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Walden reductor The is a reduction column filled with metallic silver which can be used to reduce a metal ion in aqueous solution to a lower oxidation state. It can be used "e.g." to reduce UO to U. The method is named after George H. Walden, who developed it jointly with a Ph.D. student, Sylvan M. Edmonds, at Columbia University. A copper wire is submerged in a solution of silver nitrate. Dendritic crystals of silver immediately form on the copper wire according to the following redox reaction: Cu + 2 Ag → Cu + 2 Ag The silver crystals are then removed from the copper wire, washed with pure water to remove the copper nitrate and the excess of silver nitrate and packed in a small glass column. To use the reductor, the solution to be reduced is poured at the top of the glass tube, and then drawn through it. The reactive front progresses along the column as in chromatography and the extent of reduction reaches up to 100 % as the solution passes down the tube and the product becomes completely separated from the starting material. The can be used to obtain chemical species in their low valence state if required for chemical analyses or to obtain small amounts of the compound in the appropriate form. Other reductor Other oxidizing reagents (opposite) | https://en.wikipedia.org/wiki?curid=30691605 |
Partial cube In graph theory, a partial cube is a graph that is isometric to a subgraph of a hypercube. In other words, a partial cube can be identified with a subgraph of a hypercube in such a way that the distance between any two vertices in the partial cube is the same as the distance between those vertices in the hypercube. Equivalently, a partial cube is a graph whose vertices can be labeled with bit strings of equal length in such a way that the distance between two vertices in the graph is equal to the Hamming distance between their labels. Such a labeling is called a "Hamming labeling"; it represents an isometric embedding of the partial cube into a hypercube. was the first to study isometric embeddings of graphs into hypercubes. The graphs that admit such embeddings were characterized by and , and were later named partial cubes. A separate line of research on the same structures, in the terminology of families of sets rather than of hypercube labelings of graphs, was followed by and , among others. Every tree is a partial cube. For, suppose that a tree "T" has "m" edges, and number these edges (arbitrarily) from 0 to "m" − 1. Choose a root vertex "r" for the tree, arbitrarily, and label each vertex "v" with a string of "m" bits that has a 1 in position "i" whenever edge "i" lies on the path from "r" to "v" in "T". For instance, "r" itself will have a label that is all zero bits, its neighbors will have labels with a single 1-bit, etc | https://en.wikipedia.org/wiki?curid=30697444 |
Partial cube Then the Hamming distance between any two labels is the distance between the two vertices in the tree, so this labeling shows that "T" is a partial cube. Every hypercube graph is itself a partial cube, which can be labeled with all the different bitstrings of length equal to the dimension of the hypercube. More complex examples include the following: Many of the theorems about partial cubes are based directly or indirectly upon a certain binary relation defined on the edges of the graph. This relation, first described by and given an equivalent definition in terms of distances by , is denoted by formula_1. Two edges formula_2 and formula_3 are defined to be in the relation formula_1, written formula_5, if formula_6. This relation is reflexive and symmetric, but in general it is not transitive. Winkler showed that a connected graph is a partial cube if and only if it is bipartite and the relation formula_1 is transitive. In this case, it forms an equivalence relation and each equivalence class separates two connected subgraphs of the graph from each other. A Hamming labeling may be obtained by assigning one bit of each label to each of the equivalence classes of the Djoković–Winkler relation; in one of the two connected subgraphs separated by an equivalence class of edges, all of the vertices have a 0 in that position of their labels, and in the other connected subgraph all of the vertices have a 1 in the same position | https://en.wikipedia.org/wiki?curid=30697444 |
Partial cube Partial cubes can be recognized, and a Hamming labeling constructed, in formula_8 time, where formula_9 is the number of vertices in the graph. Given a partial cube, it is straightforward to construct the equivalence classes of the Djoković–Winkler relation by doing a breadth first search from each vertex, in total time formula_10; the formula_8-time recognition algorithm speeds this up by using bit-level parallelism to perform multiple breadth first searches in a single pass through the graph, and then applies a separate algorithm to verify that the result of this computation is a valid partial cube labeling. The isometric dimension of a partial cube is the minimum dimension of a hypercube onto which it may be isometrically embedded, and is equal to the number of equivalence classes of the Djoković–Winkler relation. For instance, the isometric dimension of an formula_9-vertex tree is its number of edges, formula_13. An embedding of a partial cube onto a hypercube of this dimension is unique, up to symmetries of the hypercube. Every hypercube and therefore every partial cube can be embedded isometrically into an integer lattice. The lattice dimension of a graph is the minimum dimension of an integer lattice into which the graph can be isometrically embedded. The lattice dimension may be significantly smaller than the isometric dimension; for instance, for a tree it is half the number of leaves in the tree (rounded up to the nearest integer) | https://en.wikipedia.org/wiki?curid=30697444 |
Partial cube The lattice dimension of any graph, and a lattice embedding of minimum dimension, may be found in polynomial time by an algorithm based on maximum matching in an auxiliary graph. Other types of dimension of partial cubes have also been defined, based on embeddings into more specialized structures. Isometric embeddings of graphs into hypercubes have an important application in chemical graph theory. A "benzenoid graph" is a graph consisting of all vertices and edges lying on and in the interior of a cycle in a hexagonal lattice. Such graphs are the molecular graphs of the benzenoid hydrocarbons, a large class of organic molecules. Every such graph is a partial cube. A Hamming labeling of such a graph can be used to compute the Wiener index of the corresponding molecule, which can then be used to predict certain of its chemical properties. A different molecular structure formed from carbon, the diamond cubic, also forms partial cube graphs. | https://en.wikipedia.org/wiki?curid=30697444 |
Ethyl azide (CHN) is an explosive compound sensitive to rapid heating, shock or impact. It has exploded when heated to room temperature. When heated to decomposition it emits toxic fumes of NOx. It is irritating to eyes, respiratory system and skin. is used for organic synthesis. | https://en.wikipedia.org/wiki?curid=30697542 |
Natural bond orbital In quantum chemistry, a natural bond orbital or NBO is a calculated "bonding orbital" with maximum electron density. The NBOs are one of a sequence of natural localized orbital sets that include "natural atomic orbitals" (NAO), "natural hybrid orbitals" (NHO), "natural bonding orbitals" (NBO) and "natural (semi-)localized molecular orbitals" (NLMO). These natural localized sets are intermediate between basis atomic orbitals (AO) and molecular orbitals (MO): Natural (localized) orbitals are used in computational chemistry to calculate the distribution of electron density in atoms and in bonds between atoms. They have the "maximum-occupancy character" in localized 1-center and 2-center regions of the molecule. Natural bond orbitals (NBOs) include the highest possible percentage of the electron density, ideally close to 2.000, providing the most accurate possible “natural Lewis structure” of ψ. A high percentage of electron density (denoted %-ρ), often found to be >99% for common organic molecules, correspond with an accurate natural Lewis structure. The concept of "natural orbitals" was first introduced by Per-Olov Löwdin in 1955, to describe the unique set of orthonormal 1-electron functions that are intrinsic to the "N"-electron wavefunction. Each bonding NBO σ (the donor) can be written in terms of two directed valence hybrids (NHOs) h, h on atoms A and B, with corresponding polarization coefficients "c", "c": The bonds vary smoothly from covalent ("c" = "c") to ionic ("c" » "c") limit | https://en.wikipedia.org/wiki?curid=30703104 |
Natural bond orbital Each valence bonding NBO σ must be paired with a corresponding valence antibonding NBO σ* (the acceptor) to complete the span of the valence space: The bonding NBOs are of the "Lewis orbital"-type (occupation numbers near 2); antibonding NBOs are of the "non-Lewis orbital"-type (occupation numbers near 0). In an idealized Lewis structure, full Lewis orbitals (two electrons) are complemented by formally empty non-Lewis orbitals. Weak occupancies of the valence antibonds signal irreducible departures from an idealized localized Lewis structure, which means true "delocalization effects". With a computer program that can calculate NBOs, optimal Lewis structures can be found. An optimal Lewis structure can be defined as that one with the maximum amount of electronic charge in Lewis orbitals (Lewis charge). A low amount of electronic charge in Lewis orbitals indicates strong effects of electron delocalization. In resonance structures, major and minor contributing structures may exist. For amides, for example, NBO calculations show that the structure with a carbonyl double bond is the dominant Lewis structure. However, in NBO calculations, "covalent-ionic resonance" is not needed due to the inclusion of bond-polarity effects in the resonance structures. This is similar to other modern valence bond theory methods. | https://en.wikipedia.org/wiki?curid=30703104 |
The Journal of Chemical Thermodynamics is a monthly peer-reviewed scientific journal covering experimental thermodynamics and thermophysics including bio-thermodynamics, calorimetry, phase equilibria, equilibrium thermodynamic properties and transport properties. It is published by Elsevier. The editors-in-chief are A. Pádua, J.P.M. Trusler, and R. Weir. The journal is abstracted and indexed in Chemical Abstracts, Chemistry Citation Index, Current Contents/Physics, Chemical, & Earth Sciences, Engineered Materials Abstracts, Physics Abstracts, Reaction Citation Index, Science Citation Index, and Scopus. | https://en.wikipedia.org/wiki?curid=30708641 |
Laser diffraction analysis Laser diffraction analysis, also known as laser diffraction spectroscopy, is a technology that utilizes diffraction patterns of a laser beam passed through any object ranging from nanometers to millimeters in size to quickly measure geometrical dimensions of a particle. This process does not depend on volumetric flow rate, the amount of particles that passes through a surface over time. is based on the Fraunhofer diffraction theory, stating that the intensity of light scattered by a particle is directly proportional to the particle size. The angle of the laser beam and particle size have an inversely proportional relationship, where the laser beam angle increases as particle size decreases and vice versa. is accomplished via a red He-Ne laser, a commonly used gas laser for physics experiments that is made up of a laser tube, a high-voltage power supply, and structural packaging. Alternatively, blue laser diodes or LEDs of shorter wavelength may be used. Angling of the light energy produced by the laser is detected by having a beam of light go through a suspension and then onto a sensor. A lens is placed between the object being analyzed and the detector's focal point, causing only the surrounding laser diffraction to appear. The sizes the laser can analyze depend on the lens' focal length, the distance from the lens to its point of focus. As the focal length increases, the area the laser can detect increases as well, displaying a proportional relationship | https://en.wikipedia.org/wiki?curid=30710121 |
Laser diffraction analysis A computer can then be used to detect the object's particle sizes from the light energy produced and its layout, which the computer derives from the data collected on the particle frequencies and wavelengths. has been used to measure particle-size objects in situations such as: Since laser diffraction analysis is not the sole way of measuring particles it has been compared to the sieve-pipette method, which is a traditional technique for grain size analysis. When compared, results showed that laser diffraction analysis made fast calculations that were easy to recreate after a one-time analysis, did not need large sample sizes, and produced large amounts of data. Results can easily be manipulated because the data is on a digital surface. Both the sieve-pipette method and laser diffraction analysis are able to analyze minuscule objects, but laser diffraction analysis resulted in having better precision than its counterpart method of particle measurement. has been questioned in validity in the following areas: | https://en.wikipedia.org/wiki?curid=30710121 |
Nanofountain probe (NFP) is a device for 'drawing' micropatterns of liquid chemicals at extremely small resolution. An NFP contains a cantilevered micro-fluidic device terminated in a nanofountain. The embedded microfluidics facilitates rapid and continuous delivery of molecules from the on-chip reservoirs to the fountain tip. When the tip is brought into contact with the substrate, a liquid meniscus forms, providing a path for molecular transport to the substrate. By controlling the geometry of the meniscus through hold time and deposition speed, various inks and biomolecules could be patterned on a surface, with sub 100 nm resolution. The advent of dip-pen nanolithography (DPN) in recent years represented a revolution in nanoscale patterning technology. With sub-100-nanometer resolution and an architecture conducive to massive parallelization, DPN is capable of producing large arrays of nanoscale features. As such, conventional DPN and other probe-based techniques are generally limited in their rate of deposition and by the need for repeated re-inking during extended patterning. To address these challenges, nanofountain probe was developed by Espinosa et al. where microchannels were embedded in AFM probes to transport ink or bio-molecules from reservoirs to substrates, realizing continuous writing at the nanoscale. Integration of continuous liquid ink feeding within the NFP facilitates more rapid deposition and eliminates the need for repeated dipping, all while preserving the sub-100-nanometer resolution of DPN | https://en.wikipedia.org/wiki?curid=30721542 |
Nanofountain probe Nano fountain probes (NFPs) are fabricated on the wafer-scale using microfabrication techniques allowing for batch fabrication of numerous chips. Through the different generations of devices, design and experimentation improved the device yielding to a robust fabrication process. The highly enhanced feature dimension and shapes is expected to improve the performance in writing and imaging. NFP is used in the development of a to scale, direct-write nanomanufacturing platform. The platform is capable of constructing complex, highly-functional nanoscale devices from a diverse suite of materials (e.g., nanoparticles, catalysts (increase rate of reaction), biomolecules, and chemical solutions). Demonstrated nanopatterning capabilities include: • Biomolecules (proteins, DNA) for biodetection assays or cell adhesion studies • Functional nanoparticles for drug delivery studies and nanosystems making (fabrication) • Catalysts for carbon nanotube growth in nanodevice fabrication • Thiols for directed self-assembly of nanostructures. Taking advantage of the unique tip geometry of the NFP nanomaterials are directly injected into live cells with minimal invasiveness. This enables unique studies of nanoparticle-mediated delivery, as well as cellular pathways and toxicity. Whereas typical in vitro studies are limited to cell populations, these broadly-applicable tools enable multifaceted interrogation at a truly single cell level. | https://en.wikipedia.org/wiki?curid=30721542 |
Isostructural chemical compounds have similar chemical structures. Isomorphous when used in the relation to crystal structures is not synonymous: in addition to the same atomic connectivity that characterises isostructural compounds, isomorphous substances crystallise in the same space group and have the same unit cell dimensions. The IUCR definition used by crystallographers is: Examples include: Many minerals are isostructural when they differ only in the nature of a cation. Compounds which are isoelectronic usually have similar chemical structures. For example, methane, CH, and the ammonium ion, NH, are isoelectric and are isostructural as both have a tetrahedral structure. The C-H and N-H bond lengths are different and crystal structures are completely different because the ammonium ion only occurs in salts. | https://en.wikipedia.org/wiki?curid=30723461 |
Nanobiomechanics (also bionanomechanics) is an emerging field in nanoscience and biomechanics that combines the powerful tools of nanomechanics to explore fundamental science of biomaterials and biomechanics. Since the introduction by its founder Yuan-Cheng Fung, the field of biomechanics has become one of the branches of mechanics and bioscience. For many years, biomechanics has examined tissue. Through advancements in nanoscience, the scale of the forces that could be measured and also the scale of observation of biomaterials was reduced to "nano" and "pico" level. Consequently, it became possible to measure the mechanical properties of biological materials at nanoscale. Most of the biological materials have different hierarchical levels, and the smallest ones refer to the nanoscale. For example, bone has up to seven levels of biological organization, and the smallest level, i.e., single collagen fibril and hydroxylapatite minerals have dimensions well below 100 nm. Therefore, being able to probe properties at this small scales provides a great opportunity for better understanding the fundamental properties of these materials. For example, measurements have shown that nanomechanical heterogeneity exists even within single collagen fibrils as small as 100 nm. One of the other most relevant topics in this field is measurement of tiny forces on living cells to recognize changes caused by different diseases. For example, it has been shown that red blood cells infected by malaria are 10 times stiffer than normal cells | https://en.wikipedia.org/wiki?curid=30737379 |
Nanobiomechanics Likewise, it has been shown that cancer cells are 70 percent softer than normal cells. Early signs of aging cartilage and osteoarthritis has been shown by looking at the changes in the tissue at the nanoscale. The common methods in nanobiomechanics are atomic force microscope, optical tweezers, and magnetic twisting cytometry. Examples of relevant materials are bone and its hierarchical constituents such as single collagen fibrils, single living cells, actin filaments and microtubules, and synthetic peptide nanotubes. In addition to experimental aspect, research has been expanding through computational methods. Molecular dynamics (MD) simulations have provided a wealth of knowledge in this area. Although, the MD simulation are still limited to a small number of atoms and molecules, due to limitation in the computational performance, they have proved to be an instrumental branch of this emerging field. | https://en.wikipedia.org/wiki?curid=30737379 |
Saturated absorption spectroscopy In experimental atomic physics, saturated absorption spectroscopy or Doppler-free spectroscopy is a set-up that enables the precise determination of the transition frequency of an atom between its ground state and an optically excited state. The accuracy to which these frequencies can be determined is, ideally, limited only by the width of the excited state, which is the inverse of the lifetime of this state. However, the samples of atomic gas that are used for that purpose are generally at room temperature, where the measured frequency distribution is highly broadened due to the Doppler effect. allows precise spectroscopy of the atomic levels without having to cool the sample down to temperatures at which the Doppler broadening is no longer relevant (which would be on the order of a few millikelvins). It is also used to lock the frequency of a laser to the precise wavelength of an atomic transmission in atomic physics experiments. According to the description of an atom interacting with the electromagnetic field, the absorption of light by the atom depends on the frequency of the incident photons. More precisely, the absorption is characterized by a Lorentzian of width Γ/2 (for reference, Γ ≈ 2π×6 MHz for common Rubidium D-line transitions). If we have a cell of atomic vapour at room temperature, then the distribution of velocity will follow a Maxwell–Boltzmann distribution where formula_2 is the number of atoms, formula_3 is the Boltzmann constant, and formula_4 is the mass of the atom | https://en.wikipedia.org/wiki?curid=30737910 |
Saturated absorption spectroscopy According to the Doppler effect formula in the case of non-relativistic speeds, where formula_6 is the frequency of the atomic transition when the atom is at rest (the one which is being probed). The value of formula_7 as a function of formula_8 and formula_9 can be inserted in the distribution of velocities. The distribution of absorption as a function of the pulsation will therefore be proportional to a Gaussian with full width at half maximum For a Rubidium atom at room temperature, Therefore, without any special trick in the experimental setup probing the maximum of absorption of an atomic vapour, the uncertainty of the measurement will be limited by the Doppler broadening and not by the fundamental width of the resonance. To overcome the problem of Doppler broadening without cooling down the sample to millikelvin temperatures, a classical—and rather general—pump-probe scheme is used. A laser with a relatively high intensity is sent through the atomic vapor, known as the pump beam. Another counter-propagating weak beam is also sent through the atoms at the same frequency, known as the probe beam. The absorption of the probe beam is recorded on a photodiode for various frequencies of the beams. Although the two beams are at the same frequency, they address different atoms due to natural thermal motion | https://en.wikipedia.org/wiki?curid=30737910 |
Saturated absorption spectroscopy If the beams are red-detuned with respect to the atomic transition frequency, then the pump beam will be absorbed by atoms moving towards the beam source, while the probe beam will be absorbed by atoms moving away from that source at the same speed in the opposite direction. If the beams are blue-detuned, the opposite occurs. If, however, the laser is approximately on resonance, these two beams address the same atoms, those with velocity vectors nearly perpendicular to the direction of laser propagation. In the two-state approximation of an atomic transition, the strong pump beam will cause many of the atoms to be in the excited state; when the number of atoms in the ground state and the excited state are approximately equal, the transition is said to be saturated. When a photon from the probe beam passes through the atoms there is a good chance that, if it encounters an atom, the atom will be in the excited state and will thus undergo stimulated emission, with the photon passing through the sample. Thus, as the laser frequency is swept across the resonance, a small dip in the absorption feature will be observed at each atomic transition (generally hyperfine resonances). The stronger the pump beam, the wider and deeper the dips in the Gaussian Doppler-broadened absorption feature become. Under perfect conditions, the width of the dip can approach the natural linewidth of the transition | https://en.wikipedia.org/wiki?curid=30737910 |
Saturated absorption spectroscopy A consequence of this method of counter-propagating beams on a system with more than two states is the presence of crossover lines. When two transitions are within a single Doppler-broadened feature and share a common ground state, a crossover peak at a frequency exactly between the two transitions can occur. This is the result of moving atoms seeing the pump and probe beams resonant with two separate transitions. The pump beam can cause the ground state to be depopulated, saturating one transition, while the probe beam finds much fewer atoms in the ground state because of this saturation and its absorption falls. These crossover peaks can be quite strong, often stronger than the main saturated absorption peaks. As the pump and the probe beam must have the same exact frequency, the most convenient solution is for them to come from the same laser. The probe beam can be made of a reflection of the pump beam passed through neutral density filter to reduce its intensity. To fine-tune the frequency of the laser, a diode laser with a piezoelectric transducer that controls the cavity wavelength can be used. Due to photodiode noise, the laser frequency can be swept across the transition and the photodiode reading averaged over many sweeps. In real atoms, there are sometimes more than two relevant transitions within the sample's Doppler profile (e.g. in alkali atoms with hyperfine interactions) | https://en.wikipedia.org/wiki?curid=30737910 |
Saturated absorption spectroscopy This will generate the apparition of other dips in the absorption feature due to these new resonances in addition to crossover resonances. | https://en.wikipedia.org/wiki?curid=30737910 |
Peter Maitlis Peter Michael Maitlis, FRS (born 15 January 1933) is a retired British organometallic chemist. Maitlis was born into a middle-class Jewish family in London, England. He was educated at Hendon School (then Hendon County School) in north London 1944–50. He was awarded a Bachelor's degree in Science from the University of Birmingham, and a PhD (1956, studying under Professor Michael J. S. Dewar, who helped to develop the Dewar–Chatt–Duncanson model for bonding in organometallic compounds) and a DSc (1970) from the University of London. After completing his doctorate, Maitlis worked as an Assistant Lecturer at the University of London. He undertook postdoctoral study at Cornell University as a Fulbright Fellow (1960–1961) and then as a research fellow at Harvard University (1961–1962) under F. G. A. Stone. While working and teaching at McMaster University in Hamilton, Ontario (1962–1972), he rose from Assistant Professor to a full Professorship. Returning to the United Kingdom in 1972, Maitlis was a professor of chemistry at the University of Sheffield for 30 years until his appointment as an emeritus professor in 2002. In 1971, he published two volumes on the organic chemistry of palladium which were "widely recognised as the most authoritative account of the organo-complexes of this metal". Maitlis was elected a Fellow of the Royal Society in 1984. The citation highlights his work on the platinum group metals palladium, rhodium and iridium | https://en.wikipedia.org/wiki?curid=30738637 |
Peter Maitlis The hexafluorophosphate ion is generally considered inert and hence a suitable counterion in organometallic synthesis. However, Maitlis' work has demonstrated a solvolysis reaction of the hexafluorophosphate ion. The tris(solvent) rhodium complex [(η-CMe)Rh(MeCO)](PF) undergoes solvolysis when heated in acetone, forming a difluorophosphate-bridged complex [(η-CMe)Rh(μ-OPFO)Rh(η-CMe)]PF. Hexamethyl Dewar benzene (CMe) undergoes an unusual rearrangement reaction with hydrohalic acids to form a pentamethylcyclopentadiene derivative, and consequently can be used as a starting material for synthesising some pentamethylcyclopentadienyl organometallic compounds. Maitlis and colleagues demonstrated this synthesis and its applicability to the iridium analogue, [(η-CMe)IrCl]. His group also demonstrated a more convenient synthesis for the bright orange, air-stable diamagnetic iridium reagent using pentamethylcyclopentadiene. Isocyanides can serves as ligands in co-ordination chemistry as a result of the lone electron pair on carbon, and are especially useful with metals in the 0, +1, and +2 oxidation states. In particular, Maitlis has demonstrated that "tert"-butyl isocyanide can stabilise metals in unusual oxidation states, such as palladium(I) in the complex [("t"-BuNC)Pd(μ-Cl)] | https://en.wikipedia.org/wiki?curid=30738637 |
Peter Maitlis Metallomesogens are "metal complexes of organic ligands which exhibit liquid crystalline (mesomorphic) character [and thus they] combine the variety and range of metal-based coordination chemistry with the extraordinary physical properties exhibited by liquid crystals." They have been a research interest of Maitlis' group since the mid-1980s, and in fact Maitlis jointly directed the early investigations of these systems in the UK and actually coined the term "metallomesogen". Maitlis is Jewish. He is the father of the journalist and newsreader Emily Maitlis. The following list shows all journal articles by Maitlis which have been cited more than 200 times according to Web of Science data. The number of citations indicated is current as at 4 February 2011: | https://en.wikipedia.org/wiki?curid=30738637 |
Meprin A (, "endopeptidase-2", "meprin-a", "meprin", "N-benzoyl-L-tyrosyl-p-aminobenzoic acid hydrolase", "PABA-peptide hydrolase", "PPH") is an enzyme that cleaves protein and peptide substrates preferentially on carboxyl side of hydrophobic residues. is a dimer composed of the products transcribed from the following two genes: | https://en.wikipedia.org/wiki?curid=30740145 |
Plutonyl The plutonyl ion is an oxycation of plutonium in the oxidation state +6, with the chemical formula . It is isostructural with the uranyl ion, compared to which it has a slightly shorter M–O bond. It is easily reduced to plutonium(III). The plutonyl ion forms many complexes, particularly with ligands that have oxygen donor atoms. Complexes of the plutonyl ion are important in nuclear fuel reprocessing. The chemistry of the plutonyl ion resembles the chemistry of the uranyl ion very closely. Both ions are linear with the metal atom midway between the two oxygen atoms. Many compounds of the two ions are isostructural. The "ν" asymmetric stretching frequency is some 20 cm lower, at ca. 910 cm, in complexes with the same ligand set. From this it can be inferred that the Pu–O bond is only a little weaker than the U–O bond. The electronic structures are also similar. In aqueous solution there are some differences in hydrolysis behaviour, not only in the log "β"* values (definition of "β"*) but in nature of the polymeric species that can be formed. In the table below, 1,2 stoichiometry means a species with one actinyl ion and two hydroxide ions, etc. This is one of the few instances of notable differences between plutonyl and uranyl. Distinct optical absorbance bands at 842 and 845 nm were observed for the mononuclear and dinuclear hydrolysis species. Hydrolysis of plutonyl is important for an understanding of pollution of natural waters | https://en.wikipedia.org/wiki?curid=30740377 |
Plutonyl Another significant difference is that plutonyl is a much stronger oxidizing agent than uranyl. The standard reduction potentials for aqueous solutions are shown in the next table. Conversely, plutonyl is more easily reduced than uranyl. This difference is utilized in the separation of plutonium from uranium in the PUREX process, as described below. The plutonyl ion is always associated with other ligands. The most common arrangement is for the so-called equatorial ligands to lie in a plane perpendicular to the O–Pu–O line and passing through the plutonium atom. With four ligands, as in [PuOCl] the plutonium has a distorted octahedral environment, with a square of ligand atoms in the equatorial plane. In plutonyl nitrate, PuO(NO)2HO, as in uranyl nitrate there is a hexagon of six ligand atoms in the equatorial plane, four oxygen atoms from bidentate nitrate ions and two oxygens from the water molecules. nitrate, like uranyl nitrate, is soluble in diethyl ether. The complex that is extracted has no electrical charge. This is the most important factor in making the complex soluble in organic solvents. Also the water molecules are replaced by ether molecules. Replacing the water molecules that are bound to the plutonyl ion in aqueous solution by a second, hydrophobic, ligand increases the solubility of the neutral complex in the organic solvent. This has been called a synergic effect. The solubility of plutonyl nitrate in organic solvents is utilized in the PUREX process | https://en.wikipedia.org/wiki?curid=30740377 |
Plutonyl nitrate is extracted with tributyl phosphate, (CHCHCHCHO)PO, TBP, as the preferred second ligand, and kerosene the preferred organic solvent. It is recovered by treatment with aqueous ferrous sulfamate which selectively reduces the plutonium to the +3 oxidation state in the aqueous solution, leaving the uranium in the organic phase. complex chemistry is an active research area, for dealing with environmental contamination. | https://en.wikipedia.org/wiki?curid=30740377 |
High-refractive-index polymer A high-refractive-index polymer (HRIP) is a polymer that has a refractive index greater than 1.50. Such materials are required for anti-reflective coating and photonic devices such as light emitting diodes (LEDs) and image sensors. The refractive index of a polymer is based on several factors which include polarizability, chain flexibility, molecular geometry and the polymer backbone orientation. As of 2004, the highest refractive index for a polymer was 1.76. Substituents with high molar fractions or high-n nanoparticles in a polymer matrix have been introduced to increase the refractive index in polymers. A typical polymer has a refractive index of 1.30–1.70, but a higher refractive index is often required for specific applications. The refractive index is related to the molar refractivity, structure and weight of the monomer. In general, high molar refractivity and low molar volumes increase the refractive index of the polymer. Optical dispersion is an important property of an HRIP. It is characterized by the Abbe number. A high refractive index material will generally have a small Abbe number, or a high optical dispersion. A low birefringence has been required along with a high refractive index for many applications. It can be achieved by using different functional groups in the initial monomer to make the HRIP. Aromatic monomers both increase refractive index and decrease the optical anisotropy and thus the birefringence | https://en.wikipedia.org/wiki?curid=30747791 |
High-refractive-index polymer A high clarity (optical transparency) is also desired in a high refractive index polymer. The clarity is dependent on the refractive indexes of the polymer and of the initial monomer. When looking at thermal stability, the typical variables measured include glass transition, initial decomposition temperature, degradation temperature and the melting temperature range. The thermal stability can be measured by thermogravimetric analysis and differential scanning calorimetry. Polyesters are considered thermally stable with a degradation temperature of 410 °C. The decomposition temperature changes depending on the substituent that is attached to the monomer used in the polymerization of the high refractive index polymer. Thus, longer alkyl substituents results in lower thermal stability. Most applications favor polymers which are soluble in as many solvents as possible. Highly refractive polyesters and polyimides are soluble in common organic solvents such as dichloromethane, methanol, hexanes, acetone and toluene. The synthesis route depends on the HRIP type. The Michael polyaddition is used for a polyimide because it can be carried out at room temperature and can used for step-growth polymerization. This synthesis was first succeeded with polyimidothiethers, resulting in optically transparent polymers with high refractive index. Polycondensation reactions are also common to make high refractive index polymers, such as polyesters and polyphosphonates | https://en.wikipedia.org/wiki?curid=30747791 |
High-refractive-index polymer High refractive indices have been achieved either by introducing substituents with high molar refractions (intrinsic HRIPs) or by combining high-n nanoparticles with polymer matrixes (HRIP nanocomposites). Sulfur-containing substituents including linear thioether and sulfone, cyclic thiophene, thiadiazole and thianthrene are the most commonly used groups for increasing refractive index of a polymer. Polymers with sulfur-rich thianthrene and tetrathiaanthracene moieties exhibit n values above 1.72, depending on the degree of molecular packing. Halogen elements, especially bromine and iodine, were the earliest components used for developing HRIPs. In 1992, Gaudiana "et al." reported a series of polymethylacrylate compounds containing lateral brominated and iodinated carbazole rings. They had refractive indices of 1.67–1.77 depending on the components and numbers of the halogen substituents. However, recent applications of halogen elements in microelectronics have been severely limited by the WEEE directive and RoHS legislation adopted by the European Union to reduce potential pollution of the environment. Phosphorus-containing groups, such as phosphonates and phosphazenes, often exhibit high molar refractivity and optical transmittance in the visible light region. Polyphosphonates have high refractive indices due to the phosphorus moiety even if they have chemical structures analogous to polycarbonates. Shaver "et al | https://en.wikipedia.org/wiki?curid=30747791 |
High-refractive-index polymer " reported a series of polyphosphonates with varying backbones, reaching the highest refractive index reported for polyphosphonates at 1.66. In addition, polyphosphonates exhibit good thermal stability and optical transparency; they are also suitable for casting into plastic lenses. Organometallic components result in HRIPs with good film forming ability and relatively low optical dispersion. Polyferrocenylsilanes and polyferrocenes containing phosphorus spacers and phenyl side chains show unusually high n values (n=1.74 and n=1.72). They might be good candidates for all-polymer photonic devices because of their intermediate optical dispersion between organic polymers and inorganic glasses. Hybrid techniques which combine an organic polymer matrix with highly refractive inorganic nanoparticles could result in high n values. The factors affecting the refractive index of a high-n nanocomposite include the characteristics of the polymer matrix, nanoparticles and the hybrid technology between inorganic and organic components. The refractive index of a nanocomposite can be estimated as formula_1, where formula_2, formula_3 and formula_4 stand for the refractive indices of the nanocomposite, nanoparticle and organic matrix, respectively. formula_5 and formula_6 represent the volume fractions of the nanoparticles and organic matrix, respectively | https://en.wikipedia.org/wiki?curid=30747791 |
High-refractive-index polymer The nanoparticle load is also important in designing HRIP nanocomposites for optical applications, because excessive concentrations increase the optical loss and decrease the processability of the nanocomposites. The choice of nanoparticles is often influenced by their size and surface characteristics. In order to increase optical transparency and reduce Rayleigh scattering of the nanocomposite, the diameter of the nanoparticle should be below 25 nm. Direct mixing of nanoparticles with the polymer matrix often results in the undesirable aggregation of nanoparticles – this is avoided by modifying their surface. The most commonly used nanoparticles for HRIPs include TiO (anatase, n=2.45; rutile, n=2.70), ZrO (n=2.10), amorphous silicon (n=4.23), PbS (n=4.20) and ZnS (n=2.36). Polyimides have high refractive indexes and thus are often used as the matrix for high-n nanoparticles. The resulting nanocomposites exhibit a tunable refractive index ranging from 1.57 to 1.99. A microlens array is a key component of optoelectronics, optical communications, CMOS image sensors and displays. Polymer-based microlenses are easier to make and are more flexible than conventional glass-based lenses. The resulting devices use less power, are smaller in size and are cheaper to produce. Another application of HRIPs is in immersion lithography. In 2009 it was a new technique for circuit manufacturing using both photoresists and high refractive index fluids. The photoresist needs to have an n value of greater than 1.90 | https://en.wikipedia.org/wiki?curid=30747791 |
High-refractive-index polymer It has been shown that non-aromatic, sulfur-containing HRIPs are the best materials for an optical photoresist system. Light-emitting diodes (LEDs) are a common solid-state light source. High-brightness LEDs (HBLEDs) are often limited by the relatively low light extraction efficiency due to the mismatch of the refractive indices between the LED material (GaN, n=2.5) and the organic encapsulant (epoxy or silicone, n=1.5). Higher light outputs can be achieved by using an HRIP as the encapsulant. | https://en.wikipedia.org/wiki?curid=30747791 |
Plasma polymerization (or glow discharge polymerization) uses plasma sources to generate a gas discharge that provides energy to activate or fragment gaseous or liquid monomer, often containing a vinyl group, in order to initiate polymerization. Polymers formed from this technique are generally highly branched and highly cross-linked, and adhere to solid surfaces well. The biggest advantage to this process is that polymers can be directly attached to a desired surface while the chains are growing, which reduces steps necessary for other coating processes such as grafting. This is very useful for pinhole-free coatings of 100 picometers to 1 micrometre thickness with solvent insoluble polymers. In as early as the 1870s “polymers” formed by this process were known, but these polymers were initially thought of as undesirable byproducts associated with electric discharge, with little attention being given to their properties. It was not until the 1960s that the properties of these polymers where found to be useful. It was found that flawless thin polymeric coatings could be formed on metals, although for very thin films (<10mm) this has recently been shown to be an oversimplification. By selecting the monomer type and the energy density per monomer, known as the Yasuda parameter, the chemical composition and structure of the resulting thin film can be varied with a wide range. These films are usually inert, adhesive, and have low dielectric constants | https://en.wikipedia.org/wiki?curid=30747793 |
Plasma polymerization Some common monomers polymerized by this method include styrene, ethylene, methacrylate and pyridine, just to name a few. The 1970s brought about many advances in plasma polymerization, including the polymerization of many different types of monomers. The mechanisms of deposition however were largely ignored until more recently. Since this time most attention devoted to plasma polymerization has been in the fields of coatings, but since it is difficult to control polymer structure, it has limited applications. Plasma consists of a mixture of electrons, ions, radicals, neutrals and photons. Some of these species are in local thermodynamic equilibrium, while others are not. Even for simple gases like argon this mixture can be complex. For plasmas of organic monomers, the complexity can rapidly increase as some components of the plasma fragment, while others interact and form larger species. Glow discharge is a technique in polymerization which forms free electrons which gain energy from an electric field, and then lose energy through collisions with neutral molecules in the gas phase. This leads to many chemically reactive species, which then lead to a plasma polymerization reaction. The electric discharge process for plasma polymerization is the “low-temperature plasma” method, because higher temperatures cause degradation. These plasmas are formed by a direct current, alternating current or radio frequency generator | https://en.wikipedia.org/wiki?curid=30747793 |
Plasma polymerization There are a few designs for apparatus used in plasma polymerization, one of which is the Bell (static type), in which monomer gas is put into the reaction chamber, but does not flow through the chamber. It comes in and polymerizes without removal. This type of reactor is shown in Figure 1. This reactor has internal electrodes, and polymerization commonly takes place on the cathode side. All devices contain the thermostatic bath, which is used to regulate temperature, and a vacuum to regulate pressure. Operation: The monomer gas comes into the Bell type reactor as a gaseous species, and then is put into the plasma state by the electrodes, in which the plasma may consist of radicals, anions and cations. These monomers are then polymerized on the cathode surface, or some other surface placed in the apparatus by different mechanisms of which details are discussed below. The deposited polymers then propagate off the surface and form growing chains with seemingly uniform consistency. Another popular reactor type is the flow through reactor (continuous flow reactor), which also has internal electrodes, but this reactor allows monomer gas to flow through the reaction chamber as its name implies, which should give a more even coating for polymer film deposition. It has the advantage that more monomer keeps flowing into the reactor in order to deposit more polymer. It has the disadvantage of forming what is called “tail flame,” which is when polymerization extends into the vacuum line | https://en.wikipedia.org/wiki?curid=30747793 |
Plasma polymerization A third popular type of reactor is the electrodeless. This uses an RF coil wrapped around the glass apparatus, which then uses a radio frequency generator to form the plasma inside of the housing without the use of direct electrodes (see Inductively Coupled Plasma). The polymer can then be deposited as it is pushed through this RF coil toward the vacuum end of the apparatus. This has the advantage of not having polymer building up on the electrode surface, which is desirable when polymerizing onto other surfaces. A fourth type of system growing in popularity is the atmospheric-pressure plasma system, which is useful for depositing thin polymer films. This system bypasses the requirements for special hardware involving vacuums, which then makes it favorable for integrated industrial use. It has been shown that polymers formed at atmospheric-pressure can have similar properties for coatings as those found in the low-pressure systems. The formation of a plasma for polymerization depends on many of the following. An electron energy of 1–10 eV is required, with electron densities of 10 to 10 per cubic centimeter, in order to form the desired plasma state | https://en.wikipedia.org/wiki?curid=30747793 |
Plasma polymerization The formation of a low-temperature plasma is important; the electron temperatures are not equal to the gas temperatures and have a ratio of T/T of 10 to 100, so that this process can occur at near ambient temperatures, which is advantageous because polymers degrade at high temperatures, so if a high-temperature plasma was used the polymers would degrade after formation or would never be formed. This entails non-equilibrium plasmas, which means that charged monomer species have more kinetic energy than neutral monomer species, and cause the transfer of energy to a substrate instead of an uncharged monomer. The kinetic rate of these reactions depends mostly on the monomer gas, which must be either gaseous or vaporized. However, other parameters are also important as well, such as power, pressure, flow rate, frequency, electrode gap and reactor configuration. Low flow rates usually only depend on the amount of reactive species present for polymerization, whereas high flow rates depend on the amount of time that is spent in the reactor. Therefore, the maximum rate of polymerization is somewhere in the middle. The fastest reactions tend to be in the order of triple-bonded > double-bonded > single bonded molecules, and also lower molecular weight molecules are faster than higher ones. So acetylene is faster than ethylene, and ethylene is faster than propene, etc | https://en.wikipedia.org/wiki?curid=30747793 |
Plasma polymerization The molecular weight factor in polymer deposition is dependent on the monomer flow rate, in which a higher molecular weight monomer typically near 200 g/mol needs a much higher flow rate of 15 g/cm, whereas lower molecular weights around 50 g/mol require a flow rate of only 5 g/cm. A heavy monomer therefore needs a faster flow, and would likely lead to increased pressures, decreasing polymerization rates. Increased pressure tends to decrease polymerization rates reducing uniformity of deposition since uniformity is controlled by constant pressure. This is a reason that high-pressure plasma or atmospheric-pressure plasmas are not usually used in favor of low-pressure systems. At pressures greater than 1 torr, oligomers are formed on the electrode surface, and the monomers also on the surface can dissolve them to get a low degree of polymerization forming an oily substance. At low pressures, the reactive surfaces are low in monomer and facilitate growing high molecular weight polymers. The rate of polymerization depends on input power, until power saturation occurs and the rate becomes independent of it. A narrower electrode gap also tends to increase polymerization rates because a higher electron density per unit area is formed. Polymerization rates also depend on the type of apparatus used for the process. In general, increasing the frequency of alternating current glow discharge up to about 5 kHz increases the rate due to the formation of more free radicals | https://en.wikipedia.org/wiki?curid=30747793 |
Plasma polymerization After this frequency, inertial effects of colliding monomers inhibit polymerization. This forms the first plateau for polymerization frequencies. A second maximum in frequency occurs at 6 MHz, where side reactions are overcome again and the reaction occurs through free radicals diffused from plasma to the electrodes, at which point a second plateau is obtained. These parameters differ slightly for each monomer and must be optimized in-situ. Plasma contains many species such as ions, free radicals and electrons, so it is important to look at what contributes to the polymerization process most. The first suggested process by Westwood et al. was that of a cationic polymerization, since in a direct current system polymerization occurs mainly on the cathode. However, more investigation has led to the belief that the mechanism is more of a radical polymerization process, since radicals tend to be trapped in the films, and termination can be overcome by reinitiation of oligomers. Other kinetic studies also appear to support this theory. However, since the mid 1990s a number of papers focusing on the formation of highly functionalized plasma polymers have postulated a more significant role for cations, particularly where the plasma sheath is collosionless. The assumption that the plasma ion density is low and consequently the ion flux to surfaces is low has been challenged, pointing out that ion flux is determined according to the Bohm sheath criterion i.e | https://en.wikipedia.org/wiki?curid=30747793 |
Plasma polymerization ion flux is proportional to the square root of the electron temperature and not RT. In polymerization, both gas phase and surface reactions occur, but mechanism differs between high and low frequencies. At high frequencies it occurs in reactive intermediates, whereas at low frequencies polymerization happens mainly on surfaces. As polymerization occurs, the pressure inside the chamber decreases in a closed system, since gas phase monomers go to solid polymers. An example diagram of the ways that polymerization can take place is shown in Figure 2, wherein the most abundant pathway is shown in blue with double arrows, with side pathways shown in black. The ablation occurs by gas formation during polymerization. Polymerization has two pathways, either the plasma state or plasma induced processes, which both lead to deposited polymer. Polymers can be deposited on many substrates other than the electrode surfaces, such as glass, other organic polymers or metals, when either a surface is placed in front of the electrodes, or placed in the middle between them. The ability for them to build off of electrode surfaces is likely to be an electrostatic interaction, while on other surfaces covalent attachment is possible. Polymerization is likely to take place through either ionic and/or radical processes which are initiated by plasma formed from the glow discharge | https://en.wikipedia.org/wiki?curid=30747793 |
Plasma polymerization The classic view presented by Yasuda based upon thermal initiation of Parylene polymerization is that there are many propagating species present at any given time as shown in Figure 3. This figure shows two different pathways by which the polymerization may take place. The first pathway is a monofunctionalization process, bears resemblance to a standard free radical polymerization mechanism (M•)- although with the caveat that the reactive species may be ionic and not necessarily radical. The second pathway refers to a difunctional mechanism, which by example may contain a cationic and a radical propagating center on the same monomer (•M•). A consequence is that 'polymer' can grow in multiple directions by multiple pathways off one species, such as a surface or other monomer. This possibility let Yasuda to term the mechanism as a very rapid step-growth polymerization. In the diagram, M refers to the original monomer molecule or any of many dissociation products such as chlorine, fluorine and hydrogen. The M• species refers to those that are activated and capable of participating in reactions to form new covalent bonds. The •M• species refers to an activated difunctional monomer species. The subscripts i, j, and k show the sizes of the different species involved. Even though radicals represent the activated species, any ion or radical could be used in the polymerization. As can be seen here, plasma polymerization is a very complex process, with many parameters effecting everything from rate to chain length | https://en.wikipedia.org/wiki?curid=30747793 |
Plasma polymerization Selection, or the favouring of one particular pathway can be achieved by altering the plasma parameters. For example, pulsed plasma with selected monomers appears to favour much more regular polymer structures and it has been postulated these grow by a mechanism akin to (radical) chain growth in the plasma off-time. As can be seen in the monomer table, many simple monomers are readily polymerized by this method, but most must be smaller ionizable species because they have to be able to go into the plasma state. Though monomers with multiple bonds polymerize readily, it is not a necessary requirement, as ethane, silicones and many others polymerize also. There are also other stipulations that exist. Yasuda et al. studied 28 monomers and found that those containing aromatic groups, silicon, olefinic group or nitrogen (NH, NH, CN) were readily polymerizable, while those containing oxygen, halides, aliphatic hydrocarbons and cyclic hydrocarbons where decomposed more readily. The latter compounds have more ablation or side reactions present, which inhibit stable polymer formation. It is also possible to incorporate N, HO, and CO into copolymers of styrene. Plasma polymers can be thought of as a type of graft polymers since they are grown off of a substrate. These polymers are known to form nearly uniform surface deposition, which is one of their desirable properties. Polymers formed from this process often cross-link and form branches due to the multiple propagating species present in the plasma | https://en.wikipedia.org/wiki?curid=30747793 |
Plasma polymerization This often leads to very insoluble polymers, which gives an advantage to this process, since "hyperbranched polymers" can be deposited directly without solvent. Common polymers include: polythiophene, polyhexafluoropropylene, polytetramethyltin, polyhexamethyldisiloxane, polytetramethyldisiloxane, polypyridine, polyfuran, and poly-2-methyloxazoline. The following are listed in order of decreasing rate of polymerization: polystyrene, polymethyl styrene, polycyclopentadiene, polyacrylate, polyethyl acrylate, polymethyl methacrylate, polyvinyl acetate, polyisoprene, polyisobutene, and polyethylene. Nearly all polymers created by this method have excellent appearance, are clear, and are significantly cross-linked. Linear polymers are not formed readily by plasma polymerization methods based on propagating species. Many other polymers could be formed by this method. The properties of plasma polymers differ greatly from those of conventional polymers. While both types are dependent on the chemical properties of the monomer, the properties of plasma polymers depend more greatly on the design of the reactor and the chemical and physical characteristics of the substrate on which the plasma polymer is deposited. The location within the reactor where the deposition occurs also has an effect on the resultant polymer's properties. In fact by using plasma polymerization with a single monomer and varying the reactor, substrate, etc | https://en.wikipedia.org/wiki?curid=30747793 |
Plasma polymerization a variety of polymers, each having different physical and chemical properties, can be prepared. The large dependence of the polymer features on these factors make it difficult to assign a set of basic characteristics, but a few common properties that set plasma polymers apart from conventional polymers do exist. The most significant difference between conventional polymers and plasma polymers is that plasma polymers do not contain regular repeating units. Due to the number of different propagating species present at any one time as discussed above, the resultant polymer chains are highly branched and are randomly terminated with a high degree of cross-linking. An example of a proposed structure for plasma polymerized ethylene demonstrating a large extend of cross-linking and branching is shown in Figure 4. All plasma polymers contain free radicals as well. The amount of free radicals present varies between polymers and is dependent on the chemical structure of the monomer. Because the formation of the trapped free radicals is tied to the growth mechanism of the plasma polymers, the overall properties of the polymers directly correlate to the number of free radicals. Plasma polymers also contain an internal stress. If a thick layer (e.g. 1 µm) of a plasma polymer is deposited on a glass slide, the plasma polymer will buckle and frequently crack. The curling is attributed to an internal stress formed in the plasma polymer during the polymer deposition | https://en.wikipedia.org/wiki?curid=30747793 |
Plasma polymerization The degree of curling is dependent on the monomer as well as the conditions of the plasma polymerization. Most plasma polymers are insoluble and infusible. These properties are due to the large amount of cross-linking in the polymers, previously discussed. Consequently, the kinetic path length for these polymers must be sufficiently long, so these properties can be controlled to a point. The permeabilities of plasma polymers also differ greatly from those of conventional polymers. Because of the absence of large-scale segmental mobility and the high degree of cross-linking within the polymers, the permeation of small molecules does not strictly follow the typical mechanisms of “solution-diffusion” or molecular-level sieve for such small permeants. Really the permeability characteristics of plasma polymers falls between these two ideal cases. A final common characteristic of plasma polymers is the adhesion ability. The specifics of the adhesion ability for a given plasma polymer, such as thickness and characteristics of the surface layer, again are particular for a given plasma polymer and few generalizations can be made. offers a number of advantages over other polymerization methods and in general. The most significant advantage of plasma polymerization is its ability to produce polymer films of organic compounds that do not polymerize under normal chemical polymerization conditions | https://en.wikipedia.org/wiki?curid=30747793 |
Plasma polymerization Nearly all monomers, even saturated hydrocarbons and organic compounds without a polymerizable structure such as a double bond, can be polymerized with this technique. A second advantage is the ease of application of the polymers as coatings versus conventional coating processes. While coating a substrate with conventional polymers requires a number of steps, plasma polymerization accomplishes all these in essentially a single step. This leads to a cleaner and ‘greener’ synthesis and coating process, since no solvent is needed during the polymer preparation and no cleaning of the resultant polymer is needed either. Another ‘green’ aspect of the synthesis is that no initiator is needed for the polymer preparation since reusable electrodes cause the reaction to proceed. The resultant polymer coatings also have a number of advantages over typical coatings. These advantages include being nearly pinhole free, highly dense, and that the thickness of the coating can easily be varied. There are also a number of disadvantages relating to plasma polymerization versus conventional methods. The most significant disadvantage is the high cost of the process. A vacuum system is required for the polymerization, significantly increasing the set up price. Another disadvantage is due to the complexity of plasma processes. Because of the complexity it is not easy to achieve a good control over the chemical composition of the surface after modification | https://en.wikipedia.org/wiki?curid=30747793 |
Plasma polymerization The influence of process parameters on the chemical composition of the resultant polymer means it can take a long time to determine the optimal conditions. The complexity of the process also makes it impossible to theorize what the resultant polymer will look like, unlike conventional polymers which can be easily determined based on the monomer. The advantages offered by plasma polymerization have resulted in substantial research on the applications of these polymers. The vastly different chemical and mechanical properties offered by polymers formed with plasma polymerization means they can be applied to countless different systems. Applications ranging from adhesion, composite materials, protective coatings, printing, membranes, biomedical applications, water purification and so on have all been studied. Of particular interest since the 1980s has been the deposition of functionalized plasma polymer films. For example, functionalized films are used as a means of improving biocompatibility for biological implants6 and for producing super-hydrophobic coatings. They have also been extensively employed in biomaterials for cell attachment, protein binding and as anti-fouling surfaces. Through the use of low power and pressure plasma, high functional retention can be achieved which has led to substantial improvements in the biocompatibility of some products, a simple example being the development of extended wear contact lenses | https://en.wikipedia.org/wiki?curid=30747793 |
Plasma polymerization Due to these successes, the huge potential of functional plasma polymers is slowly being realised by workers in previously unrelated fields such as water treatment and wound management. Emerging technologies such as nanopatterning, 3D scaffolds, micro-channel coating and microencapsulation are now also utilizing functionalized plasma polymers, areas for which traditional polymers are often unsuitable A significant area of research has been on the use of plasma polymer films as permeation membranes. The permeability characteristics of plasma polymers deposited on porous substrates are different than usual polymer films. The characteristics depend on the deposition and polymerization mechanism. Plasma polymers as membranes for separation of oxygen and nitrogen, ethanol and water, and water vapor permeation have all been studied. The application of plasma polymerized thin films as reverse osmosis membranes has received considerable attention as well. Yasuda et al. have shown membranes prepared with plasma polymerization made from nitrogen containing monomers can yield up to 98% salt rejection with a flux of 6.4 gallons/ft a day. Further research has shown that varying the monomers of the membrane offer other properties as well, such as chlorine resistance. Plasma-polymerized films have also found electrical applications | https://en.wikipedia.org/wiki?curid=30747793 |
Plasma polymerization Given that plasma polymers frequently contain many polar groups, which form when the radicals react with oxygen in air during the polymerization process, the plasma polymers were expected to be good dielectric materials in thin film form. Studies have shown that the plasma polymers generally do in fact have a higher dielectric property. Some plasma polymers have been applied as chemical sensory devices due to their electrical properties. Plasma polymers have been studied as chemical sensory devices for humidity, propane, and carbon dioxide amongst others. Thus far issues with instability against aging and humidity have limited their commercial applications. The application of plasma polymers as coatings has also been studied. Plasma polymers formed from tetramethoxysilane have been studied as protective coatings and have shown to increase the hardness of polyethylene and polycarbonate. The use of plasma polymers to coat plastic lenses is increasing in popularity. Plasma depositions are able to easily coat curved materials with a good uniformity, such as those of bifocals. The different plasma polymers used can be not only scratch resistant, but also hydrophobic leading to anti-fogging effects. Plasma polymer surfaces with tunable wettability and reversibly switchable pH-responsiveness have shown the promising prospects due to their unique property in applications, such as drug delivery, biomaterial engineering, oil/water separation processes, sensors, and biofuel cells. | https://en.wikipedia.org/wiki?curid=30747793 |
Polyfluorene Polyfluorenes are a class of polymeric materials. They are of interest because similar to other conjugated polymers, they are currently being investigated for use in light-emitting diodes, field-effect transistors, and plastic solar cells. They are not a naturally occurring material, but are designed and synthesized for their applications. Modern chemistry has enabled adaptable synthesis and control over polyfluorenes that has facilitated use in many organic electronic applications. Academic and industrial research are interested in these polymers because of their optical and electrical properties. They have high photoluminescence quantum yields. They are a prototypical conjugated polymer but they are the only class of conjugated polymers which can be tuned to emit light throughout the entire visible region. Polyfluorenes are primarily interesting because of the optoelectronic properties imbued by their chromophoric constituents and their extended conjugation. The design of polyfluorene derivatives relies on the character and properties of their monomers. Thus, the discovery and development of these polymeric repeat units has had a profound influence on the development of polyfluorenes. The physical properties of polyfluorenes differ depending on their substitution pattern. Despite the similar sounding names, polyfluorene is unrelated to the element fluorine. Fluorene, a principal repeat unit in polyfluorene derivatives, was isolated from coal tar and discovered by Marcellin Berthelot prior to 1883 | https://en.wikipedia.org/wiki?curid=30747795 |
Polyfluorene Its name originates from its interesting fluorescence, fluorene became the subject of chemical-structure related color variation (visible rather than luminescent), among other things, throughout the early to mid-20th century. Since it was an interesting chromophore researchers wanted to understand which parts of the molecule were chemically reactive, and how substituting these sites influenced the color. For instance, by adding various electron donating or electron accepting moieties to fluorene, and by reacting with bases, researchers were able to change the color of the molecule. The physical properties of the fluorene molecule were recognizably desirable for polymers; as early as the 1970s researchers began incorporating this moiety into polymers. For instance, because of fluorene’s rigid, planar shape a polymer containing fluorene was shown to exhibit enhanced thermo-mechanical stability. However, more promising was integrating the optoelectronic properties of fluorene into a polymer. Reports of the oxidative polymerization of fluorene (into a fully conjugated form) exist from at least 1972. However, it was not until after the highly publicized high conductivity of doped polyacetylene, presented in 1977 by Heeger, MacDiarmid and Shirakawa, that substantial interest in the electronic properties of conjugated polymers took off. As interest in conducting plastics grew, fluorene again found application | https://en.wikipedia.org/wiki?curid=30747795 |
Polyfluorene The aromatic nature of fluorene makes it an excellent candidate component of a conducting polymer because it can stabilize and conduct a charge; in the early 1980s fluorene was electropolymerized into conjugated polymer films with conductivities of 10 S cm. The optical properties (such as variable luminescence and visible light spectrum absorption) that accompany the extended conjugation in polymers of fluorene have become increasingly attractive for device applications. Throughout the 1990s and into the 2000s, many devices such as organic light-emitting diodes (OLEDs), organic solar cells., organic thin film transistors, and biosensors have all taken advantage of the luminescent, electronic and absorptive properties of polyfluorenes. Polyfluorenes are an important class of polymers which have the potential to act as both electroactive and photoactive materials. This in part due to the shape of fluorene. Fluorene is generally planar; p-orbital overlap at the linkage between its two benzene rings results in conjugation across the molecule. This in turn allows for a reduced band gap as the excited state molecular orbitals are delocalized. Since the degree of delocalization and the spatial location of the orbitals on the molecule is influenced by the electron donating (or withdrawing) character of its substituents, the band gap energy can be varied. This chemical control over the band gap directly influences the color of the molecule by limiting the energies of light which it absorbs | https://en.wikipedia.org/wiki?curid=30747795 |
Polyfluorene Interest in polyfluorene derivatives has increased because of their high photoluminescence quantum efficiency, high thermal stability, and their facile color tunability, obtained by introducing low-band-gap co-monomers. Research in this field has increased significantly due to its potential application in tuning organic light-emitting diodes (OLEDs). In OLEDs, polyfluorenes are desirable because they are the only family of conjugated polymers that can emit colors spanning the entire visible range with high efficiency and low operating voltage. Furthermore, polyfluorenes are relatively soluble in most solvents, making them ideal for general applications. Another important quality of polyfluorenes is their thermotropic liquid crystallinity which allows the polymers to align on rubbed polyimide layers. Thermotropic liquid crystallinity refers to the polymers' ability to exhibit a phase transition into the liquid crystal phase as the temperature is changed. This is very important to the development of liquid crystal displays (LCDs) because the synthesis of liquid crystal displays requires that the liquid-crystal molecules at the two glass surfaces of the cell be aligned parallel to the two polarizer foils. This can only be done by coating the inner-surfaces of the cell with a thin, transparent film of polyamide which is then rubbed with a velvet cloth | https://en.wikipedia.org/wiki?curid=30747795 |
Polyfluorene Microscopic grooves are then generated in the polyamide layer and the liquid crystal in contact with the polyamide, the polyfluorene, can align in the rubbing direction. In addition to LCDs, polyfluorene can also be used to synthesize light-emitting diodes (LEDs). has led to LEDs that can emit polarized light with polarization ratios of more than 20 and with brightness of 100 cd m. Even though this is very impressive, it is not sufficient for general applications. Polyfluorenes often show both excimer and aggregate formation upon thermal annealing or when current is passed through them. Excimer formation involves the generation of dimerized units of the polymer which emit light at lower energies than the polymer itself. This hinders the use of polyfluorenes for most applications, including light-emitting diodes (LED). When excimer or aggregate formation occurs this lowers the efficiency of the LEDs by decreasing the efficiency of charge carrier recombination. Excimer formation also causes a red shift in the emission spectrum. Polyfluorenes can also undergo decomposition. There are two known ways in which decomposition can occur. The first involves the oxidation of the polymer that leads to the formation of an aromatic ketone, quenching the fluorescence. The second decomposition process results in aggregation leading to a red-shifted fluorescence, reduced intensity, exciton migration and relaxation through excimers | https://en.wikipedia.org/wiki?curid=30747795 |
Polyfluorene Researchers have attempted to eliminate excimer formation and enhance the efficiency of polyfluorenes by copolymerizing polyfluorene with anthracene and end-capping polyfluorenes with bulky groups which could sterically hinder excimer formation. Additionally, researchers have tried adding large substituents at the nine position of the fluorene in order to inhibit excimer and aggregate formation. Furthermore, researchers have tried to improve LEDs by synthesizing fluorene-triarylamine copolymers and other multilayer devices that are based on polyfluorenes that can be cross-linked. These have been found to have brighter fluorescence and reasonable efficiencies. Aggregation has also been combated by varying the chemical structure. For example, when conjugated polymers aggregate, which is natural in the solid state, their emission can be self-quenched, reducing luminescent quantum yields and reducing luminescent device performance. In opposition to this tendency, researchers have used tri-functional monomers to create highly branched polyfluorenes which do not aggregate due to the bulkiness of the substituents. This design strategy has achieved luminescent quantum yields of 42% in the solid state. This solution reduces the ease of processability of the material because branched polymers have increased chain entanglement and poor solubility. Another problem commonly encountered by polyfluorenes is an observed broad green, parasitic emission which detracts from the color purity and efficiency needed for an OLED | https://en.wikipedia.org/wiki?curid=30747795 |
Polyfluorene Initially attributed to excimer emission, this green emission has been shown to be due to the formation of ketone defects along the fluorene polymer backbone (oxidation of the nine position on the monomer) when there are incomplete substitution at the nine positions of the fluorene monomer. Routes to combat this involve ensuring full substitution of the monomer’s active site, or including aromatic substituents. These solutions may present structures that lack optimal bulkiness or may be synthetically difficult. Conjugated polymers, such as polyfluorene, can be designed and synthesized with different properties for a wide variety of applications. The color of the molecules can be designed through synthetic control over the electron donating or withdrawing character of the substituents on fluorene or the comonomers in polyfluorene. Solubility of the polymers are important because solution state processing is very common. Since conjugated polymers, with their planar structure, tend to aggregate, bulky side chains are added (to the 9 position of fluorene) to increase the solubility of the polymer. The earliest polymerizations of fluorene were oxidative polymerization with AlCl or FeCl, and more commonly electropolymerization. Electropolymerization is an easy route to obtain thin, insoluble conducting polymer films. However, this technique has a few disadvantages in that does not provide controlled chain growth polymerizations, and processing and characterization are difficult as a result of its insolubility | https://en.wikipedia.org/wiki?curid=30747795 |
Polyfluorene Oxidative polymerization produces a similarly poor site-selectivity on the monomer for chain growth resulting in poor control over the regularity of the polymers structure. However, oxidative polymerization does produce soluble polymers (from side-chain containing monomers) which are more easily characterized with nuclear magnetic resonance. The design of polymeric properties requires great control over the structure of the polymer. For instance, low band gap polymers require regularly alternating electron donating and electron accepting monomers. More recently, many popular cross-coupling chemistries have been applied to polyfluorenes and have enabled controlled polymerization; Palladium-catalyzed coupling reactions such as Suzuki coupling, Heck coupling, etc., as well as nickel catalyzed Yamamoto and Grignard coupling reactions have been applied to polymerization of fluorene derivatives. Such routes have enabled excellent control over the properties of polyfluorenes; the fluorene-thiophene-benzothiadiazole copolymer shown above, with a band gap of 1.78 eV when the side chains are alkoxy, appears blue because it is absorbing in the red wavelengths. Modern coupling chemistries allow other properties of polyfluorenes to be controlled through implementation of complex molecular designs | https://en.wikipedia.org/wiki?curid=30747795 |
Polyfluorene The above polymer structure pictured has excellent photoluminescent quantum yields (partly due to its fluorene monomer) excellent stability (due to its oxadiazole comonomer) good solubility (due to its many and branched alkyl side chains) and has an amine functionalized side chain for ease of tethering to other molecules or to a substrate. The luminescent color of polyfluorenes can be changed, for example, (from blue to green-yellow) by adding functional groups which participate in excited state intramolecular proton transfer. Exchanging the alkoxy side chains for alcohol side groups allows for energy dissipation (and a red-shift in emission) through reversible transfer of a proton from the alcohol to the nitrogen (on the oxadiazole). These complicated molecular structures were engineered to have these properties and were only able to be realized through careful control of their ordering and side group functionality. In recent years many industrial efforts have focused on tuning the color of lights using polyfluorenes. It was found that by doping green or red emitting materials into polyfluorenes one could tune the color emitted by the polymers. Since polyfluorene homopolymers emit higher energy blue light, they can transfer energy via Förster resonance energy transfer (FRET) to lower energy emitters. In addition to doping, color of polyfluorenes can be tuned by copolymerizing the fluorene monomers with other low band gap monomers | https://en.wikipedia.org/wiki?curid=30747795 |
Polyfluorene Researchers at the Dow Chemical Company synthesized several fluorene-based copolymers by alternating copolymerization using 5,5-dibromo-2,2-bithiophene which showed yellow emission and 4,7-dibromo-2,1,3-benzothiadiazole, which showed green emission. Other copolymerizations are also suitable; researchers at IBM performed random copolymerization of fluorene with 3,9(10)-dibromoperylene,4,4-dibromo-R-cyanostilbene, and 1,4-bis(2-(4-bromophenyl)-1-cyanovinyl)-2-(2-ethylhexyl)-5-methoxybenzene. Only a small amount of the co-monomer, approximately 5%, was needed to tune the emission of the polyfluorene from blue to yellow. This example further illustrates that by introducing monomers that have a lower band gap than the fluorene monomer, one can tune the color that is emitted by the polymer. Substitution at the nine position with various moieties has also been examined as a means to control the color emitted by polyfluorene. In the past researchers have tried putting alkyl substituents on the ninth position, however it has been found that by putting bulkier groups, such as alkoxyphenyl groups, the polymers had enhanced blue emission stability and superior polymer light-emitting diode performance (compared to polymers which have alkyl substituents at the ninth position). Polyfluorenes are also used in polymer solar cells because of their affinity for property tuning | https://en.wikipedia.org/wiki?curid=30747795 |
Polyfluorene Copolymerization of fluorene with other monomers allows researchers to optimize the absorption and electronic energy levels as a means to increase the photovoltaic performance. For instance, by lowering the band gap of polyfluorenes, the absorption spectrum of the polymer can be adjusted to coincide with the maximum photon flux region of the solar spectrum. This helps the solar cell absorb more of the sun's energy and to increase its energy conversion efficiency; donor-acceptor structured copolymers of fluorene have achieved efficiencies above 4% when their absorption edge was pushed to 700 nm. The voltage of polymer solar cells has also been increased through the design of polyfluorenes. These devices are typically produced by blending electron accepting and electron donating molecules which help separate charge to produce power. In polymer blend solar cells, the voltage produced by the device is determined by the difference between the electron donating polymer’s highest occupied molecular orbital (HOMO) energy level and the electron accepting molecules lowest unoccupied molecular orbital (LUMO) energy level. By adding electron withdrawing pendant molecules to conjugated polymers, their HOMO energy level can be lowered. For instance by adding electronegative groups on the end of conjugated side chains, researchers lowered the HOMO of a polyfluorene copolymer to −5.30 eV and increased the voltage of a solar cell to 0.99 V | https://en.wikipedia.org/wiki?curid=30747795 |
Polyfluorene Typical polymer solar cells utilize fullerene molecules as electron acceptors because of their low LUMO energy level (high electron affinity). However the tunability of polyfluorenes allows their LUMO to be lowered to a level appropriate for use as an electron acceptor. Thus, polyfluorene copolymers have also been used in polymer:polymer blend solar cells, where their electron accepting, electron conducting and light absorbing properties permit device performance. | https://en.wikipedia.org/wiki?curid=30747795 |
C13H12N2O3 The molecular formula CHNO may refer to: | https://en.wikipedia.org/wiki?curid=30747875 |
Wilhelm Schuler (27 December 1914 in Ulm, Germany – 5 June 2010 in Bad Homburg vor der Höhe, Germany) was a chemist, inventor and entrepreneur in the second half of the 20th century. Schuler studied chemistry at the Königliche Technische Hochschule zu Danzig from 1934 onwards. After he obtained the title of Diplomchemiker (~Master of Chemistry) in 1939 he was drafted to the Wehrmacht. Just after 6 months of service he was discharged after Paul Rabe (Universität Hamburg) requested Schuler for important research projects in the field of pharmaceutical ingredients at the University of Danzig. Schuler worked in the research group of Henry Albers studying organic phosphorus compounds. In 1941 he received his PhD. He remained at the University of Danzig as assistant to Henry Albers, working on alkaloids. In 1944 Schuler started working on his professorial dissertation. Shortly before the end of World War II he joined the University of Hamburg. In the research group of Ilse Esdorn (1897–1985) he worked on the analysis of natural products, mainly alkaloids, glucosides and mucus from medicinal plants. He discovered a method for the production of a gelling agent made from heather, "erica". This method became popular in the food and cosmetics industry. Schuler licensed the process to the company Spangenberg . With this income Schuler—only 32 years old—was able to start his own business. Together with a partner he founded “Dr. Schuler & Lange, Chemisches und Pharmazeutisches Laboratorium GmbH, Hamburg” | https://en.wikipedia.org/wiki?curid=30748210 |
Wilhelm Schuler Schuler & Lange produced drug ingredients and performed pharmaceutical contract research. The main client for chemical research was the company Promonta, which later merged with Byk Gulden (today Altana). In cooperation with the biologist and pharmacologist Otto Nieschulz a couple of highly successful drugs were developed, amongst them: In 1953 Degussa AG offered Schuler a rather unusual contract which allowed him to continue and expand his "private entrepreneurial activities" whilst working for Degussa. Schuler started as head of pharmaceutical research at the Degussa subsidiary (Chemiewerk Homburg). In this position Schuler was very creative and successful. In 1959 became head of the entire Degussa research and implemented new structures to their R&D-department. Schuler focused on the development of zeolithes as components in detergents, the development of automotive catalysts and various further strategic solutions, which are paramount for Evonik Degussa GmbH and other companies until today. Schuler was an inspiring mentor for the chemists Gunther Dittrich, Rudolf Fahnenstich, Axel Kleemann, Peter Kleinschmit, Heribert Offermanns and Gerd Schreyer (selection). In 1960 Schuler further expanded his “private” activities and founded Loftus Bryan Chem. Ltd. in County Wicklow, Ireland. 20 years later the company was sold to the US pharma giant Schering-Plough. In 1982 and his daughter started a new company, Iropharm Ltd., again in Ireland | https://en.wikipedia.org/wiki?curid=30748210 |
Wilhelm Schuler It focused on the production of generic pharmaceutical ingredients and intermediates. In 1997 this company was sold to Allied Signal and was later acquired by Sigma-Aldrich. Schuler lived most of his life in Bad Homburg vor der Höhe. He is also buried there. He is survived by his daughter Beate. | https://en.wikipedia.org/wiki?curid=30748210 |
Metal sulfur dioxide complex Metal sulfur dioxide complexes are complexes that contain sulfur dioxide, SO, bonded to a transition metal. Such compounds are common but are mainly of theoretical interest. Historically, the study of these compounds has provided insights into the mechanisms of migratory insertion reactions in organometallic chemistry. Sulfur dioxide forms complexes with many transition metals. Most numerous are complexes with metals i in oxidation state 0 or +1. In most cases SO binds in monodentate fashion, attaching to the metal through sulfur. Such complexes are further subdivided according to the planarity or pyramidalization at sulfur. The various bonding modes are: More exotic bonding modes are known for clusters. Complexes of the transition metals are usually generated simply by treating the appropriate metal complex with SO. The adducts are often weak. In some cases, SO displaces other ligands. A large number of labile O-bonded SO complexes arise from the oxidation of a suspension of the metals in liquid SO, an excellent solvent. The main reaction of sulfur dioxide promoted by transition metals is its reduction by hydrogen sulfide. Known as the Claus process, this reaction is conducted on a large scale as a way to remove hydrogen sulfide that arises in hydrotreating processes in refineries. Of academic interest, SO acts like a Lewis acid towards the alkyl ligand. The pathway for the insertion of SO into metal alkyl bond begins with attack of the alkyl nucleophile on the sulfur centre in SO | https://en.wikipedia.org/wiki?curid=30748324 |
Metal sulfur dioxide complex The "insertion" proceed the sulfur dioxide between the metal and the alkyl ligand leads to the "O", "O'-"sulphinate. Alternatively an "O"-sulphinate can arise. Both of these intermediates commonly convert to an "S"-sulphinate. "S"-sulphinate has sulfur–oxygen stretching frequencies from 1250–1000 cm and 1100–1000 cm. The "O", "O'-"sulphinate and "O"-sulphinate are difficult to distinguish as they have stretching frequencies from 1085–1050 cm and 1000–820 cm or lower. The pathway involving the "O", "O' "sulphinate can generally be ruled out if the original metal complex fulfilled the 18-electron rule because the two metal–oxygen bonds would exceed the 18 electron rule. The pathway by which SO inserts into a square planar alkyl complexes involves the formation of an adduct. Thereafter, the alkyl ligand migrates to the SO. | https://en.wikipedia.org/wiki?curid=30748324 |
TeraChem is the first computational chemistry software program written completely from scratch to benefit from the new streaming processors such as graphics processing units (GPUs). The computational algorithms have been completely redesigned to exploit massive parallelism of CUDA-enabled Nvidia GPUs. The original development started at the University of Illinois at Urbana-Champaign. Due to the great potential of the developed technology, this GPU-accelerated software was subsequently commercialized. Now it is distributed by PetaChem, LLC, located in the Silicon Valley. The software package is under active development and new features are released often. Very fast "ab initio" molecular dynamics and density functional theory (DFT) methods for "nanoscale" biomolecular systems with hundreds of atoms are arguably the most attractive features of TeraChem. Its affinity to extreme performance is also exemplified in the motto ""Chemistry at the Speed of Graphics"". All the methods used are based on Gaussian orbitals, a choice made to improve performance on the limited computing capacities of modern computer hardware. More comprehensive list of features can be found on the company's website or in the user guide. The software is featured in a series of clips on its own YouTube channel under "GPUChem" user. 2017 2016 2012 2011 2010 I. S. Ufimtsev, N. Luehr and T. J. Martinez Journal of Physical Chemistry Letters, Vol. 2, 1789-1793 (2011) C. M. Isborn, N. Luehr, I. S. Ufimtsev and T. J | https://en.wikipedia.org/wiki?curid=30759885 |
TeraChem Martinez Journal of Chemical Theory and Computation, Vol. 7, 1814-1823 (2011) N. Luehr, I. S. Ufimtsev, and T. J. Martinez Journal of Chemical Theory and Computation, Vol. 7, 949-954 (2011) I. S. Ufimtsev and T. J. Martinez Journal of Chemical Theory and Computation, Vol. 5, 2619-2628 (2009) I. S. Ufimtsev and T. J. Martinez Journal of Chemical Theory and Computation, Vol. 5, 1004-1015 (2009) I. S. Ufimtsev and T. J. Martinez Journal of Chemical Theory and Computation, Vol. 4, 222-231 (2008) I. S. Ufimtsev and T. J. Martinez Computing in Science and Engineering, Vol. 10, 26-34 (2008) Nirupam Aich, Joseph R V Flora and Navid B Saleh Nanotechnology, Vol. 23, 055705 (2012) Kregg D. Quarles, Cherno B. Kah, Rosi N. Gunasinghe, Ryza N. Musin, and Xiao-Qian Wang Journal of Chemical Theory Computation, Vol. 7, 2017–2020 (2011) M. P. Andersson and S. L. S. Stipp Journal of Physical Chemistry C, Vol. 115, 10044–10055 (2011) Rosi N. Gunasinghe, Cherno B. Kah, Kregg D. Quarles, and Xiao-Qian Wang Applied Physics Letters 98, 261906 (2011) Xiao-Qian Wang Physical Review B 82, 153409 (2010) Andrzej Eilmes Lecture Notes in Computer Science, 7136/2012, 276-284 (2012) Ruben Santamaria, Juan-Antonio Mondragon-Sanchez and Xim Bokhimi J. Phys. Chem. A, ASAP (2012) | https://en.wikipedia.org/wiki?curid=30759885 |
Brass mill A brass mill is a mill which processes brass. Brass mills are common in England; many date from long before the Industrial Revolution. | https://en.wikipedia.org/wiki?curid=30762173 |
Combined bisulfite restriction analysis Combined Bisulfite Restriction Analysis (or COBRA) is a molecular biology technique that allows for the sensitive quantification of DNA methylation levels at a specific genomic locus on a DNA sequence in a small sample of genomic DNA. The technique is a variation of bisulfite sequencing, and combines bisulfite conversion based polymerase chain reaction with restriction digestion. Originally developed to reliably handle minute amounts of genomic DNA from microdissected paraffin-embedded tissue samples, the technique has since seen widespread usage in cancer research and epigenetics studies. Genomic DNA of interest is treated with sodium bisulfite, which introduces methylation-dependent sequence differences. During sodium bisulfite treatment, unmethylated cytosine residues are converted to uracil, while methylated cytosine residues are unaffected. Bisulfite treated DNA is then PCR amplified, resulting in cytosine residues at originally methylated positions, and thymine residues at originally unmethylated position (that were converted to uracil). Primers used during this step do not contain CpG sites (the common target of cytosine methylation), so the amplification process does not discriminate between templates based on methylation status. PCR products are purified to ensure complete digestion in the following step | https://en.wikipedia.org/wiki?curid=30765932 |
Combined bisulfite restriction analysis The above steps lead to the methylation dependent retention or loss of CpG-containing restriction enzyme sites, such as those for TaqI (TCGA) and BstUI (CGCG), depending on whether the cytosine residue was originally methylated or not, respectively. Due to the methylation-independent amplification in the above step, the resulting PCR products will be a mixed population of fragments that have lost or retained CpG-containing restriction enzyme sites, whose respective percentages will be directly correlated to the original level of DNA methylation in the sample DNA. PCR products are then treated with a restriction enzyme ("e.g." BstUI), which will only cleave sites that were originally methylated (CGCG), while leaving sites that were originally unmethylated (TGTG). To ensure that all CpG sites are retained due to originally being methylated, and not a remnant of incomplete bisulfite conversion, a control digestion is performed, with enzymes such as Hsp92II which recognizes the sequence CATG, none of which should be remaining after bisulfite conversion (with the rare exception of non-CpG methylation) and thus no cleavage should occur if bisulfite conversion was complete. The digested fragments are then separated by polyacrylamide gel electrophoresis with the expected appearance of bands corresponding to a single large undigested fragment, and multiple smaller bands corresponding to digested fragments | https://en.wikipedia.org/wiki?curid=30765932 |
Combined bisulfite restriction analysis Quantitative amount of DNA in these bands can be determined with a device such as a phosphoimager, after which the methylation percentage of the original sample can be calculated by: COBRA has been used extensively in many research-based applications such as screening for DNA methylation changes at gene promoters in cancer studies, detecting altered methylation patterns at imprinted genes, and characterizing methylation patterns in the genome during development in mammals. In medicine, COBRA has been used as a tool to help diagnose human disease involving aberrant DNA methylation. Researchers utilized COBRA in conjunction with denaturing high performance liquid chromatography in the diagnosis of the genetic imprinting disorder Russell-Silver syndrome where of the imprinted gene H19 is responsible for the disorder in up to 50% of patients. In general, COBRA is often combined with other DNA methylation analyses and is frequently used in the initial screening of a loci of interest. If COBRA suggests altered methylation patterns, then more rigorous, labor-intensive techniques can be applied, such as bisulfite sequencing or MeDIP. Also PacBio sequencing can be used to detect DNA methylation. | https://en.wikipedia.org/wiki?curid=30765932 |
Ephaptic coupling is a form of communication within the nervous system and is distinct from direct communication systems like electrical synapses and chemical synapses. It may refer to the coupling of adjacent (touching) nerve fibers caused by the exchange of ions between the cells, or it may refer to coupling of nerve fibers as a result of local electric fields. In either case ephaptic coupling can influence the synchronization and timing of action potential firing in neurons. Myelination is thought to inhibit ephaptic interactions. The idea that the electrical activity generated by nervous tissue may influence the activity of surrounding nervous tissue is one that dates back to the late 19th century. Early experiments, like those by du Bois-Reymond, demonstrated that the firing of a primary nerve may induce the firing of an adjacent secondary nerve (termed "secondary excitation"). This effect was not quantitatively explored, however, until experiments by Katz and Schmitt in 1940, when the two explored the electric interaction of two adjacent limb nerves of the crab "Carcinus maenas". Their work demonstrated that the progression of the action potential in the active axon caused excitability changes in the inactive axon. These changes were attributed to the local currents that form the action potential. For example, the currents that caused the depolarization (excitation) of the active nerve caused a corresponding hyperpolarization (depression) of the adjacent resting fiber | https://en.wikipedia.org/wiki?curid=30767825 |
Ephaptic coupling Similarly, the currents that caused repolarization of the active nerve caused slight depolarization in the resting fiber. Katz and Schmitt also observed that stimulation of both nerves could cause interference effects. Simultaneous action potential firing caused interference and resulted in decreased conduction velocity, while slightly offset stimulation resulted in synchronization of the two impulses. In 1941 Arvanitaki explored the same topic and proposed the usage of the term "ephapse" (from the Greek ephapsis and meaning "to touch") to describe this phenomenon and distinguish it from synaptic transmission. Over time the term ephaptic coupling has come to be used not only in cases of electric interaction between adjacent elements, but also more generally to describe the effects induced by any field changes along the cell membrane. The early work performed by Katz and Schmitt demonstrated that ephaptic coupling between the two adjacent nerves was insufficient to stimulate an action potential in the resting nerve. Under ideal conditions the maximum depolarization observed was approximately 20% of the threshold stimulus. However, conditions can be manipulated in such a way that the action potential from one neuron can be spread to a neighboring neuron. This was accomplished in one study in two experimental conditions: increased calcium concentrations, which lowered the threshold potential, or by submerging the axons in mineral oil, which increased resistance | https://en.wikipedia.org/wiki?curid=30767825 |
Ephaptic coupling While these manipulations do not reflect normal conditions, they do highlight the mechanisms behind ephaptic excitation. has also been found to play an important role in inhibition of neighboring neurons. Depending on the location and identity of the neurons, various mechanisms have been found to underlie ephaptic inhibition. In one study, newly excited neighboring neurons interfered with already sustained currents, thus lowering the extracellular potential and depolarizing the neuron in relation to its surrounding environment, effectively inhibiting the action potential's propagation. Studies of ephaptic coupling have also focused on its role in the synchronization and timing of action potentials in neurons. In the simpler case of adjacent fibers that experience simultaneous stimulation the impulse is slowed because both fibers are limited to exchange ions solely with the interstitial fluid (increasing the resistance of the nerve). Slightly offset impulses (conduction velocities differing by less than 10%) are able to exchange ions constructively and the action potentials propagate slightly out of phase at the same velocity. More recent research, however, has focused on the more general case of electric fields that affect a variety of neurons. It has been observed that local field potentials in cortical neurons can serve to synchronize neuronal activity. Although the mechanism is unknown, it is hypothesized that neurons are ephaptically coupled to the frequencies of the local field potential | https://en.wikipedia.org/wiki?curid=30767825 |
Ephaptic coupling This coupling may effectively synchronize neurons into periods of enhanced excitability (or depression) and allow for specific patterns of action potential timing (often referred to as spike timing). This effect has been demonstrated and modeled in a variety of cases. A hypothesis or explanation behind the mechanism is "one-way", "master-slave", or "unidirectional synchronization" effect as mathematical and fundamental property of non-linear dynamic systems (oscillators like neurons) to synchronize under certain criteria. Such phenomenon was proposed and predicted to be possible between two HR neurons, since 2010 in simulations and modeling work by Hrg. It was also shown that such unidirectional synchronization or copy/paste transfer of neural dynamics from master to slave(s) neurons, could be exhibited in different ways. Hence the phenomenon is of not only fundamental interest but also applied one from treating epilepsy to novel learning systems. Synchronization of neurons is in principle unwanted behavior, as brain would have zero information or be simply a bulb if all neurons would synchronize. Hence it is a hypothesis that neurobiology and evolution of brain coped with ways of preventing such synchronous behavior on large scale, using it rather in other special cases. The electrical conduction system of the heart has been robustly established. However, newer research has been challenging some of the previously accepted models. The role of ephaptic coupling in cardiac cells is becoming more apparent | https://en.wikipedia.org/wiki?curid=30767825 |
Ephaptic coupling One author even goes so far as to say, “While previously viewed as a possible alternative to electrotonic coupling, ephaptic coupling has since come to be viewed as operating in tandem with gap junctions, helping sustain conduction when gap junctional coupling is compromised.” Ephaptic interactions among cardiac cells help fill in the gaps that electrical synapses alone cannot account for. There are also a number of mathematical models that more recently incorporate ephaptic coupling into predictions about electrical conductance in the heart. Experimental work suggests that sodium channel rich nanodomains located at sites of close contact between cardiac myocytes may constitute functional units of ephaptic coupling and selective disruption of these nanodomains resulted in arrhythmogenic conduction slowing, suggesting an important role for epahptic coupling in the heart. Epileptic seizures occur when there is synchrony of electrical waves in the brain. Knowing the role that ephaptic coupling plays in maintaining synchrony in electrical signals, it makes sense to look for ephaptic mechanisms in this type of pathology. One study suggested that cortical cells represent an ideal place to observe ephaptic coupling due to the tight packing of axons, which allows for interactions between their electrical fields | https://en.wikipedia.org/wiki?curid=30767825 |
Ephaptic coupling They tested the effects of changing extracellular space (which affects local electrical fields) and found that one can block epileptic synchronization independent of chemical synapse manipulation simply by increasing the space between cells. Later, a model was created to predict this phenomenon and showed scenarios with greater extracellular spacing that effectively blocked epileptic synchronization in the brain. Neurons in the olfactory system are unmyelinated and densely packed and thus the often small effects of ephaptic coupling are more easily seen. A number of studies have shown how inhibition among neurons in the olfactory system work to fine tune integration of signals in response to odor. This inhibition has been shown to occur from changes in electrical potentials alone. The addition of ephaptic coupling to olfactory neuron models adds further support to the "dedicated-line" model in which each olfactory receptor sends its signal to one neuron. The inhibition due to ephaptic coupling would help account for the integration of signals that gives rise to more nuanced perception of smells. Due to the very small electrical fields produced by neurons, mathematical models are often used in order to test a number of manipulations. Cable theory is one of the most important mathematical equations in neuroscience. It calculates electrical current using capacitance and resistance as variables and has been the main basis for many predictions about ephaptic coupling in neurons | https://en.wikipedia.org/wiki?curid=30767825 |
Ephaptic coupling However, many authors have worked to create more refined models in order to more accurately represent the environments of the nervous system. For example, many authors have proposed models for cardiac tissue that includes additional variables that account for the unique structure and geometry of cardiac cells varying scales of size, or three-dimensional electrodiffusion. In 1978, basic tests were being conducted on squid giant axons in order to find evidence of ephaptic events. It was shown that an action potential of one axon could be propagated to a neighboring axon. The level of transmission varied, from subthreshold changes to initiation of an action potential in a neighboring cell, but in all cases, it was apparent that there are implications of ephaptic coupling that are of physiological importance. One study tested the effects of ephaptic coupling by using both neurotransmitter anatagonists to block chemical synapses and gap junction blockers to block electrical synapses. It was found that rhythmic electrical discharge associated with fetal neurons in the rat spinal cord and medulla was still sustained. This suggests that connections between the neurons still exist and work to spread signals even without traditional synapses. These findings support a model in which ephaptic coupling works alongside canonical synapses to propagate signals across neuronal networks | https://en.wikipedia.org/wiki?curid=30767825 |
Ephaptic coupling One of the few known cases of a functional system in which ephaptic coupling is responsible for an observable physiological event is in the Purkinje cells of the rat cerebellum. It was demonstrated in this study that the basket cells which encapsulate some regions of Purkinje fibers can cause inhibitory effects on the Purkinje cells. The firing of these basket cells, which occurs more rapidly than in the Purkinje cells, draws current across the Purkinje cell and generates a passive hyperpolarizing potential which inhibits the activity of the Purkinje cell. Although the exact functional role of this inhibition is still unclear, it may well have a synchronizing effect in the Purkinje cells as the ephaptic effect will limit the firing time. A similar ephaptic effect has been studied in the Mauthner cells of teleosts. While the idea of non-synaptic interactions between neurons has existed since the 19th century, there has historically been a lot of skepticism in the field of neuroscience. Many people believed that the micro electrical fields produced by the neurons themselves were so small that they were negligible. While many supporters of the ephaptic coupling theory have been trying to prove its existence through experiments that block both chemical and electrical synapses, still some opponents in the field express caution | https://en.wikipedia.org/wiki?curid=30767825 |
Ephaptic coupling For example, in 2014, one scientist published a review that presents his skepticism on the idea of ephaptic coupling, saying “The agreement between their simulations and Poelzing’s data is impressive, but I will need a more definitive experimental confirmation before I can embrace the ephaptic hypothesis.” He bases his caution in wanting more distinction between gap junctions' propagation of charge and true ephaptic coupling. Whether it is a true lack of evidence or simply obstinance in the face of change, many in the field are still not entirely convinced there is unambiguous evidence of ephaptic coupling. Research continues and in 2018, surprising results were announced | https://en.wikipedia.org/wiki?curid=30767825 |
Testican is a type of proteoglycan. Testican-1 is a highly conserved, multidomain proteoglycan that is most prominently expressed in the thalamus of the brain, and is upregulated in activated astroglial cells of the cerebrum. Several functions of this gene product have now been demonstrated in vitro including membrane-type matrix metalloproteinase inhibition, cathepsin L inhibition, and low-affinity calcium binding. The purified gene product has been shown to inhibit cell attachment and neurite extensions in culture. Functions of testican in vivo have yet to be demonstrated in knockout mice or other models. has been shown to carry substantial amounts of chondroitin sulfate as well as other oligosaccharides, but the biological significance of these embellishments is not yet known. In humans there are three testicans: Testican-1 is known to play a role in lapatinib resistance, which is a drug used to treat HER2-positive gastric cancer. Testican-1 is involved in the pathway for this drug, and leads to drug resistance when upregulated. When testican-1 levels are artifically reduced, sensitivity towards lapatinib was once again increased. This shows the potential for future use in combatting drug resistance. Testican-1 has also shown to be useful when studying sepsis in patients. A study revealed that patients with more advanced sepsis exhibited higher levels of testican-1. This is useful in determining the severity of sepsis in a clinical setting and could allow healthcare professionals to formulate a better treatment plan. | https://en.wikipedia.org/wiki?curid=30768627 |
Statkraft osmotic power prototype in Hurum Statkraft osmotic power prototype is the world's first osmotic power plant, based on the energy of osmosis. The power plant is run by Statkraft. The power plant is located at Tofte in Hurum, Norway, with rooms at the factory area at Södra Cell Tofte cellulose factory. The power plant uses the osmotic gradient that occurs when fresh water and salt water meet, separated by a permeable membrane. The salt water pulls fresh water through the membrane and the pressure increases on the salt water side; this pressure increase can be used to produce electrical power with the use of a normal hydroelectric turbine/generator setup. The plant is a prototype developed together with Sintef and began test power production on 24 November 2009. Mette-Marit, Crown Princess of Norway officially opened the plant. This plant had been planned since the summer of 2008, with a water usage of 10 litres of fresh water and 20 litres of salt water per second. It is expected to give a power output of between 2-4 kW. With better membranes it is assumed that the power for a similar plant can be increased to about 10 kW. A commercial plant is expected to be built between 2012 and 2015. In 2013, Statkraft announced that is discontinuing its work to develop osmotic power technology. The larger planned pilot facility, and the future commercial plant, will not be built | https://en.wikipedia.org/wiki?curid=30771750 |
Statkraft osmotic power prototype in Hurum This type of power generation is very reliable, consisting of only slightly more moving parts than a conventional hydroelectric power plant; in this case the addition of a pair of small pumps to move the fresh and salt water to the membrane surfaces. It is very quiet when operating and requires minimal supervision. In addition, it is expected the plant could respond very quickly as an emergency power source, using the membranes to 'store' power ready in the form of high pressure water; this water could be very quickly fed to the hydroelectric turbine to generate electricity. The expected lifetime of this plant is large; with almost no moving parts (those that do move are very simple and reliable), there will be little wear occurring. The availability of spare parts and upgrade components is also good, meaning that an installed osmotic power plant could be run for many years cost-effectively. Whilst highly reliable, simple and cheap to run/maintain, this plant is very expensive to install. The permeable membrane is currently an expensive resource, and to have any meaningful output, a very large membrane area is required. The plant described in this article could reach a power output of 4 kW in ideal conditions. By comparison, an open cycle gas turbine a fraction of the size (such as the Rolls-Royce or GE aero-derivative gas turbines) could easily produce greater than 15MW for a fraction of the installation costs, although fuel and maintenance costs would be greater | https://en.wikipedia.org/wiki?curid=30771750 |
Statkraft osmotic power prototype in Hurum This is an increase in power output 3750 times greater, with a land usage that is much smaller. Comparing the ideal power output of this plant to the rough average household consumption of a modern home detailed in the article domestic energy consumption, it can be seen that this is a very limited technology - working back the figures, it equates that the average home requires 2 kW of power. Bear in mind however that advances in materials technology will likely greatly increase the power output per plant volume over time and thus make this a more useful form of power generation, particularly in remote locations where reliability is key and spare parts are difficult to come by (e.g. difficult-to-access coastal locations with a small stream or river nearby that can provide the fresh water required). | https://en.wikipedia.org/wiki?curid=30771750 |
Energy separating agent In chemical separation processes, an (ESA) is the heat or shaft work added to facilitate the separation of two chemical species. It is contrasted with a mass separating agent, which is any chemical species added to the reaction that facilitates the reaction. ESAs are used in many common separation procedures. Some important examples of procedures utilizing ESAs are vaporization (heat added), distillation (heat added), crystallization (heat evolved), and stripping (heat added). | https://en.wikipedia.org/wiki?curid=30772871 |
Mass separating agent In chemical separation processes, a mass separating agent (MSA) is a chemical species that is added to ensure that the intended separation process takes place. It is analogous to an energy separating agent, which aids separations processes via addition of energy. An MSA may be partially immiscible with one or more mixture components and frequently is the constituent of highest concentration in the added phase. Alternatively, the MSA may be miscible with a liquid feed mixture, but may selectively alter partitioning of species between liquid and vapor phases. Disadvantages of using an MSA are: (1) need for an additional separator to recover the MSA for recycle, (2) need for MSA makeup, (3) possible MSA product contamination, and (4) more difficult design procedures. Processes like absorption and stripping generally utilize various MSAs. | https://en.wikipedia.org/wiki?curid=30774139 |
Recombinant AAV mediated genome engineering Recombinant adeno-associated virus (rAAV) based genome engineering is a genome editing platform centered on the use of recombinant rAAV vectors that enables insertion, deletion or substitiution of DNA sequences into the genomes of live mammalian cells. The technique builds on Mario Capecchi and Oliver Smithies' Nobel Prize–winning discovery that homologous recombination (HR), a natural hi-fidelity DNA repair mechanism, can be harnessed to perform precise genome alterations in mice. rAAV mediated genome-editing improves the efficiency of this technique to permit genome engineering in any pre-established and differentiated human cell line, which, in contrast to mouse ES cells, have low rates of HR. The technique has been widely adopted for use in engineering human cell lines to generate isogenic human disease models. It has also been used to optimize bioproducer cell lines for the biomanufacturing of protein vaccines and therapeutics. In addition, due to the non-pathogenic nature of rAAV, it has emerged as a desirable vector for performing gene therapy in live patients. The rAAV genome is built of single-stranded deoxyribonucleic acid (ssDNA), either positive- or negative-sensed, which is about 4.7 kilobase long. These single-stranded DNA viral vectors have high transduction rates and have a unique property of stimulating endogenous HR without causing double strand DNA breaks in the genome, which is typical of other homing endonuclease mediated genome editing methods | https://en.wikipedia.org/wiki?curid=30774917 |
Recombinant AAV mediated genome engineering Users can design a rAAV vector to any target genomic locus and perform both gross and subtle endogenous gene alterations in mammalian somatic cell-types. These include gene knock-outs for functional genomics, or the ‘knock-in’ of protein tag insertions to track translocation events at physiological levels in live cells. Most importantly, rAAV targets a single allele at a time and does not result in any off-target genomic alterations. Because of this, it is able to routinely and accurately model genetic diseases caused by subtle SNPs or point mutations that are increasingly the targets of novel drug discovery programs. To date, the use of rAAV mediated genome engineering has been published in over 1300 peer reviewed scientific journals. Another emerging application of rAAV based genome editing is for gene therapy in patients, due to the accuracy and lack of off-target recombination events afforded by the approach. | https://en.wikipedia.org/wiki?curid=30774917 |
Bufothionine is a sulfur-containing compound which is present in the bufotoxins secreted by the parotoid gland of certain toads of the genera "Bufo" and "Chaunus". This specific compound can be found in the skin of certain species of toad such as the Asiatic Toad, "Chaunus arunco", "Chaunus crucifer", "Chaunus spinulosus", and "Chaunus arenarum". has been shown to suppress cell cycle growth of cancerous liver cells. Experiments were conducted in which cultured cancer cells were shown to have an increase in G2-M damage checkpoint, ensuring that growth of the cell will not continue until the damage to the DNA is corrected while also showing a drop in the G0 and G1 activity, which pertains to phase where there is cell growth and RNA production. Similarly, Buifothionine has also been shown to increase the chances of cell death and decrease cell growth of gastric cancer related cells by inhibiting the PIM3 gene, which, in cancerous cells, increases the resistance of chemotherapeutic treatments. | https://en.wikipedia.org/wiki?curid=30778322 |
Raman cooling In atomic physics, is a sub-recoil cooling technique that allows the cooling of atoms using optical methods below the limitations of Doppler cooling, Doppler cooling being limited by the recoil energy of a photon given to an atom. This scheme can be performed in simple optical molasses or in molasses where an optical lattice has been superimposed, which are called respectively free space and Raman side-band cooling. Both techniques make use of Raman scattering of laser light by the atoms. The transition between two hyperfine states of the atom can be triggered by two laser beams: the first beam excites the atom to a virtual excited state (for example because its frequency is lower than the real transition frequency), and the second beam deexcites the atom to the other hyperfine level. The frequency difference of the two beams is exactly equal to the transition frequency between the two hyperfine levels. The illustration of this process is shown in the schematic illustration of a two-photon Raman process. It enables the transition between the two levels formula_1 and formula_2. The intermediate, virtual level is represented by the dashed line, and is red-detuned with respect to the real excited level, formula_3. The frequency difference formula_4 here matches exactly the energy difference between formula_1 and formula_2. In this scheme, a pre-cooled cloud of atoms (whose temperature is of a few tens of microkelvins) undergoes a series of pulses of Raman-like processes | https://en.wikipedia.org/wiki?curid=30778796 |
Raman cooling The beams are counterpropagating, and their frequencies are just as what has been described above, except that the frequency formula_7 is now slightly red-detuned (detuning formula_8) with respect to its normal value. Thus, atoms moving towards the source of the laser 2 with a sufficient velocity will be resonant with the Raman pulses, thanks to the Doppler effect. They will be excited to the formula_2 state, and get a momentum kick decreasing the modulus of their velocity. If the propagation directions of the two lasers are interchanged, then the atoms moving in the opposite direction will be excited and get the momentum kick that will decrease the modulus of their velocities. By regularly exchanging the lasers propagating directions and varying the detuning formula_8, one can manage to have all atoms for which the initial velocity satisfies formula_11 in the state formula_2, while the atoms such that formula_13 are still in the formula_1 state. A new beam is then switched on, whose frequency is exactly the transition frequency between formula_2 and formula_3. This will optically pump the atoms from the formula_2 state to the formula_1 state, and the velocities will be randomized by this process, such that a fraction of the atoms in formula_2 will acquire a velocity formula_13. By repeating this process several times (eight in the original paper, see references), the temperature of the cloud can be lowered to less than a microkelvin. This cooling scheme starts from atoms in a magneto-optical trap | https://en.wikipedia.org/wiki?curid=30778796 |
Raman cooling An optical lattice is then ramped up, such that an important fraction of the atoms are trapped. If the lasers of the lattice are powerful enough, each site can be modelled as a harmonic trap. Since the atoms are not in their ground state, they will be trapped in one of the excited levels of the harmonic oscillator. The aim of Raman side-band cooling is to put the atoms into the ground state of the harmonic potential in the lattice site. We consider a two level atom, the ground state of which has a quantum number of F=1, such that it is three-fold degenerate with m=-1, 0 or 1. A magnetic field is added, which lifts the degeneracy in m due to the Zeeman effect. Its value is exactly tuned such that the Zeeman splitting between m=-1 and m=0 and between m=0 and m=1 is equal to the spacing of two levels in the harmonic potential created by the lattice. By means of Raman processes, an atom can be transferred to a state where the magnetic moment has decreased by one and the vibrational state has also decreased by one (red arrows on the picture). After that the atoms which are in the lowest vibrational state of the lattice potential (but with formula_21) are optically pumped to the m=1 state (role of the formula_22 and formula_23 light beams). Since the temperature of the atoms is low enough with respect to the pumping beam frequencies, the atom is very likely not to change its vibrational state during the pumping process. Thus it ends up in a lower vibrational state, which is how it is cooled | https://en.wikipedia.org/wiki?curid=30778796 |
Raman cooling In order to reach this efficient transfer to the lower vibrational state at each step, the parameters of the laser, i.e. power and timing, should be carefully tuned. In general, these parameters are different for different vibrational states because the strength of the coupling (Rabi frequency) depends on the vibrational level. Additional complication to this naive picture arises from the recoil of photons, which drive this transition. The last complication can be generally avoided by performing cooling in a so-called Lamb Dicke regime. In this regime the atom is trapped so strongly in the optical lattice that it effectively does not change its momentum due to the photon recoils. The situation is similar to the Mössbauer effect. This cooling scheme allows one to obtain a rather high density of atoms at a low temperature, using only optical techniques. Recent experiments have shown it is even sufficient to attain for example Bose–Einstein condensation. For instance, the Bose–Einstein condensation of cesium has been achieved for the first time in an experiment that used Raman side-band cooling as its first step. | https://en.wikipedia.org/wiki?curid=30778796 |
Diphenylpropylamine is a propylamine derivative and may refer to: | https://en.wikipedia.org/wiki?curid=30779709 |
Kinetic diameter is a measure applied to atoms and molecules that expresses the likelihood that a molecule in a gas will collide with another molecule. It is an indication of the size of the molecule as a target. The kinetic diameter is not the same as atomic diameter defined in terms of the size of the atom's electron shell, which is generally a lot smaller, depending on the exact definition used. Rather, it is the size of the sphere of influence that can lead to a scattering event. is related to the mean free path of molecules in a gas. Mean free path is the average distance that a particle will travel without collision. For a fast moving particle (that is, one moving much faster than the particles it is moving through) the kinetic diameter is given by, However, a more usual situation is that the colliding particle being considered is indistinguishable from the population of particles in general. Here, the Maxwell–Boltzmann distribution of energies must be considered, which leads to the modified expression, The following table lists the kinetic diameters of some common molecules; Collisions between two dissimilar particles occur when a beam of fast particles is fired into a gas consisting of another type of particle, or two dissimilar molecules randomly collide in a gas mixture. For such cases, the above formula for scattering cross section has to be modified | https://en.wikipedia.org/wiki?curid=30783530 |
Kinetic diameter The scattering cross section, σ, in a collision between two dissimilar particles or molecules is defined by the sum of the kinetic diameters of the two particles, We define an intensive quantity, the scattering coefficient α, as the product of the gas number density and the scattering cross section, The mean free path is the inverse of the scattering coefficient, For similar particles, "r" = "r" and, as before. | https://en.wikipedia.org/wiki?curid=30783530 |
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