per billion by volume. [ 51 ] [ 52 ] The absence of data on the abundance of Xe precludes us from evaluating if the abundance of Xe is close to solar values or if there is Xe paradox on Venus. The lack also prevents us from checking if the isotopic composition has been mass dependently fractionated, as in the case of Earth and Mars.
Jupiter's atmosphere has 2.5 ± 0.5 times the solar abundance values for Xenon and similarly elevated argon and krypton (2.1 ± 0.5 and 2.7 ± 0.5 times solar values separately). These signals of enrichment are due to these elements coming to Jupiter in very cold (T<30K) icy planetesimals . [ 53 ]
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Xenon octafluoride is a chemical compound of xenon and fluorine with the chemical formula Xe F 8 . [ 1 ] This is still a hypothetical compound . [ 2 ] [ 3 ] XeF 8 is reported to be unstable even under pressures reaching 200 GPa. [ 4 ]
The compound was initially predicted in 1933 by Linus Pauling —among other noble gas compounds but which, unlike other xenon fluorides, could probably never be synthesized. [ 5 ] [ 6 ] This appears to be due to the steric hindrance of the fluorine atoms around the xenon atom. However, scientists continue to try to synthesize it. [ 7 ]
The formation of xenon octafluoride has been calculated to be endothermic: [ 8 ]
The doubly charged anion octafluoroxenate XeF 2− 8 in which the oxidation number of xenon is only VI, is stable in salts.
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Xenon tetroxide is a chemical compound of xenon and oxygen with molecular formula XeO 4 , remarkable for being a relatively stable compound of a noble gas . It is a yellow crystalline solid that is stable below −35.9 ° C ; above that temperature it is very prone to exploding and decomposing into elemental xenon and oxygen (O 2 ). [ 4 ] [ 5 ]
All eight valence electrons of xenon are involved in the bonds with the oxygen, and the oxidation state of the xenon atom is +8. Oxygen is the only element that can bring xenon up to its highest oxidation state; even fluorine can only give XeF 6 (+6).
Two other short-lived xenon compounds with an oxidation state of +8, XeO 3 F 2 and XeO 2 F 4 , are accessible by the reaction of xenon tetroxide with xenon hexafluoride . XeO 3 F 2 and XeO 2 F 4 can be detected with mass spectrometry . The perxenates are also compounds where xenon has the +8 oxidation state.
At temperatures above −35.9 °C, xenon tetroxide is very prone to explosion, decomposing into xenon and oxygen gases with Δ H = −643 kJ/mol:
Xenon tetroxide dissolves in water to form perxenic acid and in alkalis to form perxenate salts:
Xenon tetroxide can also react with xenon hexafluoride to give xenon oxyfluorides:
All syntheses start from the perxenates , which are accessible from the xenates through two methods. One is the disproportionation of xenates to perxenates and xenon:
The other is oxidation of the xenates with ozone in basic solution:
Barium perxenate is reacted with sulfuric acid and the unstable perxenic acid is dehydrated to give xenon tetroxide: [ 6 ]
Any excess perxenic acid slowly undergoes a decomposition reaction to xenic acid and oxygen:
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Xenon trioxide is an unstable compound of xenon in its +6 oxidation state . It is a very powerful oxidizing agent , and liberates oxygen from water slowly, accelerated by exposure to sunlight. It is dangerously explosive upon contact with organic materials. When it detonates, it releases xenon and oxygen gas.
Synthesis of xenon trioxide is by aqueous hydrolysis of XeF 6 : [ 2 ]
The resulting xenon trioxide crystals are a strong oxidising agent and can oxidise most substances that are at all oxidisable. However, it is slow-acting and this reduces its usefulness. [ 3 ]
Above 25 °C, xenon trioxide is very prone to violent explosion:
When it dissolves in water, an acidic solution of xenic acid is formed:
This solution is stable at room temperature and lacks the explosive properties of xenon trioxide. It oxidises carboxylic acids quantitatively to carbon dioxide and water . [ 4 ]
Alternatively, 15-crown-5 coordinates to xenon trioxide to give a complex stable at room-temperature against mechanical shock. [ 5 ]
Alternatively, it dissolves in alkaline solutions to form xenates . The HXeO − 4 anion is the predominant species in xenate solutions. [ 6 ] These are not stable and begin to disproportionate into perxenates (+8 oxidation state) and xenon and oxygen gas. [ 7 ] Solid perxenates containing XeO 4− 6 have been isolated by reacting XeO 3 with an aqueous solution of hydroxides. Xenon trioxide reacts with inorganic fluorides such as KF, RbF, or CsF to form stable solids of the form MXeO 3 F . [ 8 ]
Hydrolysis of xenon hexafluoride or xenon tetrafluoride yields a solution from which colorless XeO 3 crystals can be obtained by evaporation. [ 2 ] The crystals are stable for days in dry air, but readily absorb water from humid air to form a concentrated solution. The crystal structure is orthorhombic with a = 6.163 Å, b = 8.115 Å, c = 5.234 Å, and 4 molecules per unit cell. The density is 4.55 g/cm 3 . [ 9 ]
XeO 3 should be handled with great caution. Samples have detonated when undisturbed at room temperature. Dry crystals react explosively with cellulose. [ 9 ] [ 10 ]
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Xenophagy (Greek "strange" + "eating") and allotrophy (Greek "other" + "nutrient") are changes in established patterns of biological consumption, by individuals or groups.
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A xenosome is a bacterium that lives in the body of some marine protozoans . It primarily refers to bacterial invaders of the cytoplasm of a single genus of marine scuticociliates . [ 1 ] They are found in ciliates , sometimes with a methanogenic role inside anaerobic ciliates. [ 2 ]
In 1985, researcher John Corliss proposed to expand the definition of the term to include all DNA -containing, membrane-bounded bodies or organelles — prokaryotic or eukaryotic in original nature—found within the cytoplasm or nucleus of eukaryotic cells of any or all kinds, regardless of whether the occupation was temporary or permanent. [ 1 ]
This biology article is a stub . You can help Wikipedia by expanding it .
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Xenotransplantation ( xenos- from the Greek meaning "foreign" or strange [ 1 ] [ 2 ] ), or heterologous transplant , is the transplantation of living cells , tissues or organs from one species to another. [ 3 ] Such cells, tissues or organs are called xenografts or xenotransplants . It is contrasted with allotransplantation (from other individual of same species), syngeneic transplantation or isotransplantation (grafts transplanted between two genetically identical individuals of the same species), and autotransplantation (from one part of the body to another in the same person). [ citation needed ] Xenotransplantation is an artificial method of creating an animal- human chimera , that is, a human with a subset of animal cells. In contrast, an individual where each cell contains genetic material from a human and an animal is called a human–animal hybrid . [ 4 ]
Patient derived xenografts are created by xenotransplantation of human tumor cells into immunocompromised mice, and is a research technique frequently used in pre-clinical oncology research. [ 5 ]
Human xenotransplantation offers a potential treatment for end-stage organ failure , a significant health problem in parts of the industrialized world . It also raises many novel medical, legal and ethical issues. [ 6 ] A continuing concern is that many animals, such as pigs , have a shorter lifespan than humans, meaning that their tissues age at a quicker rate. (Pigs have a maximum life span of about 27 years. [ 7 ] ) Disease transmission ( xenozoonosis ) and permanent alteration to the genetic code of animals are also causes for concern. Similarly to objections to animal testing , animal rights activists have also objected to xenotransplantation on ethical grounds. [ 8 ] A few temporarily successful cases of xenotransplantation are published. [ 9 ]
Bioprosthetic artificial heart valves are generally pig or bovine -derived, but the cells are killed by glutaraldehyde treatment before insertion, [ 10 ] therefore technically not fulfilling the WHO definition of xenotransplantation of being live cells. [ 3 ]
The first serious attempts at xenotransplantation (then called heterotransplantation) appeared in the scientific literature in 1905, when slices of rabbit kidney were transplanted into a child with chronic kidney disease . [ 11 ] In the first two decades of the 20th century, several subsequent efforts to use organs from lambs, pigs, and primates were published. [ 11 ]
Scientific interest in xenotransplantation declined when the immunological basis of the organ rejection process was described. The next waves of studies on the topic came with the discovery of immunosuppressive drugs . Even more studies followed Joseph Murray 's first successful renal transplantation in 1954 and scientists, facing the ethical questions of organ donation for the first time, accelerated their effort in looking for alternatives to human organs. [ 11 ]
On February 16, 1963, the first transplant of a non-human animal's organ into a human being took place in Minneapolis when surgeons led by Dr. Claude R. Hitchcock and R. Joseph Kiser "tried grafting a baboon kidney " into "a woman in whom previously implanted human kidney (from a corpse) was doing poorly", and the kidney "immediately began functioning normally and cleared her blood of wastes". Her body rejected the kidney five days afterward and she died in March, three weeks later. [ 12 ]
Starting in October 1963, doctors at Tulane University attempted renal transplantations from non-human primates in six people who were near death. The first person, a 32 year old woman with a chronic kidney disease received the kidneys of a rhesus monkey and the kidneys "functioned well for seven days, then failed," and the patient died later from her illness. [ 13 ] The first successful attempt (one in which the patient was able to leave the hospital and return home) with a chimpanzee was performed on November 5 at Charity Hospital in New Orleans by a 12-man team of Tulane physicians, led by Dr. Keith Reemtsma , and the patient, a 44-year-old dock worker named Jefferson Davis, left the hospital on December 17 after a six-week recuperation. [ 14 ] ; after this and several subsequent unsuccessful attempts to use primates as organ donors and the development of a working cadaver organ procuring program, interest in xenotransplantation for kidney failure dissipated. [ 11 ] Out of 13 such transplants performed by Keith Reemtsma , one kidney recipient lived for nine months. [ 15 ]
An American infant girl known as " Baby Fae " with hypoplastic left heart syndrome was the first infant recipient of a xenotransplantation, when she received a baboon heart on October 26, 1984. [ 16 ] The procedure was performed by Leonard Lee Bailey at Loma Linda University Medical Center in Loma Linda, California . Fae died 21 days later, on November 15, [ 17 ] due to a humoral-based graft rejection thought to be caused mainly by an ABO blood type mismatch, considered unavoidable due to the rarity of type O baboons. The graft was meant to be temporary, but unfortunately a suitable allograft replacement could not be found in time. While the procedure itself did not advance the progress on xenotransplantation, it did shed a light on the insufficient amount of organs for infants. The story made such an impact that the crisis of infant organ shortage improved for that time. [ 18 ] [ 15 ]
The first heart transplant in a human ever performed was by Hardy in 1964, using a chimpanzee heart, but the patient died within 2 hours. [ 15 ] The first transplant of a non-genetically modified [ 19 ] [ 20 ] pig's heart, lungs and kidneys into a human was performed in Sonapur, Assam , in India in mid-December 1996, and was announced in January 1997. [ 19 ] The recipient was Purno Saikia, a 32-year-old terminally-ill man; he died of multiple infections shortly after the operation. [ 19 ] [ 21 ] The Indian cardiothoracic surgeon Dhani Ram Baruah and two of his associates, Jonathan Ho Kei-shing (of the Hong Kong-based Prince of Wales Medical Institute) [ 22 ] and C.S. James, performed the surgeries. [ 19 ] Baruah claimed that Saikia had failed to respond to conventional surgery, and that the patient and his family had consented to the procedure. [ 23 ]
All three involved in the surgery were arrested on January 9, 1997, [ 19 ] for the alleged violation of the Transplantation of Human Organs and Tissues Act of 1994 . [ 19 ] [ 24 ] Baruah was dismissed in medical circles as a "mad scientist" and the procedure was dubbed a "hoax". Baruah himself signed a statement saying he had done no transplant, but then alleged that the confession was forced from him. [ 23 ] [ 20 ] They were found guilty of unethical procedure and culpable homicide and imprisoned for 40 days. [ 25 ] Dhani Ram Baruah's surgical institute was also found to be without necessary registration. [ 26 ]
Critics said Dhani Bam Baruah's claims and medical procedures were neither taken seriously nor accepted by the scientific community because he never got his findings scientifically peer-reviewed . [ 27 ] Past complaints of ethics violations during surgeries in Hong Kong by Baruah and Ho had occurred in 1992, when they had implanted heart valves, developed by Baruah, made of animal tissue. A year later, six patients died. The Asian Medical News reported that "grave concerns" were expressed "over the procedure and ethics of the implementation". [ 20 ]
In September 2021, surgeons led by Robert Montgomery performed the first genetically engineered pig kidney xenotransplant to a brain-dead human at NYU Langone Health with no sign of immediate rejection (partly because the pig thymus gland was transplanted as well). [ 28 ] The kidney was procured from a pig with only a single gene modification: the removal of alpha-gal . [ 29 ]
In July 2023, surgeons from the NYU Langone Transplant Institute completed a transplant of a genetically modified pig kidney (along with the pig's thymus gland underneath it) into a patient declared brain dead but maintained on a respirator. [ 30 ] The patient had previously consented to be an organ donor, but his tissues were not considered suitable for transplant. The kidney came from an animal with a knocked-out gene for the production of alpha gal sugars, which has been implicated in immune response to mammalian tissue. [ 31 ] In order to ensure that renal function was only supported by the pig kidney, the team removed both of the patient's kidneys. The team has reported that the kidney has maintained optimal functioning for over a month, as evidenced by routine testing of creatinine and weekly biopsies. The team plans to monitor the patient for another month, pending approval by ethics board and his family. [ citation needed ]
In March 2024, Richard Slayman , a patient whose transplanted human kidney had failed, received a genetically engineered pig kidney xenotransplant from surgeons at Massachusetts General Hospital . [ 32 ] [ 33 ] This kidney has 69 genomic edits (3 gene knockout, 7 human gene insertion and 59 copies of the porcine retrovirus knockout) made by eGenesis, Inc. [ 34 ] Mr. Slayman died a few months later of unrelated causes, with no apparent rejection of the kidney. [ 35 ] Meanwhile, in April 2024, Lisa Pisano became the second person to receive such a kidney transplant. [ 35 ] [ 36 ] Because of "unique challenges" related to a mechanical heart pump she received along with the kidney, her kidney had to be removed due to "insufficient blood flow" late in May. [ 37 ] Medication also deteriorated the kidney, which led to the organs rejection. [ 38 ]
In January 2022, doctors led by cardiothoracic surgeon Bartley P. Griffith and Muhammad M. Mohiuddin [ 39 ] at the University of Maryland Medical Center and University of Maryland School of Medicine performed a heart transplant from a genetically modified pig to a terminally ill patient, David Bennett Sr., who was ineligible for a standard human heart transplant. The pig had undergone specific gene editing to remove enzymes responsible for producing sugar antigens that lead to hyperacute organ rejection in humans. The US medical regulator gave special dispensation to carry out the procedure under compassionate use criteria. [ 40 ] The recipient died two months after the transplantation. [ 41 ]
In June and July 2022, surgeons at NYU Langone Health performed two genetically modified pig heart transplants into recently deceased humans. [ 42 ] The hearts were from pigs that had the identical 10 genetic modifications used in the University of Maryland Medical Center heart xenotransplantation in January 2022. All three hearts came from Revivicor, Inc., a facility based in Blacksburg, Va., and a subsidiary of United Therapeutics . [ 43 ]
On 20 September 2023, surgeons at the University of Maryland Medical Center in Baltimore performed a heart transplant from a genetically modified pig to Lawrence Faucette, a patient with terminal heart disease who was ineligible for a traditional heart transplant. On 30 October 2023, Faucette died after showing signs of organ rejection. [ 44 ]
A worldwide shortage of organs for clinical implantation causes about 20–35% of patients who need replacement organs to die on the waiting list. [ 45 ] Certain procedures, some of which are being investigated in early clinical trials, aim to use cells or tissues from other species to treat life-threatening and debilitating illnesses such as cancer , diabetes , liver failure and Parkinson's disease . If vitrification can be perfected, it could allow for long-term storage of xenogenic cells, tissues and organs so that they would be more readily available for transplant. [ citation needed ]
Xenotransplants could save thousands of patients waiting for donated organs. [ citation needed ] The animal organ, probably from a pig or baboon could be genetically altered with human genes to trick a patient's immune system into accepting it as a part of its own body. [ 46 ] They have re-emerged because of the lack of organs available and the constant battle to keep immune systems from rejecting allotransplants. Xenotransplants are thus potentially a more effective alternative. [ 47 ] [ 48 ] [ 49 ]
Xenotransplantation of human tumor cells into immunocompromised mice is a research technique frequently used in oncology research. [ 50 ] It is used to predict the sensitivity of the transplanted tumor to various cancer treatments; several companies offer this service, including the Jackson Laboratory . [ 51 ]
Human organs have been transplanted into animals as a powerful research technique for studying human biology without harming human patients. This technique has also been proposed as an alternative source of human organs for future transplantation into human patients. [ 52 ] For example, researchers from the Ganogen Research Institute transplanted human fetal kidneys into rats which demonstrated life supporting function and growth. [ 5 ]
Since they are the closest relatives to humans, non-human primates were first considered as a potential organ source for xenotransplantation to humans. Chimpanzees were originally considered the best option since their organs are of similar size, and they have good blood type compatibility with humans, which makes them potential candidates for xenotransfusions . However, since chimpanzees are listed as an endangered species , other potential donors were sought. Baboons are more readily available, but impractical as potential donors. Problems include their smaller body size, the infrequency of blood group O (the universal donor), their long gestation period, and their typically small number of offspring. In addition, a major problem with the use of nonhuman primates is the increased risk of disease transmission, since they are so closely related to humans. [ 53 ]
Pigs ( Sus scrofa domesticus ) are currently thought to be the best candidates for organ donation. The risk of cross-species disease transmission is decreased because of their increased phylogenetic distance from humans. [ 1 ] Pigs have relatively short gestation periods, large litters, and are easy to breed, making them readily available. [ 54 ] They are inexpensive and easy to maintain in pathogen-free facilities, and current gene editing tools are adapted to pigs to combat rejection and potential zoonoses . [ 54 ] Pig organs are anatomically comparable in size, and new infectious agents are less likely since they have been in close contact with humans through domestication for many generations. [ 55 ] Treatments sourced from pigs have proven to be successful such as porcine-derived insulin for patients with diabetes mellitus. [ 56 ] Increasingly, genetically engineered pigs are becoming the norm, which raises moral qualms, but also increases the success rate of the transplant. [ 57 ] Current experiments in xenotransplantation most often use pigs as the donor, and baboons as human models. In 2020, the U.S. Food and Drug Administration approved a genetic modification of pigs so they do not produce alpha-gal sugars. [ 58 ] Pig organs have been used for kidney and heart transplants into humans. [ 40 ] [ 28 ] [ 32 ] [ 36 ] [ 44 ]
To date, [ citation needed ] no xenotransplantation trials have been entirely successful due to the many obstacles arising from the response of the recipient's immune system . Xenozoonoses are one of the biggest threats to rejections, as they are xenogeneic infections. The introduction of these microorganisms are a big issue that lead to the fatal infections and then rejection of the organs. [ 59 ] This response, which is generally more extreme than in allotransplantations, ultimately results in rejection of the xenograft, and can in some cases result in the immediate death of the recipient. There are several types of rejection organ xenografts are faced with, these include hyperacute rejection, acute vascular rejection, cellular rejection, and chronic rejection. [ citation needed ]
A rapid, violent, and hyperacute response comes as a result of antibodies present in the host organism. These antibodies are known as xenoreactive natural antibodies (XNAs). [ 1 ]
This rapid and violent type of rejection occurs within minutes to hours from the time of the transplant. It is mediated by the binding of XNAs (xenoreactive natural antibodies) to the donor endothelium, causing activation of the human complement system , which results in endothelial damage, inflammation, thrombosis and necrosis of the transplant. XNAs are first produced and begin circulating in the blood in neonates, after colonization of the bowel by bacteria with galactose moieties on their cell walls. Most of these antibodies are the IgM class, but also include IgG , and IgA . [ 55 ]
The epitope XNAs target is an α-linked galactose moiety, galactose-alpha-1,3-galactose (also called the α-Gal epitope), produced by the enzyme alpha- galactosyltransferase . [ 60 ] Most non-primates contain this enzyme thus, this epitope is present on the organ epithelium and is perceived as a foreign antigen by primates, which lack the galactosyl transferase enzyme. In pig to primate xenotransplantation, XNAs recognize porcine glycoproteins of the integrin family. [ 55 ]
The binding of XNAs initiate complement activation through the classical complement pathway . Complement activation causes a cascade of events leading to: destruction of endothelial cells, platelet degranulation, inflammation, coagulation, fibrin deposition, and hemorrhage. The result is thrombosis and necrosis of the xenograft. [ 55 ]
Hyperacute rejection is a severe, immediate immune response that occurs when a transplanted organ, such as a pig kidney, is rapidly attacked and destroyed by the recipient's immune system. In the context of pig kidney xenotransplantation, this type of rejection is triggered by pre-existing antibodies in the recipient's blood that recognize and bind to antigens on the surface of the pig kidney cells. These antigens, which are foreign to the human immune system, include certain carbohydrates and proteins that are not present in human tissues. The binding of these antibodies activates the complement system, leading to a cascade of events that cause widespread clotting and inflammation in the transplanted organ's blood vessels. As a result, the kidney quickly becomes ischemic (lacking adequate blood flow) and undergoes acute damage, often resulting in the organ's immediate loss.
Hyperacute rejection can severely affect the recipient’s body by leading to the rapid and complete failure of the transplanted kidney. This failure not only undermines the purpose of the transplant, which is to restore kidney function, but also poses serious health risks to the recipient. The sudden loss of kidney function can result in the accumulation of waste products and fluids in the body, causing symptoms such as swelling, electrolyte imbalances, and potential life-threatening complications. Furthermore, hyperacute rejection necessitates immediate medical intervention, often leading to the removal of the rejected kidney and the need to explore alternative treatment options, such as returning to dialysis or seeking another transplant. [ 61 ]
Since hyperacute rejection presents such a barrier to the success of xenografts, several strategies to overcome it are under investigation:
Interruption of the complement cascade
Transgenic organs (Genetically engineered pigs)
Also known as delayed xenoactive rejection, this type of rejection occurs in discordant xenografts within 2 to 3 days, if hyperacute rejection is prevented. The process is much more complex than hyperacute rejection and is currently not completely understood. Acute vascular rejection requires de novo protein synthesis and is driven by interactions between the graft endothelial cells and host antibodies, macrophages, and platelets. The response is characterized by an inflammatory infiltrate of mostly macrophages and natural killer cells (with small numbers of T cells ), intravascular thrombosis, and fibrinoid necrosis of vessel walls. [ 60 ]
Binding of the previously mentioned XNAs to the donor endothelium leads to the activation of host macrophages as well as the endothelium itself. The endothelium activation is considered type II since gene induction and protein synthesis are involved. The binding of XNAs ultimately leads to the development of a procoagulant state, the secretion of inflammatory cytokines and chemokines , as well as expression of leukocyte adhesion molecules such as E-selectin , intercellular adhesion molecule-1 ( ICAM-1 ), and vascular cell adhesion molecule-1 ( VCAM-1 ). [ 55 ]
This response is further perpetuated as normally binding between regulatory proteins and their ligands aid in the control of coagulation and inflammatory responses. However, due to molecular incompatibilities between the molecules of the donor species and recipient (such as porcine major histocompatibility complex molecules and human natural killer cells), this may not occur. [ 60 ]
Due to its complexity, the use of immunosuppressive drugs along with a wide array of approaches are necessary to prevent acute vascular rejection, and include administering a synthetic thrombin inhibitor to modulate thrombogenesis, depletion of anti-galactose antibodies (XNAs) by techniques such as immunoadsorption, to prevent endothelial cell activation, and inhibiting activation of macrophages (stimulated by CD4 + T cells) and NK cells (stimulated by the release of Il-2). Thus, the role of MHC molecules and T cell responses in activation would have to be reassessed for each species combo. [ 60 ]
Accommodation, which is the survival of the xenograft despite the presence of circulating XNAs, is possible if hyperacute and acute vascular rejection are avoided. The graft is given a break from humoral rejection [ 65 ] when the complement cascade is interrupted, circulating antibodies are removed, their function is changed, or there is a change in the expression of surface antigens on the graft. This allows the xenograft to up-regulate and express protective genes, which aid in resistance to injury, such as heme oxygenase-1 (an enzyme that catalyzes the degradation of heme). [ 55 ]
Rejection of the xenograft in hyperacute and acute vascular rejection is due to the response of the humoral immune system , since the response is elicited by the XNAs. Cellular rejection is based on cellular immunity , and is mediated by natural killer cells that accumulate in and damage the xenograft and T-lymphocytes which are activated by MHC molecules through both direct and indirect xenorecognition. [ citation needed ]
In direct xenorecognition, antigen presenting cells from the xenograft present peptides to recipient CD4 + T cells via xenogeneic MHC class II molecules, resulting in the production of interleukin 2 (IL-2). Indirect xenorecognition involves the presentation of antigens from the xenograft by recipient antigen presenting cells to CD4 + T cells. Antigens of phagocytosed graft cells can also be presented by the host's class I MHC molecules to CD8 + T cells. [ 1 ] [ 66 ]
The strength of cellular rejection in xenografts remains uncertain, however, it is expected to be stronger than in allografts due to differences in peptides among different animals. This leads to more antigens potentially recognized as foreign, thus eliciting a greater indirect xenogenic response. [ 1 ]
A proposed strategy to avoid cellular rejection is to induce donor non-responsiveness using hematopoietic chimerism. [ 46 ] Donor stem cells are introduced into the bone marrow of the recipient, where they coexist with the recipient's stem cells. The bone marrow stem cells give rise to cells of all hematopoietic lineages, through the process of hematopoiesis . Lymphoid progenitor cells are created by this process and move to the thymus where negative selection eliminates T cells found to be reactive to self. The existence of donor stem cells in the recipient's bone marrow causes donor reactive T cells to be considered self-reactive and undergo apoptosis . [ 1 ]
Chronic rejection is slow and progressive, and usually occurs in transplants that survive the initial rejection phases. [ 60 ] Scientists are still unclear how chronic rejection exactly works, research in this area is difficult since xenografts rarely survive past the initial acute rejection phases. Nonetheless, it is known that XNAs and the complement system are not primarily involved. [ 60 ] Fibrosis in the xenograft occurs as a result of immune reactions, cytokines (which stimulate fibroblasts), or healing (following cellular necrosis in acute rejection). Perhaps the major cause of chronic rejection is arteriosclerosis . Lymphocytes, which were previously activated by antigens in the vessel wall of the graft, activate macrophages to secrete smooth muscle growth factors. This results in a build up of smooth muscle cells on the vessel walls, causing the hardening and narrowing of vessels within the graft. Chronic rejection leads to pathologic changes of the organ, and is why transplants must be replaced after so many years. [ 66 ] It is also anticipated that chronic rejection will be more aggressive in xenotransplants as opposed to allotransplants. [ 67 ]
Successful efforts have been made to create knockout mice without α1,3GT; the resulting reduction in the highly immunogenic αGal epitope has resulted in the reduction of the occurrence of hyperacute rejection, but has not eliminated other barriers to xenotransplantation such as dysregulated coagulation, also known as coagulopathy . [ 68 ]
Different organ xenotransplants result in different responses in clotting. For example, kidney transplants result in a higher degree of coagulopathy , or impaired clotting, than cardiac transplants, whereas liver xenografts result in severe thrombocytopenia , causing recipient death within a few days due to bleeding. [ 68 ] An alternate clotting disorder, thrombosis , may be initiated by preexisting antibodies that affect the protein C anticoagulant system. Due to this effect, porcine donors must be extensively screened before transplantation. Studies have also shown that some porcine transplant cells are able to induce human tissue factor expression, thus stimulating platelet and monocyte aggregation around the xenotransplanted organ, causing severe clotting. [ 69 ] Additionally, spontaneous platelet accumulation may be caused by contact with pig von Willebrand factor. [ 69 ]
Just as the α1,3G epitope is a major problem in xenotransplantation, so too is dysregulated coagulation a cause of concern. Transgenic pigs that can control for variable coagulant activity based on the specific organ transplanted would make xenotransplantation a more readily available solution for the 70,000 patients per year who do not receive a human donation of the organ or tissue they need. [ 69 ]
Extensive research is required to determine whether animal organs can replace the physiological functions of human organs. Many issues include:
Xenozoonosis, also known as zoonosis or xenosis, is the transmission of infectious agents between species via xenograft. Animal to human infection is normally rare, but has occurred in the past. An example of such is the avian influenza , when an influenza A virus was passed from birds to humans. [ 70 ] Xenotransplantation may increase the chance of disease transmission for 3 reasons: (1) implantation breaches the physical barrier that normally helps to prevent disease transmission, (2) the recipient of the transplant will be severely immunosuppressed, and (3) human complement regulators (CD46, CD55, and CD59) expressed in transgenic pigs have been shown to serve as virus receptors, and may also help to protect viruses from attack by the complement system. [ 71 ]
Examples of viruses carried by pigs include porcine herpesvirus , rotavirus , parvovirus , and circovirus . Porcine herpesviruses and rotaviruses can be eliminated from the donor pool by screening, however others (such as parvovirus and circovirus) may contaminate food and footwear then re-infect the herd. Thus, pigs to be used as organ donors must be housed under strict regulations and screened regularly for microbes and pathogens. Unknown viruses, as well as those not harmful in the animal, may also pose risks. [ 71 ] Of particular concern are PERVS (porcine endogenous retroviruses), vertically transmitted microbes that embed in swine genomes. The risks with xenosis are twofold, as not only could the individual become infected, but a novel infection could initiate an epidemic in the human population. Because of this risk, the FDA has suggested any recipients of xenotransplants shall be closely monitored for the remainder of their life, and quarantined if they show signs of xenosis. [ 72 ]
Baboons and pigs carry myriad transmittable agents that are harmless in their natural host, but extremely toxic and deadly in humans. HIV is an example of a disease believed to have jumped from monkeys to humans. Researchers also do not know if an outbreak of infectious diseases could occur and if they could contain the outbreak even though they have measures for control. Another obstacle facing xenotransplants is that of the body's rejection of foreign objects by its immune system. These antigens (foreign objects) are often treated with powerful immunosuppressive drugs that could, in turn, make the patient vulnerable to other infections and actually aid the disease. This is the reason the organs would have to be altered to fit the patients' DNA ( histocompatibility ). [ citation needed ]
In 2005, the Australian National Health and Medical Research Council (NHMRC) declared an eighteen-year moratorium on all animal-to-human transplantation, concluding that the risks of transmission of animal viruses to patients and the wider community had not been resolved. [ 73 ] This was repealed in 2009 after an NHMRC review stated "... the risks, if appropriately regulated, are minimal and acceptable given the potential benefits.", citing international developments on the management and regulation of xenotransplantation by the World Health Organisation and the European Medicines Agency. [ 74 ]
Endogenous retroviruses are remnants of ancient viral infections, found in the genomes of most, if not all, mammalian species. Integrated into the chromosomal DNA, they are vertically transferred through inheritance. [ 67 ] Due to the many deletions and mutations they accumulate over time, they usually are not infectious in the host species, however the virus may become infectious in another species. [ 55 ] PERVS were originally discovered as retrovirus particles released from cultured porcine kidney cells. [ 75 ] Most breeds of swine harbor approximately 50 PERV genomes in their DNA. [ 76 ] Although it is likely that most of these are defective, some may be able to produce infectious viruses so every proviral genome must be sequenced to identify which ones pose a threat. In addition, through complementation and genetic recombination, two defective PERV genomes could give rise to an infectious virus. [ 77 ] There are three subgroups of infectious PERVs (PERV-A, PERV-B, and PERV-C). Experiments have shown that PERV-A and PERV-B can infect human cells in culture. [ 76 ] [ 78 ] To date no experimental xenotransplantations have demonstrated PERV transmission, yet this does not mean PERV infections in humans are impossible. [ 71 ] Pig cells have been engineered to inactivate all 62 PERVs in the genome using CRISPR Cas9 genome editing technology, [ 79 ] and eliminated infection from the pig to human cells in culture. [ 80 ] [ 81 ] [ 82 ]
Xenografts have been a controversial procedure since they were first attempted. Many, including animal rights groups, strongly oppose killing animals to harvest their organs for human use. [ 83 ] In the 1960s, many organs came from the chimpanzees, and were transferred into people that were deathly ill, and in turn, did not live much longer afterwards. [ 84 ] Modern scientific supporters of xenotransplantation argue that the potential benefits to society outweigh the risks, making pursuing xenotransplantation the moral choice. [ 85 ] None of the major religions object to the use of genetically modified pig organs for life-saving transplantation. [ 86 ] Religions such as Buddhism and Jainism , however, have long espoused non-violence towards all living creatures. [ 57 ] In general, the use of pig and cow tissue in humans has been met with little resistance, save some religious beliefs and a few philosophical objections. Experimentation without consent doctrines are now followed, which was not the case in the past, which may lead to new religious guidelines to further medical research on pronounced ecumenical guidelines. The "Common Rule" is the United States bio-ethics mandate as of 2011 [update] . [ 87 ]
At the beginning of the 20th century when studies in xenotransplantation were just beginning, few questioned the morality of it, turning to animals as a "natural" alternative to allografts . [ 88 ] While satirical plays mocked xenografters such as Serge Voronoff , and some images showing emotionally distraught primates – whom Voronoff had deprived of their testicles – appeared, no serious attempts were yet made to question the science based on animal rights concerns. [ 88 ] Xenotransplantation was not taken seriously, at least in France, during the first half of the 20th century. [ 88 ]
With the Baby Fae incident of 1984 as the impetus, animal rights activists began to protest, gathering media attention and proving that some people felt that it was unethical and a violation of the animal's own rights to use its organs to preserve a sick human's life. [ 88 ] Treating animals as mere tools for the slaughter on demand by human will would lead to a world they would not prefer. [ 88 ] Supporters of the transplant pushed back, claiming that saving a human life justifies the sacrifice of an animal one. [ 88 ] Most animal rights activists found the use of primate organs more reprehensible than those of, for example, pigs. [ 88 ] As Peter Singer et al. have expressed, many primates exhibit greater social structure, communication skills, and affection than mentally deficient humans and human infants. [ 89 ] Despite this, it is considerably unlikely that animal suffering will provide sufficient impetus for regulators to prevent xenotransplantation. [ 57 ]
Autonomy and informed consent are important when considering the future uses of xenotransplantation. A patient undergoing xenotransplantation should be fully aware of the procedure and should have no outside force influencing their choice. [ 90 ] The patient should understand the risks and benefits of such a transplantation. A public health dimension can also be considered. [ 91 ]
The Ethics Committee of the International Xenotransplantation Association pointed out in 2003 that one major ethical issue is the societal response to such a procedure. [ 92 ]
The application of the four bioethics principles is standardized in the moral conduct of laboratories. [ 93 ] The four principles emphasize informed consent, the Hippocratic Oath to do no harm, using skills to help others, and protecting the right to quality care. [ 94 ]
Though xenotransplantation may have future medical benefits, it also has the serious risk of introducing and spreading the infectious diseases, into the human population. [ 95 ] Guidelines have been drafted by governments with the purpose of forming the foundation of infectious disease surveillance. [ 95 ] United Kingdom guidelines state that patients have to agree to "the periodic provision of bodily samples that would then be archived for epidemiological purposes", "post-mortem analysis in case of death, the storage of samples post-mortem, and the disclosure of this agreement to their family", "refrain from donating blood, tissue or organs", "the use of barrier contraception when engaging in sexual intercourse", "keep both name and current address on register and to notify the relevant health authorities when moving abroad" and "divulge confidential information, including one's status as a xenotransplantation recipient to researchers, all health care professionals from whom one seeks professional services, and close contacts such as current and future sexual partners." [ 95 ] The patient must abide by these rules throughout their lifetime or until the government determines that there is no need for public health safeguards. [ 95 ]
The Food and Drug Administration (FDA) has also stated that if a transplantation takes place, the recipient must undergo monitoring for the rest of their lifetime and waive their right to withdraw. The reason for requiring lifelong monitoring is due to the risk of acute infections that may occur. The FDA suggests that a passive screening program should be implemented and should extend for the life of the recipient. [ 96 ]
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A xerophile (from Greek ξηρός : xērós ' dry ' and φίλος : phílos ' loving ' ) [ 1 ] is an extremophilic organism that can grow and reproduce in conditions with a low availability of water, also known as water activity .
Water activity, a thermodynamical value denoted a w , is defined as the partial water vapor pressure p in equilibrium with the substance relative to (divided by) the (partial) vapor pressure of pure water p* at the same temperature: a w ≡ p p ∗ {\displaystyle a_{w}\equiv {\frac {p}{p^{*}}}} The thermodynamical water activity is thus equal to the relative humidity (RH), and the chemical activity of pure water is equal to one: a w = 1.0.
When the atmosphere above a substance, or a solution , is undersaturated in water vapor (p < p * ) , its water activity is lower than one.
Xerophiles are "xerotolerant", meaning tolerant of dry conditions. They can often survive in environments with water activity below 0.8; above which is typical for most life on Earth. Typically xerotolerance is used with respect to matrix drying, where a substance has a low water concentration. These environments include arid desert soils. The term osmophile , or osmotolerant, is typically applied to microorganisms that can grow in solutions with high solute concentrations ( salts , sugars ), such as halophiles .
Eukaryotic and most prokaryotic life will collect or create compatible solutes, also called osmolytes , which establish a counter balance to the osmotic pressures. An example would be some bacteria accumulate KCl to counter-balance NaCl osmotic pressures. Fungi appear to use glycerol as an osmolyte since when cultures are grown in glycerol high concentrations that become better adapted to surviving low water activities. [ 2 ]
All taxonomic kingdoms posses examples of xerophiles. Microbial xerophiles will usually inhabit environments that are sugar-rich or salt-rich, and xerophilic bacteria will most commonly be found in salt rich areas. [ 2 ] Because xerophiles often live in salt-rich environments many halophilic species such as H. halophila , Bacillus halophilus , and H. salina are often also xerophilic.
A xerophilic archaea would be Natronococcus .
Xerophilic fungi will usually be found in environments that are sugar rich, and some xerophilic fungi have shown extremely low water activity, as low as .61. Xerophilic fungi include Trichosporonoides nigrescens , [ 3 ] Zygosaccharomyces , and Aspergillus penicillioides .
Among multi plant life an example of a xerophilic plant group is cacti .
Xerophilic micro organisms can be utilized in efforts of bioremediation. This is especially the case when the environment needing bioremediation has low water activity. Xerotolerant bacteria isolated from areas in Chile have expressed traits allowing it to be used as to begin bioremediation. [ 4 ]
For plants to properly grow in dry areas they will need a usable xerotolerant microbiome. In dessert plants xerophiles are set in a plants microbiome helping with its water management. [ 5 ]
Xerophiles are a concern to food storage industry due to their ability to bypass common food preservation methods. Many foods are preserved by creating high osmotic pressures that dry out and kill any microbes that attempt to culture in the food. Foods such as honey or jam have such high levels of sugar and low levels of water normal micro organisms can not grow on them. However, xerophilic organisms can grow in these mediums posing a threat to food safety. [ 2 ]
The common food preservation methods of reducing water activity ( food drying ) may not always be sufficient to prevent the growth of xerophilic organisms, often resulting in food spoilage . Some mold and yeast species are xerophilic. Mold growth on bread is an example of food spoilage by xerophilic organisms. [ citation needed ]
Complete dehydration based on the freeze-drying technique with effective protection inside a tight packaging system, strictly impervious to water and atmospheric gases ( O 2 and CO 2 ), may be required for long-term preservation of food and pharmacochemical substances ( antibiotics , vaccines …). Freeze drying can limit the microbial activity on the long term, as long as the product remains perfectly dry in a hermetically sealed and intact package, but it is not a sterilisation technique per se , because after rehydration, even if many dehydrated cells suffer irreversible and lethal damages, some resistant spores and bacterial endospores can still be revived again, and multiplied, by means of microbiological cultures if the product was not initially sterilized by applying a proven technique.
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The Xerox Dover laser printer was an early laser printer manufactured at Xerox PARC in the late 1970s. [ 1 ] Around 35 were built. [ 1 ] It was a successor to the EARS printer, itself a successor to the Xerox Graphics Printer . [ 2 ]
The Dover was developed by Gary Starkweather . [ 3 ] The printer was based on a stripped down Xerox 7000 reduction duplicator chassis. [ 4 ]
Dover printers were in use at several high-profile computer science research labs. A Dover printer was installed at Stanford University's computer science department in 1980, [ 5 ] and a Dover printer was available at the MIT AI Lab in 1982, hosted by a Xerox Alto computer. [ 6 ]
This computing article is a stub . You can help Wikipedia by expanding it .
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The Xi baryons or cascade particles are a family of subatomic hadron particles which have the symbol Ξ and may have an electric charge ( Q ) of +2 e , +1 e , 0, or −1 e , where e is the elementary charge .
Like all conventional baryons , Ξ particles contain three quarks . Ξ baryons, in particular, contain either one up or one down quark and two other, more massive quarks. The two more massive quarks are any two of strange , charm , or bottom (doubles allowed). For notation, the assumption is that the two heavy quarks in the Ξ are both strange ; subscripts "c" and "b" are added for each even heavier charm or bottom quark that replaces one of the two presumed strange quarks .
They are historically called the cascade particles because of their unstable state; they are typically observed to decay rapidly into lighter particles, through a chain of decays (cascading decays). [ 2 ] The first discovery of a charged Xi baryon was in cosmic ray experiments by the Manchester group in 1952. [ 3 ] The first discovery of the neutral Xi particle was at Lawrence Berkeley Laboratory in 1959. [ 4 ] It was also observed as a daughter product from the decay of the omega baryon ( Ω − ) observed at Brookhaven National Laboratory in 1964. [ 2 ] The Xi spectrum is important to nonperturbative quantum chromodynamics (QCD), such as lattice QCD . [ why? ]
The Ξ − b particle is also known as the cascade B particle and contains quarks from all three families. It was discovered by DØ and CDF experiments at Fermilab . The discovery was announced on 12 June 2007. It was the first known particle made of quarks from all three quark generations – namely, a down quark , a strange quark , and a bottom quark . The DØ and CDF collaborations reported the consistent masses of the new state. The Particle Data Group world average mass is 5.7924 ± 0.0030 GeV/ c 2 .
For notation, the assumption is that the two heavy quarks are both strange , denoted by a simple Ξ ; a subscript "c" is added for each constituent charm quark , and a "b" for each bottom quark . Hence Ξ c , Ξ b , Ξ cc , Ξ cb , etc. Unless specified , the non-up/down quark content of Xi baryons is strange (i.e. there is one up or down quark and two strange quarks). However a Ξ 0 b contains one up, one strange, and one bottom quark, while a Ξ 0 bb contains one up and two bottom quarks.
In 2012, the CMS experiment at the Large Hadron Collider detected a Ξ ∗0 b baryon (reported mass 5945 ± 2.8 MeV/ c 2 ). [ 5 ] [ 6 ] (Here,"*" indicates a baryon decuplet .) The LHCb experiment at CERN discovered two new Xi baryons in 2014: Ξ′ − b and Ξ ∗− b . [ 7 ]
In 2017, the LHCb researchers reported yet another Xi baryon: the double charmed Ξ ++ cc baryon, consisting of two heavy charm quarks and one up quark. The mass of Ξ ++ cc is about 3.8 times that of a proton . [ 8 ] [ 9 ]
Isospin and spin values in parentheses have not been firmly established by experiments, but are predicted by the quark model and are consistent with the measurements.
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Xiangfeng wu ( Chinese : 相風烏 ; pinyin : xiāngfēngwū ; lit. 'wind-indicating bird') [ 1 ] were wind surveying instruments used to gather and measure the direction of the wind in ancient China. [ 2 ]
Prior to the invention of Xiangfeng wu, the ancient Chinese used pieces of silk or cloth that was hung on a pole to measure wind direction. [ 2 ] Epigraphic evidence attributing to the discovery of weather crow on a wall painting in a tomb dating to the Eastern Han dynasty in 1972. [ 2 ] The Sanfu huangtu (三輔黃圖, Description of the Three Districts in the Capital), a 3rd-century book written by Miao Changyan about the palaces at Chang'an, describes a copper bird-shaped wind vane situated on a tower roof for the measurement of wind direction. [ 1 ] [ 2 ]
Xiangfeng wu composed of copper slices that were fixed on the top of a pole which could be revolved if the wind was blowing in a certain direction. Xiangfeng wu were first used in meteorological observatories and were later installed in government towers and private houses. [ 3 ]
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Xiao Cheng Zeng is a scientist in physical chemistry and materials science. He currently serves as the Head of Department of Materials Science & Engineering and a Chair Professor of Materials Chemistry and Chemical Engineering at City University of Hong Kong . He is also an Emeritus Chancellor's University Professor at University of Nebraska–Lincoln (USA). He is a fellow of the American Association for the Advancement of Science , a fellow of the American Physical Society , a fellow of US Materials Research Society , a fellow of Royal Society of Chemistry , and a foreign fellow of the European Academy of Sciences. He had also held a John Simon Guggenheim fellowship .
He received his bachelor degree in Physics from Peking University in 1984. He then pursued postgraduate study in US via CUSPEA program (created by Nobel Laureate in Physics Professor Tsung-Dao Lee ), and received his Ph.D. in Condensed Matter Physics from the Ohio State University in 1989. Thereafter, he did his postdoctoral research in Physical Chemistry at University of Chicago (1989–1992) and UCLA (1992–1993).
As a computational physical chemist, Zeng has made original contributions to thermodynamics and phase transition of nanoconfined water/ice, and water/surface interaction and wetting; original contributions to gold-cluster science and nanocatalysis; atmospheric reactions; and computational design of low-dimensional materials.
Low-dimensional ice/ice hydrates: Zeng opened the field of low-dimensional ice/ice hydrates. In 1997, he predicted 2D bilayer hexagonal ice, nicknamed "the Nebraska ice"., [ 1 ] a phase of water ice that is two-dimensional. [ 2 ] And it was confirmed by two experiments: one by the Pacific Northwest National Laboratory in 2009 [ 3 ] and another by the Peking University in 2020. [ 4 ] Thereafter, this 2D bilayer hexagonal ice is named 2D ice I. Moreover, his theoretical predictions of the 1D ices (1D ice-nanotubes I-III), [ 5 ] "DNA-ice", [ 6 ] ferroelectric ice-χ, [ 7 ] and 2D amorphous, [ 8 ] plastic, and superionic ices [ 9 ] enriched ice family.
Gold-cluster science & nanocatalysis: In 2006, he discovered the first all-metal cage molecules (Au 16-18 ). [ 10 ] Later, he resolved size/structure/catalytic-activity relationship of 20+ gold clusters; [ 11 ] [ 12 ] he also developed a grand-unified model to decipher structures of 70+ ligand-covered gold clusters. [ 13 ]
Atmospheric reactions: Since 2015, he has found several new chemical reactions on water/cloud droplets with implications to atmospheric new particle formation and haze chemistry. [ 14 ] [ 15 ] [ 16 ] [ 17 ]
Computational design of low-dimensional materials: In 2011, he predicted 20+ metallic boron monolayer structures and created systematic naming series for these monolayers, including α, β, χ, and δ series; [ 18 ] two in these series, χ 3 -borophene and β 12 -borophene were later confirmed by experiments. [ 19 ] [ 20 ]
Zeng's scientific accomplishments are documented in more than 700 journal publications with 59000+ citations in Google scholar (H-index 122) and 50000+ in Web of Science (H-index 109). These publications include 7 in Nature/Science , 23 in Nature/Science Sister Journals, 26 in Proc. Natl. Acad. Sci. (PNAS) , 73 in Journal of the American Chemical Society (JACS) , 30 in Angewandte Chemie/Advanced Materials , 9 in Phys. Rev. Lett./Phys. Rev. X , and 4 in Joule/Chem .
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50 MP, f/2.0, 75mm (telephoto), PDAF (10cm - ∞), OIS, 3.2x optical zoom
The Xiaomi 14 is a series of Android -based smartphones manufactured by Xiaomi that succeeds the Xiaomi 13 series. The series is the company's flagship [ 4 ] range. The Xiaomi 14 and 14 Pro, the first phones to feature a Qualcomm Snapdragon 8 Gen 3 System On Chip , [ 5 ] were announced on 26 October 2023 and were released in China on 1 November 2023. The Xiaomi 14 Ultra was released on February 22, 2024, in China alongside the Xiaomi Pad 6S Pro. [ 6 ]
The global launch of the Xiaomi 14 series happened at MWC 2024 on February 25, 2024, in Barcelona , Spain . [ 7 ]
The Xiaomi 14 features a flat front made of Corning Gorilla Glass Victus , meanwhile the Xiaomi 14 Pro has a front with curved side edges made of in-house Xiaomi Shield (Longjing) Glass. [ 8 ] The Xiaomi 14 Ultra, as the Pro model, features a front made of Shield Glass but it is curved around the perimeter, though not as prominently as on the sides of the Pro model.
The back of the Xiaomi 14 is made of a silicone polymer material in the Snow Mountain Powder color option and glass in other color options, the Xiaomi 14 Pro features the back made of glass in all color options, while on the Xiaomi 14 Ultra it is made of glass in the Dragon Crystal Blue color and eco leather in other color options.
The frame of all three models is generally made of aluminium but the Xiaomi 14 Pro and Xiaomi 14 Ultra offer Titanium Special Editions with the titanium frame. [ 8 ]
The only noticeable design change of the Xiaomi 14 compared to the Xiaomi 13 is the camera island, which has no dividing lines and features a different arrangement of elements, such as three camera lenses, LED flash and Leica logo. The Xiaomi 14 Pro, compared to the Xiaomi 13 Pro, features a flatter back and a consistent frame width on all sides. Meanwhile, the Xiaomi 14 Ultra retains mostly identical to its predecessor design but includes the bezel around the camera island [ 9 ] and lacks a bump on the upper part of the back.
All modes have IP68 dust and water resistance.
On the bottom side, there is a dual SIM tray, a microphone , a USB-C port and a speaker . On the top of the Xiaomi 14 Pro and Xiaomi 14 Ultra there is a second speaker. On the right side, there is the volume rocker and the power button. Elements such as the IR port , an additional microphone have been moved to the camera island. [ 10 ]
On the global market, the Xiaomi 14 is available in black , white , and Jade Green color options. In China , the Xiaomi 14 is also available in Snow Mountain Powder ( pink ).
The Xiaomi 14 Pro is available in black, white, Jade Green, and Titanium Special Edition color options.
On the global market, the Xiaomi 14 Ultra is available in black and white color options. In China, the Xiaomi 14 Ultra is also available in Dragon Crystal Blue and Titanium Special Edition.
The Xiaomi 14 and Xiaomi 14 Pro are the first smartphones to receive the flagship Qualcomm Snapdragon 8 Gen 3 SoC with an Adreno 750 GPU . [ 11 ] The Xiaomi 14 Ultra uses the same platform.
The smartphones feature non-removable Li-Po batteries with a capacity of 4610 mAh in the Xiaomi 14, 4880 mAh in the Xiaomi 14 Pro, 5000 mAh in the global model of the Xiaomi 14 Ultra and 5300 mAh in the Chinese model of the Xiaomi 14 Ultra.
The Xiaomi 14 and 14 Ultra support 90 W fast wired charging and the Xiaomi 14 Pro supports 120 W fast wired charging. The Xiaomi 14 and Xiaomi 14 Pro also support 50 W fast wireless charging and the Xiaomi 14 Ultra supports an 80 W fast wireless charging, which, at the time of release, is the most powerful wireless charging since the Xiaomi Mi 11 Ultra . All three models feature a 5 W reverse wireless charging support.
Xiaomi 14 and Xiaomi 14 Pro feature a triple camera setup consisting of the following lenses:
On the other hand, the Xiaomi 14 Ultra features quad-camera setup consisting of the following lenses:
In addition, the Xiaomi 14 Ultra features a TOF 3D camera for depth measurement. All models also use LEICA SUMMILUX optics.
The main cameras of the Xiaomi 14 and Xiaomi 14 Pro can record video at up to 8K @24 fps , while the main camera of the Xiaomi 14 Ultra can record video at up to 8K@30fps.
All three models also feature a 32 MP front-facing camera with an f/2.5 aperture and the ability to record video at up to 4K @60fps.
The models feature an LTPO AMOLED display with an adaptive 1–120 Hz refresh rate, Dolby Vision, HDR10+, a 20:9 aspect ratio , a centered circular cutout for the front-facing camera, and an under-display optical fingerprint sensor. The display of the Xiaomi 14 has 6.36" diagonal , 2670 × 1200 resolution , and a 460 ppi pixel density , while the display of the Xiaomi 14 Pro and Xiaomi 14 Ultra have 6.73" diagonal, 3200 × 1440 ( QHD+ ) resolution, and a 522 ppi pixel density.
The smartphones feature stereo speakers with Dolby Atmos support. In the Xiaomi 14 the earpiece doubles as the second speaker, while the Xiaomi 14 Pro and Xiaomi 14 Ultra feature a dedicated second speaker on the top of the frame.
The Xiaomi 14 is available in 8 GB /256 GB, 12 GB/256 GB, 16 GB/512 GB, and 16 GB/1 TB memory configurations, while the Xiaomi 14 Pro and Xiaomi 14 Ultra are available in 12 GB/256 GB, 16 GB/512 GB, and 16 GB/1 TB memory configurations
The Xiaomi 14 and Xiaomi 14 Pro are the first smartphones to receive the new Xiaomi HyperOS operating system, that succeeds MIUI. On both models, Xiaomi HyperOS is based on Android 14 . Xiaomi 14 Ultra features the same software. Later, all three models were updated to Xiaomi HyperOS 2 based on Android 15 .
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https://en.wikipedia.org/wiki/Xiaomi_14
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50 MP, f/2.0, 75mm (telephoto), PDAF (10cm - ∞), OIS, 3.2x optical zoom
The Xiaomi 14 is a series of Android -based smartphones manufactured by Xiaomi that succeeds the Xiaomi 13 series. The series is the company's flagship [ 4 ] range. The Xiaomi 14 and 14 Pro, the first phones to feature a Qualcomm Snapdragon 8 Gen 3 System On Chip , [ 5 ] were announced on 26 October 2023 and were released in China on 1 November 2023. The Xiaomi 14 Ultra was released on February 22, 2024, in China alongside the Xiaomi Pad 6S Pro. [ 6 ]
The global launch of the Xiaomi 14 series happened at MWC 2024 on February 25, 2024, in Barcelona , Spain . [ 7 ]
The Xiaomi 14 features a flat front made of Corning Gorilla Glass Victus , meanwhile the Xiaomi 14 Pro has a front with curved side edges made of in-house Xiaomi Shield (Longjing) Glass. [ 8 ] The Xiaomi 14 Ultra, as the Pro model, features a front made of Shield Glass but it is curved around the perimeter, though not as prominently as on the sides of the Pro model.
The back of the Xiaomi 14 is made of a silicone polymer material in the Snow Mountain Powder color option and glass in other color options, the Xiaomi 14 Pro features the back made of glass in all color options, while on the Xiaomi 14 Ultra it is made of glass in the Dragon Crystal Blue color and eco leather in other color options.
The frame of all three models is generally made of aluminium but the Xiaomi 14 Pro and Xiaomi 14 Ultra offer Titanium Special Editions with the titanium frame. [ 8 ]
The only noticeable design change of the Xiaomi 14 compared to the Xiaomi 13 is the camera island, which has no dividing lines and features a different arrangement of elements, such as three camera lenses, LED flash and Leica logo. The Xiaomi 14 Pro, compared to the Xiaomi 13 Pro, features a flatter back and a consistent frame width on all sides. Meanwhile, the Xiaomi 14 Ultra retains mostly identical to its predecessor design but includes the bezel around the camera island [ 9 ] and lacks a bump on the upper part of the back.
All modes have IP68 dust and water resistance.
On the bottom side, there is a dual SIM tray, a microphone , a USB-C port and a speaker . On the top of the Xiaomi 14 Pro and Xiaomi 14 Ultra there is a second speaker. On the right side, there is the volume rocker and the power button. Elements such as the IR port , an additional microphone have been moved to the camera island. [ 10 ]
On the global market, the Xiaomi 14 is available in black , white , and Jade Green color options. In China , the Xiaomi 14 is also available in Snow Mountain Powder ( pink ).
The Xiaomi 14 Pro is available in black, white, Jade Green, and Titanium Special Edition color options.
On the global market, the Xiaomi 14 Ultra is available in black and white color options. In China, the Xiaomi 14 Ultra is also available in Dragon Crystal Blue and Titanium Special Edition.
The Xiaomi 14 and Xiaomi 14 Pro are the first smartphones to receive the flagship Qualcomm Snapdragon 8 Gen 3 SoC with an Adreno 750 GPU . [ 11 ] The Xiaomi 14 Ultra uses the same platform.
The smartphones feature non-removable Li-Po batteries with a capacity of 4610 mAh in the Xiaomi 14, 4880 mAh in the Xiaomi 14 Pro, 5000 mAh in the global model of the Xiaomi 14 Ultra and 5300 mAh in the Chinese model of the Xiaomi 14 Ultra.
The Xiaomi 14 and 14 Ultra support 90 W fast wired charging and the Xiaomi 14 Pro supports 120 W fast wired charging. The Xiaomi 14 and Xiaomi 14 Pro also support 50 W fast wireless charging and the Xiaomi 14 Ultra supports an 80 W fast wireless charging, which, at the time of release, is the most powerful wireless charging since the Xiaomi Mi 11 Ultra . All three models feature a 5 W reverse wireless charging support.
Xiaomi 14 and Xiaomi 14 Pro feature a triple camera setup consisting of the following lenses:
On the other hand, the Xiaomi 14 Ultra features quad-camera setup consisting of the following lenses:
In addition, the Xiaomi 14 Ultra features a TOF 3D camera for depth measurement. All models also use LEICA SUMMILUX optics.
The main cameras of the Xiaomi 14 and Xiaomi 14 Pro can record video at up to 8K @24 fps , while the main camera of the Xiaomi 14 Ultra can record video at up to 8K@30fps.
All three models also feature a 32 MP front-facing camera with an f/2.5 aperture and the ability to record video at up to 4K @60fps.
The models feature an LTPO AMOLED display with an adaptive 1–120 Hz refresh rate, Dolby Vision, HDR10+, a 20:9 aspect ratio , a centered circular cutout for the front-facing camera, and an under-display optical fingerprint sensor. The display of the Xiaomi 14 has 6.36" diagonal , 2670 × 1200 resolution , and a 460 ppi pixel density , while the display of the Xiaomi 14 Pro and Xiaomi 14 Ultra have 6.73" diagonal, 3200 × 1440 ( QHD+ ) resolution, and a 522 ppi pixel density.
The smartphones feature stereo speakers with Dolby Atmos support. In the Xiaomi 14 the earpiece doubles as the second speaker, while the Xiaomi 14 Pro and Xiaomi 14 Ultra feature a dedicated second speaker on the top of the frame.
The Xiaomi 14 is available in 8 GB /256 GB, 12 GB/256 GB, 16 GB/512 GB, and 16 GB/1 TB memory configurations, while the Xiaomi 14 Pro and Xiaomi 14 Ultra are available in 12 GB/256 GB, 16 GB/512 GB, and 16 GB/1 TB memory configurations
The Xiaomi 14 and Xiaomi 14 Pro are the first smartphones to receive the new Xiaomi HyperOS operating system, that succeeds MIUI. On both models, Xiaomi HyperOS is based on Android 14 . Xiaomi 14 Ultra features the same software. Later, all three models were updated to Xiaomi HyperOS 2 based on Android 15 .
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https://en.wikipedia.org/wiki/Xiaomi_14_Pro
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50 MP, f/2.0, 75mm (telephoto), PDAF (10cm - ∞), OIS, 3.2x optical zoom
The Xiaomi 14 is a series of Android -based smartphones manufactured by Xiaomi that succeeds the Xiaomi 13 series. The series is the company's flagship [ 4 ] range. The Xiaomi 14 and 14 Pro, the first phones to feature a Qualcomm Snapdragon 8 Gen 3 System On Chip , [ 5 ] were announced on 26 October 2023 and were released in China on 1 November 2023. The Xiaomi 14 Ultra was released on February 22, 2024, in China alongside the Xiaomi Pad 6S Pro. [ 6 ]
The global launch of the Xiaomi 14 series happened at MWC 2024 on February 25, 2024, in Barcelona , Spain . [ 7 ]
The Xiaomi 14 features a flat front made of Corning Gorilla Glass Victus , meanwhile the Xiaomi 14 Pro has a front with curved side edges made of in-house Xiaomi Shield (Longjing) Glass. [ 8 ] The Xiaomi 14 Ultra, as the Pro model, features a front made of Shield Glass but it is curved around the perimeter, though not as prominently as on the sides of the Pro model.
The back of the Xiaomi 14 is made of a silicone polymer material in the Snow Mountain Powder color option and glass in other color options, the Xiaomi 14 Pro features the back made of glass in all color options, while on the Xiaomi 14 Ultra it is made of glass in the Dragon Crystal Blue color and eco leather in other color options.
The frame of all three models is generally made of aluminium but the Xiaomi 14 Pro and Xiaomi 14 Ultra offer Titanium Special Editions with the titanium frame. [ 8 ]
The only noticeable design change of the Xiaomi 14 compared to the Xiaomi 13 is the camera island, which has no dividing lines and features a different arrangement of elements, such as three camera lenses, LED flash and Leica logo. The Xiaomi 14 Pro, compared to the Xiaomi 13 Pro, features a flatter back and a consistent frame width on all sides. Meanwhile, the Xiaomi 14 Ultra retains mostly identical to its predecessor design but includes the bezel around the camera island [ 9 ] and lacks a bump on the upper part of the back.
All modes have IP68 dust and water resistance.
On the bottom side, there is a dual SIM tray, a microphone , a USB-C port and a speaker . On the top of the Xiaomi 14 Pro and Xiaomi 14 Ultra there is a second speaker. On the right side, there is the volume rocker and the power button. Elements such as the IR port , an additional microphone have been moved to the camera island. [ 10 ]
On the global market, the Xiaomi 14 is available in black , white , and Jade Green color options. In China , the Xiaomi 14 is also available in Snow Mountain Powder ( pink ).
The Xiaomi 14 Pro is available in black, white, Jade Green, and Titanium Special Edition color options.
On the global market, the Xiaomi 14 Ultra is available in black and white color options. In China, the Xiaomi 14 Ultra is also available in Dragon Crystal Blue and Titanium Special Edition.
The Xiaomi 14 and Xiaomi 14 Pro are the first smartphones to receive the flagship Qualcomm Snapdragon 8 Gen 3 SoC with an Adreno 750 GPU . [ 11 ] The Xiaomi 14 Ultra uses the same platform.
The smartphones feature non-removable Li-Po batteries with a capacity of 4610 mAh in the Xiaomi 14, 4880 mAh in the Xiaomi 14 Pro, 5000 mAh in the global model of the Xiaomi 14 Ultra and 5300 mAh in the Chinese model of the Xiaomi 14 Ultra.
The Xiaomi 14 and 14 Ultra support 90 W fast wired charging and the Xiaomi 14 Pro supports 120 W fast wired charging. The Xiaomi 14 and Xiaomi 14 Pro also support 50 W fast wireless charging and the Xiaomi 14 Ultra supports an 80 W fast wireless charging, which, at the time of release, is the most powerful wireless charging since the Xiaomi Mi 11 Ultra . All three models feature a 5 W reverse wireless charging support.
Xiaomi 14 and Xiaomi 14 Pro feature a triple camera setup consisting of the following lenses:
On the other hand, the Xiaomi 14 Ultra features quad-camera setup consisting of the following lenses:
In addition, the Xiaomi 14 Ultra features a TOF 3D camera for depth measurement. All models also use LEICA SUMMILUX optics.
The main cameras of the Xiaomi 14 and Xiaomi 14 Pro can record video at up to 8K @24 fps , while the main camera of the Xiaomi 14 Ultra can record video at up to 8K@30fps.
All three models also feature a 32 MP front-facing camera with an f/2.5 aperture and the ability to record video at up to 4K @60fps.
The models feature an LTPO AMOLED display with an adaptive 1–120 Hz refresh rate, Dolby Vision, HDR10+, a 20:9 aspect ratio , a centered circular cutout for the front-facing camera, and an under-display optical fingerprint sensor. The display of the Xiaomi 14 has 6.36" diagonal , 2670 × 1200 resolution , and a 460 ppi pixel density , while the display of the Xiaomi 14 Pro and Xiaomi 14 Ultra have 6.73" diagonal, 3200 × 1440 ( QHD+ ) resolution, and a 522 ppi pixel density.
The smartphones feature stereo speakers with Dolby Atmos support. In the Xiaomi 14 the earpiece doubles as the second speaker, while the Xiaomi 14 Pro and Xiaomi 14 Ultra feature a dedicated second speaker on the top of the frame.
The Xiaomi 14 is available in 8 GB /256 GB, 12 GB/256 GB, 16 GB/512 GB, and 16 GB/1 TB memory configurations, while the Xiaomi 14 Pro and Xiaomi 14 Ultra are available in 12 GB/256 GB, 16 GB/512 GB, and 16 GB/1 TB memory configurations
The Xiaomi 14 and Xiaomi 14 Pro are the first smartphones to receive the new Xiaomi HyperOS operating system, that succeeds MIUI. On both models, Xiaomi HyperOS is based on Android 14 . Xiaomi 14 Ultra features the same software. Later, all three models were updated to Xiaomi HyperOS 2 based on Android 15 .
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https://en.wikipedia.org/wiki/Xiaomi_14_Ultra
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Xiaomi 15 is a series of Android -based smartphones manufactured by Xiaomi . It succeeds the Xiaomi 14 series and introduces advancements in display technology, camera system, and performance. The devices run on Xiaomi HyperOS 2, based on Android 15 , and are powered by the Qualcomm Snapdragon 8 Elite chipset. The Xiaomi 15 and Xiaomi 15 Pro were introduced on October 29, 2024, while the Xiaomi 15 Ultra was introduced on February 27, 2025. The global versions of the Xiaomi 15 and Xiaomi 15 Ultra were announced on March 3, 2025 at MWC and feature eSIM support and smaller batteries compared to the Chinese counterparts. [ 1 ] [ 2 ] [ 3 ] [ 4 ]
The Xiaomi 15 features a 6.36-inch OLED display with a 1.5K resolution, a 120 Hz refresh rate, and a peak brightness of 3200 nits. The phone is built with an aviation-grade aluminum frame and has a compact design measuring 152.3 × 71.2 × 8.08 mm, weighing 181 g. It is rated IP68 for dust and water resistance, offering durability for daily use. [ 5 ] [ 6 ]
Powered by the Qualcomm Snapdragon 8 Elite chipset, the Xiaomi 15 lineup is built on a 3 nm process. It includes 12 GB or 16 GB of LPDDR5X RAM and offers storage options of 256 GB, 512 GB or 1 TB using UFS 4.0 technology in the base and Pro models and UFS 4.1 technology in the Ultra model. The devices are equipped with an advanced cooling system to manage thermal performance during high-intensity tasks. They support 5G connectivity, Wi-Fi 7, Bluetooth 6.0, NFC, and USB-C. [ 7 ] [ 8 ]
Xiaomi collaborated with Leica Camera to enhance the imaging capabilities of the Xiaomi 15 series. The Xiaomi 15 includes a triple-camera system, consisting of a 50 MP primary sensor with optical image stabilization, a 50 MP ultra-wide sensor, and a 50 MP telephoto sensor. The front-facing camera features a 32 MP OmniVision OV32B40 sensor designed for high-resolution selfies and video calls. [ 9 ] [ 10 ]
The Xiaomi 15 series features a silicon-carbon batteries , [ 11 ] which has higher energy density compared to the lithium-ion batteries. That allows manufacturers to include batteries with bigger capacity in smartphones without changing their size.
The battery of the Xiaomi 15 Pro has a capacity of 6100 mAh, while the battery of the Xiaomi 15 has a capacity of 5240 mAh in the global market and 5400 mAh in the Chinese market, and the battery of the Xiaomi 15 Ultra has a capacity of 5410 mAh in the global market and 6000 mAh in the Chinese market.
All three models support 90W wired and 50W wireless charging. They include Xiaomi’s proprietary Surge P3 charging chipset and Surge G2 battery management system for optimized power efficiency. [ 12 ] [ 13 ]
The Xiaomi 15 series run on Xiaomi HyperOS 2, which is based on Android 15. It includes various optimizations for system fluidity and battery management while offering deep integration with Xiaomi’s ecosystem of devices. [ 14 ]
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https://en.wikipedia.org/wiki/Xiaomi_15
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Xiaomi 15 is a series of Android -based smartphones manufactured by Xiaomi . It succeeds the Xiaomi 14 series and introduces advancements in display technology, camera system, and performance. The devices run on Xiaomi HyperOS 2, based on Android 15 , and are powered by the Qualcomm Snapdragon 8 Elite chipset. The Xiaomi 15 and Xiaomi 15 Pro were introduced on October 29, 2024, while the Xiaomi 15 Ultra was introduced on February 27, 2025. The global versions of the Xiaomi 15 and Xiaomi 15 Ultra were announced on March 3, 2025 at MWC and feature eSIM support and smaller batteries compared to the Chinese counterparts. [ 1 ] [ 2 ] [ 3 ] [ 4 ]
The Xiaomi 15 features a 6.36-inch OLED display with a 1.5K resolution, a 120 Hz refresh rate, and a peak brightness of 3200 nits. The phone is built with an aviation-grade aluminum frame and has a compact design measuring 152.3 × 71.2 × 8.08 mm, weighing 181 g. It is rated IP68 for dust and water resistance, offering durability for daily use. [ 5 ] [ 6 ]
Powered by the Qualcomm Snapdragon 8 Elite chipset, the Xiaomi 15 lineup is built on a 3 nm process. It includes 12 GB or 16 GB of LPDDR5X RAM and offers storage options of 256 GB, 512 GB or 1 TB using UFS 4.0 technology in the base and Pro models and UFS 4.1 technology in the Ultra model. The devices are equipped with an advanced cooling system to manage thermal performance during high-intensity tasks. They support 5G connectivity, Wi-Fi 7, Bluetooth 6.0, NFC, and USB-C. [ 7 ] [ 8 ]
Xiaomi collaborated with Leica Camera to enhance the imaging capabilities of the Xiaomi 15 series. The Xiaomi 15 includes a triple-camera system, consisting of a 50 MP primary sensor with optical image stabilization, a 50 MP ultra-wide sensor, and a 50 MP telephoto sensor. The front-facing camera features a 32 MP OmniVision OV32B40 sensor designed for high-resolution selfies and video calls. [ 9 ] [ 10 ]
The Xiaomi 15 series features a silicon-carbon batteries , [ 11 ] which has higher energy density compared to the lithium-ion batteries. That allows manufacturers to include batteries with bigger capacity in smartphones without changing their size.
The battery of the Xiaomi 15 Pro has a capacity of 6100 mAh, while the battery of the Xiaomi 15 has a capacity of 5240 mAh in the global market and 5400 mAh in the Chinese market, and the battery of the Xiaomi 15 Ultra has a capacity of 5410 mAh in the global market and 6000 mAh in the Chinese market.
All three models support 90W wired and 50W wireless charging. They include Xiaomi’s proprietary Surge P3 charging chipset and Surge G2 battery management system for optimized power efficiency. [ 12 ] [ 13 ]
The Xiaomi 15 series run on Xiaomi HyperOS 2, which is based on Android 15. It includes various optimizations for system fluidity and battery management while offering deep integration with Xiaomi’s ecosystem of devices. [ 14 ]
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https://en.wikipedia.org/wiki/Xiaomi_15_Pro
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Xiaomi 15 is a series of Android -based smartphones manufactured by Xiaomi . It succeeds the Xiaomi 14 series and introduces advancements in display technology, camera system, and performance. The devices run on Xiaomi HyperOS 2, based on Android 15 , and are powered by the Qualcomm Snapdragon 8 Elite chipset. The Xiaomi 15 and Xiaomi 15 Pro were introduced on October 29, 2024, while the Xiaomi 15 Ultra was introduced on February 27, 2025. The global versions of the Xiaomi 15 and Xiaomi 15 Ultra were announced on March 3, 2025 at MWC and feature eSIM support and smaller batteries compared to the Chinese counterparts. [ 1 ] [ 2 ] [ 3 ] [ 4 ]
The Xiaomi 15 features a 6.36-inch OLED display with a 1.5K resolution, a 120 Hz refresh rate, and a peak brightness of 3200 nits. The phone is built with an aviation-grade aluminum frame and has a compact design measuring 152.3 × 71.2 × 8.08 mm, weighing 181 g. It is rated IP68 for dust and water resistance, offering durability for daily use. [ 5 ] [ 6 ]
Powered by the Qualcomm Snapdragon 8 Elite chipset, the Xiaomi 15 lineup is built on a 3 nm process. It includes 12 GB or 16 GB of LPDDR5X RAM and offers storage options of 256 GB, 512 GB or 1 TB using UFS 4.0 technology in the base and Pro models and UFS 4.1 technology in the Ultra model. The devices are equipped with an advanced cooling system to manage thermal performance during high-intensity tasks. They support 5G connectivity, Wi-Fi 7, Bluetooth 6.0, NFC, and USB-C. [ 7 ] [ 8 ]
Xiaomi collaborated with Leica Camera to enhance the imaging capabilities of the Xiaomi 15 series. The Xiaomi 15 includes a triple-camera system, consisting of a 50 MP primary sensor with optical image stabilization, a 50 MP ultra-wide sensor, and a 50 MP telephoto sensor. The front-facing camera features a 32 MP OmniVision OV32B40 sensor designed for high-resolution selfies and video calls. [ 9 ] [ 10 ]
The Xiaomi 15 series features a silicon-carbon batteries , [ 11 ] which has higher energy density compared to the lithium-ion batteries. That allows manufacturers to include batteries with bigger capacity in smartphones without changing their size.
The battery of the Xiaomi 15 Pro has a capacity of 6100 mAh, while the battery of the Xiaomi 15 has a capacity of 5240 mAh in the global market and 5400 mAh in the Chinese market, and the battery of the Xiaomi 15 Ultra has a capacity of 5410 mAh in the global market and 6000 mAh in the Chinese market.
All three models support 90W wired and 50W wireless charging. They include Xiaomi’s proprietary Surge P3 charging chipset and Surge G2 battery management system for optimized power efficiency. [ 12 ] [ 13 ]
The Xiaomi 15 series run on Xiaomi HyperOS 2, which is based on Android 15. It includes various optimizations for system fluidity and battery management while offering deep integration with Xiaomi’s ecosystem of devices. [ 14 ]
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https://en.wikipedia.org/wiki/Xiaomi_15_Ultra
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The Xiaomi Mi Smart Band 5 ( Xiaomi Mi Band 5 in China) is a wearable activity tracker produced by Xiaomi Inc . It was announced in China on 11 June 2020, and went on sale on 18 June 2020 in China, [ 1 ] with a Global version released on 15 July 2020 [ 2 ] as Xiaomi Mi Smart Band 5. It was released in India on 29 September 2020. [ 3 ] It has a 1.1-inch, 126x294 resolution capacitive AMOLED display and 24/7 heart rate monitoring claiming a 50% more accurate PPG sensor than its predecessor. It supports a charging dock that locates using a magnet which is said to be easier to use than previous generation chargers. The NFC-enabled version also has an in-built microphone for Xiaomi's in-built assistant Xiao.
The Global Mi Smart Band does not have features such as Alexa Support, NFC and SpO2 tracking. [ 4 ]
Heart rate monitor (24/7), Step counter , 11 workout modes (Outdoor running, Indoor running, Walking, Cycling, Indoor cycling, Pool swimming, Freestyle, Elliptical, jump rope, yoga, rowing machine), personal activity intelligence), camera controller, weather alerts, near-field communication (in some models), timer, stopwatch, rapid eye movement sleep + sleep monitor, women's health tracking, alarms, reminders, notifications, call mute and Call decline, Find phone, Silent phone, Brightness +/- and other minor features, Music controller, sedentary reminder , event reminder, breathing exercises, stress counter. Chinese version only: virtual assistant , microphone.
This mobile technology –related article is a stub . You can help Wikipedia by expanding it .
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https://en.wikipedia.org/wiki/Xiaomi_Mi_Smart_Band_5
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Ximera (pronounced “chimera”) is an open-source, interactive textbook platform, most commonly used in teaching math. [ 1 ] It was originally an massive open online course by Ohio State University on Coursera and YouTube . [ 2 ] The system was originally known as MOOCulus and Calculus One . [ 3 ]
The course features over 25 hours of video and exercises. The instructor is Jim Fowler, an associate professor of mathematics at the Ohio State University. [ 4 ] The course was available for the first time on Coursera during the Spring Semester of 2012–13. More than 47,000 students enrolled in the course, and several thousand successfully completed the 15-week course, [ 5 ] which has been favorably reviewed. [ 6 ]
The course begins with an introduction to functions and limits , and goes on to explain derivatives . By the end of this course, the student will have learnt the fundamental theorem of calculus , chain rule , derivatives of transcendental functions , integration , and applications of all these in the real world. This course is followed by Calculus Two.
Ximera course was initially released on Coursera in the Spring Semester of 2012–13 under the name Calculus One. [ 7 ] MOOCulus , an online platform that lets you practice Calculus was developed at the Ohio State University to provide students a place to practice Calculus problems. The platform, which was built using Ruby on Rails was built because Coursera didn't offer an engaging way to practice problems. [ 8 ] [ 9 ] The whole course, which consists of 200+ videos, was typeset as a textbook on April 10, 2014. The textbook, which is licensed under Creative Commons Attribution Non-commercial Share Alike License, incorporated some of its example and exercise problems from Elementary calculus: An approach using Infinitesimals . [ 10 ]
In 2023, Ximera received a $2.1 million grant from the Department of Education. Students are expected to save $4 million to $10 a year. [ 11 ] As part of the funding, accessibility (defined by Web Content Accessibility Guidelines 2.1AA) is a requirement, and they partnered with "Tailor Swift Bot" for that work. [ 12 ]
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https://en.wikipedia.org/wiki/Ximera
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XINU Is Not Unix ( XINU , a recursive acronym ), is an operating system for embedded systems , [ 2 ] originally developed by Douglas Comer for educational use at Purdue University in the 1980s. The name is both recursive, and is Unix spelled backwards. It has been ported to many hardware platforms, including the DEC PDP-11 and VAX systems, Motorola 68k ( Sun-2 and Sun-3 workstations , AT&T UNIX PC , MECB ), Intel x86 , PowerPC G3 , MIPS , ARM architecture and AVR (atmega328p/Arduino). Xinu was also used for some models of Lexmark printers. [ 2 ]
Despite its name suggesting some similarity to Unix, Xinu is a different type of operating system, written with no knowledge of the Unix source code, or compatibility goals. It uses different abstractions, and system calls , some with names matching those of Unix, but different semantics. [ 2 ]
Xinu first ran on the LSI-11 platform. A Motorola 68000 port was done by Derrick Burns in 1983. A VAX port was done in 1986 by Comer and Tom Stonecypher, an IBM PC compatible port in 1988 by Comer and Timothy Fossum, a second Motorola 68000 (Sun 3) port circa 1988 by Shawn Ostermann and Steve Chapin, a Macintosh platform port in 1989 by Comer and Steven Munson, an Intel 80486 version by John Lin in 1995, a SPARC port by Jim Griffioen, and a PowerPC port in 2005 and MIPS port of Embedded Xinu in 2006 by Dennis Brylow.
Dennis Brylow at Marquette University has ported Xinu to both the PowerPC and MIPSEL (little-endian MIPS) processor architectures . Porting Xinu to reduced instruction set computing (RISC) architectures greatly simplified its implementation, increasing its ability to be used as a tool for teaching and research.
MIPSEL was chosen as a target architecture due to the proliferation of the MIPSEL-based WRT54GL router and the cool incentive that motivates some students to become involved in projects. The first embedded Xinu systems laboratory based on the WRT54GL router was developed at Marquette University. In collaboration with the Marquette Xinu team, an embedded Xinu laboratory was formed at the University of Mississippi, laying the groundwork for further work on developing a Virtual Xinu Laboratory.
Embedded Xinu is a fresh reimplementation of the Xinu design, in ANSI C , on an embedded RISC architecture. The MIPS port of Embedded Xinu was developed from 2006 to 2010 at Marquette University, under the direction of Dr. Dennis Brylow. The Embedded Xinu operating system is copyright (c) 2007, 2008, 2009, and 2010 by Douglas Comer and Dennis Brylow.
The Xinu Laboratory in the University of Mississippi's Department of Computer and Information Science was formed during the summer of 2008 by Dr. Paul Ruth. Assisting him in the project were Jianshu Zhao and Patrick Hoover, who were both graduate students at the time. Also assisting him were Chelsea Norman and Kevin Kent, who were undergraduates at the time. The initial laboratory is based on the Marquette University Embedded Xinu Laboratory.
Located in the server room of Weir Hall on the campus of the University of Mississippi, is composed of a dozen modified Linksys WRT54GL wireless routers, a 32 port Digi Etherlite serial annex, a 24 port 10/100 Mbit/s Ethernet switch , a BayTech serial controlled power strip, and quite a few wires. The system is controlled by a standard PC running Debian Linux . The whole system cost less than $3000, not including the PC.
The WRT54G routers use the MIPSEL architecture and are used as backend devices on which the Xinu embedded operating system runs. The PC runs several daemons which enable and manage the users ability to access the backends.
The Nexos Project is a joint effort between Marquette University, the University at Buffalo , and the University of Mississippi to build curriculum materials and a supporting experimental laboratory for hands-on projects in computer systems courses. The approach focuses on low cost, flexible, commodity embedded hardware, freely available development and debugging tools, and a fresh implementation of a classic operating system, Embedded Xinu, that is ideal for student exploration. Virtual Xinu addresses two challenges that limit the effectiveness of Nexos. First, potential faculty adopters have clearly indicated that even with the current minimal monetary cost of installation, the hardware modifications, and time investment remain troublesome factors that scare off interested educators. Second, overcoming inherent complications that arise due to the shared subnet that allow student projects to interfere with each other in ways that are difficult to recreate, debug, and understand. Ultimately, porting the Xinu operating systems to QEMU virtual hardware, and developing the virtual networking platform have produced results showing success using Virtual Xinu in the classroom during one semester of the Operating Systems course at the University of Mississippi by Dr Ruth.
There are several versions of Xinu available for platforms such as an x86, ARM, AVR (Arduino) and MIPS. The last versions by Douglas Comer is for both the Intel x86 (Galileo) and ARM Cortex-A8 (BeagleBone Black). [ 3 ] The version for AVR requires 32 KB of flash, thus Arduino Uno boards and alike are supported. [ 4 ] The source code of these versions is freely available. [ 5 ]
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https://en.wikipedia.org/wiki/Xinu
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Xirang ( Chinese : 息壤 ; pinyin : xírǎng ) [ a ] was a magical soil in Chinese mythology with the ability to self-expand and grow continuously. Its properties made it particularly effective for use by Gun and Yu the Great in fighting the rising waters of the Great Flood . [ 1 ]
This Chinese word compounds xí 息 "breathe; cease; rest; grow; multiply" and rǎng 壤 "soil; earth". Noting similarities with earth-diver creation myths, Anne Birrell translates xirang as "self-renewing soil", and compares other translations of "breathing earth" ( Wolfram Eberhard ), "swelling mold" ( Derk Bodde ), "idle soil" (Roger Greatrex), and "living earth" or "breathing earth" (Rémi Mathieu). [ 2 ]
In some versions of the myths, Gun stole the xirang from the Shangdi , who sent the deity Zhu Rong to execute him in punishment, on Feather Mountain . [ 3 ] : 87 According to some accounts, Yu, on the other hand, went up to Heaven. After begging Shangdi, he received from him a gift of as much xirang as his magical black tortoise could carry on its back, thus allowing Yu to successfully block up the 233,559 springs , the sources of the flood waters. [ 3 ] : 87–88 In other versions of these myths, xirang was stolen or obtained from the Primordial Divinity , or Gun's executioner was other than Zhu Rong. [ 4 ]
A historical basis has been suggested for both the Great Flood [ 5 ] and for xirang. Sinologist David Hawkes proposes that the myths are a symbolic interpretation of a societal transition. In this case, Gun represents a society at an earlier technological stage, which engages in small-scale agriculture which involves raising areas of arable land sufficiently above the level of the marshes. The "magically-expanding" xirang soil may represent a type of friable raised garden, made up of soil, brushwood, and similar materials. Yu and his work in controlling the flood would symbolize a later type of society, which allowed a much larger scale approach to transforming wetlands to arable fields. [ 6 ]
A less mythical explanation could be sought in various forms of expansive clay . Generally impervious to water, clays are useful in creating the core of earthen dams . Expansive clays, in particular, slowly expand when wetted, thus matching the "swelling" translation. When dried, they take on a puffy "popcorn" look, which could be interpreted as "breathing" or airy. Such clays are abundant in the Shaanxi Province where many of these events are thought to have occurred. [ citation needed ]
The xirang mythology has interesting parallels to the mythologies of the indigenous peoples of the Americas , particularly the earth-diver creation myth . In the earth-diver myth, the primordial waters cover all, until after overcoming great perils, a certain creature is able to dive down into the waters and retrieve a small bit of magical soil. This xirang -like soil then magically expands into the land areas of today. [ 2 ]
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https://en.wikipedia.org/wiki/Xirang
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Xsupplicant is a supplicant that allows a workstation to authenticate with a RADIUS server using 802.1X and the Extensible Authentication Protocol (EAP). It can be used for computers with wired or wireless LAN connections to complete a strong authentication before joining the network and supports the dynamic assignment of WEP keys.
Xsupplicant up to version 1.2.8 was designed to run on Linux clients as a command line utility. Version 1.3.X and greater are designed to run on Windows XP and are currently being ported to Linux/BSD systems, and include a robust graphical user interface , and also includes network access control (NAC) functionality from Trusted Computing Group 's Trusted Network Connect NAC.
Xsupplicant was chosen by the OpenSea Alliance, dedicated to developing, promoting, and distributing an open source 802.1X supplicant. [ 1 ]
Xsupplicant supports the following EAP types:
Xsupplicant is primarily maintained by Chris Hessing.
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https://en.wikipedia.org/wiki/Xsupplicant
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Xtracycle is the name of a company [ 1 ] and the name commonly used for the variety of load-carrying bicycle that results from use of the company's products: the FreeRadical kit. [ 2 ] Web forums and blogs often use the shorthand Xtrabike , Xtra , or simply X to refer to either the FreeRadical extension or the entire extended bicycle.
The FreeRadical was conceived by Ross Evans at Stanford University and developed during his work in the mid-1990s managing a " Bikes Not Bombs " project in Nicaragua , where having a bicycle enhances a person's employment opportunities. In 1998 Evans and his friend Kipchoge Spencer created Xtracycle Inc to manufacture and market the invention, as well as a nonprofit organization, Worldbike, devoted to encourage a bicycle-centric lifestyle and culture. [ 3 ]
Despite the fact that the FreeRadical qualifies as an aftermarket bike accessory, its growing acceptance has sparked an Xtracycle aftermarket not formally connected with Xtracycle Inc: varieties of specialized kickstands , electric-assist motors , and even bike-mounted blenders have come to market, even though each requires the prior purchase of a FreeRadical or other Xtracycle-compatible frame to function properly.
Xtracycle Inc has worked with various bicycle manufacturers to build purpose-built extended bicycles compatible with their accessories. The first to actually produce and market an integrated Xtracycle frameset was Surly Bikes with the Big Dummy. [ 4 ] XInc continues to form similar covenants with manufacturers in all price ranges, with the goal of making the Xtracycle less of a niche product and more mainstream. XInc is also working on FreeRadical attachments sized for children's and youths' bicycles.
Other applications for the FreeRadical have included linking two Xtracycles to support a mobile stage for use in parades and street fairs, and a computerized chalkpowder-printer device mounted on an Xtracycle that leaves a dot-matrix trail of messages on the street. [ 5 ]
In 2008, Xtracycle put their longtail bike frame specifications online [ 6 ] as part of their project to open source their longtail frame design. They’ve created a Longtail Standard and logoing to allow vendors to design their products to fit in the Xtracycle FreeRadical ecosystem. The "Longtail Technology" logo can be used on bikes, accessories or packaging. The open sourcing of the patented technology was meant to stimulate the cargo bike movement, while developing a standard for "longtail" frames and accessories. Several frame and accessory makers have adopted the standards, while others have developed competing and incompatible long-frame cargo bike designs.
However, the documents are no longer freely available, and now require an agreement with Xtracycle first. [ 7 ]
The FreeRadical is an extender for a bicycle.
In 2009 the Radish was launched by Xtracycle. It is a production long-tail bicycle with a low-standover height frame and matching FreeRadical.
In 2013 the EdgeRunner is a second generation cargo bicycle with a 20-inch rear wheel. The EdgeRunner has been called the "Best longtail ever. No contest." [ 8 ]
In 2013 the CargoJoe is a folding cargo bike developed in a partnership between Xtracycle and Tern .
In 2011 Xtracycle created a sidecar for cargo transport that can carry up to 250 pounds. [ 9 ]
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https://en.wikipedia.org/wiki/Xtracycle
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Xuan tu or Hsuan thu ( simplified Chinese : 弦图 ; traditional Chinese : 絃圖 ; pinyin : xuántú ; Wade–Giles : hsüan 2 tʻu 2 ) is a diagram given in the ancient Chinese astronomical and mathematical text Zhoubi Suanjing indicating a proof of the Pythagorean theorem . [ 1 ] Zhoubi Suanjing is one of the oldest Chinese texts on mathematics. The exact date of composition of the book has not been determined. Some estimates of the date range as far back as 1100 BCE, while others estimate the date as late as 200 CE. However, from astronomical evidence available in the book it would appear that much of the material in the book is from the time of Confucius , that is, the 6th century BCE. [ 2 ] [ 3 ] Hsuan thu represents one of the earliest known proofs of the Pythagorean theorem and also one of the simplest. The text in Zhoubi Suanjing accompanying the diagram has been translated as follows: [ 3 ] [ 2 ]
The art of numbering proceeds from the circle and the square. The circle is derived from the square and the square from the rectangle (literally, the T-square or the carpenter's square). The rectangle originates from the fact that 9x9 = 81 (that is, the multiplication table or properties of numbers as such). Thus, let us cut a rectangle (diagonally) and make the width 3 (units) wide and the height 4 (units) long. The diagonal between the two corners will then be 5 (units) long. Now after drawing a square on the diagonal, circumscribe it by half-rectangles like that which has been left outside, so as to form a (square) plate. Thus the (four) outer half-rectangles of width 3, length 4 and diagonal 5, together make two rectangles (of area 24); then (when this is subtracted from the square plate of area 24) the remainder is of area 25. This (process) is called "piling up the rectangles" ( chi chu ).
The hsuan thu diagram makes use of the 3,4,5 right triangle to demonstrate the Pythagorean theorem. However the Chinese people seems to have generalized its conclusion to all right triangles. [ 1 ] [ 3 ]
The hsuan thu diagram, in its generalized form can be found in the writings of the Indian mathematician Bhaskara II (c. 1114–1185). The description of this diagram appears in verse 129 of Bijaganita of Bhaskara II. [ 4 ] There is a legend that Bhaskara's proof of the Pythagorean theorem consisted of only just one word, namely, "Behold!". However, using the notations of the diagram, the theorem follows from the following equation:
c 2 = ( a − b ) 2 + 4 ( 1 2 a b ) = a 2 + b 2 . {\displaystyle c^{2}=(a-b)^{2}+4({\tfrac {1}{2}}ab)=a^{2}+b^{2}.}
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https://en.wikipedia.org/wiki/Xuan_tu
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Xuntian ( Chinese : 巡天 ; pinyin : Xúntiān ; lit: Tour of Heaven), [ a ] also known as the Chinese Space Station Telescope [ 5 ] ( CSST ) ( Chinese : 巡天空间望远镜 ; pinyin : Xúntiān Kōngjiān Wàngyuǎnjìng ) is a planned Chinese space telescope currently under development. [ 6 ]
It will feature a 2-meter (6.6 foot) diameter primary mirror and is expected to have a field of view 300–350 times larger than the Hubble Space Telescope . [ 7 ] This will allow the telescope to image up to 40 percent of the sky using its 2.5 gigapixel camera.
As of 2024, Xuntian is scheduled for launch no earlier than late 2026 [ 2 ] [ 8 ] [ 9 ] on a Long March 5B rocket to co-orbit with the Tiangong space station in slightly different orbital phases , which will allow for periodic docking with the station. [ 10 ]
This state-of-the-art telescope, characterized by its off-axis design without any obstruction, sidesteps diffraction challenges associated with mirror support structures. As a result, its point spread function (PSF) remains unscathed, presenting a valuable asset for weak-lensing shear measurements .
The primary mission of the CSST revolves around high-resolution large-area multiband imaging and slitless spectroscopy surveys, spanning the wavelength range of 255–1,000 nm. Precise cosmology serves as the principal scientific driver behind this ambitious endeavor, with a focus on observing regions at median-to-high Galactic and ecliptic latitudes. Over a period of 10 years, the survey camera is slated to cover approximately 17,500 square degrees of the sky in various bands, reaching point-source 5σ limiting magnitudes of about 26 (AB mag) in g and r bands.
The CSST's spectral resolution (R=λ/Δλ) for the slitless spectrograph averages no less than 200, attaining wide-band-equivalent limiting magnitudes in GV (400–620 nm) and GI (620–1,000 nm) bands at about 23 mag. Beyond its wide-area survey, the CSST will target specific deep fields, aiming for observations that surpass the depth of the broader survey by at least one magnitude. The collective strengths of its angular resolution , depth, wavelength range, and capacity for both imaging and spectroscopy, coupled with extensive sky coverage, render the CSST survey highly competitive.
Notably, the CSST's observations are poised to complement and enhance other contemporaneous large-scale projects, including the Vera C. Rubin Observatory , the Euclid Space Telescope , and the Nancy Grace Roman Space Telescope .
Xuntian is equipped with five first-generation instruments, including a survey camera, a terahertz receiver, a multichannel imager, an integral field spectrograph , and a cool planet imaging coronagraph . [ 11 ]
The survey camera is also known as the multi-color photometry and slitless spectroscopy survey module. The module is located at the main focal plane and divided into the multi-color photometry submodule of 7 bands (NUV, u, g, r, i, z, y) and the slitless spectroscopy submodule of 3 bands (GU, GV, GI). The multi-color photometry submodule includes 18 filters, covering 60% of the area of this module. The slitless spectroscopy submodule includes 12 gratings, covering the other 40% of the area.
The terahertz receiver, also known as the high sensitivity terahertz detection module (HSTDM), enables terahertz (THz) astronomical observations from space. Conducting THz observations in space eliminates Earth's atmospheric absorption. HSTDM is a high-resolution spectrometer and the first space heterodyne receiver using niobium nitride (NbN) -based superconducting tunnel junction (Superconductor-Insulator-Superconductor (SIS)) mixer (the NbN SIS mixer). [ 12 ]
The Multichannel imager (MCI) has three channels covering the same wavelength range as the survey camera from the NUV to NIR bands, and these channels can work simultaneously. Three sets of filters, i.e., narrow-, medium-, and wide-band filters, will be installed on the MCI to perform extreme-deep field surveys with a field of view of 7.5′×7.5′. The magnitude limit can be stacked to a depth of 29–30 AB mag in three channels. It will study the formation and evolution of high- z galaxies, properties of dark matter and dark energy, and also can be used to calibrate the photo- z measurements with its nine medium-band filters for the main surveys. [ 13 ]
The CSST-IFS ( Integral Field Spectrograph ) is one of the 5 instruments onboard the CSST. The key advantages of the CSST-IFS are the high spatial resolution of 0.2" and the full range optical wavelength coverage (0.35-1.0 μm ). Considering the limitation of the 2-meter aperture of the CSST, the CSST-IFS is optimal for targeting compact and bright sources, which therefore will be irreplaceable for studying galactic central regions ( AGN feedback) and star-forming regions . [ 14 ]
The cool planet imaging coronagraph (CPI-C) aims to realize high-contrast (< 10 −8 ) direct imaging of exoplanets with an inner working angle (IWA) of 0.35′′ in the visible (0.6328 μm ). It plans to follow up exoplanets discovered by radial velocity observations, study planet formation and evolution, and probe protoplanetary disks . [ 15 ] CPI-C works at 0.53-1.6 μm and is equipped with 7 broad passbands.
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https://en.wikipedia.org/wiki/Xuntian
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ΧyMTeΧ is a macro package for TeX which renders high-quality chemical structure diagrams. Using the typesetting system, the name is styled as X ϒ M T e X . It was originally written by Shinsaku Fujita ( 藤田眞作 , Fujita Shinsaku ) . Molecules are defined by TeX markup .
The following code produces the image for corticosterone below.
This article about chemistry software is a stub . You can help Wikipedia by expanding it .
This digital typography article is a stub . You can help Wikipedia by expanding it .
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https://en.wikipedia.org/wiki/XyMTeX
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Xylan ( / ˈ z aɪ l æ n / ; [ 3 ] / ˈ z aɪ l ən / [ 4 ] ) ( CAS number : 9014-63-5) is a type of hemicellulose , a polysaccharide consisting mainly of xylose residues. It is found in plants , in the secondary cell walls of dicots and all cell walls of grasses . [ 5 ] Xylan is the third most abundant polysaccharide on Earth, after cellulose and chitin . [ citation needed ]
Xylans are polysaccharides made up of β-1,4-linked xylose (a pentose sugar ) residues with side branches of α-arabinofuranose and/or α-glucuronic acids. On the basis of substituted groups xylan can be categorized into three classes i) glucuronoxylan (GX) ii) neutral arabinoxylan (AX) and iii) glucuronoarabinoxylan (GAX). [ 6 ] In some cases contribute to cross-linking of cellulose microfibrils and lignin through ferulic acid residues. [ 7 ]
Xylans play an important role in the integrity of the plant cell wall and increase cell wall recalcitrance to enzymatic digestion ; [ 8 ] [ 9 ] thus, they help plants to defend against herbivores and pathogens (biotic stress). Xylan also plays a significant role in plant growth and development. Typically, xylans content in hardwoods is 10-35%, whereas they are 10-15% in softwoods . The main xylan component in hardwoods is O-acetyl-4-O-methylglucuronoxylan, whereas arabino-4-O-methylglucuronoxylans are a major component in softwoods. In general, softwood xylans differ from hardwood xylans by the lack of acetyl groups and the presence of arabinose units linked by α-(1,3)-glycosidic bonds to the xylan backbone. [ 10 ]
Some macrophytic green algae contain xylan (specifically homoxylan [ 11 ] ) especially those within the Codium and Bryopsis genera [ 12 ] where it replaces cellulose in the cell wall matrix. Similarly, it replaces the inner fibrillar cell-wall layer of cellulose in some red algae .
The quality of cereal flours and the hardness of dough are affected by their xylan content, [ 6 ] thus, playing a significant role in bread industry. The main constituent of xylan can be converted into xylitol (a xylose derivative), which is used as a natural food sweetener, which helps to reduce dental cavities and acts as a sugar substitute for diabetic patients. Poultry feed has a high percentage of xylan. [ 6 ]
Xylan is one of the foremost anti-nutritional factors in common use feedstuff raw materials. Xylooligosaccharides produced from xylan are considered as "functional food" or dietary fibers [ 13 ] due their potential prebiotic properties. [ 14 ]
The regular branching patterns of xylans may facilitate their co-crystallization with cellulose in the plant cell wall. [ 15 ] Xylan also tends to crystallize from aqueous solution. [ 16 ] Additional polymorphs of (1→4)-β-D-xylan have been obtained by crystallization from non-aqueous environments. [ 17 ]
Several glycosyltransferases are involved in the biosynthesis of xylans. [ 18 ] [ 19 ]
In eukaryotes, GTs represent about 1% to 2% of gene products. [ 20 ] GTs are assembled into complexes existing in the Golgi apparatus. However, no xylan synthase complexes have been isolated from Arabidopsis tissues (dicot). The first gene involved in the biosynthesis of xylan was revealed on xylem mutants (irx) in Arabidopsis thaliana because of some mutation affecting xylan biosynthesis genes. As a result, abnormal plant growth due to thinning and weakening of secondary xylem cell walls was seen. [ 21 ] Arabidopsis mutant irx9 (At2g37090), irx14 (At4g36890), irx10/gut2 (At1g27440), irx10-L/gut1 (At5g61840) showed defect in xylan backbone biosynthesis. [ 21 ] Arabidopsis mutants irx7 , irx8 , and parvus are thought to be related to the reducing end oligosaccharide biosynthesis. [ 22 ] Thus, many genes have been associated with xylan biosynthesis but their biochemical mechanism is still unknown. Zeng et al . (2010) immuno-purified xylan synthase activity from etiolated wheat ( Triticum aestivum ) microsomes. [ 23 ] Jiang et al . (2016) reported a xylan synthase complex (XSC) from wheat that has a central core formed of two members of the GT43 and GT47 families (CAZy database). They purified xylan synthase activity from wheat seedlings through proteomics analysis and showed that two members of TaGT43 and TaGT47 are sufficient for the synthesis of a xylan-like polymer in vitro. [ 24 ]
Xylanase converts xylan into xylose . Given that plants contain up to 30% xylan, xylanase is important to the nutrient cycle . [ 25 ] The degradation of xylan and other hemicelluloses is relevant to the production of biofuels . Being less crystalline and more highly branched, these hemicelluloses are particularly susceptible to hydrolysis . [ 26 ] [ 27 ]
As a major component of plants, xylan is potentially a significant source of renewable energy especially for second generation biofuels. [ 28 ] However, xylose (backbone of xylan) is a pentose sugar that is hard to ferment during biofuel conversion because microorganisms like yeast cannot ferment pentose naturally. [ 29 ]
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https://en.wikipedia.org/wiki/Xylan
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Xylan is a fluoropolymer -based industrial coating, most commonly used in non-stick cookware. Generally, it is applied in a thin film to the target material to improve its durability and non-stick properties.
Consumer demand for non-stick coatings with improved durability drove research in the 1960s. DuPont , at that time focused on improving housewares, developed important properties of nonstick coatings through research. DuPont scientists found that adding Fluorinated ethylene propylene to the hard, adhesion-promoting binder resins produced one-coat products that were more durable than earlier Teflon formulations. In 1969, when Xylan was under developed, Tefal and Teflon accounted for nearly all of the fluoropolymer coatings industry. [ 1 ]
Xylan is generally used to reduce friction , improve wear resistance , and for non-stick applications. Additionally, it can be used to protect a metal from corrosion . The most commonly known application is in non-stick cookware but Xylan coatings have also been used extensively in the automotive industry and for corrosion protection in the oil and gas industry. [ 2 ]
Xylan is the umbrella trademark for most of the Whitford Corporation fluoropolymer coatings line. Xylan is made of low friction, wear resistant composites of fluoropolymers and reinforcing binder resins . Xylan coatings can be one-, two- (primer and top-coat), and three- (primer, mid-coat, top-coat) coat conventional and reinforced (filled) coating systems.
The fluoropolymers utilized in Xylan coatings consist of PTFE , PFA , and FEP . The properties listed below may not apply to all Xylan coatings as the fluoropolymer and resin content and type can have significant effects on each property. [ 3 ]
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https://en.wikipedia.org/wiki/Xylan_(coating)
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In organic chemistry , xylene or xylol (from Greek ξύλον (xylon) ' wood ' ; [ 1 ] [ 2 ] IUPAC name : dimethylbenzene ) are any of three organic compounds with the formula (CH 3 ) 2 C 6 H 4 . They are derived from the substitution of two hydrogen atoms with methyl groups in a benzene ring; which hydrogens are substituted determines which of three structural isomers results. It is a colorless, flammable, slightly greasy liquid of great industrial value. [ 3 ]
The mixture is referred to as both xylene and, more precisely, xylenes. Mixed xylenes refers to a mixture of the xylenes plus ethylbenzene . The four compounds have identical molecular formulas C 8 H 10 . Typically the four compounds are produced together by various catalytic reforming and pyrolysis methods. [ 4 ]
Xylenes are an important petrochemical produced by catalytic reforming and also by coal carbonisation in the manufacture of coke fuel . They also occur in crude oil in concentrations of about 0.5–1%, depending on the source. Small quantities occur in gasoline and aircraft fuels .
Xylenes are produced mainly as part of the BTX aromatics ( benzene , toluene , and xylenes) extracted from the product of catalytic reforming known as reformate .
Several million tons are produced annually. [ 3 ] In 2011, a global consortium began construction of one of the world's largest xylene plants in Singapore . [ 5 ]
Xylene was first isolated and named in 1850 by the French chemist Auguste Cahours (1813–1891), having been discovered as a constituent of wood tar . [ 6 ]
Xylenes are produced by the methylation of toluene and benzene . [ 3 ] [ 7 ] Commercial or laboratory-grade xylene produced usually contains about 40–65% of m -xylene and up to 20% each of o -xylene , p -xylene and ethylbenzene . [ 8 ] [ 9 ] [ 10 ] The ratio of isomers can be shifted to favor the highly valued p -xylene via the patented UOP- Isomar process [ 11 ] or by transalkylation of xylene with itself or trimethylbenzene. These conversions are catalyzed by zeolites . [ 3 ]
ZSM-5 is used to facilitate some isomerization reactions leading to mass production of modern plastics.
The physical properties of the isomers of xylene differ slightly. The melting point ranges from −47.87 °C (−54.17 °F) ( m -xylene) to 13.26 °C (55.87 °F) ( p -xylene)—as usual, the para isomer's melting point is much higher because it packs more readily in the crystal structure. The boiling point for each isomer is around 140 °C (284 °F). The density of each isomer is around 0.87 g/mL (7.3 lb/US gal; 8.7 lb/imp gal) and thus is less dense than water . The odor of xylene is detectable at concentrations as low as 0.08 to 3.7 ppm (parts of xylene per million parts of air) and can be tasted in water at 0.53 to 1.8 ppm. [ 9 ]
P210 , P233 , P240 , P241 , P242 , P243 , P261 , P264 , P271 , P273 , P280 , P301+P310 , P302+P352 , P303+P361+P353 , P304+P312 , P304+P340 , P305+P351+P338 , P312 , P321 , P322 , P331 , P332+P313 , P337+P313 , P362 , P363 , P370+P378 , P403+P233 , P403+P235 , P405 , P501
Xylenes form azeotropes with water and a variety of alcohols. The azeotrope with water consists of 60% xylenes and boils at 94.5 °C. [ 3 ] As with many alkylbenzene compounds, xylenes form complexes with various halocarbons . [ 12 ] The complexes of different isomers often have dramatically different properties from each other. [ 13 ]
p -Xylene is the principal precursor to terephthalic acid and dimethyl terephthalate , both monomers used in the production of polyethylene terephthalate (PET) plastic bottles and polyester clothing. 98% of p -xylene production, and half of all xylenes produced is consumed in this manner. [ 10 ] [ 14 ] o -Xylene is an important precursor to phthalic anhydride . The demand for isophthalic acid is relatively modest, so m -xylene is rarely sought (and hence the utility of its conversion to the o - and p -isomers).
Xylenes are used as a solvent in printing , rubber , and leather industries. It is a common component of ink, rubber , and adhesives . [ 15 ] In thinning paints and varnishes , it can be substituted for toluene where slower drying is desired, and thus is used by conservators of art objects in solubility testing. [ 16 ] Similarly it is a cleaning agent , e.g., for steel , silicon wafers , and integrated circuits . In dentistry, xylene can be used to dissolve gutta percha , a material used for endodontics (root-canal treatments). In the petroleum industry, xylene is also a frequent component of paraffin solvents, used when the tubing becomes clogged with paraffin wax.
Xylene is used in the laboratory to make baths with dry ice to cool reaction vessels, [ 17 ] and as a solvent to remove synthetic immersion oil from the microscope objective in light microscopy . [ 18 ] In histology , xylene is the most widely used clearing agent. [ 19 ] Xylene is used to remove paraffin from dried microscope slides prior to staining. After staining, microscope slides are put in xylene prior to mounting with a coverslip.
In one large-scale application, para-xylene is converted to terephthalic acid . The major application of ortho-xylene is as a precursor to phthalate esters , used as plasticizer . Meta-xylene is converted to isophthalic acid derivatives, which are components of alkyd resins . [ 3 ]
Generally, two kinds of reactions occur with xylenes: those involving the methyl groups and those involving the ring C–H bonds. Being benzylic and hence weakened, the C–H bonds of the methyl groups are susceptible to free-radical reactions, including halogenation to the corresponding xylene dichlorides (bis(chloromethyl)benzenes), while mono-bromination yields xylyl bromide , a tear gas agent. Oxidation and ammoxidation also target the methyl groups, affording dicarboxylic acids and the dinitriles. Electrophiles attack the aromatic ring, leading to chloro- and nitroxylenes. [ 3 ]
Xylene is flammable but of modest acute toxicity, with LD 50 ranges from 200 to 5000 mg/kg for animals. Oral LD 50 for rats is 4300 mg/kg. The principal mechanism of detoxification is oxidation to methylbenzoic acid and hydroxylation to hydroxylene. [ 3 ]
The main effect of inhaling xylene vapor is depression of the central nervous system (CNS), with symptoms such as headache, dizziness, nausea and vomiting. At an exposure of 100 ppm, one may experience nausea or a headache. At an exposure between 200 and 500 ppm, symptoms can include feeling "high", dizziness, weakness, irritability, vomiting, and slowed reaction time. [ 20 ] [ 21 ]
The side effects of exposure to low concentrations of xylene ( < 200 ppm ) are reversible and do not cause permanent damage. Long-term exposure may lead to headaches, irritability, depression, insomnia, agitation, extreme tiredness, tremors, hearing loss, impaired concentration and short-term memory loss. [ 22 ] [ clarification needed ] A condition called chronic solvent-induced encephalopathy , commonly known as "organic-solvent syndrome" has been associated with xylene exposure. There is very little information available that isolates xylene from other solvent exposures in the examination of these effects. [ 20 ]
Hearing disorders have been also linked to xylene exposure, both from studies with experimental animals, [ 23 ] [ 24 ] as well as clinical studies. [ 25 ] [ 26 ] [ 27 ]
Xylene is also a skin irritant and strips the skin of its oils, making it more permeable to other chemicals. The use of impervious gloves and masks, along with respirators where appropriate, is recommended to avoid occupational health issues from xylene exposure. [ 20 ]
Xylenes are metabolized to methylhippuric acids . [ 28 ] [ 29 ] The presence of methylhippuric acid can be used as a biomarker to determine exposure to xylene. [ 29 ] [ 30 ]
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https://en.wikipedia.org/wiki/Xylene
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Xylenol orange is an organic reagent , most commonly used as a tetrasodium salt as an indicator for metal titrations . When used for metal titrations , it will appear red in the titrand and become yellow once it reaches its endpoint. Historically, commercial preparations of it have been notoriously impure, [ 2 ] sometimes consisting of as little as 20% xylenol orange, and containing large amounts of semi-xylenol orange and iminodiacetic acid . Purities as high as 90% are now available.
It is fluorescent, and has excitation maximums of 440 & 570 nm and an emission maximum of 610 nm. [ 3 ]
This article about an organic compound is a stub . You can help Wikipedia by expanding it .
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https://en.wikipedia.org/wiki/Xylenol_orange
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Xylit (from xylon , "silk") is a waste product generated by the mining of lignite . As in peat , embedded iron structures do not become completely sedimented . Its density is around 250 kg/m 3 . [ 1 ]
Its very low heat content , even in a dried state, makes it a good fuel for heat generation. It has been used in France as compost , and is sometimes used in potting soil and as a substrate for horticulture . Because it is more elastic and robust than wood, [ 2 ] it can be used as a good substitute for bricks.
Its unique structure that traps nutrients and pollutants, as well as its high specific surface area (encouraging trickling filter development) and its exceptional longevity (30 years), allow to be used as filter media in some decentralized wastewater systems .
This article is partially translated from German and French Wikipedia articles.
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https://en.wikipedia.org/wiki/Xylit
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Xylitol pentanitrate ( XPN ) is a nitrated ester primary explosive [ 3 ] [ 4 ] first synthesized in 1891 by Gabriel Bertrand . [ 5 ] [ 6 ] Law enforcement has taken an interest in XPN along with erythritol tetranitrate (ETN) and pentaerythritol tetranitrate (PETN) due to their ease of synthesis, which makes them accessible to amateur chemists and terrorists . [ 7 ] [ 8 ]
At room temperature XPN exists as a white crystalline solid. When heated to 163 °C, liquid xylitol pentanitrate begins to crackle and produce a dark vapour. When decomposed, a gram of XPN produces 200 mL of gas, which makes it a high performance explosive. [ 3 ]
Rotter impact analysis of XPN found a figure of insensitiveness of 25 ( RDX = 80). XPN displayed a similar sensitivity to static discharge to ETN and PETN. [ 3 ]
Xylitol pentanitrate is formed by nitration of xylitol pentaacetate . Nowadays, fuming nitric acid and glacial acetic acid is often used, [ 9 ] but Bertrand originally employed a cheaper nitrating agent, the mixture of nitric and sulfuric acids (he called it mélange nitrosulfurique , the common English name is "mixed acid"). [ 6 ]
Much like ETN, XPN has a positive oxygen balance , which means the carbon and hydrogen in the molecule can be fully oxidized without another oxidizing agent being added:
The decomposition of four molecules of XPN releases three O 2 . The free oxygen molecules can be used to oxidize an added metal dust or negative oxygen balanced explosive such as TNT .
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https://en.wikipedia.org/wiki/Xylitol_pentanitrate
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Xylomannan is an antifreeze molecule, found in the freeze-tolerant Alaskan beetle Upis ceramboides . [ 1 ] Unlike antifreeze proteins , xylomannan is not a protein. Instead, it is a combination of a sugar ( saccharide ) and a fatty acid that is found in cell membranes . [ 2 ] As such is expected to work in a different manner than AFPs. It is believed to work by incorporating itself directly into the cell membrane and preventing the freezing of water molecules within the cell. [ 3 ]
Xylomannan is also found in the red seaweed Nothogenia fastigiata ( Scinaiaceae family). Fraction F6 of a sulphated xylomannan from Nothogenia fastigiata was found to inhibit replication of a variety of viruses, including Herpes simplex virus types 1 and 2 (HSV-1, HSV-2), Human cytomegalovirus (HCMV, HHV-5), Respiratory syncytial virus (RSV), Influenzavirus A , Influenzavirus B , Junin and Tacaribe virus, Simian immunodeficiency virus , and (weakly) Human immunodeficiency virus types 1 and 2. [ 4 ]
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https://en.wikipedia.org/wiki/Xylomannan
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A xylopodium is a underground storage growth which is multibranched and may cover a circle thirty feet (nine meters) in diameter. [ 1 ] They differ from lignotubers which are more compact in form, like a tuber . They are most common in the cerrados of Brazil even including a monocot ( Smilax goyazana ), [ 1 ] but the mallee roots of Australia are more like xylopodia than lignotubers sensu stricto .
This plant article is a stub . You can help Wikipedia by expanding it .
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https://en.wikipedia.org/wiki/Xylopodium
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D- Xylose is a five-carbon aldose ( pentose , monosaccharide ) that can be catabolized or metabolized into useful products by a variety of organisms.
There are at least four different pathways for the catabolism of D-xylose: An oxido-reductase pathway is present in eukaryotic microorganisms. Prokaryotes typically use an isomerase pathway, and two oxidative pathways, called Weimberg and Dahms pathways respectively, are also present in prokaryotic microorganisms.
This pathway is also called the “Xylose Reductase-Xylitol Dehydrogenase” or XR-XDH pathway. Xylose reductase (XR) and xylitol dehydrogenase (XDH) are the first two enzymes in this pathway. XR is reducing D-xylose to xylitol using NADH or NADPH . Xylitol is then oxidized to D-xylulose by XDH, using the cofactor NAD . In the last step D-xylulose is phosphorylated by an ATP utilising kinase, XK, to result in D-xylulose-5-phosphate which is an intermediate of the pentose phosphate pathway .
In this pathway the enzyme xylose isomerase converts D-xylose directly into D-xylulose. D-xylulose is then phosphorylated to D-xylulose-5-phosphate as in the oxido-reductase pathway. At equilibrium, the isomerase reaction results in a mixture of 83% D-xylose and 17% D-xylulose because the conversion of xylose to xylulose is energetically unfavorable. [ 1 ]
The Weimberg pathway [ 2 ] is an oxidative pathway where the D-xylose is oxidized to D-xylono-lactone by a D-xylose dehydrogenase followed by a lactonase to hydrolyze the lactone to D-xylonic acid. A xylonate dehydratase is splitting off a water molecule resulting in 2-keto 3-deoxy-xylonate . 2-keto-3-deox-D-xylonate dehydratase forms the α-ketoglutarate semialdehyde. This is subsequently oxidised via α-ketoglutarate semialdehyde dehydrogenase to yield 2-ketoglutarate which serves as a key intermediate in the citric acid cycle. [ 3 ]
The Dahms pathway [ 4 ] starts as the Weimberg pathway but the 2-keto-3 deoxy-xylonate is split by an aldolase to pyruvate and glycolaldehyde .
It is desirable to ferment D-xylose to ethanol. This can be accomplished either by native xylose fermenting yeasts such as Scheffersomyces Pichia stipitis or by metabolically engineered strains of Saccharomyces cerevisiae . Pichia stipitis is not as ethanol tolerant as the traditional ethanol producing yeast Saccharomyces cerevisiae . S. cerevisiae on the other hand can not ferment D-xylose to ethanol. In attempts to generate S. cerevisiae strains that are able to ferment D-xylose the XYL1 and XYL2 genes of P. stipitis coding for the D-xylose reductase (XR) and xylitol dehydrogenase (XDH), respectively were introduced in S. cerevisiae by means of genetic engineering. [ 5 ] XR catalyze the formation of xylitol from D-xylose and XDH the formation of D-xylulose from xylitol. Saccharomyces cerevisiae can naturally ferment D-xylulose through the pentose phosphate pathway .
In another approach, bacterial xylose isomerases have been introduced into S. cerevisiae . This enzyme catalyze the direct formation of D-xylulose from D-xylose. Many attempts at expressing bacterial isomerases were not successful due to misfolding or other problems, but a xylose isomerase from the anaerobic fungus Piromyces Sp. has proven effective. [ 6 ] One advantage claimed for S. cerevisiae engineered with the xylose isomerase is that the resulting cells can grow anaerobically on xylose after evolutionary adaptation.
Studies on flux through the oxidative pentose phosphate pathway during D-xylose metabolism have revealed that limiting the rate of this step may be beneficial to the efficiency of fermentation to ethanol. Modifications to this flux that may improve ethanol production include deleting the GND1 gene, or the ZWF1 gene. [ 7 ] Since the pentose phosphate pathway produces additional NADPH during metabolism, limiting this step will help to correct the already evident imbalance between NAD(P)H and NAD+ cofactors and reduce xylitol byproduct formation.
Another experiment comparing the two D-xylose metabolizing pathways revealed that the XI pathway was best able to metabolize D-xylose to produce the greatest ethanol yield, while the XR-XDH pathway reached a much faster rate of ethanol production. [ 8 ]
Overexpression of the four genes encoding non-oxidative pentose phosphate pathway enzymes Transaldolase , Transketolase , Ribulose-5-phosphate epimerase and Ribose-5-phosphate ketol-isomerase [ 9 ] led to both higher D-xylulose [ 10 ] and D-xylose [ 11 ] fermentation rate.
The aim of this genetic recombination in the laboratory is to develop a yeast strain that efficiently produces ethanol. However, the effectiveness of D-xylose metabolizing laboratory strains do not always reflect their metabolism abilities on raw xylose products in nature. Since D-xylose is mostly isolated from agricultural residues such as wood stocks then the native or genetically altered yeasts will need to be effective at metabolizing these less pure natural sources.
Varying expression of the XR and XDH enzyme levels have been tested in the laboratory in the attempt to optimize the efficiency of the D-xylose metabolism pathway. [ 12 ]
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https://en.wikipedia.org/wiki/Xylose_metabolism
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Yttrium(III) nitrate is an inorganic compound , a salt with the formula Y(NO 3 ) 3 . The hexahydrate is the most common form commercially available.
Yttrium(III) nitrate can be prepared by dissolving corresponding metal oxide in 6 mol/L nitric acid : [ 1 ]
Yttrium(III) nitrate hexahydrate loses crystallized water at relatively low temperature. Upon further heating, basic salt YONO 3 is formed. [ 2 ] At 600 C, the thermal decomposition is complete. Y 2 O 3 is the final product. [ 3 ]
Y(NO 3 ) 3 ·3TBP is formed when tributyl phosphate is used as the extracting solvent. [ 4 ]
Yttrium(III) nitrate is mainly used as a source of Y 3+ cations. It is a precursor of some yttrium-containing materials, such as Y 4 Al 2 O 9 , [ 3 ] YBa 2 Cu 3 O 6.5+x [ 2 ] and yttrium-based metal-organic frameworks . [ 5 ] It can also be used as a catalyst in organic synthesis. [ 6 ]
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https://en.wikipedia.org/wiki/Y(NO3)3
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A Y-SNP is a single-nucleotide polymorphism on the Y chromosome . Y-SNPs are often used in paternal genealogical DNA testing . [ 1 ]
A single nucleotide polymorphism (SNP) is a change to a single nucleotide in a DNA sequence . [ 2 ] The relative mutation rate for an SNP is extremely low. [ 3 ] This makes them ideal for marking the history of the human genetic tree. SNPs are named with a letter code and a number. The letter indicates the lab or research team that discovered the SNP. The number indicates the order in which it was discovered. For example, M173 is the 173rd SNP documented by the Human Population Genetics Laboratory at Stanford University , which uses the letter M.
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https://en.wikipedia.org/wiki/Y-SNP
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In organic chemistry , aromaticity is a chemical property describing the way in which a conjugated ring of unsaturated bonds , lone pairs , or empty orbitals exhibits a stabilization stronger than would be expected from conjugation alone. The earliest use of the term was in an article by August Wilhelm Hofmann in 1855. [ 1 ] There is no general relationship between aromaticity as a chemical property and the olfactory properties of such compounds.
Aromaticity can also be considered a manifestation of cyclic delocalization and of resonance . [ 2 ] [ 3 ] [ 4 ] This is usually considered to be because electrons are free to cycle around circular arrangements of atoms that are alternately single- and double- bonded to one another. This commonly seen model of aromatic rings, namely the idea that benzene was formed from a six-membered carbon ring with alternating single and double bonds (cyclohexatriene), was developed by Kekulé (see History section below). Each bond may be seen as a hybrid of a single bond and a double bond, every bond in the ring identical to every other. The model for benzene consists of two resonance forms, which corresponds to the double and single bonds superimposing to give rise to six one-and-a-half bonds. Benzene is a more stable molecule than would be expected without accounting for charge delocalization.
As is standard for resonance diagrams , a double-headed arrow is used to indicate that the two structures are not distinct entities, but merely hypothetical possibilities. Neither is an accurate representation of the actual compound, which is best represented by a hybrid (average) of these structures, which can be seen at right. A C=C bond is shorter than a C−C bond, but benzene is perfectly hexagonal—all six carbon-carbon bonds have the same length , intermediate between that of a single and that of a double bond .
A better representation is that of the circular π bond (Armstrong's inner cycle ), in which the electron density is evenly distributed through a π-bond above and below the ring. This model more correctly represents the location of electron density within the aromatic ring.
The single bonds are formed with electrons in line between the carbon nuclei — these are called σ-bonds . Double bonds consist of a σ-bond and a π-bond. The π-bonds are formed from overlap of atomic p-orbitals above and below the plane of the ring. The following diagram shows the positions of these p-orbitals:
Since they are out of the plane of the atoms, these orbitals can interact with each other freely, and become delocalized. This means that, instead of being tied to one atom of carbon, each electron is shared by all six in the ring. Thus, there are not enough electrons to form double bonds on all the carbon atoms, but the "extra" electrons strengthen all of the bonds on the ring equally. The resulting molecular orbital has π symmetry.
The first known use of the word "aromatic" as a chemical term — namely, to apply to compounds that contain the phenyl radical — occurs in an article by August Wilhelm Hofmann in 1855. [ 1 ] If this is indeed the earliest introduction of the term, it is curious that Hofmann says nothing about why he introduced an adjective indicating olfactory character to apply to a group of chemical substances only some of which have notable aromas . Also, many of the most odoriferous organic substances known are terpenes , which are not aromatic in the chemical sense. But terpenes and benzenoid substances do have a chemical characteristic in common, namely higher unsaturation indices than many aliphatic compounds , and Hofmann may not have been making a distinction between the two categories.
In the 19th century, chemists found it puzzling that benzene could be so unreactive toward addition reactions, given its presumed high degree of unsaturation. The cyclohexatriene structure for benzene was first proposed by August Kekulé in 1865. Over the next few decades, most chemists readily accepted this structure, since it accounted for most of the known isomeric relationships of aromatic chemistry.
Between 1897 and 1906, J. J. Thomson , the discoverer of the electron, proposed three equivalent electrons between each carbon atom in benzene.
An explanation for the exceptional stability of benzene is conventionally attributed to Sir Robert Robinson , who was apparently the first (in 1925) [ 6 ] to coin the term aromatic sextet as a group of six electrons that resists disruption.
In fact, this concept can be traced further back, via Ernest Crocker in 1922, [ 7 ] to Henry Edward Armstrong , who in 1890 wrote "the (six) centric affinities act within a cycle ... benzene may be represented by a double ring ( sic ) ... and when an additive compound is formed, the inner cycle of affinity suffers disruption, the contiguous carbon-atoms to which nothing has been attached of necessity acquire the ethylenic condition". [ 8 ] [ verification needed ]
Here, Armstrong is describing at least four modern concepts. First, his "affinity" is better known nowadays as the electron , which was to be discovered only seven years later by J. J. Thomson. Second, he is describing electrophilic aromatic substitution , proceeding (third) through a Wheland intermediate , in which (fourth) the conjugation of the ring is broken. He introduced the symbol C centered on the ring as a shorthand for the inner cycle , thus anticipating Erich Clar 's notation. It is argued that he also anticipated the nature of wave mechanics , since he recognized that his affinities had direction, not merely being point particles, and collectively having a distribution that could be altered by introducing substituents onto the benzene ring ( much as the distribution of the electric charge in a body is altered by bringing it near to another body ).
The quantum mechanical origins of this stability, or aromaticity, were first modelled by Hückel in 1931. He was the first to separate the bonding electrons into sigma and pi electrons.
An aromatic (or aryl ) compound contains a set of covalently bound atoms with specific characteristics:
Whereas benzene is aromatic (6 electrons, from 3 double bonds), cyclobutadiene is not, since the number of π delocalized electrons is 4, which of course is a multiple of 4. The cyclobutadienide (2−) ion, however, is aromatic (6 electrons). An atom in an aromatic system can have other electrons that are not part of the system, and are therefore ignored for the 4n + 2 rule. In furan , the oxygen atom is sp² hybridized. One lone pair is in the π system and the other in the plane of the ring (analogous to C-H bond on the other positions). There are 6 π electrons, so furan is aromatic.
Aromatic molecules typically display enhanced chemical stability, compared to similar non-aromatic molecules. A molecule that can be aromatic will tend to alter its electronic or conformational structure to be in this situation. This extra stability changes the chemistry of the molecule. Aromatic compounds undergo electrophilic aromatic substitution and nucleophilic aromatic substitution reactions, but not electrophilic addition reactions as happens with carbon-carbon double bonds.
Many of the earliest-known examples of aromatic compounds, such as benzene and toluene, have distinctive pleasant smells. This property led to the term "aromatic" for this class of compounds, and hence the term "aromaticity" for the eventually discovered electronic property.
The circulating π electrons in an aromatic molecule produce ring currents that oppose the applied magnetic field in NMR . [ 9 ] The NMR signal of protons in the plane of an aromatic ring are shifted substantially further down-field than those on non-aromatic sp² carbons. This is an important way of detecting aromaticity. By the same mechanism, the signals of protons located near the ring axis are shifted up-field.
Aromatic molecules are able to interact with each other in so-called π-π stacking : The π systems form two parallel rings overlap in a "face-to-face" orientation. Aromatic molecules are also able to interact with each other in an "edge-to-face" orientation: The slight positive charge of the substituents on the ring atoms of one molecule are attracted to the slight negative charge of the aromatic system on another molecule.
Planar monocyclic molecules containing 4n π electrons are called antiaromatic and are, in general, destabilized. Molecules that could be antiaromatic will tend to alter their electronic or conformational structure to avoid this situation, thereby becoming non-aromatic. For example, cyclooctatetraene (COT) distorts itself out of planarity, breaking π overlap between adjacent double bonds. Relatively recently, cyclobutadiene was discovered to adopt an asymmetric, rectangular configuration in which single and double bonds indeed alternate; there is no resonance and the single bonds are markedly longer than the double bonds, reducing unfavorable p-orbital overlap. This reduction of symmetry lifts the degeneracy of the two formerly non-bonding molecular orbitals, which by Hund's rule forces the two unpaired electrons into a new, weakly bonding orbital (and also creates a weakly antibonding orbital). Hence, cyclobutadiene is non-aromatic; the strain of the asymmetric configuration outweighs the anti-aromatic destabilization that would afflict the symmetric, square configuration.
Aromatic compounds play key roles in the biochemistry of all living things. The four aromatic amino acids histidine , phenylalanine , tryptophan , and tyrosine each serve as one of the 20 basic building-blocks of proteins. Further, all 5 nucleotides ( adenine , thymine , cytosine , guanine , and uracil ) that make up the sequence of the genetic code in DNA and RNA are aromatic purines or pyrimidines . The molecule heme contains an aromatic system with 22 π electrons. Chlorophyll also has a similar aromatic system.
Aromatic compounds are important in industry. Key aromatic hydrocarbons of commercial interest are benzene , toluene , ortho -xylene and para -xylene . About 35 million tonnes are produced worldwide every year. They are extracted from complex mixtures obtained by the refining of oil or by distillation of coal tar, and are used to produce a range of important chemicals and polymers, including styrene , phenol , aniline , polyester and nylon .
The overwhelming majority of aromatic compounds are compounds of carbon, but they need not be hydrocarbons.
Benzene , as well as most other annulenes ( cyclodecapentaene excepted) with the formula C n H n where n ≥ 4 and is an even number, such as cyclotetradecaheptaene .
In heterocyclic aromatics ( heteroaromats ), one or more of the atoms in the aromatic ring is of an element other than carbon. This can lessen the ring's aromaticity, and thus (as in the case of furan ) increase its reactivity. Other examples include pyridine , pyrazine , imidazole , pyrazole , oxazole , thiazole , thiophene , and their benzannulated analogs ( benzimidazole , for example).
Polycyclic aromatic hydrocarbons are molecules containing two or more simple aromatic rings fused together by sharing two neighboring carbon atoms (see also simple aromatic rings ). Examples are naphthalene , anthracene , and phenanthrene .
Many chemical compounds are aromatic rings with other functional groups attached. Examples include trinitrotoluene (TNT), acetylsalicylic acid (aspirin), paracetamol , and the nucleotides of DNA .
Aromaticity is found in ions as well: the cyclopropenyl cation (2e system), the cyclopentadienyl anion (6e system), the tropylium ion (6e), and the cyclooctatetraene dianion (10e). Aromatic properties have been attributed to non-benzenoid compounds such as tropone . Aromatic properties are tested to the limit in a class of compounds called cyclophanes .
A special case of aromaticity is found in homoaromaticity where conjugation is interrupted by a single sp ³ hybridized carbon atom.
When carbon in benzene is replaced by other elements in borabenzene , silabenzene , germanabenzene , stannabenzene , phosphorine or pyrylium salts the aromaticity is still retained. Aromaticity also occurs in compounds that are not carbon-based at all. Inorganic 6-membered-ring compounds analogous to benzene have been synthesized. Hexasilabenzene (Si 6 H 6 ) and borazine (B 3 N 3 H 6 ) are structurally analogous to benzene, with the carbon atoms replaced by another element or elements. In borazine, the boron and nitrogen atoms alternate around the ring. Quite recently, the aromaticity of planar Si 5 6- rings occurring in the Zintl phase Li 12 Si 7 was experimentally evidenced by Li solid state NMR. [ 10 ]
Metal aromaticity is believed to exist in certain metal clusters of aluminium. [ citation needed ]
Möbius aromaticity occurs when a cyclic system of molecular orbitals, formed from p π atomic orbitals and populated in a closed shell by 4n (n is an integer) electrons, is given a single half-twist to correspond to a Möbius strip . A π system with 4n electrons in a flat (non-twisted) ring would be anti-aromatic, and therefore highly unstable, due to the symmetry of the combinations of p atomic orbitals. By twisting the ring, the symmetry of the system changes and becomes allowed (see also Möbius–Hückel concept for details). Because the twist can be left-handed or right-handed , the resulting Möbius aromatics are dissymmetric or chiral .
As of 2012, there is no proof that a Möbius aromatic molecule was synthesized. [ 11 ] [ 12 ] Aromatics with two half-twists corresponding to the paradromic topologies were first suggested by Johann Listing . [ 13 ] In carbo-benzene the ring bonds are extended with alkyne and allene groups.
Y-aromaticity is a concept which was developed to explain the extraordinary stability and high basicity of the guanidinium cation. Guanidinium does not have a ring structure but has six π-electrons which are delocalized over the molecule. However, this concept is controversial and some authors have stressed different effects. [ 14 ] [ 15 ] [ 16 ]
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y-cruncher is a computer program for the calculation of some mathematical constant with theoretical accuracy limited only by computing time and available storage space. It was originally developed to calculate the Euler-Mascheroni constant γ ; the y is derived from it in the name.
Since 2010, y-cruncher has been used for all record calculations of the number pi and other constants.
The software is downloadable from the website of the developers for Microsoft Windows and Linux . It does not have a graphical interface , but works on the command line . Calculation options are selected or entered via the text menu, the results are saved as a file.
Some popular uses of y-cruncher are running hardware benchmarks to measure performance of computer system. An example of such benchmark is HWBOT . y-cruncher can also be used for stress-tests , as performed computations are sensitive to RAM errors and the program can automatically detect such errors. [ 1 ] [ 2 ]
Alexander J. Yee started developing in high school a Java library for arbitrary-precision arithmetic called "BigNumber". With this he was able together with his roommate Raymond Chan on 8 December 2006 set the world record for the most number of calculated decimal places for the Euler-Mascheroni constant with 116 580 041 decimal places. [ 3 ] In January 2009, they broke their own record and calculated 14 922 244 782 decimal places. At this point, the program was renamed to "y-cruncher" and ported to C and C++ . [ 4 ]
In the aftermath, Shigeru Kondo with the help of y-cruncher calculated π {\displaystyle \pi } to 5 trillion digits on 2 August 2010. [ 5 ]
Next year, Yee and Kondo calculated 10 trillion decimal places and broke the then-valid world record for decimal places of π . [ 6 ] After that, Yee decided to completely overhaul the program and rewrite it from scratch in version v0.6.1. [ 7 ] This enabled determining π {\displaystyle \pi } with 12.1 trillion digits in just 94 days compared to 371 days that were spent for the previous record. [ 8 ]
y-cruncher has the following characteristic properties: [ 9 ] [ 10 ]
Since 2009, most of the world record-level calculations of mathematical constants have been performed with y-cruncher. The technical challenge does not (any longer) lie in the calculation itself, but in providing an environment that enables a comparatively efficient execution. [ 11 ]
The tool can serve several purposes. On the one hand, it allows the capabilities of CPUs and RAM to be determined and compared with other models. On the other hand, these hardware components can also be tested for stability and error susceptibility through stress testing . An alternative program for this would be Prime95 . The advantage of the program lies in the fact that (partial) calculations can be carried out on an old Pentium PC, an up-to-date workstation , and theoretically even supercomputers , without measured performance falling off a measurement scale (or complex benchmarks becoming incompatible due to new hardware and interfaces). Setting new computing records also represents a contemporary feasibility study and can serve as an indicator of computer performance improvement over time when regularly performed and with similar parameters. [ citation needed ]
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The Y-factor method is a widely used technique for measuring the gain and noise temperature of an amplifier . It is based on the Johnson–Nyquist noise of a resistor at two different, known temperatures. [ 1 ]
Consider a microwave amplifier with a 50- ohm impedance with a 50-ohm resistor connected to the amplifier input. If the resistor is at a physical temperature T R , then the Johnson–Nyquist noise power coupled to the amplifier input is P J = k B T R B , where k B is the Boltzmann constant , and B is the bandwidth. The noise power at the output of the amplifier (i.e. the noise power coupled to an impedance-matched load that is connected to the amplifier output) is P out = Gk B ( T R + T amp ) B , where G is the amplifier power gain, and T amp is the amplifier noise temperature . In the Y-factor technique, P out is measured for two different, known values of T R . P out is then converted to an effective temperature T out (in units of kelvin ) by dividing by k B and the measurement bandwidth B . The two values of T out are then plotted as a function of T R (also in units of kelvin), and a line is fit to these points (see figure). The slope of this line is equal to the amplifier power gain. The x intercept of the line is equal to the negative of the amplifier noise temperature − T amp in kelvins. The amplifier noise temperature can also be determined from the y intercept, which is equal to T amp multiplied by the gain.
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https://en.wikipedia.org/wiki/Y-factor
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In mathematics , the y-homeomorphism , or crosscap slide , is a special type of auto- homeomorphism in non-orientable surfaces .
It can be constructed by sliding a Möbius strip included on the surface
around an essential 1-sided closed curve until the original position; thus it is necessary that the surfaces have genus greater than one. The projective plane R P 2 {\displaystyle {\mathbb {R} P}^{2}} has no y-homeomorphism.
This geometry-related article is a stub . You can help Wikipedia by expanding it .
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https://en.wikipedia.org/wiki/Y-homeomorphism
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In analytic geometry , using the common convention that the horizontal axis represents a variable x {\displaystyle x} and the vertical axis represents a variable y {\displaystyle y} , a y {\displaystyle y} -intercept or vertical intercept is a point where the graph of a function or relation intersects the y {\displaystyle y} -axis of the coordinate system . [ 1 ] As such, these points satisfy x = 0 {\displaystyle x=0} .
If the curve in question is given as y = f ( x ) , {\displaystyle y=f(x),} the y {\displaystyle y} -coordinate of the y {\displaystyle y} -intercept is found by calculating f ( 0 ) {\displaystyle f(0)} . Functions which are undefined at x = 0 {\displaystyle x=0} have no y {\displaystyle y} -intercept.
If the function is linear and is expressed in slope-intercept form as f ( x ) = a + b x {\displaystyle f(x)=a+bx} , the constant term a {\displaystyle a} is the y {\displaystyle y} -coordinate of the y {\displaystyle y} -intercept. [ 2 ]
Some 2-dimensional mathematical relationships such as circles , ellipses , and hyperbolas can have more than one y {\displaystyle y} -intercept. Because functions associate x {\displaystyle x} -values to no more than one y {\displaystyle y} -value as part of their definition, they can have at most one y {\displaystyle y} -intercept.
Analogously, an x {\displaystyle x} -intercept is a point where the graph of a function or relation intersects with the x {\displaystyle x} -axis. As such, these points satisfy y = 0 {\displaystyle y=0} . The zeros, or roots, of such a function or relation are the x {\displaystyle x} -coordinates of these x {\displaystyle x} -intercepts. [ 3 ]
Functions of the form y = f ( x ) {\displaystyle y=f(x)} have at most one y {\displaystyle y} -intercept, but may contain multiple x {\displaystyle x} -intercepts. The x {\displaystyle x} -intercepts of functions, if any exist, are often more difficult to locate than the y {\displaystyle y} -intercept, as finding the y {\displaystyle y} -intercept involves simply evaluating the function at x = 0 {\displaystyle x=0} .
The notion may be extended for 3-dimensional space and higher dimensions, as well as for other coordinate axes, possibly with other names. For example, one may speak of the I {\displaystyle I} -intercept of the current–voltage characteristic of, say, a diode . (In electrical engineering , I {\displaystyle I} is the symbol used for electric current .)
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Indian National Science Academy
Bhabha Atomic Research Centre, Mumbai
Yogendra Pathak Viyogi ( Y. P. Viyogi ) is an Indian physicist at Indian National Science Academy . He is specialized in the field of experimental nuclear physics . [ 1 ] [ 2 ] [ 3 ]
"It is certainly a matter of great pride for all of us to be a part of the discovery of anti-alpha, the heaviest anti-matter to have been seen in terrestrial experiments."
He born at Madhubani in the year 1948. [ 4 ] He completed his primary education at his own village.
He received his post graduate degree in physics from Bihar University in Muzaffarpur . [ 5 ]
He joined the 15th batch of Training School Programme of Bhabha Atomic Research Centre , Mumbai in 1971. He was trained in experimental nuclear physics at BARC and at Lawrence Berkeley laboratory , USA. He moved to Kolkata to work at the Variable Energy Cyclotron Centre , a unit of the Department of Atomic Energy and obtained his PhD in 1984 from the University of Calcutta . He was also a postdoctoral fellow at GANIL Laboratory in France from 1984 to 1986. He was Director of Institute of Physics , Bhubaneswar during June 2006 – June 2009. He retired from service in October 2012 as Outstanding Scientist at VECC Kolkata . [ 1 ]
Y P Viyogi has studied projectile fragmentation reactions involving intermediate energy of heavy nuclei at Berkeley . He has been also involved in the study of quark gluon plasma using indigenous photon multiplicity detector (PMD) at CERN in Geneva and Brookhaven National Laboratory in USA . He led Indian group of physicists at the ALICE experiment in CERN. [ 5 ] [ 6 ] In July 1993, Y P Viyogi published an article on heavy ion collisions having title "Ultra - relativistic heavy ion experiments: a perspective" at Pramana Journal of Physics . [ 7 ]
In 2011, he was the leader of Indian physicists in STAR experiment at Relativistic Heavy Ion Collider (RHIC) in Brookhaven National Laboratory , USA. There he was involved in the observation and detection of the antimatter helium-4 nucleus or anti-alpha. He is one of the physicists who witnessed the discovery of the heaviest anti-matter known as anti-alpha particle. [ 8 ] [ 9 ]
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Yttrium oxide , also known as yttria , is Y 2 O 3 . It is an air-stable, white solid substance .
The thermal conductivity of yttrium oxide is 27 W/(m·K). [ 5 ]
Yttrium oxide is widely used to make Eu:YVO 4 and Eu:Y 2 O 3 phosphors that give the red color in color TV picture tubes.
Y 2 O 3 is a prospective solid-state laser material. In particular, lasers with ytterbium as dopant allow the efficient operation both in continuous operation [ 6 ] and in pulsed regimes. [ 7 ] At high concentration of excitations (of order of 1%) and poor cooling, the quenching of emission at laser frequency and avalanche broadband emission takes place. [ 8 ] (Yttria-based lasers are not to be confused with YAG lasers using yttrium aluminium garnet , a widely used crystal host for rare earth laser dopants).
The original use of the mineral yttria and the purpose of its extraction from mineral sources was as part of the process of making gas mantles and other products for turning the flames of artificially-produced gases (initially hydrogen, later coal gas, paraffin, or other products) into human-visible light. This use is almost obsolete - thorium and cerium oxides are larger components of such products these days.
Yttrium oxide is used to stabilize the Zirconia in late-generation porcelain-free metal-free dental ceramics. This is a very hard ceramic used as a strong base material in some full ceramic restorations. [ 9 ] The zirconia used in dentistry is zirconium oxide which has been stabilized with the addition of yttrium oxide . The full name of zirconia used in dentistry is "yttria-stabilized zirconia" or YSZ.
Yttrium oxide is also used to make yttrium iron garnets , which are very effective microwave filters. [ 10 ]
Y 2 O 3 is used to make the high temperature superconductor YBa 2 Cu 3 O 7 , known as "1-2-3" to indicate the ratio of the metal constituents:
This synthesis is typically conducted at 800 °C.
Yttrium oxide is an important starting point for inorganic compounds. For organometallic chemistry it is converted to YCl 3 in a reaction with concentrated hydrochloric acid and ammonium chloride .
Y 2 O 3 is used in specialty coatings and pastes that can withstand high temperatures and act as a barrier for reactive metals such as uranium. [ 11 ]
NASA developed a material it dubbed Solar White that it is exploring for use as a radiator in deep space, where it is expected to reflect more than 99.9% of the sun’s energy (low solar radiation absorption and high infrared emittance). [ 12 ] A sphere covered with a 10 mm coating sited far from the Earth and 1 astronomical unit from the sun could keep temperatures below 50 K. One use is long-term cryogenic storage. [ 13 ]
It's also used to create red phosphors for LED screens and TV tubes, as well as in anti-reflective coatings to enhance light transmission. [ 14 ]
Yttriaite-(Y) , approved as a new mineral species in 2010, is the natural form of yttria. It is exceedingly rare, occurring as inclusions in native tungsten particles in a placer deposit of the Bol’shaja Pol’ja ( Russian : Большая Полья ) river, Prepolar Ural , Siberia . As a chemical component of other minerals, the oxide yttria was first isolated in 1789 by Johan Gadolin , from rare-earth minerals in a mine at the Swedish town of Ytterby , near Stockholm . [ 15 ]
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Yttrium(III) sulfide ( Y 2 S 3 ) is an inorganic chemical compound . It is a compound of yttrium and sulfur .
This inorganic compound –related article is a stub . You can help Wikipedia by expanding it .
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Yttrium aluminium garnet ( YAG , Y 3 Al 5 O 12 ) is a synthetic crystalline material of the garnet group. It is a cubic yttrium aluminium oxide phase, with other examples being YAlO 3 (YAP [ 2 ] ) in a hexagonal or an orthorhombic, perovskite -like form, and the monoclinic Y 4 Al 2 O 9 (YAM [ 3 ] ). [ 4 ]
Due to its broad optical transparency, [ 5 ] low internal stress, high hardness, chemical and heat resistance, YAG is used for a variety of optics. [ 6 ] Its lack of birefringence (unlike sapphire) makes it an interesting material for high-energy/high-power laser systems. Laser damage levels of YAG ranged from 1.1 to 2.2 kJ/cm 2 (1064 nm, 10 ns). [ 7 ]
YAG, like garnet and sapphire , has no uses as a laser medium when pure. However, after being doped with an appropriate ion, YAG is commonly used as a host material in various solid-state lasers . [ 8 ] Rare earth elements such as neodymium and erbium can be doped into YAG as active laser ions, yielding Nd:YAG and Er:YAG lasers, respectively. Cerium -doped YAG (Ce:YAG) is used as a phosphor in cathode-ray tubes and white light-emitting diodes , and as a scintillator .
YAG for a period [ when? ] was used in jewelry as a diamond and other gemstone simulant. Colored variants and their doping elements include: [ 1 ] green ( chromium ), blue ( cobalt ), red ( manganese ), yellow ( titanium ), blue/pink/purple ( neodymium , depending on light source), pink, and orange. As faceted gems they are valued (as synthetics) for their clarity, durability, high refractive index and dispersion , and occasionally properties like simulating alexandrite 's color-changing property. The critical angle of YAG is 33 degrees. YAG cuts like natural garnet , with polishing being performed with alumina or diamond (50,000 or 100,000 grit) on common polishing laps. YAG has low heat sensitivity. [ 9 ]
As a synthetic gemstone YAG has numerous varietal and trade names, as well as a number of misnomers. Synonymous names include: alexite , amamite , circolite , dia-bud , diamite , diamogem , diamonair , diamone , diamonique , diamonite , diamonte , di'yag , geminair , gemonair , kimberly , Linde simulated diamond , nier-gem , regalair , replique , somerset , triamond , YAIG , and yttrium garnet . Production for the gem trade decreased after the introduction of synthetic cubic zirconia ; as of 1995 [update] there was little production. [ 1 ] Some demand exists as synthetic garnet, and for designs where the very high refractive index of cubic zirconia is not desirable. [ citation needed ]
Neodymium - doped YAG ( Nd:YAG ) was developed in the early 1960s, and the first working Nd:YAG laser was invented in 1964. Neodymium-YAG is the most widely used active laser medium in solid-state lasers , being used for everything from low-power continuous-wave lasers to high-power Q-switched (pulsed) lasers with power levels measured in the kilowatts. [ 10 ] The thermal conductivity of Nd:YAG is higher and its fluorescence lifetime is about twice as long as that of Nd:YVO 4 crystals, however it is not as efficient and is less stable, requiring more precisely controlled temperatures. The best absorption band of Nd:YAG for pumping the laser is centered at 807.5 nm, and is 1 nm wide. [ 11 ]
Most Nd:YAG lasers produce infrared light at a wavelength of 1064 nm. Light at this wavelength is rather dangerous to vision, since it can be focused by the eye's lens onto the retina , but the light is invisible and does not trigger the blink reflex . Nd:YAG lasers can also be used with frequency doubling or frequency tripling crystals , to produce green light with a wavelength of 532 nm or ultraviolet light at 355 nm, respectively.
The dopant concentration in commonly used Nd:YAG crystals usually varies between 0.5 and 1.4 molar percent. Higher dopant concentration is used for pulsed lasers; lower concentration is suitable for continuous-wave lasers. Nd:YAG is pinkish-purple, with lighter-doped rods being less intensely colored than heavier-doped ones. Since its absorption spectrum is narrow, the hue depends on the light under which it is observed.
YAG doped with neodymium and chromium ( Nd:Cr:YAG or Nd/Cr:YAG ) has absorption characteristics which are superior to Nd:YAG. This is because energy is absorbed by the broad absorption bands of the Cr 3+ dopant and then transferred to Nd 3+ by dipole-dipole interactions. [ 12 ] This material has been suggested for use in solar-pumped lasers , which could form part of a solar power satellite system. [ 13 ]
Erbium -doped YAG ( Er:YAG ) is an active laser medium lasing at 2940 nm. Its absorption bands suitable for pumping are wide and located between 600 and 800 nm, allowing for efficient flashlamp pumping. The dopant concentration used is high: about 50% of the yttrium atoms are replaced. The Er:YAG laser wavelength couples well into water and body fluids, making this laser especially useful for medicine and dentistry uses; it is used for treatment of tooth enamel and in cosmetic surgery. Er:YAG is used for noninvasive monitoring of blood sugar . The mechanical properties of Er:YAG are essentially the same as Nd:YAG. Er:YAG operates at wavelengths where the threshold for eye damage is relatively high (since the light is absorbed before striking the retina ), works well at room temperature, and has high slope efficiency . Er:YAG is pink. [ 14 ]
Ytterbium -doped YAG ( Yb:YAG ) is an active laser medium lasing at 1030 nm, with a broad, 18 nm wide absorption band at 940 nm. [ 15 ] It is one of the most useful media for high-power diode-pumped solid state lasers . The dopant levels used range between 0.2% and 30% of replaced yttrium atoms. Yb:YAG has very low fractional heating, very high slope efficiency , [ 16 ] and no excited-state absorption or up-conversion, high mechanical strength and high thermal conductivity. Yb:YAG can be pumped by reliable InGaAs laser diodes at 940 or 970 nm.
Yb:YAG is a good substitute for 1064 nm Nd:YAG in high-power applications, and its frequency-doubled 515 nm version can replace the 514 nm argon lasers .
Neodymium - cerium double-doped YAG ( Nd:Ce:YAG , or Nd,Ce:YAG ) is an active laser medium material very similar to Nd:YAG. The added cerium atoms strongly absorb in the ultraviolet region and transfer their energy to the neodymium atoms, increasing the pumping efficiency; the result is lower thermal distortion and higher power output than Nd:YAG at the same pumping level. The lasing wavelength, 1064 nm, is the same as for Nd:YAG. The material has a good resistance to damage caused by UV from the pump source, and low lasing threshold . Usually 1.1–1.4% of Y atoms are replaced with Nd, and 0.05–0.1% with Ce.
Holmium - chromium - thulium triple-doped YAG ( Ho:Cr:Tm:YAG , or Ho,Cr,Tm:YAG ) is an active laser medium material with high efficiency. It lases at 2080 nm and can be pumped by a flashlamp or a laser diode. [ 17 ] It is widely used in military, medicine, and meteorology. It works well at room temperature, has high slope efficiency , and operates at a wavelength where the threshold for eye damage is relatively high. When pumped by a diode, the 785 nm band for Tm 3+ ion can be used. [ 17 ] Other major pump bands are located between 400 and 800 nm. The dopant levels used are 0.35 atom.% Ho, 5.8 atom.% Tm, and 1.5 at.% Cr. The rods have green color, imparted by chromium(III).
Thulium -doped YAG ( Tm:YAG ) is an active laser medium that operates between 1930 and 2040 nm. It is suitable for diode pumping. A dual-mode Tm:YAG laser emits two frequencies separated by 1 GHz.
Chromium (IV)-doped YAG ( Cr:YAG ) provides a large absorption cross section in the 0.9-1.2 micrometer spectral region, which makes it an attractive choice as a passive Q-switch for Nd-doped lasers. The resulting devices are solid-state, compact and low-cost. Cr:YAG has high damage threshold, good thermal conductivity, good chemical stability, resists ultraviolet radiation, and is easily machinable. It is replacing more traditional Q-switching materials like lithium fluoride and organic dyes . The dopant levels used range between 0.5 and 3 percent (molar). Cr:YAG can be used for passive Q-switching of lasers that operate at wavelengths between 1000 and 1200 nm, such as those based on Nd:YAG, Nd:YLF , Nd:YVO 4 , and Yb:YAG.
Cr:YAG can be also used as a laser gain medium itself, producing tunable lasers with outputs adjustable between 1350 and 1550 nm. The Cr:YAG laser can generate ultrashort pulses (in the femtoseconds range) when it is pumped at 1064 nm by a Nd:YAG laser. [ 18 ]
Cr:YAG has been demonstrated in an application of non-linear optics as a self-pumped phase-conjugate mirror in a Nd:YAG "loop resonator". [ citation needed ] Such a mirror provides compensation of both phase and polarization aberrations induced into the loop resonator.
Dysprosium -doped YAG ( Dy:YAG ) is a temperature-sensitive phosphor used in temperature measurements. [ 19 ] The phosphor is excited by a laser pulse and its temperature-dependent fluorescence is observed. Dy:YAG is sensitive in ranges of 300–1700 K . [ 20 ] The phosphor can be applied directly to the measured surface, or to an end of an optical fiber . It has also been studied as a single-phase white emitting phosphor in phosphor-converted white light-emitting diodes. [ 21 ]
Samarium -doped YAG ( Sm:YAG ) is a temperature-sensitive phosphor similar to Dy:YAG.
Terbium -doped YAG ( Tb:YAG ) is a phosphor used in cathode-ray tubes . It emits at yellow-green color, at 544 nm.
Cerium (III)-doped YAG ( Ce:YAG or YAG:Ce ) is a phosphor, or a scintillator when in pure single-crystal form, with a wide range of uses. It emits yellow light when subjected to blue or ultraviolet light or to x-rays. [ 22 ] It is used in white light-emitting diodes as a coating on a high-brightness blue InGaN diode, converting part of the blue light into yellow, which together then appear as white. Such an arrangement gives less than ideal color rendering . The output brightness decreases with increasing temperature, further altering device color output. [ citation needed ]
Ce:YAG is also used in some mercury-vapor lamps as one of the phosphors, often together with Eu:Y(P,V)O 4 (yttrium phosphate-vanadate). It is also used as a phosphor in cathode-ray tubes, where it emits green (530 nm) to yellow-green (550 nm) light. When excited by electrons, it has virtually no afterglow (70 ns decay time). It is suitable for use in photomultipliers .
Ce:YAG is used in PET scanners , high-energy gamma radiation and charged particle detectors, and high-resolution imaging screens for gamma, x-rays , beta radiation and ultraviolet radiation .
Ce:YAG can be further doped with gadolinium .
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Yambo is a computer software package for studying many-body theory aspects of solids and molecule systems. [ 1 ] [ 2 ] It calculates the excited state properties of physical systems from first principles , e.g., from quantum mechanics law without the use of empirical data. It is an open-source software released under the GNU General Public License (GPL). However the main development repository is private and only a subset of the features available in the private repository are cloned into the public repository and thus distributed. [ 3 ]
Yambo can calculate:
Yambo can treat molecules and periodic systems (both metallic an insulating) in three dimensions (crystalline solids)
two dimensions (surfaces) and one dimension (e.g., nanotubes , nanowires , polymer chains). It can also handle collinear (i.e., spin-polarized wave functions ) and non-collinear ( spinors ) magnetic systems.
Typical systems are of the size of 10-100 atoms, or 10-400 electrons, per unit cell in the case of periodic systems.
Yambo relies on many-body perturbation theory and time-dependent density functional theory . [ 13 ] [ 14 ] Quasiparticle energies are calculated within the GW approximation [ 15 ] for the self energy. Optical properties are calculated either by solving the Bethe–Salpeter equation [ 16 ] [ 17 ] or by using the adiabatic local density approximation within time-dependent density functional theory.
Yambo uses a plane waves basis set to represent the electronic (single-particle) wavefunctions. Core electrons are described with norm-conserving pseudopotentials .
The choice of a plane-wave basis set enforces the periodicity of the systems. Isolated systems, and systems that are periodic in only one or two directions can be treated by using a supercell approach.
For such systems Yambo offers two numerical techniques for the treatment of the Coulomb integrals: the cut-off [ 18 ] and the random-integration method.
The Yambo team provides a wiki web-page with a list of tutorials and lecture notes .
On the yambo web-site there is also a list of all thesis done with the code.
Part of the YAMBO code is kept under a private repository.
These are the features implemented and not yet distributed:
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Yet Another Scientific Artificial Reality Application ( YASARA ) is a computer program for molecular visualising, modelling, and dynamics. It has many scientific uses, as expressed by the large number of scientific articles mentioning the software. [ 1 ] The free version of YASARA [ 2 ] is well suited to bioinformatics education. A series of freely available bioinformatics courses exist that use this software.
See the Center for Molecular and Biomolecular Informatics (CMBI) education pages for a series of examples. [ 3 ]
Official website
This article about molecular modelling software is a stub . You can help Wikipedia by expanding it .
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Yttrium barium copper oxide ( YBCO ) is a family of crystalline chemical compounds that display high-temperature superconductivity ; it includes the first material ever discovered to become superconducting above the boiling point of liquid nitrogen [77 K (−196.2 °C; −321.1 °F)] at about 93 K (−180.2 °C; −292.3 °F). [ 3 ]
Many YBCO compounds have the general formula Y Ba 2 Cu 3 O 7− x (also known as Y123), although materials with other Y:Ba:Cu ratios exist, such as Y Ba 2 Cu 4 O y (Y124) or Y 2 Ba 4 Cu 7 O y (Y247). At present, there is no singularly recognised theory for high-temperature superconductivity.
It is part of the more general group of rare-earth barium copper oxides (ReBCO) in which, instead of yttrium, other rare earths are present.
In April 1986, Georg Bednorz and Karl Müller , working at IBM in Zurich , discovered that certain semiconducting oxides became superconducting at relatively high temperature, in particular, a lanthanum barium copper oxide becomes superconducting at 35 K. This oxide was an oxygen-deficient perovskite -related material that proved promising and stimulated the search for related compounds with higher superconducting transition temperatures. In 1987, Bednorz and Müller were jointly awarded the Nobel Prize in Physics for this work.
Following Bednorz and Müller's discovery, a team led by Paul Ching Wu Chu at the University of Alabama in Huntsville and University of Houston discovered that YBCO has a superconducting transition critical temperature ( T c ) of 93 K. [ 3 ] The first samples were Y 1.2 Ba 0.8 Cu O 4 , but this was an average composition for two phases, a black and a green one. Workers at Bell Laboratories identified the black phase as the superconductor, determined its composition YBa 2 Cu 3 O 7−δ and synthesized it in single phase [ 4 ]
YBCO was the first material found to become superconducting above 77 K, the boiling point of liquid nitrogen , whereas the majority of other superconductors require more expensive cryogens. Nonetheless, YBCO and its many related materials have yet to displace superconductors requiring liquid helium for cooling.
Relatively pure YBCO was first synthesized by heating a mixture of the metal carbonates at temperatures between 1000 and 1300 K. [ 5 ] [ 6 ]
Modern syntheses of YBCO use the corresponding oxides and nitrates. [ 6 ]
The superconducting properties of YBa 2 Cu 3 O 7− x are sensitive to the value of x , its oxygen content. Only those materials with 0 ≤ x ≤ 0.65 are superconducting below T c , and when x ~ 0.07 , the material superconducts at the highest temperature of 95 K , [ 6 ] or in highest magnetic fields: 120 T for B perpendicular and 250 T for B parallel to the CuO 2 planes. [ 7 ]
In addition to being sensitive to the stoichiometry of oxygen, the properties of YBCO are influenced by the crystallization methods used. Care must be taken to sinter YBCO. YBCO is a crystalline material, and the best superconductive properties are obtained when crystal grain boundaries are aligned by careful control of annealing and quenching temperature rates.
Numerous other methods to synthesize YBCO have developed since its discovery by Wu and his co-workers, such as chemical vapor deposition (CVD), [ 5 ] [ 6 ] sol-gel , [ 8 ] and aerosol [ 9 ] methods. These alternative methods, however, still require careful sintering to produce a quality product.
However, new possibilities have been opened since the discovery that trifluoroacetic acid ( TFA ), a source of fluorine, prevents the formation of the undesired barium carbonate (BaCO 3 ). Routes such as CSD (chemical solution deposition) have opened a wide range of possibilities, particularly in the preparation of long YBCO tapes. [ 10 ] This route lowers the temperature necessary to get the correct phase to around 700 °C (973 K; 1,292 °F). This, and the lack of dependence on vacuum, makes this method a very promising way to get scalable YBCO tapes.
YBCO crystallizes in a defect perovskite structure . It can be viewed as a layered structure: the boundary of each layer is defined by planes of square planar CuO 4 units sharing 4 vertices. The planes can sometimes be slightly puckered. [ 5 ] Perpendicular to these CuO 4 planes are CuO 2 ribbons sharing 2 vertices. The yttrium atoms are found between the CuO 4 planes, while the barium atoms are found between the CuO 2 ribbons and the CuO 4 planes. This structural feature is illustrated in the figure to the right.
Although YBa 2 Cu 3 O 7 is a well-defined chemical compound with a specific structure and stoichiometry, materials with fewer than seven oxygen atoms per formula unit are non-stoichiometric compounds . The structure of these materials depends on the oxygen content. This non-stoichiometry is denoted by the x in the chemical formula YBa 2 Cu 3 O 7− x . When x = 1, the O(1) sites in the Cu(1) layer (as labelled in the unit cell ) are vacant and the structure is tetragonal . The tetragonal form of YBCO is insulating and does not superconduct. Increasing the oxygen content slightly causes more of the O(1) sites to become occupied. For x < 0.65, Cu-O chains along the b axis of the crystal are formed. Elongation of the b axis changes the structure to orthorhombic , with lattice parameters of a = 3.82, b = 3.89, and c = 11.68 Å. [ 12 ] Optimum superconducting properties occur when x ~ 0.07, i.e., almost all of the O(1) sites are occupied, with few vacancies.
In experiments where other elements are substituted on the Cu and Ba [ why? ] sites, evidence has shown that conduction occurs in the Cu(2)O planes while the Cu(1)O(1) chains act as charge reservoirs, which provide carriers to the CuO planes. However, this model fails to address superconductivity in the homologue Pr123 ( praseodymium instead of yttrium). [ 13 ] This (conduction in the copper planes) confines conductivity to the a - b planes and a large anisotropy in transport properties is observed. Along the c axis, normal conductivity is 10 times smaller than in the a - b plane. For other cuprates in the same general class, the anisotropy is even greater and inter-plane transport is highly restricted.
Furthermore, the superconducting length scales show similar anisotropy, in both penetration depth (λ ab ≈ 150 nm, λ c ≈ 800 nm) and coherence length, (ξ ab ≈ 2 nm, ξ c ≈ 0.4 nm). Although the coherence length in the a - b plane is 5 times greater than that along the c axis it is quite small compared to classic superconductors such as niobium (where ξ ≈ 40 nm). This modest coherence length means that the superconducting state is more susceptible to local disruptions from interfaces or defects on the order of a single unit cell, such as the boundary between twinned crystal domains. This sensitivity to small defects complicates fabricating devices with YBCO, and the material is also sensitive to degradation from humidity.
Many possible applications of this and related high temperature superconducting materials have been discussed. For example, superconducting materials are finding use as magnets in magnetic resonance imaging , magnetic levitation , and Josephson junctions . (The most used material for power cables and magnets is BSCCO .) [ citation needed ]
YBCO has yet to be used in many applications involving superconductors for two primary reasons:
The most promising method developed to utilize this material involves deposition of YBCO on flexible metal tapes coated with buffering metal oxides. This is known as coated conductor . Texture (crystal plane alignment) can be introduced into the metal tape (the RABiTS process) or a textured ceramic buffer layer can be deposited, with the aid of an ion beam, on an untextured alloy substrate (the IBAD process). Subsequent oxide layers prevent diffusion of the metal from the tape into the superconductor while transferring the template for texturing the superconducting layer. Novel variants on CVD, PVD, and solution deposition techniques are used to produce long lengths of the final YBCO layer at high rates. Companies pursuing these processes include American Superconductor , Superpower (a division of Furukawa Electric ), Sumitomo , Fujikura , Nexans Superconductors, Commonwealth Fusion Systems , and European Advanced Superconductors. A much larger number of research institutes have also produced YBCO tape by these methods. [ citation needed ]
The superconducting tape is used for SPARC , a tokamak fusion reactor design that can achieve breakeven energy production. [ 15 ]
Surface modification of materials has often led to new and improved properties. Corrosion inhibition, polymer adhesion and nucleation, preparation of organic superconductor/insulator/high- T c superconductor trilayer structures, and the fabrication of metal/insulator/superconductor tunnel junctions have been developed using surface-modified YBCO. [ 16 ]
These molecular layered materials are synthesized using cyclic voltammetry . Thus far, YBCO layered with alkylamines, arylamines, and thiols have been produced with varying stability of the molecular layer. It has been proposed that amines act as Lewis bases and bind to Lewis acidic Cu surface sites in YBa 2 Cu 3 O 7 to form stable coordination bonds .
In 1987, shortly after it was discovered, physicist and science author Paul Grant published in the U.K. Journal New Scientist a straightforward guide for synthesizing YBCO superconductors using widely-available equipment. [ 17 ] Thanks in part to this article and similar publications at the time, YBCO has become a popular high-temperature superconductor for use by hobbyists and in education, as the magnetic levitation effect can be easily demonstrated using liquid nitrogen as coolant.
In 2021, SuperOx, a Russian and Japanese company, developed a new manufacturing process for making YBCO wire for fusion reactors. This new wire was shown to conduct between 700 and 2000 Amps per square millimeter. The company was able to produce 186 miles of wire in 9 months, between 2019 and 2021, dramatically improving the production capacity. The company used a plasma-laser deposition process, on a electropolished substrate to make 12-mm width tape and then slit it into 3-mm tape. [ 18 ]
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Yttrium(III) bromide is an inorganic compound with the chemical formula YBr 3 . It is a white solid. Anhydrous yttrium(III) bromide can be produced by reacting yttrium oxide or yttrium(III) bromide hydrate and ammonium bromide . The reaction proceeds via the intermediate (NH 4 ) 3 YBr 6 . [ 3 ] Another method is to react yttrium carbide (YC 2 ) and elemental bromine . [ 4 ] Yttrium(III) bromide can be reduced by yttrium metal to YBr or Y 2 Br 3 . [ 5 ] It can react with osmium to produce Y 4 Br 4 Os. [ 6 ]
This inorganic compound –related article is a stub . You can help Wikipedia by expanding it .
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Yttrium(III) chloride is an inorganic compound of yttrium and chloride . It exists in two forms, the hydrate (YCl 3 (H 2 O) 6 ) and an anhydrous form (YCl 3 ). Both are colourless salts that are highly soluble in water and deliquescent .
Solid YCl 3 adopts a cubic [ citation needed ] structure with close-packed chloride ions and yttrium ions filling one third of the octahedral holes and the resulting YCl 6 octahedra sharing three edges with adjacent octahedra, giving it a layered structure. [ 5 ] [ 1 ] This structure is shared by a range of compounds, notably AlCl 3 .
YCl 3 is often prepared by the " ammonium chloride route," starting from either Y 2 O 3 or hydrated chloride or oxychloride. [ 6 ] [ 7 ] or YCl 3 ·6H 2 O. [ 8 ] These methods produce (NH 4 ) 2 [YCl 5 ]:
The pentachloride decomposes thermally according to the following equation:
The thermolysis reaction proceeds via the intermediacy of (NH 4 )[Y 2 Cl 7 ].
Treating Y 2 O 3 with aqueous HCl produces the hydrated chloride (YCl 3 ·6H 2 O). When heated, this salt yields yttrium oxychloride rather than reverting to the anhydrous form.
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YDS is a scheduling algorithm for dynamic speed scaling processors which minimizes the total energy consumption. It was named after and developed by Yao et al. [ 1 ] There is both an online and an offline version of the algorithm.
Definitions:
The algorithm then works as follows:
In other terms it's a recursive algorithm that will follow these steps until all jobs are scheduled:
For any Job instance, the algorithm computes an optimal schedule minimizing the total energy consumption. [ 2 ]
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Yttrium(III) fluoride is an inorganic chemical compound with the chemical formula Y F 3 . It is not known naturally in 'pure' form. The fluoride minerals containing essential yttrium include tveitite-(Y) (Y,Na) 6 Ca 6 Ca 6 F 42 and gagarinite-(Y) NaCaY(F,Cl) 6 . Sometimes mineral fluorite contains admixtures of yttrium. [ 1 ] [ 2 ]
YF 3 can be produced by reacting fluorine with yttria or yttrium hydroxide with hydrofluoric acid . [ 3 ]
Yttrium(III) fluoride has a refractive index of 1.51 at 500 nm [ 4 ] and is transparent in the range from 193 nm to 14,000 nm (i.e. from the UV to IR range).
Pure yttrium can be obtained from yttrium(III) fluoride by reduction with calcium .
Yttrium(III) fluoride crystallizes in the orthorhombic crystal system , with space group Pnma (space group no. 62), with the lattice parameters a = 6.3537 Å, b = 6.8545 Å, c = 4.3953 Å. [ 5 ] Yttrium is nine times coordinated by fluorine atoms.
It occurs as the mineral waimirite-(Y) . [ 6 ]
Yttrium(III) fluoride can be used for the production of metallic yttrium, [ 7 ] thin films, glasses [ 8 ] and ceramics.
Conditions/substances to avoid are: acids , active metals and moisture .
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Yttrium hydride is a compound of hydrogen and yttrium . It is considered to be a part of the class of rare-earth metal hydrides . It exists in several forms, the most common being a metallic compound with formula YH 2 . YH 2 has a face-centred cubic structure, and is a metallic compound . Under great pressure, extra hydrogen can combine to yield an insulator with a hexagonal structure, with a formula close to YH 3 . [ 1 ] Hexagonal YH 3 has a band gap of 2.6 eV. Under pressure of 12 GPa YH 3 transforms to an intermediate state, and when the pressure increases to 22 GPa another metallic face-centred cubic phase is formed. [ 2 ]
In 1996, it was shown that the metal - insulator transition when going from YH 2 to YH 3 can be used to change the optical state of windows from non-transparent to transparent. [ 3 ] This report spurred a wave of research on metal hydride -based chromogenic materials and smart windows ; gasochromic windows reacting to hydrogen gas and electrochromic structures where the transparency can be regulated by applying an external voltage. [ 4 ] When containing a substantial amount of oxygen, yttrium hydride is also found to exhibit reversible photochromic properties. [ 5 ] This switchable optical property enables their utilization in many technological applications, such as sensors, goggles, and medical devices in addition to the smart windows. According to a research results, the strength of the photochromic response is found to decrease with increasing oxygen concentration in the film accompanied by an optical band gap widening. [ 6 ]
Yttrium hydride is being looked at as a high temperature superconductor. [ 7 ]
Yttrium hydride is being looked at as a neutron moderator [ 8 ] for use in new nuclear reactor designs.
This inorganic compound –related article is a stub . You can help Wikipedia by expanding it .
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https://en.wikipedia.org/wiki/YH2
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Yttrium hydride is a compound of hydrogen and yttrium . It is considered to be a part of the class of rare-earth metal hydrides . It exists in several forms, the most common being a metallic compound with formula YH 2 . YH 2 has a face-centred cubic structure, and is a metallic compound . Under great pressure, extra hydrogen can combine to yield an insulator with a hexagonal structure, with a formula close to YH 3 . [ 1 ] Hexagonal YH 3 has a band gap of 2.6 eV. Under pressure of 12 GPa YH 3 transforms to an intermediate state, and when the pressure increases to 22 GPa another metallic face-centred cubic phase is formed. [ 2 ]
In 1996, it was shown that the metal - insulator transition when going from YH 2 to YH 3 can be used to change the optical state of windows from non-transparent to transparent. [ 3 ] This report spurred a wave of research on metal hydride -based chromogenic materials and smart windows ; gasochromic windows reacting to hydrogen gas and electrochromic structures where the transparency can be regulated by applying an external voltage. [ 4 ] When containing a substantial amount of oxygen, yttrium hydride is also found to exhibit reversible photochromic properties. [ 5 ] This switchable optical property enables their utilization in many technological applications, such as sensors, goggles, and medical devices in addition to the smart windows. According to a research results, the strength of the photochromic response is found to decrease with increasing oxygen concentration in the film accompanied by an optical band gap widening. [ 6 ]
Yttrium hydride is being looked at as a high temperature superconductor. [ 7 ]
Yttrium hydride is being looked at as a neutron moderator [ 8 ] for use in new nuclear reactor designs.
This inorganic compound –related article is a stub . You can help Wikipedia by expanding it .
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https://en.wikipedia.org/wiki/YH3
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The Yarkovsky–O'Keefe–Radzievskii–Paddack effect , or YORP effect for short, changes the rotation state of a small astronomical body – that is, the body's spin rate and the obliquity of its pole (s) – due to the scattering of solar radiation off its surface and the emission of its own thermal radiation .
The YORP effect is typically considered for asteroids with their heliocentric orbit in the Solar System . The effect is responsible for the creation of binary and tumbling asteroids as well as for changing an asteroid's pole towards 0 ° , 90°, or 180° relative to the ecliptic plane and so modifying its heliocentric radial drift rate due to the Yarkovsky effect .
The term was coined by David P. Rubincam in 2000 [ 1 ] to honor four important contributors to the concepts behind the so-named YORP effect. In the 19th century, Ivan Yarkovsky realized that the thermal radiation escaping from a body warmed by the Sun carries off momentum as well as heat . Translated into modern physics, each emitted photon possesses a momentum p = E/c where E is its energy and c is the speed of light . Vladimir Radzievskii applied the idea to rotation based on changes in albedo [ 2 ] and Stephen Paddack realized that shape was a much more effective means of altering a body's spin rate. [ 3 ] Stephen Paddack and John O'Keefe suggested that the YORP effect leads to rotational bursting and by repeatedly undergoing this process, small asymmetric bodies are eventually reduced to dust. [ 4 ] [ 5 ]
In principle, electromagnetic radiation interacts with the surface of an asteroid in three significant ways: radiation from the Sun is (1) absorbed and (2) diffusively reflected by the surface of the body and the body's internal energy is (3) emitted as thermal radiation . Since photons possess momentum , each of these interactions leads to changes in the angular momentum of the body relative to its center of mass . If considered for only a short period of time, these changes are very small, but over longer periods of time, these changes may integrate to significant changes in the angular momentum of the body. For bodies in a heliocentric orbit , the relevant long period of time is the orbital period (i.e. year), since most asteroids have rotation periods (i.e. days) shorter than their orbital periods. Thus, for most asteroids, the YORP effect is the secular change in the rotation state of the asteroid after averaging the solar radiation torques over first the rotational period and then the orbital period.
In 2007 there was direct observational confirmation of the YORP effect on the small asteroids 54509 YORP (then designated 2000 PH 5 ) [ 6 ] [ 7 ] and 1862 Apollo . [ 8 ] The spin rate of 54509 YORP will double in just 600,000 years, and the YORP effect can also alter the axial tilt and precession rate, so that the entire suite of YORP phenomena can send asteroids into interesting resonant spin states, and helps explain the existence of binary asteroids . [ 9 ]
Observations show that asteroids larger than 125 km in diameter have rotation rates that follow a Maxwellian frequency distribution , while smaller asteroids (in the 50 to 125 km size range) show a small excess of fast rotators. The smallest asteroids (size less than 50 km) show a clear excess of very fast and slow rotators, and this becomes even more pronounced as smaller-sized populations are measured. These results suggest that one or more size-dependent mechanisms are depopulating the centre of the spin rate distribution in favour of the extremes. The YORP effect is a prime candidate. It is not capable of significantly modifying the spin rates of large asteroids by itself, so a different explanation must be sought for objects such as 253 Mathilde .
In late 2013 asteroid P/2013 R3 was observed breaking apart, likely because of a high rotation speed from the YORP effect. [ 10 ]
Assume a rotating spherical asteroid has two wedge-shaped fins attached to its equator, irradiated by parallel rays of sunlight. The reaction force from photons departing from any given surface element of the spherical core will be normal to the surface, such that no torque is produced (the force vectors all pass through the centre of mass).
Thermally-emitted photons reradiated from the sides of the wedges, however, can produce a torque, as the normal vectors do not pass through the centre of mass. Both fins present the same cross section to the incoming light (they have the same height and width), and so absorb and reflect the same amount of energy each and produce an equal force. Due to the fin surfaces being oblique, however, the normal forces from the reradiated photons do not cancel out. In the diagram, fin A's outgoing radiation produces an equatorial force parallel to the incoming light and no vertical force, but fin B's force has a smaller equatorial component and a vertical component. The unbalanced forces on the two fins lead to torque and the object spins. The torque from the outgoing light does not average out, even over a full rotation, so the spin accelerates over time. [ 11 ]
An object with some "windmill" asymmetry can therefore be subjected to minuscule torque forces that will tend to spin it up or down as well as make its axis of rotation precess . The YORP effect is zero for a rotating ellipsoid if there are no irregularities in surface temperature or albedo .
In the long term, the object's changing obliquity and rotation rate may wander randomly, chaotically or regularly, depending on several factors. For example, assuming the Sun remains on its equator , asteroid 951 Gaspra , with a radius of 6 km and a semi-major axis of 2.21 AU , would in 240 Ma (240 million years) go from a rotation period of 12 h to 6 h and vice versa. If 243 Ida were given the same radius and orbit values as Gaspra, it would spin up or down twice as fast, while a body with Phobos' shape would take several billion years to change its spin by the same amount.
Size as well as shape affects the amount of the effect. Smaller objects will spin up or down much more quickly. If Gaspra were smaller by a factor of 10 (to a radius of 500 m), its spin will halve or double in just a few million years. Similarly, the YORP effect intensifies for objects closer to the Sun. At 1 AU, Gaspra would double/halve its spin rate in a mere 100,000 years. After one million years, its period may shrink to ~2 h, at which point it could start to break apart. [ citation needed ] According to a 2019 model, the YORP effect is likely to cause "widespread fragmentation of asteroids" as the Sun expands into a luminous red giant , and may explain the dust disks and apparent infalling matter observed at many white dwarfs . [ 12 ] [ 13 ]
This is one mechanism through which binary asteroids may form, and it may be more common than collisions and planetary near-encounter tidal disruption as the primary means of binary formation.
Asteroid 2000 PH 5 was later named 54509 YORP to honor its part in the confirmation of this phenomenon.
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https://en.wikipedia.org/wiki/YORP_effect
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83719
66090
ENSG00000090238
ENSMUSG00000042675
P61236
P61237
NM_001145524 NM_031477
NM_025347 NM_026875
NP_001138996 NP_113665
NP_001366225 NP_001366226
Yippee-like 3 (Drosophila) is a protein that in humans is encoded by the YPEL3 gene . [ 5 ] [ 6 ] YPEL3 has growth inhibitory effects in normal and tumor cell lines. [ 7 ] One of five family members (YPEL1-5), YPEL3 was named in reference to its Drosophila melanogaster orthologue . [ 6 ] Initially discovered in a gene expression profiling assay of p53 activated MCF7 cells, [ 8 ] induction of YPEL3 has been shown to trigger permanent growth arrest or cellular senescence in certain human normal and tumor cell types. [ 7 ] DNA methylation of a CpG island near the YPEL3 promoter as well as histone acetylation may represent possible epigenetic mechanisms leading to decreased gene expression in human tumors. [ 7 ]
Human YPEL3 is located on the short arm of chromosome 16 (p1611.2) and covers 4.62kb from 30015754 to 30011130 on the reverse strand. [ 6 ] [ 9 ] The Drosophila Yippee protein was identified as a putative zinc finger motif containing protein exhibiting a high degree of conservation among the cysteines and histidines . [ 10 ] Zinc fingers function as structural platforms for DNA binding.
YPEL3 was first identified as murine SUAP, named for small unstable apoptotic protein because of its apparent role in cellular growth inhibition via apoptosis when studied in myeloid precursor cell lines . [ 11 ] SUAP later attained its current designation as YPEL3 (Yippee like three), after it was discovered to be one of five human genes possessing homology with the Drosophila Yippee protein. [ 6 ]
The Drosophila Yippee protein was originally discovered in a yeast interaction trap screen when it was found to physically interact with Hyalophora cecropia Hemolin . After subsequent cloning and sequencing experiments Yippee was found to be a conserved gene family of proteins present in a diverse range of eukaryotic organisms, ranging from fungi to humans. [ 10 ] When analyzed at the amino acid level, Drosophila melanogaster Yippee and YPEL1 displayed a high level of homology (76%). During later sequence analysis of human chromosome 22 , researchers identified a gene family YPEL1-YPEL5 , which had high homology with the Drosophila Yippee gene. [ 6 ]
YPEL3’s role as a novel tumor suppressor and its involvement in cellular proliferation were discovered during experiments to investigate p53 dependent cell cycle arrest. While investigating the p53 tumor suppressor protein, microarray studies which targeted Hdmx and Hdm2 , both p53 negative regulators, revealed YPEL3 as a potential p53 regulated gene in MCF7 breast cancer cells. [ 8 ] Investigation into its function led to the discovery of YPEL3 being a novel protein whose growth suppressive activity is thought to be mediated through a cellular senescence pathway. [ 7 ]
p53 is a tumor suppressor protein encoded by the human gene TP53 whose function is to prevent unregulated cell growth. p53 can be activated in response to a wide variety of cellular stressors, both oncogenic and non-oncogenic. An important checkpoint in a complex pathway, activated p53 has been shown to bind DNA and transcriptionally regulate genes that can mediate a variety of cellular growth processes including DNA repair , growth arrest, cellular senescence and apoptosis . [ 12 ] The importance of functioning p53 in the regulation of the cell cycle is evident in that 55% of human cancers exhibit p53 mutations. [ 13 ]
YPEL3 was discovered to be a possible p53 target after a screen for such genes was performed in MCF7 breast cancer cells following RNAi knockdown of p53 negative inhibitors. [ 8 ] In both human normal and tumor cell lines, YPEL3 has been shown to be a p53-inducible gene. Two putative p53 binding sites have been identified, one 1.3-Kbp 5' of the YPEL3 promoter and another upstream of the YPEL3 promoter . [ 6 ]
As a part of the p53 pathway response and its anti-proliferation role, cellular senescence has gained attention for its working relationship with tumor suppressor genes. [ 14 ] Characterized by the limited ability of cultured normal cells to divide, senescence has been shown to be triggered through oncogenic activation( premature senescence) as well as telomere shortening as the result of successive rounds of DNA replication (replicative senescence). [ 15 ] Recognized hallmarks of cellular senescence include senescence associated(SA)beta galactosidase staining and the appearance of senescence-associated heterochromatic foci(SAHF) within the nuclei of senescent cells. [ 16 ] [ 17 ]
Although studies in murine myeloid precursor cell lines indicated YPEL3 to have a role in apoptosis, human YPEL3 failed to demonstrate an apoptotic response using sub-G1 or poly ADP ribose polymerase cleavage as accepted indicators of programmed cell death. [ 11 ] YPEL3 has been shown to trigger premature senescence when studied in IMR90 primary human fibroblasts . Studies in U2OS osteosarcoma cells and MCF7 breast cancer cells have also demonstrated increased cellular senescence upon YPEL3 induction. [ 7 ] As further possible evidence to its function, reduced expression of YPEL3 has been observed in ovarian, lung, and colon tumor cell lines. [ 7 ] [ 18 ]
Epigenetics is the study of changes in gene activity that do not involve alterations to genetic code, or DNA . Instead, just above the genome sits various epigenetic markers which serve to provide instructions to activate or inactivate genes to varying degrees. This silencing or activation of genes has been recognized to play an important role in the differentiation of nascent cells and several human disease states including cancer . Unlike genetic mutations , epigenetic changes are considered reversible, although further study is needed.
Two common methods of epigenetic modification are DNA methylation and histone modification. Specifically, hypermethylation of CpG islands ( guanine and cytosine rich spans of DNA) near the promoters of tumor suppressor genes have been documented in specific tumor cell lines. In the case of the tumor suppressors VHL (associated with von Hippel–Lindau disease), p16 , hMLH1, and BRCA1 (a gene associated with breast cancer susceptibility), hypermethylation of the CpG-island has been shown to be a method of gene inactivation. [ 19 ]
Both histone acetylation and DNA methylation have been studied as possible epigenetic means of regulating YPEL3 expression. When studied in Cp70 ovarian carcinoma cells, hypermethylation of a CpG island immediately upstream of the YPEL3 promoter has been seen to down regulate YPEL3 expression. [ 7 ] Hypermethylation seen in the promoters of tumor suppressor genes are cancer type specific, allowing each tumor type to be identifiable with an individual pattern. [ 20 ] Such discoveries have led researchers to investigate epigenetic markers as potential diagnostic tools, prognostic factors, and indicators for the responsiveness to treatment of human cancers, although continued study is needed. [ 19 ]
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https://en.wikipedia.org/wiki/YPEL3
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Yttrium phosphate , YPO 4 , is the phosphate salt of yttrium . [ 1 ] It occurs in nature as minerals xenotime [ 2 ] and weinschenkite. [ 3 ]
Yttrium phosphate can be obtained by reacting yttrium chloride and sodium phosphate , or by reacting yttrium nitrate and diammonium hydrogen phosphate in solution: [ 2 ]
Yttrium phosphate can also be prepared by the reaction of yttrium(III) oxide and diammonium hydrogen phosphate: [ 4 ]
Yttrium chloride and phosphoric acid are mixed at 35~40°C, and then ammonia solution is added dropwise to react: [ 5 ]
Yttrium phosphate belongs to the tetragonal crystal system , and the unit cell parameters are a =0.68832 nm, c =0.60208 nm. It can exist as a monohydrate, dihydrate or the anhydrous form. The dihydrate belongs to the monoclinic crystal system , the space group is B 2/b , and the unit cell parameters are a =0.648 nm, b =1.512 nm, c =0.628 nm, β=129.4°, Z=4. [ 6 ]
Yttrium phosphate reacts with concentrated alkali to form yttrium hydroxide . [ 7 ]
Yttrium phosphate is used as a catalyst and is a potential containment material for nuclear waste. [ 2 ] Ce 3+ -doped yttrium phosphate shows luminescence in the UV range and can be used for tanning lamps. [ 8 ] [ 9 ] Double-doped materials such as Ce 3+ -Tb 3+ have also been reported.
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https://en.wikipedia.org/wiki/YPO4
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