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Depletion force A depletion force is an effective attractive force that arises between large colloidal particles that are suspended in a dilute solution of "depletants", which are smaller solutes that are preferentially excluded from the vicinity of the large particles. One of the earliest reports of depletion forces that lead to particle coagulation is that of Bondy, who observed the separation or 'creaming' of rubber latex upon addition of polymer depletant molecules (sodium alginate) to solution. More generally, depletants can include polymers, micelles, osmolytes, ink, mud, or paint dispersed in a continuous phase. Depletion forces are often regarded as entropic forces, as was first explained by the established Asakura–Oosawa model. In this theory the depletion force arises from an increase in osmotic pressure of the surrounding solution when colloidal particles get close enough such that the excluded cosolutes (depletants) cannot fit in between them. Because the particles were considered as hard-core (completely rigid) particles, the emerging picture of the underlying mechanism inducing the force was necessarily entropic. The system of colloids and depletants in solution is typically modeled by treating the large colloids and small depletants as dissimilarly sized hard spheres. Hard spheres are characterized as non-interacting and impenetrable spheres. These two fundamental properties of hard spheres are described mathematically by the "hard-sphere potential" | https://en.wikipedia.org/wiki?curid=39639151 |
Depletion force The hard-sphere potential imposes steric constraint around large spheres which in turn gives rise to "excluded volume", that is, volume that is unavailable for small spheres to occupy. In a colloidal dispersion, the colloid-colloid interaction potential is approximated as the interaction potential between two hard spheres. For two hard spheres of diameter of formula_1, the interaction potential as a function of interparticle separation is: called the hard-sphere potential where formula_3 is the center-to-center distance between the spheres. If both colloids and depletants are in a dispersion, there is interaction potential between colloidal particles and depletant particles that is described similarly by the hard-sphere potential. Again, approximating the particles to be hard-spheres, the interaction potential between colloids of diameter formula_4 and depletant sols of diameter formula_5 is: where formula_3 is the center-to-center distance between the spheres. Typically, depletant particles are very small compared to the colloids so formula_8 The underlying consequence of the hard-sphere potential is that dispersed colloids cannot penetrate each other and have no mutual attraction or repulsion. When both large colloidal particles and small depletants are in a suspension, there is a region which surrounds every large colloidal particle that is unavailable for the centers of the depletants to occupy. This steric restriction is due to the colloid-depletant hard-sphere potential | https://en.wikipedia.org/wiki?curid=39639151 |
Depletion force The volume of the excluded region is where formula_4 is the diameter of the large spheres and formula_5 is the diameter of the small spheres. When the large spheres get close enough, the excluded volumes surrounding the spheres intersect. The overlapping volumes result in a reduced excluded volume, that is, an increase in the total free volume available to small spheres. The reduced excluded volume, formula_12 can be written where formula_14 is half the width of the lens-shaped region of overlap volume formed by spherical caps. The volume available formula_15 for small spheres is the difference between the total volume of the system and the excluded volume. To determine the available volume for small spheres, there are two distinguishable cases: first, the separation of the large spheres is big enough so small spheres can penetrate in between them; second, the large spheres are close enough so that small spheres cannot penetrate between them. For each case, the available volume for small spheres is given by In the latter case small spheres are depleted from the interparticle region between large spheres and a depletion force ensues. The depletion force is described as an entropic force because it is fundamentally a manifestation of the second law of thermodynamics, which states that a system tends to increase its entropy. The gain in translational entropy of the depletants, owing to the increased available volume, is much greater than the loss of entropy from flocculation of the colloids | https://en.wikipedia.org/wiki?curid=39639151 |
Depletion force The positive change in entropy lowers the Helmholtz free energy and causes colloidal flocculation to happen spontaneously. The system of colloids and depletants in a solution is modeled as a canonical ensemble of hard spheres for statistical determinations of thermodynamic quantities. However, recent experiments and theoretical models found that depletion forces can be enthalpically driven. In these instances, the intricate balance of interactions between the solution components results in the net exclusion of cosolute from macromolecule. This exclusion results in an effective stabilization of the macromolecule self-association, which can be not only enthalpically dominated, but also entropically unfavorable. The total volume available for small spheres increases when the excluded volumes around large spheres overlap. The increased volume allotted for small spheres allows them greater translational freedom which increases their entropy. Because the canonical ensemble is an athermal system at a constant volume the Helmholtz free energy is written where formula_18 is the Helmholtz free energy, formula_19 is the entropy and formula_20 is the temperature. The system's net gain in entropy is positive from increased volume, thus the Helmholtz free energy is negative and depletion flocculation happens spontaneously. The free energy of the system is obtained from a statistical definition of Helmholtz free energy where formula_22 is the partition function for the canonical ensemble | https://en.wikipedia.org/wiki?curid=39639151 |
Depletion force The partition function contains statistical information that describes the canonical ensemble including its total volume, the total number of small spheres, the volume available for small spheres to occupy, and the de Broglie wavelength. If hard-spheres are assumed, the partition function formula_22 is The volume available for small spheres,formula_15 was calculated above.formula_26 is the number of small spheres and formula_27 is the de Broglie wavelength. Substituting formula_22 into the statistical definition, the Helmholtz free energy now reads The magnitude of the depletion force, formula_30 is equal to the change in Helmholtz free energy with distance between two large spheres and is given by The entropic nature of depletion forces was proven experimentally in some cases. For example, some polymeric crowders induce entropic depletion forces that stabilize proteins in their native state. Other examples include many systems with hard-core only interactions. The depletion force is an effect of increased osmotic pressure in the surrounding solution. When colloids get sufficiently close, that is when their excluded volumes overlap, depletants are expelled from the interparticle region. This region between colloids then becomes a phase of pure solvent. When this occurs, there is a higher depletant concentration in the surrounding solution than in the interparticle region | https://en.wikipedia.org/wiki?curid=39639151 |
Depletion force The resulting density gradient gives rise to an osmotic pressure that is anisotropic in nature, acting on the outer sides of the colloids and promoting flocculation. If the hard-sphere approximation is employed, the osmotic pressure is: where formula_33 is osmotic pressure and formula_34 is number density of small spheres and formula_35 is Boltzmann's constant. Depletion forces were first described by Sho Asakura and Fumio Oosawa in 1954. In their model, the force is always considered to be attractive. Additionally, the force is considered to be proportional to the osmotic pressure. The Asakura–Oosawa model assumes low macromolecule densities and that the density distribution, formula_36, of the macromolecules is constant. Asakura and Oosawa described four cases in which depletion forces would occur. They first described the most general case as two solid plates in a solution of macromolecules. The principles for the first case were then extended to three additional cases. In the Asakura–Oosawa model for depletion forces, the change in free-energy imposed by an excluded cosolute, formula_37, is: where formula_39 is the osmotic pressure, and formula_40 is the change in excluded volume (which is related to molecular size and shape). The very same result can be derived using the Kirkwood-Buff solution theory. In the first case, two solid plates are placed in a solution of rigid spherical macromolecules | https://en.wikipedia.org/wiki?curid=39639151 |
Depletion force If the distance between two plates, formula_41, is smaller than the diameter of solute molecules, formula_5, then no solute can enter between the plates. This results in pure solvent existing between the plates. The difference in concentration of macromolecules in the solution between the plates and the bulk solution causes a force equal to the osmotic pressure to act on the plates. In a very dilute and monodisperse solution the force is defined by where formula_44 is the force, and formula_26 is the total number of solute molecules. The force causes the entropy of the macromolecules to increase and is attractive when formula_46 Asakura and Oosawa described the second case as consisting of two plates in a solution of rod like macromolecules. The rod like macromolecules are described as having a length, formula_47, where formula_48, the area of the plates. As the length of the rods increases, the concentration of the rods between the plates is decreased as it becomes more difficult for the rods to enter between the plates due to steric hindrances. As a result, the force acting on the plates increases with the length of the rods until it becomes equal to the osmotic pressure. In this context, it is worth mentioning that even the isotropic-nematic transition of lyotropic liquid crystals, as first explained in Onsager's theory, can in itself be considered a special case of depletion forces. The third case described by Asakura and Oosawa is two plates in a solution of polymers | https://en.wikipedia.org/wiki?curid=39639151 |
Depletion force Due to the size of the polymers, the concentration of polymers in the neighborhood of the plates is reduced, which result the conformational entropy of the polymers being decreased. The case can be approximated by modeling it as diffusion in a vessel with walls which absorb diffusing particles. The force, formula_44, can then be calculated according to: In this equation formula_51 is the attraction from the osmotic effect. formula_52 is the repulsion due to chain molecules confined between plates. formula_44 is on order of formula_54, the mean end-to-end distance of chain molecules in free space. The final case described by Asakura and Oosawa describes two large, hard spheres of diameter formula_4, in a solution of small, hard spheres of diameter formula_5. If the distance between the center of the spheres, formula_3, is less than formula_58, then the small spheres are excluded from the space between the large spheres. This results in the area between the large spheres having a reduced concentration of small spheres and therefore reduced entropy. This reduced entropy causes a force to act upon the large spheres pushing them together. This effect was convincingly demonstrated in experiments with vibrofluidized granular materials where attraction can be directly visualized. Asakura and Oosawa assumed low concentrations of macromolecules. However, at high concentrations of macromolecules, structural correlation effects in the macromolecular liquid become important | https://en.wikipedia.org/wiki?curid=39639151 |
Depletion force Additionally, the repulsive interaction strength strongly increases for large values of formula_59 (large radius/small radius). In order to account for these issues, the Derjaguin approximation, which is valid for any type of force law, has been applied to depletion forces. The Derjaguin approximation relates the force between two spheres to the force between two plates. The force is then integrated between small regions on one surface and the opposite surface, which is assumed to be locally flat. If there are two spheres of radii formula_60 and formula_61 on the formula_62 axis, and the spheres are formula_63 distance apart, where formula_3 is much smaller than formula_60 and formula_61, then the force, formula_67, in the formula_68 direction is In this equation, formula_70, and formula_71 is the normal force per unit area between two flat surfaces distance formula_68 apart. When the Derjaguin approximation is applied to depletion forces, and 0<h<2Rs, then the depletion force given by the Derjaguin approximation is In this equation, formula_74 is the geometrical factor, which is set to 1, and formula_75, the interfacial tension at the wall-fluid interface. Asakura and Oosawa assumed a uniform particle density, which is true in a homogenous solution. However, if an external potential is applied to a solution, then the uniform particle density is disrupted, making Asakura and Oosawa's assumption invalid. Density functional theory accounts for variations in particle density by using the grand canonical potential | https://en.wikipedia.org/wiki?curid=39639151 |
Depletion force The grand canonical potential, which is a state function for the grand canonical ensemble, is used to calculate the probability density for microscopic states in macroscopic state. When applied to depletion forces, the grand canonical potential calculates the local particle densities in a solution. Density functional theory states that when any fluid is exposed to an external potential, formula_76, then all equilibrium quantities become functions of number density profile, formula_77. As a result, the total free energy is minimized. The Grand canonical potential, formula_78, is then written where formula_80 is the chemical potential, formula_20 is the temperature, and formula_82 is the helmholtz free energy. The original Asakura–Oosawa model considered only hard-core interactions. In such an athermal mixture the origin of depletion forces is necessarily entropic. If the intermolecular potentials also include repulsive and/or attractive terms, and if the solvent is considered explicitly, the depletion interaction can have additional thermodynamic contributions. The notion that depletion forces can also be enthalpically driven has surfaced due to recent experiments regarding protein stabilization induced by compatible osmolytes, such as trehalose, glycerol, and sorbitol. These osmolytes are preferentially excluded from protein surfaces, forming a layer of preferential hydration around the proteins. When the protein folds - this exclusion volume diminishes, making the folded state lower in free energy | https://en.wikipedia.org/wiki?curid=39639151 |
Depletion force Hence the excluded osmolytes shift the folding equilibrium towards the folded state. This effect was generally thought to be an entropic force, in the spirit of the original Asakura–Oosawa model and of macromolecular crowding. However, thermodynamic breakdown of the free-energy gain due to osmolyte addition showed the effect is in fact enthalpically driven, whereas entropy can even be disfavorable. For many cases, the molecular origin of this enthalpically driven depletion force can be traced to an effective "soft" repulsion in the potential of mean force between macromolecule and cosolute. Both Monte-Carlo simulations and a simple analytic model demonstrate that when the hard-core potential (as in Asakura and Oosawa's model) is supplemented with an additional repulsive "softer" interaction, the depletion force can become enthalpically dominated. Depletion forces have been observed and measured using a variety of instrumentation including atomic force microscopy, optical tweezers, and hydrodynamic force balance machines. Atomic force microscopy (AFM) is commonly used to directly measure the magnitude of depletion forces. This method uses the deflection of a very small cantilever contacting a sample which is measured by a laser. The force required to cause a certain amount of beam deflection can be determined from the change in angle of the laser. The small scale of AFM allows for dispersion particles to be measured directly yielding a relatively accurate measurement of depletion forces | https://en.wikipedia.org/wiki?curid=39639151 |
Depletion force The force required to separate two colloid particles can be measured using optical tweezers. This method uses a focused laser beam to apply an attractive or repulsive force on dielectric micro and nanoparticles. This technique is used with dispersion particles by applying a force which resists depletion forces. The displacement of the particles is then measured and used to find the attractive force between the particles. HFB machines measure the strength of particle interactions using liquid flow to separate the particles. This method is used to find depletion force strength by adhering to a static plate one particle in a dispersion particle doublet and applying shear force through fluid flow. The drag created by the dispersion particles resists the depletion force between them, pulling the free particle away from the adhered particle. A force balance of the particles at separation can be used to determine the depletion force between the particles. Depletion forces are used extensively as a method of destabilizing colloids. By introducing particles into a colloidal dispersion, attractive depletion forces can be induced between dispersed particles. These attractive interactions bring the dispersed particles together resulting in flocculation. This destabilizes the colloid as the particles are no longer dispersed in the liquid but concentrated in floc formations. Flocs are then easily removed through filtration processes leaving behind a non-dispersed, pure liquid | https://en.wikipedia.org/wiki?curid=39639151 |
Depletion force The use of depletion forces to initiate flocculation is a common process in water treatment. The relatively small size of dispersed particles in waste water renders typical filtration methods ineffective. However, if the dispersion was to be destabilized and flocculation occur, particles can then be filtered out to produce pure water. Therefore, coagulants and flocculants are typically introduced to waste water which create these depletion forces between the dispersed particles. Some wine production methods also use depletion forces to remove dispersed particles from wine. Unwanted colloidal particles can be found in wine originating from the must or produced during the winemaking process. These particles typically consist of carbohydrates, pigmentation molecules, or proteins which may adversely affect the taste and purity of the wine. Therefore, flocculants are often added to induce floc precipitation for easy filtration. The table below lists common flocculants along with their chemical formulas, net electrical charge, molecular weight and current applications. There are suggestions that depletion forces may be a significant contributor in some biological systems, specifically in membrane interactions between cells or any membranous structure. With concentrations of large molecules such as proteins or carbohydrates in the extracellular matrix, it is likely some depletion force effects are observed between cells or vesicles that are very close | https://en.wikipedia.org/wiki?curid=39639151 |
Depletion force However, due to the complexity of most biological systems, it is difficult to determine how much these depletion forces influence membrane interactions. Models of vesicle interactions with depletion forces have been developed, but these are greatly simplified and their applicability to real biological systems is questionable. Depletion forces in colloid-polymer mixtures drive colloids to form aggregates that are densely packed locally. This local dense packing is also observed in colloidal systems without polymer depletants. Without polymer depletants the mechanism is similar, because the particles in dense colloidal suspension act, effectively, as depletants for one another This effect is particularly striking for anisotropically shaped colloidal particles, where the anisotropy of the shape leads to the emergence of directional entropic forces that are responsible for the ordering of hard anisotropic colloids into a wide range of crystal structures. | https://en.wikipedia.org/wiki?curid=39639151 |
Gold laundering is the process whereby illegally obtained gold is melted and recast into another form. The recasting is performed to obscure or conceal the true origin of the gold. The recast gold is then sold, thus laundering it into cash. It may also refer to a money laundering transaction in which the exchanged good is gold. There are several stages in the gold laundering process. The first is the acquisition of the gold, which may be from any combination of sources, at least one of which is illegal. This is then treated as scrap metal, melted, then cast into a mold. This results in a transportable ingot, gold bar or other bulk form bullion. In some cases, there is no gold involved in the laundering scheme. Colombia reported 70 tons of gold exports for 2012 despite production of only 15 tons. This is a result of fictitious gold exports filed with the Colombian customs agency by drug cartels in order to import "cash from international drug deals". Acquisition of the gold occurs in many forms. Small-scale gold mining operations, particularly those with no permits or licences, extract gold from areas unsuitable for large mining operations. Gold obtained in this fashion that is exported may not be traced or authenticated. This includes artisanal mines operated by few individuals, who then sell the gold to brokers. Another form of acquisition is theft. In the 1940s, Nazi Germany forcibly obtained the possessions of Jews, sometimes in collaboration with other parties | https://en.wikipedia.org/wiki?curid=39639653 |
Gold laundering Most victims who died in death camps and Nazi concentration camps were robbed of all valuable property by the state, which was then sold or, in the case of bullion, sent to the Reichsbank. For example, in 1945 the Hungarian Gold Train was established to transport the property of Hungarian Jews to Berlin. Once the Jews had been deported to German concentration camps, the valuables were sorted into categories such that their owners could no longer be identified. In the late 1930s and throughout World War II, Germany would increase its unofficial gold reserves by expropriating gold from foreign governments, including $223m from Belgium and $193m from the Netherlands. Some of the Nazi gold was exchanged for cash, and some deposited in financial institutions, including $316 million of the looted gold at the Swiss National Bank. Of the nearly 100 tons of gold laundered through Swiss banks, only 4 tons was returned. The Banco de Portugal, the central bank of Portugal, would regularly purchase Nazi gold held at the Swiss National Bank, according to a 1944 document from the American Overseas Special Services. The scrap gold is then sold by brokers to smelters or refiners, who heat the gold to at least , its melting point. This may include gold from numerous sources, including recycled and newly mined gold. Once melted, it may be purified by parting, which separates out silver impurities, and the Miller process or Wohlwill process, depending on the desired level of purity and scale of operation | https://en.wikipedia.org/wiki?curid=39639653 |
Gold laundering Once refined, it is cast into a transportable form for further processing. Gold fingerprinting, that is identifying a particular source of gold based on its impurities or trace elements, may be possible if the impurities have not been removed via refining. To prevent laundering of gold and other metals (such as tantalum, tin, and tungsten), some nations have established systems and regulations. For example, Rwanda introduced regulations for mineral trade that requires all ore extraction be tagged, in part a response to the Dodd–Frank Wall Street Reform and Consumer Protection Act in the United States that requires companies to disclose the use of conflict minerals by publishing a supply chain audit. However, with only 100 government monitors overseeing 450 mining sites during typical business hours, smuggling still occurs, exacerbated by corrupt monitors illegally selling tags or failing to record transactions in logbooks. In 2013, Ghanaian president John Dramani Mahama announced that the government of Ghana would establish a task force to regulate the small-scale mining sector. This was to ensure that gold produced in small-scale mines could be authenticated, and also to eliminate the use of heavy machinery that had become prevalent on such sites, which have caused environmental damage. The government of Uganda obtained $200 million in trade revenue from the sale of gold in 2012, but expects that to decrease in 2013 as a result of illegal mining operations | https://en.wikipedia.org/wiki?curid=39639653 |
Gold laundering The group InSight Crime claims that illegal mining represents up to 30% of the revenues of the Revolutionary Armed Forces of Colombia (FARC). On 22 November 2000, the World Jewish Congress lawsuit against Swiss banks was settled, providing a fund of $1.25 billion for restitution to individuals whose property was confiscated by the Nazis during World War II. | https://en.wikipedia.org/wiki?curid=39639653 |
Hoffman nucleation theory is a theory developed by John D. Hoffman and coworkers in the 1970s and 80s that attempts to describe the crystallization of a polymer in terms of the kinetics and thermodynamics of polymer surface nucleation. The theory introduces a model where a surface of completely crystalline polymer is created and introduces surface energy parameters to describe the process. is more of a starting point for polymer crystallization theory and is better known for its fundamental roles in the Hoffman-Weeks Lamellar Thickening and Lauritzen-Hoffman Growth Theory. Polymers contain different morphologies on the molecular level which give rise to their macro properties. Long range disorder in the polymer chain is representative of amorphous solids, and the chain segments are considered amorphous. Long range polymer order is similar to crystalline material, and chain segments are considered crystalline. The thermal characteristics of polymers are fundamentally different from those of most solid materials. Solid materials typically have one melting point, the T, above which the material loses internal molecular ordering and becomes a liquid. Polymers have both a melting temperature T and a glass transition temperature T. Above the T, the polymer chains lose their molecular ordering and exhibit reptation, or mobility. Below the T, but still above the T, the polymer chains lose some of their long-range mobility and can form either crystalline or amorphous regions | https://en.wikipedia.org/wiki?curid=39642591 |
Hoffman nucleation theory In this temperature range, as the temperature decreases, amorphous regions can transition into crystalline regions, causing the bulk material to become more crystalline over all. Below the T, molecular motion is stopped and the polymer chains are essentially frozen in place. In this temperature range, amorphous regions can no longer transition into crystalline regions, and the polymer as a whole has reached its maximum crystallinity. addresses the amorphous to crystalline polymer transition, and this transition can only occur in the temperature range between the T and T. The transition from an amorphous to a crystalline single polymer chain is related to the random thermal energy required to align and fold sections of the chain to form ordered regions titled lamellae, which are a subset of even bigger structures called spherulites. The Crystallization of polymers can be brought about by several different methods, and is a complex topic in itself. Nucleation is the formation and growth of a new phase with or without the presence of external surface. The presence of this surface results in heterogeneous nucleation whereas in its absence homogeneous nucleation occurs. Heterogeneous nucleation occurs in cases where there are pre-existing nuclei present, such as tiny dust particles suspended in a liquid or gas or reacting with a glass surface containing . For the process of Hoffman nucleation and its progression to Lauritzen-Hoffman Growth Theory, homogeneous nucleation is the main focus | https://en.wikipedia.org/wiki?curid=39642591 |
Hoffman nucleation theory Homogeneous nucleation occurs where no such contaminants are present and is less commonly seen. Homogeneous nucleation begins with small clusters of molecules forming from one phase to the next. As the clusters grow, they aggregate through the condensation of other molecules. The size continues to increase and ultimately form macroscopic droplets (or bubbles depending on the system). Nucleation is often described mathematically through the change in Gibbs free energy of n moles of vapor at vapor pressure P that condenses into a drop. Also the nucleation barrier, in polymer crystallization, consists of both enthalpic and entropic components that must be over come. This barrier consists of selection processes taking place in different length and time scales which relates to the multiple regimes later on. This barrier is the free energy required to overcome in order to form nuclei. It is the formation of the nuclei from the bulk to a surface that is the interfacial free energy. The interfacial free energy is always a positive term and acts to destabilize the nucleus allowing the continuation of the growing polymer chain. The nucleation continues as a favorable reaction. The Lauritzen–Hoffman plot (right) models the three different regimes when (logG) + U*/k(T-T) is plotted against (TΔT). It can be used to describe the rate at which secondary nucleation competes with lateral addition at the growth front among the different temperatures | https://en.wikipedia.org/wiki?curid=39642591 |
Hoffman nucleation theory This theory can be used to help understand the preferences of nucleation and growth based on the polymer's properties including its standard melting temperature. For many polymers, the change between the initial lamellar thickness at T is roughly the same as at T and can thus be modeled by the Gibbs–Thomson equation fairly well. However, since it implies that the lamellar thickness over the given supercooling range (T-T) is unchanged, and many homogeneous nucleation of polymers implies a change of thickness at the growth front, Hoffman and Weeks pursued a more accurate representation. In this regard, the Hoffman-Weeks plot was created and can be modeled through the equation formula_1 where β is representative of a thickening factor given by L = L β and Tand T are the crystallization and melting temperatures, respectively. Applying this experimentally for a constant β allows for the determination of the equilibrium melting temperature, T° at the intersection of Tand T. The crystallization process of polymers does not always obey simple chemical rate equations. Polymers can crystallize through a variety of different regimes and unlike simple molecules, the polymer crystal lamellae have two very different surfaces. The two most prominent theories in polymer crystallization kinetics are the Avrami equation and Lauritzen-Hoffman Growth Theory. The Lauritzen-Hoffman growth theory breaks the kinetics of polymer crystallization into ultimately two rates | https://en.wikipedia.org/wiki?curid=39642591 |
Hoffman nucleation theory The model breaks down into the addition of monomers onto a growing surface. This initial step is generally associated with the nucleation of the polymer. From there, the kinetics become the rate which the polymer grows on the surface, or the lateral growth rate, in comparison with the growth rate onto the polymer extending the chain, the secondary nucleation rate. These two rates can result in three situations. For Regime I, the growth rate on the front laterally, referred to as "g", is the rate-determining step (RDS) and exceeds the secondary nucleation rate, "i". In this instance of "g" » "i", monolayers are formed one at a time so that if the substrate has a length of L and thickness, "b", the overall linear growth can be described through the equation formula_2 and the rate of nucleation in specific can further be described by formula_3 with K equal to formula_4 where This shows that in Region I, lateral nucleation along the front successfully dominates at temperatures close to the melting temperature, however at more extreme temperatures other forces such as diffusion can impact nucleation rates. In Regime II, the lateral growth rate is either comparable or smaller than the nucleation rate "g" ≤ "i", which causes secondary (or more) layers to form before the initial layer has been covered | https://en.wikipedia.org/wiki?curid=39642591 |
Hoffman nucleation theory This allows the linear growth rate to be modeled by formula_5 Using the assumption that "g" and "i" are independent of time, the rate at which new layers are formed can be approximated and the rate of nucleation in regime II can be expressed as formula_6 with K equal to about 1/2 of the K from Regime I, formula_7 Lastly, Regime III in the L-H model depicts the scenario where lateral growth is inconsequential to the overall rate, since the nucleation of multiple sites causes "i » g". This means that the growth rate can be modeled by the same equation as Regime I, <math> G_\text{III} = biL_\text{p} = G_\text{III}^\circ e^ | https://en.wikipedia.org/wiki?curid=39642591 |
COSMO-RS (short for COnductor like Screening MOdel for Real Solvents) is a quantum chemistry based equilibrium thermodynamics method with the purpose of predicting chemical potentials µ in liquids. It processes the screening charge density σ on the surface of molecules to calculate the chemical potential µ of each species in solution. Perhaps in dilute solution a constant potential must be considered. As an initial step a quantum chemical COSMO calculation for all molecules is performed and the results (e.g. the screening charge density) are stored in a database. In a separate step uses the stored COSMO results to calculate the chemical potential of the molecules in a liquid solvent or mixture. The resulting chemical potentials are the basis for other thermodynamic equilibrium properties such as activity coefficients, solubility, partition coefficients, vapor pressure and free energy of solvation. The method was developed to provide a general prediction method with no need for system specific adjustment. Due to the use of σ from COSMO calculations, does not require functional group parameters. Quantum chemical effects like group-group interactions, mesomeric effects and inductive effects also are incorporated into by this approach. The method was first published in 1995 by A. Klamt. A refined version of was published in 1998 and is the basis for newer developments and reimplementations. The below description is a simplified overview of the version published in 1998 | https://en.wikipedia.org/wiki?curid=39644075 |
COSMO-RS As long as the above assumptions hold, the chemical potential µ in solution can be calculated from the interaction energies of pairwise surface contacts. Within the basic formulation of COSMO-RS, interaction terms depend on the screening charge density σ. Each molecule and mixture can be represented by the histogram p(σ), the so-called σ-profile. The σ-profile of a mixture is the weighted sum of the profiles of all its components. Using the interaction energy E(σ,σ') and the σ-profile of the solvent p(σ'), the chemical potential µ(σ) of a surface piece with screening charge σ is determined as: Due to the fact that µ(σ) is present on both sides of the equation, it needs to be solved iteratively. By combining the above equation with p(σ) for a solute x, and adding the σ-independent combinatorial and dispersive contributions, the chemical potential for a solute X in a solvent S results in: In analogy to activity coefficient models used in chemical engineering, such as NRTL, UNIQUAC or UNIFAC, the final chemical potential can be split into a combinatorial and a residual (non ideal) contribution. The interaction energies E(σ,σ') of two surface pieces are the crucial part for the final performance of the method and different formulations are used within the various implementations. In addition to the liquid phase terms a chemical potential estimate for the ideal gas phase µ has been added to to enable the prediction of vapor pressure, free energy of solvation and related quantities | https://en.wikipedia.org/wiki?curid=39644075 |
COSMO-RS The residual part is the sum of three different contributions, where E and E are part of E and µ is added directly to the chemical potential. In the E expression α is an adjustable parameter and σ and σ' refer to the screening charge densities of the two surface patches in contact. This term has been labeled "misfit" energy, because it results from the mismatch of the charged surface pieces in contact. It represents the Coulomb interaction relative to the state in a perfect conductor. A molecule in a perfect conductor (COSMO state) is perfectly shielded electronically; each charge on the molecular surface is shielded by a charge of the same size but of opposite sign. If the conductor is replaced by surface pieces of contacting molecules the screening of the surface will not be perfect any more. Hence an interaction energy from this misfit of σ on the surface patches will arise. In the E expression σ and σ are the screening charge densities of the hydrogen bond acceptor and donor respectively. The hydrogen bonding threshold σ and the prefactor c are adjustable parameters. The max[] and min[] construction ensures that the screening charge densities of the acceptor and donor exceeds the threshold for hydrogen bonding. The dispersion energy of a solute depends on an element (k) specific prefactor γ and the amount of exposed surface A of this element. It is not part of the interaction energy but enters the chemical potential directly | https://en.wikipedia.org/wiki?curid=39644075 |
COSMO-RS Though the use of quantum chemistry reduces the need for adjustable parameters, some fitting to experimental data is inevitable. The basic parameters are α, c, σ as used in the interaction energies, and one general parameter for the effective contact area. In addition, one adjustable van der Waals parameter γ per element is required. All parameters either are general or element specific, which is a distinctive feature of as compared to group contribution methods like UNIFAC. The original streamline of was continuously developed and extended by A. Klamt in his company COSMOlogic (now part of BIOVIA), and the most advanced software for is the COSMOtherm software, now available from BIOVIA. They also offer a huge database (COSMObase) with more than 12000 COSMO files. COSMOtherm proved its prediction accuracy by delivering the most accurate physicochemical property predictions in the recent SAMPL5 and SAMPL6 challenges. LVPP maintains an open sigma-profile database with COSMO-SAC ("Segment Activity Coefficient") parameterizations. Gaussian (software) claims to support via an external program. SCM licenses a commercial implementation in the Amsterdam Modeling Suite, which also includes COSMO-SAC, UNIFAC and QSPR models. "COSMO-RS: From quantum Chemistry to Fluid Phase Thermodynamics and Drug Design", A. Klamt, Elsevier: Amsterdam, 2005, "COSMO-RS: An Alternative to Simulation for Calculating Thermodynamic Properties of Liquid Mixtures", A. Klamt, F. Eckert and W | https://en.wikipedia.org/wiki?curid=39644075 |
COSMO-RS Arlt, Annual Review of Chemical and Biomolecular Engineering, 1, 101-122, (2010), DOI: 10.1146/annurev-chembioeng-073009-100903 | https://en.wikipedia.org/wiki?curid=39644075 |
Metal carbonyl cluster In chemistry, a metal carbonyl cluster is a compound that contains two or more metals linked in part by metal-metal bonds and containing carbon monoxide (CO) as the exclusive or predominant ligand. Simple examples include Fe(CO), Fe(CO), Mn(CO). High nuclearity clusters include [Rh(CO)H] and the stacked Pt triangules [Pt(CO)] (n = 2–6). The first metal carbonyl clusters, Fe(CO), Ir(CO), and Rh(CO), were reported starting in the 1930's, often by Walter Hieber. The structures weresubsequently established by X-ray crystallography.. Paolo Chini (1928-1980) was a pioneer for the synthesis and characterization of high nuclearity metal carbonyl clusters. His first studies started in 1958, in the attempt to repeat a patent that claimed an improved selectivity in hydroformylation. From a mixture of iron and cobalt carbonyls the first bimetallic carbonyl cluster HFeCo(CO) was obtained. Binary carbonyl clusters consist only of metal and CO. They are the most widely studied and used metal carbonyl clusters. They arise in general by the condensation of unsaturated metal carbonyls. Dissociation of CO from Ru(CO) would give Ru(CO), which could trimerize to Ru(CO). The reaction mechanisms are more complicated than this simple scenario. Condensation of low molecular weight metal carbonyls requires decarbonylation, which can be induced thermally, photochemically, or using various reagents. The nuclearity (number of metal centers) of binary metal carbonyl clusters is usually no greater than six | https://en.wikipedia.org/wiki?curid=39644483 |
Metal carbonyl cluster The synthesis and characterization of the platinum carbonyl dianions [Pt(CO)] (n = 1-10), also known as Chini Clusters or more correctly Chini-Longoni clusters, are recognized by the scientific community as the most spectacular result of Chini’s work. Chini clusters follow the general formula of [Pt(CO)],1 < n < 10. These clusters are prepared by reduction of hexachloroplatinate with strongly basic methanol under an atmosphere of CO. These clusters consist of stacks of triangularly shaped Pt subunits. Although these clusters were first reported in 1969 by Chatt and Booth, their structure were not established until Chini and Longoni’s work in 1976. Chini clusters are based on a planar triangular building block that can be condensed as multiple units forming chains usually anywhere from two to ten units long. The chains are formed by stacking of the planar units, extending through platinum to platinum bonds forming trigonal prismatic clusters. Within a triangular unit, the platinum-platinum bond lengths are 2.65 Å and between units the Pt---Pt bond lengths are 3.05 Å. Cluster structure is easily disrupted by deposition onto surfaces such as carbon or silicon, where the chains are broken, but the triangular subunits remain intact. The tetramer [Pt(CO)] is the most common member of this series of clusters. These clusters undergo reversible redox. They catalyze the hydrogenation of alkenes, ketones, aldehydes | https://en.wikipedia.org/wiki?curid=39644483 |
Metal carbonyl cluster Chini clusters can also be converted heterometal clusters and catalyze pH driven redox reactions and transport. First, the Chini clusters are the source of platinum atoms for the mixed metal cluster synthesis. For instance, the reaction [Pt(CO)] with [Ag(PPh)] produces heterometal cluster [PtAg(CO)(PPh)]. Second, the Chini clusters with redox properties act as a catalyst that helps transport sodium ions and electrons in the same direction across a liquid membrane, driven by pH-gradient. The [Pt(CO)] platinum clusters, where n = 4 – 6, are reduced by HO-. (n-1)[Pt(CO)] + 2OH ↔ n[Pt(CO)] + HO + 1/2O Although the nuclearity of binary metal carbonyl clusters is usually six or fewer, carbido clusters often have higher nuclearities. Metal carbonyls of the iron and cobalt triads are well known to form carbido derivatives. Examples include [RhC(CO)] and [RuC(CO)]. Carbonyl carbides exist not only with fully encapsulated carbon (e.g., [FeC(CO)]) but also with exposed carbon centres as in FeC(CO) and FeC(CO). For low nuclearity clusters, bonding is often described as if it is localized. For this purpose, the eighteen electron rule is used. Thus, 34 electrons in an organometallic complex predicts a dimetallic complex with a metal-metal bond. For higher nuclearity clusters, more elaborate rules are invoked including Jemmis mno rules and Polyhedral skeletal electron pair theory. Although clusters are often written with discrete M-M bonds, the nature of this bonding is unclear, especially when there are bridging ligands. | https://en.wikipedia.org/wiki?curid=39644483 |
C23H26O4 The molecular formula CHO may refer to: | https://en.wikipedia.org/wiki?curid=39646231 |
Graphene foam is a solid, open-cell foam made of single-layer sheets of graphene. It is a candidate substrate for the electrode of lithium-ion batteries. The foam can be manufactured using vapor deposition to coat a metal foam, a three-dimensional mesh of metal filaments. The metal is then removed. A physically flexible battery was created using the foam for electrodes. The anode was made by coating the foam with a lithium-titanium compound () and the cathode by coating the foam with . Both electrodes were lightweight and their large surface area provided high energy density of 110 Wh/kg, comparable to commercial batteries. Power density was much greater than a typical battery. At a rate that completely discharged the material in 18 seconds, power delivered was 80 percent of what it produced during an hour-long discharge. Performance remained stable through 500 charge/discharge cycles. In 2017 researchers used carbon nanotubes to reinforce a foam. The latter material supports 3,000 times its own weight and can return to its original shape when unweighted. Nanotubes, a powdered nickel catalyst and sugar were mixed. Dried pellets of the substance were then compressed in a steel die in the shape of a screw. The nickel was removed, leaving a screw-shaped piece of foam. The nanotubes' outer layers split and bonded with the graphene. | https://en.wikipedia.org/wiki?curid=39650838 |
Cheon Jinwoo is the H.G. Underwood Professor at Yonsei University and the Director of the Center for Nanomedicine, Institute for Basic Science (IBS). As a leading chemist in inorganic materials chemistry and nanomedicine Cheon and his group research chemical principles for the preparation of complex inorganic materials. He has been a Clarivate Analytics Highly Cited Researcher both in the field of chemistry in 2014, 2015, 2016 and cross-field in 2018. He is a fellow of the American Chemical Society, Royal Society of Chemistry, and Korean Academy of Science and Technology, a senior editor of "Accounts of Chemical Research" and an editorial advisory board member of "Journal of Materials Chemistry", "Nano Letters" and "Materials Horizons". Cheon enrolled in Yonsei University in 1981 majoring in chemistry. He later obtained a Bachelor of Science and Master of Science in 1985 and 1987, respectively. Studying under Professor Gregory S. Girolami, Cheon received a Ph.D. in Chemistry from the University of Illinois at Urbana-Champaign in 1993. Staying in the U.S., he did postdoc work in the University of California Berkeley Staying in the U.S., Cheon was a postdoc in the University of California Berkeley. For the next three years, he was a staff research associate at UCLA before returning to Korea to work as an assistant and then associate professor at KAIST. His research at KAIST focused on geometrical shape control of nanoparticles and magnetic particles | https://en.wikipedia.org/wiki?curid=39652486 |
Cheon Jinwoo This also marked his first publication on nanocrystals which is a reoccurring interest in his research career and a source of multiple highly cited articles. He started working at Yonsei University as a full professor in 2002 and later became the Horace G. Underwood Professor in 2008. His research at Yonsei on nanoscale phenomena has led to nanomaterial applications in biology, including highly sensitive MRI contrast agents and nanoscale toolkits for cells. A notable study is from 2004, when he demonstrated the principle of size-dependent MRI contrast effects using nanoparticles which enabled the development of magnetism-engineered iron oxide (MEIO) as an ultra-sensitive nanoparticle MRI contrast agent which might help detect early stage cancer. From 2010 to 2016, Cheon was the director of the National Creative Research Initiative Center for Evolutionary Nanoparticles. From 2015, he became the director of the newly established Yonsei-Institute of Basic Science Center for Nanomedicine at the Yonsei University Sinchon campus. | https://en.wikipedia.org/wiki?curid=39652486 |
Spherical basis In pure and applied mathematics, particularly quantum mechanics and computer graphics and their applications, a spherical basis is the basis used to express spherical tensors. The spherical basis closely relates to the description of angular momentum in quantum mechanics and spherical harmonic functions. While spherical polar coordinates are one orthogonal coordinate system for expressing vectors and tensors using polar and azimuthal angles and radial distance, the spherical basis are constructed from the standard basis and use complex numbers. A vector A in 3D Euclidean space can be expressed in the familiar Cartesian coordinate system in the standard basis e, e, e, and coordinates "A", "A", "A": or any other coordinate system with associated basis set of vectors. From this extend the scalars to allow multiplication by complex numbers, so that we are now working in formula_1 rather than formula_2 | https://en.wikipedia.org/wiki?curid=39653582 |
Spherical basis In the spherical bases denoted e, e, e, and associated coordinates with respect to this basis, denoted "A", "A", "A", the vector A is: where the spherical basis vectors can be defined in terms of the Cartesian basis using complex-valued coefficients in the "xy" plane: \mathbf{e}_{-} & = +\frac{1}{\sqrt{2}}\mathbf{e}_x - \frac{i}{\sqrt{2}}\mathbf{e}_y \\ in which "i" denotes the imaginary unit, and one normal to the plane in the "z" direction: The inverse relations are: \mathbf{e}_y & = + \frac{i}{\sqrt{2}} \mathbf{e}_+ + \frac{i}{\sqrt{2}}\mathbf{e}_{-} \\ \mathbf{e}_z & = \mathbf{e}_0 While giving a basis in a 3-dimensional space is a valid definition for a spherical tensor, it only covers the case for when the rank formula_4 is 1. For higher ranks, one may use either the commutator, or rotation definition of a spherical tensor. The commutator definition is given below, any operator formula_5 that satisfies the following relations is a spherical tensor : formula_6 formula_7 Analogously to how the spherical harmonics transform under a rotation, a general spherical tensor transforms as follows, when the states transform under the unitary Wigner D-matrix formula_8, where is a (3×3 rotation) group element in SO(3). That is, these matrices represent the rotation group elements. With the help of its Lie algebra, one can show these two definitions are equivalent | https://en.wikipedia.org/wiki?curid=39653582 |
Spherical basis For the spherical basis, the coordinates are complex-valued numbers "A", "A", "A", and can be found by substitution of () into (), or directly calculated from the inner product , (): A_{-} & = \left\langle \mathbf{A}, \mathbf{e}_- \right\rangle = +\frac{A_x}{\sqrt{2}} + \frac{iA_y}{\sqrt{2}} \\ with inverse relations: A_y & = - \frac{i}{\sqrt{2}} A_+ - \frac{i}{\sqrt{2}} A_{-} \\ A_z & = A_0 In general, for two vectors with complex coefficients in the same real-valued orthonormal basis e, with the property e·e = "δ", the inner product is: where · is the usual dot product and the complex conjugate * must be used to keep the magnitude (or "norm") of the vector positive definite. The spherical basis is an orthonormal basis, since the inner product , () of every pair vanishes meaning the basis vectors are all mutually orthogonal: and each basis vector is a unit vector: hence the need for the normalizing factors of 1/. The defining relations () can be summarized by a transformation matrix U: with inverse: It can be seen that U is a unitary matrix, in other words its Hermitian conjugate U (complex conjugate and matrix transpose) is also the inverse matrix U. For the coordinates: and inverse: Taking cross products of the spherical basis vectors, we find an obvious relation: where "q" is a placeholder for +, −, 0, and two less obvious relations: The inner product between two vectors A and B in the spherical basis follows from the above definition of the inner product: | https://en.wikipedia.org/wiki?curid=39653582 |
DNA-directed RNA interference (ddRNAi) is a gene-silencing technique that utilizes DNA constructs to activate an animal cell's endogenous RNA interference (RNAi) pathways. DNA constructs are designed to express self-complementary double-stranded RNAs, typically short-hairpin RNAs (shRNA), that once processed bring about silencing of a target gene or genes. Any RNA, including endogenous mRNAs or viral RNAs, can be silenced by designing constructs to express double-stranded RNA complementary to the desired mRNA target. This mechanism has great potential as a novel therapeutic to silence disease-causing genes. Proof-of-concept has been demonstrated across a range of disease models, including viral diseases such as HIV, hepatitis B or hepatitis C, or diseases associated with altered expression of endogenous genes such as drug-resistant lung cancer, neuropathic pain, advanced cancer and retinitis pigmentosa. As seen in Figure 1, a ddRNAi construct encoding an shRNA is packaged into a delivery vector or reagent tailored to target specific cells. Inside the cell, the DNA is transported to the nucleus where transcription machinery continually manufactures the encoded RNAs. The shRNA molecules are then processed by endogenous systems and enter the RNAi pathway and silence the desired genes | https://en.wikipedia.org/wiki?curid=39654121 |
DNA-directed RNA interference Unlike small interfering RNA (siRNA) therapeutics that turn over within a cell and consequently only silence genes transiently, DNA constructs are continually transcribed, replenishing the cellular ‘dose’ of shRNA, thereby enabling long-term silencing of targeted genes. The ddRNAi mechanism, therefore, offers the potential for ongoing clinical benefit with reduced medical intervention. Figure 2 illustrates the most common type of ddRNAi DNA construct, which is designed to express a shRNA. This consists of a promoter sequence, driving expression of sense and antisense sequences separated by a loop sequence, followed by a transcriptional terminator. The antisense species processed from the shRNA can bind to the target RNA and specify its degradation. shRNA constructs typically encode sense and antisense sequences of 20 – 30 nucleotides. Flexibility in construct design is possible: for example, the positions of sense and antisense sequences can be reversed, and other modifications and additions can alter intracellular shRNA processing. Moreover, a variety of promoter loop and terminator sequences can be used. A particularly useful variant is a multi-cassette (Figure 2b). Designed to express two or more shRNAs, they can target multiple sequences for degradation simultaneously. This is a particularly useful strategy for targeting viruses. Natural sequence variations can render a single shRNA-target site unrecognizable preventing RNA degradation | https://en.wikipedia.org/wiki?curid=39654121 |
DNA-directed RNA interference Multi-cassette constructs that target multiple sites within the same viral RNA circumvent this issue. Delivery of ddRNAi DNA constructs is simplified by the existence of a number of clinically-approved and well-characterized gene therapy vectors developed for the purpose. Delivery is a major challenge for RNAi-based therapeutics with new modifications and reagents continually being developed to optimize target cell delivery. Two broad strategies to facilitate delivery of DNA constructs to the desired cells are available: these use either viral vectors or one of a number of classes of transfection reagents. " In vivo " delivery of ddRNAi constructs has been demonstrated using a range of vectors and reagents with different routes of administration (ROA). ddRNAi constructs have also been successfully delivered into host cells ex vivo, and then transplanted back into the host. For example, in a Phase I clinical trial at the City of Hope National Medical Center, California, US, four HIV-positive patients with non-Hodgkin's lymphoma were successfully treated with autologous hematopoietic progenitor cells pre-transduced ex vivo with ddRNAi constructs using lentiviral vectors. This construct was designed to express three therapeutic RNAs, one of which was a shRNA, thereby combating HIV replication in three different ways: Ongoing expression of the shRNA has been confirmed in T cells, monocytes and B cells more than one year after transplantation | https://en.wikipedia.org/wiki?curid=39654121 |
DNA-directed RNA interference Nervana is an investigational ddRNAi construct that knocks down the expression of protein kinase C gamma (PKCγ) known to be associated with neuropathic pain and morphine tolerance. Two conserved PKCγ sequences found across all key model species and humans have been identified, and both single and double DNA cassettes designed. In vitro, expression of PKCγ was silenced by 80%. When similar ddRNAi constructs were delivered intrathecally using a lentiviral vector, pain relief in a neuropathic-rat model was demonstrated. The development of resistance to chemotherapies such as paclitaxel and cisplatin in non-small-cell lung cancer (NSCLC) is strongly associated with over expression of beta III tubulin. Investigations by the Children's Cancer Institute Australia (University of NSW, Lowy Cancer Research Centre) demonstrated that beta III-tubulin knockdown by ddRNAi delayed tumor growth and increased chemosensitivity in mouse models. Tributarna is a triple DNA cassette expressing three shRNA molecules that each separately target beta III tubulin and strongly inhibit its expression. Studies in an orthotopic-mouse model, where the construct is delivered by a modified polyethylenimine vector, jetPEI, that targets lung tissue are in progress. The hepatitis B virus (HBV) genome encodes its own DNA polymerase for replication | https://en.wikipedia.org/wiki?curid=39654121 |
DNA-directed RNA interference Biomics Biotechnologies has evaluated around 5000 siRNA sequences of this gene for effective knockdown; five sequences were chosen for further investigation and shown to have potent silencing activity when converted into shRNA expression cassettes. A multi-cassette construct, Hepbarna, is under preclinical development for delivery by an adeno-associated virus 8 (AAV-8) liver-targeting vector. Classified as an orphan disease, there is currently no therapy for OPMD, caused by a mutation in the poly(A) binding protein nuclear 1 (PABPN1) gene. Silencing the mutant gene using ddRNAi offers a potential therapeutic approach. Besides the ex vivo approach by the City of Hope National Medical Center discussed above, the Center for Infection and Immunity Amsterdam (CINIMA), University of Amsterdam, the Netherlands, is extensively researching the composition of multi-cassette DNA constructs to tackle HIV. As with all gene therapies, a number of safety and toxicity issues need to be evaluated during the development of ddRNAi therapeutics: Oncogene activation by viral insertion: Some gene therapy vectors integrate into the host genome, thereby acting as insertional mutagens. This was a particular issue with early retroviral vectors where insertions adjacent to oncogenes resulted in the development of lymphoid tumors | https://en.wikipedia.org/wiki?curid=39654121 |
DNA-directed RNA interference AAV vectors are considered a low risk for host-genome integration, as adeno-associated virus infection has not been associated with the induction of cancers in humans despite widespread prevalence across the general population. Moreover, extensive clinical use of AAV vectors has provided no evidence of carcinogenicity. While lentiviral vectors do integrate into the genome they do not appear to show a propensity to activate oncogene expression. Immune response to gene therapy vectors: An immunological response to an adenoviral vector resulted in the death of a patient in an early human trial. Careful monitoring of potential toxicities in preclinical testing and analyses of pre-existing antibodies to gene therapy vectors in patients minimizes such risks. Innate immune response: siRNAs have been shown to activate immune responses through interaction with Toll-like receptors leading to interferon responses. These receptors reside on the cells surface and so ddRNAi constructs – delivered directly into intracellular space – are not expected to induce this response. Toxic effects due to over-expression of shRNAs: High level expression of shRNAs has been shown to be toxic. Strategies to minimize levels of shRNA expression or promote precise processing of shRNAs can overcome this problem. Off-target effects: Unintended silencing of genes that share sequence homology with expressed shRNAs can theoretically occur | https://en.wikipedia.org/wiki?curid=39654121 |
DNA-directed RNA interference Careful selection of shRNA sequences and thorough preclinical testing of constructs can circumvent this issue. | https://en.wikipedia.org/wiki?curid=39654121 |
Tensor operator In pure and applied mathematics, quantum mechanics and computer graphics, a tensor operator generalizes the notion of operators which are scalars and vectors. A special class of these are spherical tensor operators which apply the notion of the spherical basis and spherical harmonics. The spherical basis closely relates to the description of angular momentum in quantum mechanics and spherical harmonic functions. The coordinate-free generalization of a tensor operator is known as a representation operator. In quantum mechanics, physical observables that are scalars, vectors, and tensors, must be represented by scalar, vector, and tensor operators, respectively. Whether something is a scalar, vector, or tensor depends on how it is viewed by two observers whose coordinate frames are related to each other by a rotation. Alternatively, one may ask how, for a single observer, a physical quantity transforms if the state of the system is rotated. Consider, for example, a system consisting of a molecule of mass formula_1, traveling with a definite center of mass momentum, formula_2, in the formula_3 direction. If we rotate the system by formula_4 about the formula_5 axis, the momentum will change to formula_6, which is in the formula_7 direction. The center-of-mass kinetic energy of the molecule will, however, be unchanged at formula_8. The kinetic energy is a scalar and the momentum is a vector, and these two quantities must be represented by a scalar and a vector operator, respectively | https://en.wikipedia.org/wiki?curid=39654522 |
Tensor operator By the latter in particular, we mean an operator whose expected values in the initial and the rotated states are formula_2 and formula_6. The kinetic energy on the other hand must be represented by a scalar operator, whose expected value must be the same in the initial and the rotated states. In the same way, tensor quantities must be represented by tensor operators. An example of a tensor quantity (of rank two) is the electrical quadrupole moment of the above molecule. Likewise, the octupole and hexadecapole moments would be tensors of rank three and four, respectively. Other examples of scalar operators are the total energy operator (more commonly called the Hamiltonian), the potential energy, and the dipole-dipole interaction energy of two atoms. Examples of vector operators are the momentum, the position, the orbital angular momentum, formula_11, and the spin angular momentum, formula_12. (Fine print: Angular momentum is a vector as far as rotations are concerned, but unlike position or momentum it does not change sign under space inversion, and when one wishes to provide this information, it is said to be a pseudovector.) Scalar, vector and tensor operators can also be formed by products of operators. For example, the scalar product formula_13 of the two vector operators, formula_11 and formula_12, is a scalar operator, which figures prominently in discussions of the spin-orbit interaction | https://en.wikipedia.org/wiki?curid=39654522 |
Tensor operator Similarly, the quadrupole moment tensor of our example molecule has the nine components Here, the indices formula_17 and formula_18 can independently take on the values 1, 2, and 3 (or formula_7, formula_5, and formula_3) corresponding to the three Cartesian axes, the index formula_22 runs over all particles (electrons and nuclei) in the molecule, formula_23 is the charge on particle formula_22, and formula_25 is the formula_17th component of the position of this particle. Each term in the sum is a tensor operator. In particular, the nine products formula_27 together form a second rank tensor, formed by taking the direct product of the vector operator formula_28 with itself. The rotation operator about the unit vector n (defining the axis of rotation) through angle "θ" is where J = ("J", "J", "J") are the rotation generators (also the angular momentum matrices): and let formula_31 be a rotation matrix. According to the Rodrigues' rotation formula, the rotation operator then amounts to An operator formula_33 is invariant under a unitary transformation "U" if in this case for the rotation formula_35, The orthonormal basis set for total angular momentum is formula_37, where "j" is the total angular momentum quantum number and "m" is the magnetic angular momentum quantum number, which takes values −"j", −"j" + 1, ..., "j" − 1, "j" | https://en.wikipedia.org/wiki?curid=39654522 |
Tensor operator A general state in the space rotates to a new state formula_37 by: Using the completeness condition: we have Introducing the Wigner D matrix elements: gives the matrix multiplication: For one basis ket: For the case of orbital angular momentum, the eigenstates formula_46 of the orbital angular momentum operator L and solutions of Laplace's equation on a 3d sphere are spherical harmonics: where "P" is an associated Legendre polynomial, is the orbital angular momentum quantum number, and "m" is the orbital magnetic quantum number which takes the values −, − + 1, ... − 1, The formalism of spherical harmonics have wide applications in applied mathematics, and are closely related to the formalism of spherical tensors, as shown below. Spherical harmonics are functions of the polar and azimuthal angles, "ϕ" and "θ" respectively, which can be conveniently collected into a unit vector n("θ", "ϕ") pointing in the direction of those angles, in the Cartesian basis it is: So a spherical harmonic can also be written formula_49. Spherical harmonic states formula_50 rotate according to the inverse rotation matrix "U"("R"), while formula_46 rotates by the initial rotation matrix formula_35 | https://en.wikipedia.org/wiki?curid=39654522 |
Tensor operator We define the Rotation of an operator by requiring that the expectation value of the original operator formula_54 with respect to the initial state be equal to the expectation value of the rotated operator with respect to the rotated state, Now as, we have, since, formula_56 is arbitrary, A scalar operator is invariant under rotations: This is equivalent to saying a scalar operator commutes with the rotation generators: Examples of scalar operators include Vector operators (as well as pseudovector operators) are a set of 3 operators that can be rotated according to: from this and the infinitesimal rotation operator and its Hermitian conjugate, and ignoring second order term in formula_69,one can derive the commutation relation with the rotation generator: where "ε" is the Levi-Civita symbol, which all vector operators must satisfy, by construction. As the symbol "ε" is a pseudotensor, pseudovector operators are invariant up to a sign: +1 for proper rotations and −1 for improper rotations. Vector operators include and peusodovector operators include In Dirac notation: and since | "Ψ" > is any quantum state, the same result follows: Note that here, the term "vector" is used two different ways: kets such as are elements of abstract Hilbert spaces, while the vector operator is defined as a quantity whose components transform in a certain way under rotations | https://en.wikipedia.org/wiki?curid=39654522 |
Tensor operator A vector operator in the spherical basis is where the components are: and the commutators with the rotation generators are: where "q" is a placeholder for the spherical basis labels (+1, 0, −1), and: (some authors may place a factor of 1/2 on the left hand side of the equation) and raise ("J") or lower ("J") the total magnetic quantum number "m" by one unit. In the spherical basis the generators are: The rotation transformation in the spherical basis (originally written in the Cartesian basis) is then: One can generalize the "vector" operator concept easily to "tensorial operators", shown next. A tensor operator can be rotated according to: Consider a dyadic tensor with components "T" = "ab", this rotates infinitesimally according to: Cartesian dyadic tensors of the form where a and b are two vector operators: are reducible, which means they can be re-expressed in terms of a and b as a rank 0 tensor (scalar), plus a rank 1 tensor (an antisymmetric tensor), plus a rank 2 tensor (a symmetric tensor with zero trace): where the first term includes just one component, a scalar equivalently written (a·b)/3, the second includes three independent components, equivalently the components of (a×b)/2, and the third includes five independent components. Throughout, "δ" is the Kronecker delta, the components of the identity matrix. The number in the superscripted brackets denotes the tensor rank | https://en.wikipedia.org/wiki?curid=39654522 |
Tensor operator These three terms are irreducible, which means they cannot be decomposed further and still be tensors satisfying the defining transformation laws under which they must be invariant. These also correspond to the number of spherical harmonic functions 2 + 1 for = 0, 1, 2, the same as the ranks for each tensor. Each of the irreducible representations T, T ... transform like angular momentum eigenstates according to the number of independent components. Example of a Tensor operator, in general, "Note:" This is just an example, in general, a tensor operator cannot be written as the product of two Tensor operators as given in the above example. Continuing the previous example of the second order dyadic tensor T = a ⊗ b, casting each of a and b into the spherical basis and substituting into T gives the spherical tensor operators of the second order, which are: Using the infinitesimal rotation operator and its Hermitian conjugate, one can derive the commutation relation in the spherical basis: and the finite rotation transformation in the spherical basis is: In general, tensor operators can be constructed from two perspectives. One way is to specify how spherical tensors transform under a physical rotation - a group theoretical definition. A rotated angular momentum eigenstate can be decomposed into a linear combination of the initial eigenstates: the coefficients in the linear combination consist of Wigner rotation matrix entries | https://en.wikipedia.org/wiki?curid=39654522 |
Tensor operator Spherical tensor operators are sometimes defined as the set of operators that transform just like the eigenkets under a rotation. A spherical tensor "T" of rank "k" is defined to rotate into "T" according to: where "q" = "k", "k" − 1, ..., −"k" + 1, −"k". For spherical tensors, "k" and "q" are analogous labels to and "m" respectively, for spherical harmonics. Some authors write "T" instead of "T", with or without the parentheses enclosing the rank number "k". Another related procedure requires that the spherical tensors satisfy certain commutation relations with respect to the rotation generators "J", "J", "J" - an algebraic definition. The commutation relations of the angular momentum components with the tensor operators are: For any 3d vector, not just a unit vector, and not just the position vector: a spherical tensor is a spherical harmonic as a function of this vector a, and in Dirac notation: (the super and subscripts switch places for the corresponding labels "" ↔ "k" and "m" ↔ "q" which spherical tensors and spherical harmonics use). Spherical harmonic states and spherical tensors can also be constructed out of the Clebsch–Gordan coefficients. Irreducible spherical tensors can build higher rank spherical tensors; if "A" and "B" are two spherical tensors of ranks "k" and "k" respectively, then: is a spherical tensor of rank | https://en.wikipedia.org/wiki?curid=39654522 |
Tensor operator The Hermitian adjoint of a spherical tensor may be defined as There is some arbitrariness in the choice of the phase factor: any factor containing will satisfy the commutation relations. The above choice of phase has the advantages of being real and that the tensor product two commuting Hermitian operators is still Hermitian. Some authors define it with a different sign on , without the , or use only the floor of . Orbital angular momentum operators have the ladder operators: which raise or lower the orbital magnetic quantum number "m" by one unit. This has almost exactly the same form as the spherical basis, aside from constant multiplicative factors. Spherical tensors can also be formed from algebraic combinations of the spin operators "S", "S", "S", as matrices, for a spin system with total quantum number "j" = + "s" (and = 0). Spin operators have the ladder operators: which raise or lower the spin magnetic quantum number "m" by one unit. Spherical bases have broad applications in pure and applied mathematics and physical sciences where spherical geometries occur. The transition amplitude is proportional to matrix elements of the dipole operator between the initial and final states. We use an electrostatic, spinless model for the atom and we consider the transition from the initial energy level E to final level E. These levels are degenerate, since the energy does not depend on the magnetic quantum number m or m′ | https://en.wikipedia.org/wiki?curid=39654522 |
Tensor operator The wave functions have the form, The dipole operator is proportional to the position operator of the electron, so we must evaluate matrix elements of the form, where, the initial state is on the right and the final one on the left. The position operator r has three components, and the initial and final levels consist of 2ℓ + 1 and 2ℓ′ + 1 degenerate states, respectively. Therefore if we wish to evaluate the intensity of a spectral line as it would be observed, we really have to evaluate 3(2ℓ′+ 1)(2ℓ+ 1) matrix elements, for example, 3×3×5 = 45 in a 3d → 2p transition. This is actually an exaggeration, as we shall see, because many of the matrix elements vanish, but there are still many non-vanishing matrix elements to be calculated. A great simplification can be achieved by expressing the components of r, not with respect to the Cartesian basis, but with respect to the spherical basis. First we define, Next, by inspecting a table of the Y′s, we find that for ℓ = 1 we have, where, we have multiplied each Y by the radius r. On the right hand side we see the spherical components r of the position vector r. The results can be summarized by, for q = 1, 0, −1, where q appears explicitly as a magnetic quantum number. This equation reveals a relationship between vector operators and the angular momentum value ℓ = 1, something we will have more to say about presently | https://en.wikipedia.org/wiki?curid=39654522 |
Tensor operator Now the matrix elements become a product of a radial integral times an angular integral, We see that all the dependence on the three magnetic quantum numbers (m′,q,m) is contained in the angular part of the integral. Moreover, the angular integral can be evaluated by the three-Y formula, whereupon it becomes proportional to the Clebsch-Gordan coefficient, The radial integral is independent of the three magnetic quantum numbers (m′, q, m), and the trick we have just used does not help us to evaluate it. But it is only one integral, and after it has been done, all the other integrals can be evaluated just by computing or looking up Clebsch-Gordan coefficients. The selection rule m′ = q + m in the Clebsch-Gordan coefficient means that many of the integrals vanish, so we have exaggerated the total number of integrals that need to be done. But had we worked with the Cartesian components r of r, this selection rule might not have been obvious. In any case, even with the selection rule, there may still be many nonzero integrals to be done (nine, in the case 3d → 2p). The example we have just given of simplifying the calculation of matrix elements for a dipole transition is really an application of the Wigner-Eckart theorem, which we take up later in these notes. The spherical tensor formalism provides a common platform for treating coherence and relaxation in nuclear magnetic resonance | https://en.wikipedia.org/wiki?curid=39654522 |
Tensor operator In NMR and EPR, spherical tensor operators are employed to express the quantum dynamics of particle spin, by means of an equation of motion for the density matrix entries, or to formulate dynamics in terms of an equation of motion in Liouville space. The Liouville space equation of motion governs the observable averages of spin variables. When relaxation is formulated using a spherical tensor basis in Liouville space, insight is gained because the relaxation matrix exhibits the cross-relaxation of spin observables directly. | https://en.wikipedia.org/wiki?curid=39654522 |
Dimethoxycoumarin may refer to: | https://en.wikipedia.org/wiki?curid=39657878 |
Anthocyanone A is a degradation product of malvidin 3-O-glucoside under acidic conditions. It is found in wine. | https://en.wikipedia.org/wiki?curid=39658504 |
Lead tetrafluoride is a compound of lead and fluorine. The yellow solid (melting point 600 °C) is the only room-temperature stable tetrahalide of lead. is isostructural with tin(IV) fluoride and contains planar layers of octahedrally coordinated lead, where the octahedra share four corners and there are two terminal, unshared, fluorine atoms "trans" to one another. | https://en.wikipedia.org/wiki?curid=39665033 |
Lanthanum trifluoride is a refractory ionic compound of lanthanum and fluorine. Bonding is ionic with lanthanum highly coordinated. The cation sits at the center of a trigonal prism. Nine fluorides are close: three at the bottom corners of the trigonal prism, three in the faces of the trigonal prism, and three at top corners of the trigonal prism. There are also two fluorides a little further away above and below the prism. The cation can be considered 9-coordinate or 11-coordinate. The larger sized rare earth elements (lanthanides), which are those with smaller atomic number, also form trifluorides with the LaF structure. Some actinides do as well. Lanthanum fluoride is sometimes used as the "high-index" component in multilayer optical elements such as ultraviolet dichroic and narrowband mirrors. Fluorides are among the most commonly used compounds for UV optical coatings due to their relative inertness and transparency in the far ultraviolet (FUV, 100 nm <λ <200 nm). Multilayer reflectors and antireflection coatings are typically composed of pairs of transparent materials, one with a low index of refraction, the other with a high index. There are very few high-index materials in the far UV. LaF3 is one. The material is also a component of multimetal fluoride glasses such as ZBLAN. It is also used (with europium fluoride, EuF2) in fluoride selective electrodes. | https://en.wikipedia.org/wiki?curid=39665579 |
Carborane acid Carborane acids H(CXBYZ) (X, Y, Z = H, Alk, F, Cl, Br, CF) are a class of superacids, some of which are estimated to be at least one million times stronger than 100% sulfuric acid in terms of their Hammett acidity function values ("H" ≤ –18) and possess computed p"K" values well below –20, establishing them as some of the strongest known Brønsted acids. The most well studied example is the highly chlorinated derivative H(CHBCl). The acidity of H(CHBCl) was found to vastly exceed that of triflic acid, CFSOH, and bistriflimide, (CFSO)NH, compounds previously regarded as the strongest isolable acids. Their high acidities stem from the extensive delocalization of their conjugate bases, carboranate anions (CXBYZ), which are usually further stabilized by electronegative groups like Cl, F, and CF. Due to the lack of oxidizing properties and the exceptionally low nucleophilicity and high stability of their conjugate bases, they are the only superacids known to protonate C fullerene without decomposing it. Additionally, they form stable, isolable salts with protonated benzene, CH, the parent compound of the Wheland intermediates encountered in electrophilic aromatic substitution reactions. The fluorinated carborane acid, H(CHBF), is even stronger than chlorinated carborane acid. It is able to protonate butane to form "tert"-butyl cation at room temperature and is the only known acid to protonate carbon dioxide to give the bridged cation, [H(CO)], making it possibly the strongest known acid | https://en.wikipedia.org/wiki?curid=39667666 |
Carborane acid In particular, CO does not undergo observable protonation when treated with the mixed superacids HF-SbF or HSOF-SbF. As a class, the carborane acids form the most acidic group of well-defined, isolable substances known, far more acidic than previously known single-component strong acids like triflic acid or perchloric acid. In certain cases, like the nearly perhalogenated derivatives mentioned above, their acidities rival (and possibly exceed) those of the traditional mixed Lewis-Brønsted superacids like magic acid and fluoroantimonic acid. (However, a head-to-head comparison has not been possible thus far, due to the lack of a measure of acidity that is suitable for both classes of acids: p"K" values are ill-defined for the chemically complex mixed acids while "H" values cannot be measured for the very high melting carborane acids). A Brønsted-Lowry acid’s strength corresponds with its ability to release a hydrogen ion. One common measure of acid strength for concentrated, superacidic liquid media is the Hammett acidity function, "H". Based on its ability to quantitatively protonate benzene, the chlorinated carborane acid H(CHBCl) was conservatively estimated to have an "H" value at or below −18, leading to the common assertion that carborane acids are at least a million times stronger than 100% sulfuric acid ("H" = −12). However, since the "H" value measures the protonating ability of a "liquid" medium, the crystalline and high-melting nature of these acids precludes direct measurement of this parameter | https://en.wikipedia.org/wiki?curid=39667666 |
Carborane acid In terms of p"K", a slightly different measure of acidity defined as the ability of a given solute to undergo ionization in a solvent, carborane acids are estimated to have p"K" values below −20, even without electron-withdrawing substituents on the boron atoms (e.g., H(CHBH) is estimated to have a p"K" of −24), with the (yet unknown) fully fluorinated analog H(CFBF) having a calculated p"K" of −46. The known acid H(CHBF) with one fewer fluorine is expected to be only slightly weaker (p"K" < −40). In the gas phase, H(CHBF) has a computed acidity of 216 kcal/mol, compared to an experimentally determined acidity of 241 kcal/mol (in reasonable agreement with the computed value of 230 kcal/mol) for H(CHBCl). In contrast, HSbF (a simplified model for the proton donating species in fluoroantimonic acid) has a computed gas phase acidity of 255 kcal/mol, while the previous experimentally determined record holder was (CFSO)NH, a congener of bistriflimide, at 291 kcal/mol. Thus, H(CHBF) is likely the most acidic substance so far synthesized in bulk, in terms of its gas phase acidicity. In view of its unique reactivity, it is also a strong contender for being the most acidic substance in the condensed phase (see above). Some even more strongly acidic derivatives have been predicted, with gas phase acidities < 200 kcal/mol. Carborane acids differ from classical superacids in being well-defined one component substances. In contrast, classical superacids are often mixtures of a Brønsted acid and Lewis acid (e.g. HF/SbF) | https://en.wikipedia.org/wiki?curid=39667666 |
Carborane acid Despite being the strongest acid, the boron-based carborane acids are described as being "gentle", cleanly protonating weakly basic substances without further side reactions. Whereas conventional superacids decompose fullerenes due to their strongly oxidizing Lewis acidic component, carborane acid has the ability to protonate fullerenes at room temperature to yield an isolable salt. Furthermore, the anion that forms as a result of proton transfer is nearly completely inert. This property is what makes the carborane acids the only substances that are comparable in acidity to the mixed superacids that can also be stored in a glass bottle, as various fluoride-donating species (which attack glass) are not present or generated. was first discovered and synthesized by Professor Christopher Reed and his colleagues in 2004 at the University of California, Riverside. Prior to carborane acid's discovery, the long-standing record of “strongest acids as single isolable compounds” was held by the two superacids, fluorosulfonic acid and trifluoromethanesulfonic acid, with p"K"s of −14 and −16 respectively. The parent molecule from which carborane acid is derived, an icosahedral carboranate anion, , was first synthesized at DuPont in 1967 by Walter Knoth. Research into this molecule's properties was put on hiatus until the mid 1980s when the Czech group of boron scientists, Plešek, Štíbr, and Heřmánek improved the process for halogenation of carborane molecules | https://en.wikipedia.org/wiki?curid=39667666 |
Carborane acid These findings were instrumental in developing the current procedure for carborane acid synthesis. The process consists of treating Cs[HCBH] with , refluxing under dry argon to fully chlorinate the molecule yielding carborane acid, but this has been shown to fully chlorinate only under select conditions. In 2010, Reed published a guide giving detailed procedures for the synthesis of carborane acids and their derivatives. Nevertheless, the synthesis of carborane acids remains lengthy and difficult and requires a well-maintained glovebox and some specialized equipment. The starting material is commercially available decaborane(14), a highly toxic substance. The most well-studied carborane acid H(CHBCl) is prepared in 13 steps. The last few steps are especially sensitive and require a glovebox at < 1 ppm HO without any weakly basic solvent vapors, since bases as weak as benzene or dichloromethane will react with carborane-based electrophiles and Brønsted acids. The final step of the synthesis is the metathesis of the μ-hydridodisilylium carboranate salt with excess liquid, anhydrous hydrogen chloride, presumably driven by the formation of strong Si–Cl and H–H bonds in the volatile byproducts: The product was isolated by evaporation of the byproducts and was characterized by its infrared (ν = 3023 cm) and nuclear magnetic resonance (δ 4.55 (s, 1H, CH), 20.4 (s, 1H, H) in liquid SO) spectra (note the extremely downfield chemical shift of the acidic proton) | https://en.wikipedia.org/wiki?curid=39667666 |
Carborane acid Although the reactions used in the synthesis are analogous, obtaining a pure sample of the more acidic H(CHBF) turned out to be even more difficult, requiring extremely rigorous procedures to exclude traces of weakly basic impurities. consists of 11 boron atoms; each boron atom is bound to a chlorine atom. The chlorine atoms serve to enhance acidity and act as shields against attacks from the outside due to the steric hindrance they form around the cluster. The cluster, consisting of the 11 borons, 11 chlorines, and a single carbon atom, is paired with a hydrogen atom, bound to the carbon atom. The boron and carbon atoms are allowed to form six bonds due to boron’s ability to form three-center, two-electron bonds. Although the structure of the carborane acid differs greatly from conventional acids, both distribute charge and stability in a similar fashion. The carboranate anion distributes its charge by delocalizing the electrons throughout the 12 cage atoms. This was shown in a single crystal X-ray diffraction study revealing shortened bond lengths in the heterocyclic portion of the ring suggesting electronic delocalization. The chlorinated carba-"closo"-dodecaborate anion is an outstandingly stable anion with what has previously been described as “substitutionally inert” B–Cl vertices. The descriptor "closo" indicates that the molecule is formally derived (by B-to-C replacement) from a borane of stoichiometry and charge [BH""] ("n =" 12 for known carborane acids) | https://en.wikipedia.org/wiki?curid=39667666 |
Carborane acid The cagelike structure formed by the 11 boron atoms and 1 carbon atom allows the electrons to be highly delocalized through the 3D cage (the special stabilization of the carborane system has been termed "σ-aromaticity"), and the high energy required to disrupt the boron cluster portion of the molecule is what gives the anion its remarkable stability. Because the anion is extremely stable, it will not behave as a nucleophile toward the protonated substrate, while the acid itself is completely non-oxidizing, unlike the Lewis acidic components of many superacids like antimony pentafluoride. Hence, sensitive molecules like C can be protonated without decomposition. There are many proposed applications for the boron-based carborane acids. For instance, they have been proposed as catalysts for hydrocarbon cracking and isomerization of "n"-alkanes to form branched isoalkanes ("isooctane", for example). Carborane acids may also be used as strong, selective Brønsted acids for fine chemical synthesis, where the low nucleophilicity of the counteranion may be advantageous. In mechanistic organic chemistry, they may be used in the study of reactive cationic intermediates. In inorganic synthesis, their unparalleled acidity may allow for the isolation of exotic species like salts of protonated xenon. | https://en.wikipedia.org/wiki?curid=39667666 |
Reversible-deactivation polymerization (RDP) is a form of polymerization propagated by chain carriers the some of which at any instant are held in a state of dormancy through an equilibrium process involving other species. An example of reversible deactivation anionic polymerization (RDAP) is group transfer polymerization of alkyl methacrylates, where the initiator and the dormant state is a silyl ketene acetal. In the case of reversible deactivation radical polymerization (RDRP), a majority of chain must be held in a dormant state to ensure that the concentration of active carriers is sufficiently low as to render chain termination reactions negligible. Despite having some common features, RDP is distinct from living polymerization which requires a complete absence of termination and irreversible chain transfer. | https://en.wikipedia.org/wiki?curid=39687960 |
C11H6O4 The molecular formula CHO (molar mass: 202.16 g/mol, exact mass: 202.026609 u) may refer to : | https://en.wikipedia.org/wiki?curid=39692445 |
Zero liquid discharge Zero Liquid Discharge (ZLD) is a treatment process designed to remove all the liquid waste from a system. The focus of ZLD is to reduce wastewater economically and produce clean water that is suitable for reuse (e.g. irrigation), thereby saving money and being beneficial to the environment. ZLD systems employ advanced wastewater/desalination treatment technologies to purify and recycle virtually all of the wastewater produced. Also ZLD technologies help plants meet discharge and water reuse requirements, enabling businesses to: The conventional way to reach ZLD is with thermal technologies such as evaporators (multi stage flash (MSF), multi effect distillation (MED) and mechanical vapor compression (MCV)) and crystallizers and recover their condensate. Thus, ZLD plants produce solid waste. ZLD technology includes pre-treatment and evaporation of the industrial effluent until the dissolved solids precipitate as crystals. These crystals are removed and dewatered with a filter press or a centrifuge. The water vapor from evaporation is condensed and returned to the process. In the last decades though, there has been an effort from the water treatment industry to revolutionize the high water recovery and ZLD technologies . This has led to processes like electrodialysis (ED/EDR), forward osmosis (FO) and membrane distillation (MD). A quick overview and comparison can be seen by the following table , Despite the variable sources of a wastewater stream, a ZLD system is generally comprised by two steps | https://en.wikipedia.org/wiki?curid=39695061 |
Felicity effect The is an effect observed during acoustic emission in a structure undergoing repeated mechanical loading. It negates the effect of emission silence in the structure that is often observed from the related Kaiser effect at high loads. A material demonstrating the gives off acoustic emission at a lower load than one previously reached in an increasing load cycle regime. | https://en.wikipedia.org/wiki?curid=39697622 |
Kaiser effect (material science) The Kaiser effect is a phenomenon observed in geology and material science that describes a pattern of acoustic emission (AE) or seismicity in a body of rock or other material subjected to repeated cycles of mechanical stress. In material that exhibits an initial seismic response under a certain load, the Kaiser effect describes the absence of acoustic emission or seismic events until that load is exceeded. The Kaiser effect results from discontinuities (fractures) created in material during previous steps that do not move, expand, or propagate until the former stress is exceeded. The Kaiser effect is named after Joseph Kaiser, who first studied this behavior in materials in the 1950s. He discovered the phenomenon when he was studying AE response of metals, finding that the materials retain a "memory" of previously applied stresses. The German researcher found that a stressed metal sample is zero if the applied stress is less than the previously applied maximum stress. Similar effect was also found in rock samples deformed in the course of acoustic emission, particularly as a result of cyclic thermal loadings of carboniferous sandstone and mudstone samples. The Kaiser effect became useful in estimating complete stress tensors based on a capacity to determine reliably the magnitudes of the preceding normal stresses applied to the specimen in various directions | https://en.wikipedia.org/wiki?curid=39697749 |
Kaiser effect (material science) Induced seismicity associated with fluid pumping in boreholes and wells often exhibits the Kaiser effect, whereby seismicity may be observed shortly following an initial fluid injection, but further seismicity is limited if the fluid flow remains at a constant pressure. If the fluid pressure at the injection site is later increased, renewed seismicity may be observed due to the greater ease of fracturing caused by higher pore fluid pressure in the rock. The Kaiser effect has also been observed in relation to recharge of magma chambers below active volcanic systems. | https://en.wikipedia.org/wiki?curid=39697749 |
International Copper Study Group The (ICSG) is an intergovernmental organisation of copper producing and consuming states that functions as the international commodity board for copper. Its main purpose is to increase copper market transparency and promote international discussions and cooperation on issues related to copper. As of 2019, ICSG Share in the World Copper Market represents 82 percent of world copper mine production, 87 percent of world copper refined production and 83 percent of the world copper refined usage. The creation of the ICSG was negotiated in 1989 in Geneva and was agreed to in a multilateral treaty known as the 'Agreement establishing the Terms of Reference of the International Copper Study Group'. The ICSG came into existence on 23 January 1992, with headquarters in Lisbon, Portugal. In order to fulfill its mandate, the Study Group has three main objectives: ICSG publishes monthly Bulletins with statistical data on copper mine, smelter and refinery production, copper usage, stocks, prices and trade for copper products (sample available on ICSG website). ICSG also publishes a Directory on the current and planned development of world mines, smelters and refineries. (sample available on ICSG website). An annual Directory is also published on the current status of copper semis plants. ICSG maintains one of the world's most complete historical and current databases with copper statistics. For more information on ICSG publications please go to ICSG website | https://en.wikipedia.org/wiki?curid=39698577 |
International Copper Study Group ICSG published a monthly press release on the state of the copper market. To be included in the distribution list please contact ICSG. ICSG also undertakes regular studies on topics of interest to the copper industry such as: Membership in the ICSG is open to any state that is involved in the production, trade, or consumption of copper. As of 2019, there are 24 members, including the European Union: There are six states that were previously full members of ICSG, but have withdrawn from the organisation: | https://en.wikipedia.org/wiki?curid=39698577 |
C21H27ClO5 The molecular formula CHClO may refer to: | https://en.wikipedia.org/wiki?curid=39699961 |
INT (chemical) INT (iodonitrotetrazolium or 2-(4-iodophenyl)-3-(4-nitrophenyl)-5-phenyl-2"H"-tetrazolium) is a commonly used tetrazolium salt (usually prepared with chloride ions), similar to tetrazolium chloride that on reduction produces a red formazan dye that can be used for quantitative redox assays. It is also toxic to prokaryotes. INT is an artificial electron acceptor which can be utilized in a colorimetric assay to determine the concentration of protein in a solution. It can be reduced by succinate dehydrogenase to furazan, the formation of which can be measured by absorbance at 490 nm. The activity of succinate dehydrogenase is readily observed by the naked eye as the solution turns from colorless to rusty red. | https://en.wikipedia.org/wiki?curid=39701308 |
Ramboll Studio Dreiseitl is one of the leading landscape architecture practices of Germany specialising in the integration of art, urban hydrology, environmental engineering, and landscape architecture within an urban context. The practise was founded in 1980 by the German landscape architect Herbert Dreiseitl with a goal to promote sustainable projects with a high aesthetic and social value. Today it has offices in Germany, Singapore and Beijing. In May 2013, Atelier Dreiseitl (now Ramboll Studio Dreiseitl) GmbH formed a new partnership with the international engineering consultancy, the Ramboll Group A/S, based in Copenhagen. The multidisciplinary practice seeks to raise awareness of the social and ecological value of water in urban design. The scope of the practice's work includes strategic catchment-based urban masterplans, urban parks, rivers, civic space and water playgrounds. Over the past 30 years, has accumulated experience in technical water systems, including water storage, treatment and reuse, retention and infiltration techniques, grey and black water systems, climatisation and green roofs. has been described as “the hidden champion of the German design scene”. are responsible for the waterscape on Potsdamer Platz in Berlin. Water was central to Renzo Piano and Christoph Kohlbecker's original design, but it was that conceived and developed the scheme for rainwater harvest, circulation and display and created the many opportunities for engagement with water | https://en.wikipedia.org/wiki?curid=39708285 |
Ramboll Studio Dreiseitl The scheme is one of the largest urban rainwater harvesting projects in the world and in 2011, it became one of the first city quarters to be retrospectively awarded the DGNB Certificate of the German Sustainable Building Council (DGNB) in silver. Another major project in Germany is Arkadien Winnenden, the ecological city design which was named winner of the Green Dot Award ‘Build’ category in 2011. The firm turned the abandoned factory site into an eco-friendly development which combines dense layout with green space, includes permeable paving and waterways which provide natural flood control and a lake which filters rainwater. In 2018 was awarded the German Design Award in the category "Urban Space an Infrastructure" for their design and planning of the Hafen Offenbach in Offenbach am Main, Germany. Studio Dreiseitl has transformed the initial urban plan by reconnecting public open spaces with their scenic context, creating a liveable as well as ecologically enriching neighbourhood. This project is one of the great examples of integrating great design into built reality. In Asia, projects include the blue green infrastructure in the Tianjin Cultural Park near Beijing and the water strategy for the central catchment for the city of Singapore, together with the engineers CH2M Hill, as well as the design of the 60ha Bishan-Ang Mo Kio Park | https://en.wikipedia.org/wiki?curid=39708285 |
Ramboll Studio Dreiseitl The rehabilitation of the previously concreted Kallang River (which became a dangerous torrent in the rainy season) employed techniques of water collection and flood control which were entirely new to Singapore. built a test area and held workshops to explain the concepts. The designers gave the river gentle banks and recycled the concrete from the old drainage channel to create stairs. Now Bishan Park is one of the most popular parks in Singapore, where people can have a new and direct connection to nature. In 2012, the design was awarded the Presidents Design Award Singapore and the World Architecture Festival “Landscape of the Year”. 2012 German Urban Planning Award – BUGA Koblenz, Germany 2012 President's Design Award – Bishan-Ang Mo Kio Park, Singapore 2012 WAF Landscape of the Year – Kallang River Bishan-Ang Mo Kio Park, Singapore 2011 Green Dot Award Ecological City – Arkadien Winnenden, Germany 2011 LivCom Environmental Best Practice Eco Quartier – Pfaffenhofen, Germany 2011 LivCom Award for Liveable Communities – Pfaffenhofen, Germany 2011 BCA Greenmark “Platinum” Award for New Parks – JTC CleanTech Park, Singapore 2011 DGNB Silver Sustainable City District – Berlin Potsdamer Platz, Germany 2002 ASLA Merit Award – Chicago City Hall Green Roof, USA 2001 Earth Society Foundation “Environmental Award” – Solar City Linz, Austria 1989 United Nations “Best Practice Award” – Solar City Linz, Austria | https://en.wikipedia.org/wiki?curid=39708285 |
C16H16O8 The molecular formula CHO (molar mass: 336.29 g/mol, exactMass = 336.084517 u) may refer to: | https://en.wikipedia.org/wiki?curid=39710393 |
Dynamical heterogeneity describes the behavior of glass-forming materials when undergoing a phase transition from the liquid state to the glassy state. In dynamical heterogeneity, the dynamics of cooling to a glassy state show variation within the material. Polymer properties include viscoelasticity and may be synthetic or natural. When a polymeric liquid is cooled below its freezing temperature without crystallizing, it becomes a supercooled liquid. When the supercooled liquid is further cooled, it becomes a glass. The temperature at which a polymer becomes a glass by fast cooling is called the glass transition temperature T. At this temperature, viscosity reaches up to 10 poise depending upon cooling-rate. It is possible for a phase transition from polymer to glassy state to take place. Polymer glass transitions have many determinants including relaxation time, viscosity and cage size. At low temperatures the dynamics become very slow (sluggish) and relaxation time increases from picoseconds to seconds, minutes, or more. At high temperatures, the correlation function has a ballistic regime for very short times (when particles do not interact) and a microscopic regime. In the microscopic regime, the correlation functions decay exponentially at high temperatures. At low temperatures the correlation functions have an intermediate regime in which particles have both slow and fast relaxations. The slow relaxation is an indication of cages in the glassy system. In glassy state density is not homogeneous i.e | https://en.wikipedia.org/wiki?curid=39716485 |
Dynamical heterogeneity particles are localized in different density distributions in space. It means that density fluctuations are present in the system. Particle dynamics become very slow because temperature is directly proportional to kinetic energy causing the particles trapped in local regions by each other. Particles are doing rattling motion inside these cages and cooperate with each other. These regions in the glassy polymer are called cages. In the intermediate regime each particle has its own and different relaxation time. The dynamics in all these cases are different, so at a small scale, there are a large number of cages in the system relative to the size of the whole system. This is known as dynamical heterogeneity in the glassy state of the system. A measurement of dynamical heterogeneity can be done by calculating correlation functions like Non-Gaussian parameter, four point correlation functions (Dynamic Susceptibility) and three time correlation functions. | https://en.wikipedia.org/wiki?curid=39716485 |
Collision-induced absorption and emission In spectroscopy, collision-induced absorption and emission refers to spectral features generated by inelastic collisions of molecules in a gas. Such inelastic collisions (along with the absorption or emission of photons) may induce quantum transitions in the molecules, or the molecules may form transient supramolecular complexes with spectral features different from the underlying molecules. is particularly important in dense gases, such as hydrogen and helium clouds found in astronomical systems. is distinguished from collisional broadening in spectroscopy in that collisional broadening comes from elastic collisions of molecules, whereas collision-induced absorption and emission is an inherently inelastic process. Ordinary spectroscopy is concerned with the spectra of single atoms or molecules. Here we outline the very different spectra of complexes consisting of two or more interacting atoms or molecules: the "interaction-induced" or "collision-induced" spectroscopy. Both ordinary and collision-induced spectra may be observed in emission and absorption and require an electric or magnetic multipole moment - in most cases an electric dipole moment - to exist for an optical transition to take place from an initial to a final quantum state of a molecule or a molecular complex. (For brevity of expression we will use here the term "molecule" interchangeably for atoms as well as molecules) | https://en.wikipedia.org/wiki?curid=39720117 |
Collision-induced absorption and emission A complex of interacting molecules may consist of two or more molecules in a collisional encounter, or else of a weakly bound van der Waals molecule. On first sight, it may seem strange to treat optical transitions of a collisional complex, which may exist just momentarily, for the duration of a fly-by encounter (roughly 10 seconds), in much the same way as this was long done for molecules in ordinary spectroscopy. But even transient complexes of molecules may be viewed as a new, "supermolecular" system which is subject to the same spectroscopic rules as ordinary molecules. Ordinary molecules may be viewed as complexes of atoms that have new and possibly quite different spectroscopic properties than the individual atoms the molecule consists of, when the atoms are not bound together as a molecule (or are not "interacting"). Similarly, complexes of interacting molecules may (and usually do) acquire new optical properties, which often are absent in the non-interacting, well separated individual molecules. Collision-induced absorption (CIA) and emission (CIE) spectra are well known in the microwave and infrared regions of the electromagnetic spectrum, but they occur in special cases also in the visible and near ultraviolet regions. Collision-induced spectra have been observed in nearly all dense gases, and also in many liquids and solids. CIA and CIE are due to the intermolecular interactions, which generate electric dipole moments | https://en.wikipedia.org/wiki?curid=39720117 |
Collision-induced absorption and emission We note that an analogous collision-induced light scattering (CILS) or Raman process also exists, which is well studied and is in many ways completely analogous to CIA and CIE. CILS arises from interaction-induced polarizability increments of molecular complexes; the excess polarizability of a complex, relative the sum of polarizabilities of the noninteracting molecules. Molecules interact at close range through intermolecular forces (the "van der Waals forces"), which cause minute shifts of the electron density distributions (relative the distributions of electrons when the molecules are not interacting). Intermolecular forces are repulsive at near range, where electron exchange forces dominate the interaction, and attractive at somewhat greater separations, where the dispersion forces are active. (If separations are further increased, all intermolecular forces fall off rapidly and may be totally neglected.) Repulsion and attraction are due, respectively, to the small defects or excesses of electron densities of molecular complexes in the space between the interacting molecules, which often result in interaction-induced electric dipole moments that contribute some to interaction-induced emission and absorption intensities. The resulting dipoles are referred to as exchange force-induced dipole and dispersion force-induced dipoles, respectively. Other dipole induction mechanisms also exist in molecular (as opposed to monatomic) gases and in mixtures of gases, when molecular gases are present | https://en.wikipedia.org/wiki?curid=39720117 |
Collision-induced absorption and emission Molecules have centers of positive charge (the nuclei), which are surrounded by a cloud of electrons. Molecules thus may be thought of being surrounded by various electric multipolar fields which will polarize any collisional partner momentarily in a fly-by encounter, generating the so-called multipole-induced dipoles. In diatomic molecules such as H and N, the lowest-order multipole moment is the quadrupole, followed by a hexadecapole, etc., hence the quadrupole-induced, hexadecapole-induced... dipoles. Especially the former is often the strongest, most significant of the induced dipoles contributing to CIA and CIE. Other induced dipole mechanisms exist. In collisional systems involving molecules of three or more atoms (CO, CH...), collisional frame distortion may be an important induction mechanism. Collision-induced emission and absorption by simultaneous collisions of three or more particles generally do involve pairwise-additive dipole components, as well as important irreducible dipole contributions and their spectra. Collision-induced absorption was first reported in compressed oxygen gas in 1949 by Harry Welsch and associates at frequencies of the fundamental band of the O molecule. (Note that an unperturbed O molecule, like all other diatomic homonuclear molecules, is infrared inactive on account of the inversion symmetry and does thus not possess a "dipole allowed" rotovibrational spectrum at any frequency). Molecular fly-by collisions take little time, something like 10 s | https://en.wikipedia.org/wiki?curid=39720117 |
Collision-induced absorption and emission Optical transition of collisional complexes of molecules generate spectral "lines" that are very broad - roughly five orders of magnitude broader than the most familiar "ordinary" spectral lines (Heisenberg's uncertainty relation). The resulting spectral "lines" usually strongly overlap so that collision-induced spectral bands typically appear as continua (as opposed to the bands of often discernible lines of ordinary molecules). Collision-induced spectra appear at the frequencies of the rotovibrational and electronic transition bands of the unperturbed molecules, and also at sums and differences of such transition frequencies: simultaneous transitions in two (or more) interacting molecules are well known to generate optical transitions of molecular complexes. Intensities of spectra of individual atoms or molecules typically vary linearly with the numerical gas density. However, if gas densities are sufficiently increased, quite generally contributions may also be observed that vary as density squared, cubed... These are the collision-induced spectra of two-body (and quite possibly three-body...) collisional complexes. The collision-induced spectra have sometimes been separated from the continua of individual atoms and molecules, based on the characteristic density dependences. In other words, a virial expansion in terms of powers of the numerical gas density is often observable, just as this is widely known for the virial expansion of the equation of state of compressed gases | https://en.wikipedia.org/wiki?curid=39720117 |
Collision-induced absorption and emission The first term of the expansion, which is linear in density, represents the ideal gas (or "ordinary") spectra where these exist. (This first term vanishes for the infrared inactive gases,) And the quadratic, cubic... terms of the virial expansions arise from optical transitions of binary, ternary... intermolecular complexes, which are (often unjustifyably) neglected in the ideal gas approximation of spectroscopy. Two kinds of complexes of molecules exist: the collisional complexes discussed above, which are short lived. Besides, bound (i.e. relatively stable) complexes of two or more molecules exist, the so-called van der Waals molecules. These exist usually for much longer times than the collisional complexes and, under carefully chosen experimental conditions (low temperature, moderate gas density), their rotovibrational band spectra show "sharp" (or resolvable) lines (Heisenberg uncertainty principle), much like ordinary molecules. If the parent molecules are nonpolar, the same induced dipole mechanisms, which are discussed above, are responsible for the observable spectra of van der Waals molecules. Figure 1 (to be included) Figure 1 shows an example of a collision-induced absorption spectra of H-He complexes at a variety of temperatures. The spectra were computed from the fundamental theory, using quantum chemical methods, and were shown to be in close agreement with laboratory measurements at temperatures, where such measurements exist (for temperatures around 300 K and lower) | https://en.wikipedia.org/wiki?curid=39720117 |
Collision-induced absorption and emission The intensity scale of the figure is highly compressed. At the lowest temperature (300 K), a series of six striking maxima is seen, with deep minima between them. The broad maxima roughly coincide with the H vibrational bands. With increasing temperature, the minima become less striking and disappear at the highest temperature (curve at the top, for the temperature of 9000 K). A similar picture is to be expected for the CIA spectra of pure hydrogen gas (i.e. without admixed gases) and, in fact for the CIA spectra of many other gases. The main difference, say if nitrogen CIA spectra are considered instead of those of hydrogen gas, would be a much closer spacing, if not a total overlapping, of the diverse CIA bands which appear roughly at the frequencies of the vibrational bands of the N molecule. The significance of CIA for astrophysics was recognized early-on, especially where dense atmospheres of mixtures of molecular hydrogen and helium gas exist. Herzberg pointed out direct evidence of H molecules in the atmospheres of the outer planets. The atmospheres of the inner planets (including Earth) and of Saturn's big moon Titan show strong CIA in the infrared, due to concentrations of molecular gases such as nitrogen, oxygen, carbon dioxide, etc. exist. In more recent years extrasolar planets were discovered, whose atmospheres are hot (thousand kelvin or more), but resemble otherwise Jupiter's atmosphere (mixtures of mostly H and He), where strong CIA exists | https://en.wikipedia.org/wiki?curid=39720117 |
Collision-induced absorption and emission Stars that burn hydrogen are called main sequence (MS) stars - these are by far the most common objects in the night sky. When the hydrogen fuel is exhausted and temperatures begin to fall, the object undergoes various transformations and a white dwarf star is eventually born, the ember of the expired MS star. Temperatures of a new-born white dwarf may be in the hundreds of thousand kelvin, but if the mass of the white dwarf is less than just a few solar masses, burning of He to C and O is not possible and the star will slowly cool down forever. The coolest white dwarfs observed have temperatures of roughly 4000 K, which must mean that the universe is not old enough so that lower temperature stars cannot be found. The emission spectra of "cool" white dwarfs does not at all look like a Planck blackbody spectrum. Instead, nearly the whole infrared is attenuated or missing altogether from the star's emission, owing to CIA in the hydrogen-helium atmospheres surrounding their cores. The impact of CIA on the observed spectral energy distribution is well understood and accurately modeled for most cool white dwarfs. For white dwarfs with a mix H/He atmosphere, the intensity of the H-He CIA can be used to infer the hydrogen abundance at the white dwarf photosphere. However, predicting CIA in the atmospheres of the coolest white dwarfs is more challenging, in part because of the formation of many-body collisional complexes. The atmospheres of low metallicity cool stars are composed primarily of hydrogen and helium | https://en.wikipedia.org/wiki?curid=39720117 |
Collision-induced absorption and emission Collision-induced absorption by H-H and H-He transient complexes will be a more or less important opacity source of their atmospheres. For example, CIA in the H fundamental band, which falls on top of an opacity window between HO/CH or HO/CO (depending on the temperature), plays an important role in shaping brown dwarf spectra. Higher gravity brown dwarf stars often show even stronger CIA, owing to the density squared dependence of CIA intensities, when other "ordinary" opacity sources are linearly dependent on density. CIA is also important in low-metallicity brown dwarfs, since "low metallicity" means reduced CNO (and other) elemental abundances compared to H and He, and thus stronger CIA compared to HO, CO, and CH absorption. CIA absorption of H-X collisional complexes is thus an important diagnostic of high-gravity and low-metallicity brown dwarfs. All of this is also true of the M dwarfs, but to a lesser extent. M dwarf atmospheres are hotter so that some increased portion of the H molecules is in the dissociated state, which weakens CIA by H--X complexes. The significance of CIA for cool astronomical objects was long suspected or known to some degree. Attempts to model the formation of the "first" star from the pure hydrogen and helium gas clouds below about 10,000 K show that the heat generated in the gravitational contraction phase must be somehow radiatively released for further cooling to be possible | https://en.wikipedia.org/wiki?curid=39720117 |
Collision-induced absorption and emission This is no problem as long as temperatures are still high enough so that free electrons exist: electrons are efficient emitters when interacting with neutrals (bremsstrahlung). However, at the lower temperatures in neutral gases, the recombination of hydrogen atoms to H molecules is a process that generates enormous amounts of heat that must somehow be radiated away in CIE processes; if CIE were non-existing, molecule formation could not take place and temperatures could not fall further. Only CIE processes permit further cooling, so that molecular hydrogen will accumulate. A dense, cool environment will thus develop so that a gravitational collapse and star formation can actually proceed. Because of the great importance of many types of CIA spectra in planetary and astrophysical research, a well known spectroscopy data base has recently been expanded to include a number of CIA spectra in various frequency bands and for a variety of temperatures. | https://en.wikipedia.org/wiki?curid=39720117 |
Deoxyinosine monophosphate (dIMP) is a nucleoside monophosphate and a derivative of inosinic acid. It can be formed by the deamination of the purine base in deoxyadenosine monophosphate (dAMP). The enzyme deoxyribonucleoside triphosphate pyrophosphohydrolase, encoded by YJR069C in "S. cerevisiae" and containing (d)ITPase and (d)XTPase activities, hydrolyses dITP, resulting in the release of pyrophosphate and dIMP. | https://en.wikipedia.org/wiki?curid=39721195 |
Atmospheric optics ray-tracing codes Atmospheric optics ray tracing codes - this article list codes for light scattering using ray-tracing technique to study atmospheric optics phenomena such as rainbows and halos. Such particles can be large raindrops or hexagonal ice crystals. Such codes are one of many approaches to calculations of light scattering by particles. Ray tracing techniques can be applied to study light scattering by spherical and non-spherical particles under the condition that the size of a particle is much larger than the wavelength of light. The light can be considered as collection of separate rays with width of rays much larger than the wavelength but smaller than a particle. Rays hitting the particle undergoes reflection, refraction and diffraction. These rays exit in various directions with different amplitudes and phases. Such ray tracing techniques are used to describe optical phenomena such as rainbow of halo on hexagonal ice crystals for large particles. Review of several mathematical techniques is provided in series of publications. The 46° halo was first explained as being caused by refractions through ice crystals in 1679 by the French physicist Edmé Mariotte (1620–1684) in terms of light refraction Jacobowitz in 1971 was the first to apply the ray-tracing technique to hexagonal ice crystal. Wendling et al. (1979) extended Jacobowitz's work from hexagonal ice particle with infinite length to finite length and combined Monte Carlo technique to the ray-tracing simulations | https://en.wikipedia.org/wiki?curid=39723835 |
Atmospheric optics ray-tracing codes The compilation contains information about the electromagnetic scattering by hexagonal ice crystals, large raindrops, and relevant links and applications. | https://en.wikipedia.org/wiki?curid=39723835 |
Sensitive flame A sensitive flame is a gas flame which under suitable adjustment of pressure resonates readily with sounds or air vibrations in the vicinity. Noticed by both the American scientist John LeConte and the English physicist William Fletcher Barrett, they recorded the effect that a shrill note had upon a gas flame issuing from a tapering jet. The phenomenon caught the attention of the Irish physicist John Tyndall who gave a lecture on the process to the Royal Institution in January 1867. While not necessary to observe the effect, a higher flame temperature allows for easier observation. Sounds at lower to mid-range frequencies have little to no effect on the flame. However, shrill noises at high frequencies produce noticeable effects on the flame. Even the ticking of a pocket watch was observed as producing a high enough frequency to affect the flame. | https://en.wikipedia.org/wiki?curid=39728832 |
Faiza Al-Kharafi Faiza Mohammed Al-Kharafi ( "Fāyzah al-Kharāfī"; born 1946) is a Kuwaiti chemist and academic. She was the president of Kuwait University from 1993 to 2002, and the first woman to head a major university in the Middle East. She is the vice president of the World Academy of Sciences. was born to a wealthy family in Kuwait in 1946 and developed an interest in science from a young age. She attended Al Merkab High School. She received her BSc from Ain Shams University in Cairo in 1967. She then attended Kuwait University where she founded the Corrosion and Electrochemistry Research Laboratory while in graduate school. She received her master's in 1972 and her PhD in 1975. Al-Kharafi worked in Kuwait University's Department of Chemistry from 1975 to 1981. In 1984 she became chair of the department and served as Dean of the Faculty of Science from 1986 to 1989. She became a professor of chemistry at Kuwait University in 1987. On 5 July 1993, Emir Jaber Al-Ahmad Al-Jaber Al-Sabah issued a decree appointing Al-Kharafi as rector of the University, and she became the first woman to head a major university in the Middle East. Al-Kharafi helped reconstruct Kuwait University after the First Gulf War, which ended in 1991. She served as president from 1993 to 2002 where she oversaw 1,500 staff members, over 5,000 employees, and over 20,000 students. Al- Kharafi has demonstrated to be an advocate for research in Kuwait | https://en.wikipedia.org/wiki?curid=39731386 |
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