Rare reactivity

The typical time scale for the Car-Parinello MD simulation is presently of the order of picoseconds. This time scale is usually not sufficient to directly observe a chemical reaction in a single free dynamics simulation, due to relatively high activation-energy barriers. Thus, many approaches have been proposed to simulate such rare reactive events. 

VandeVondele J, Rothlisberger U, Accelerating Rare Reactive Events by Means of a Finite Electronic Temperature, J Am Chem Soc, 124, 8163-8171 (2002)... 

The relatively rare reactive events have an extremely low statistical weight. To study them one has to force the model, bringing into contact two local structures based on molecules A and B, in our example, and then studying the evolution of such local structures in the whole liquid. We are so led once again to consider a microscopic event, i.e. the chemical reaction, which is now set in a wider and more detailed model for the liquid. 

TPS was originally developed for studying rare reactive events. The most difficult problem in studying reactive events, which is hidden by the wide acceptance of TST, is to define an appropriate reaction coordinate and find the location of the TS. In addition, if one tries to simulate rare reaction events using a molecular dynamics simulation, most of the trajectories that start from the reactants will not cross to the products, and as a consequence the calculation becomes computationally not feasible, because the time step that must be used in the MD simulation is much smaller than the timescale of interest. 

Matsuki and colleagues evaluated histidine decarboxylase, an enzyme of the amine precursor uptake and decarboxylation system known to be distributed in mast cells and enterochromaffin-like cells, with the hypothesis that this enzyme was a marker for neuroendocrine differentiation. The authors found that the anti-histidine decarboxylase antibody stained most small cell lung cancers (18 of 23 sensitivity 0.78) and was rarely reactive with non-neuroendocrine lung tumors... 

The techniques used by Edberg et al. 2 and Brown and Clarke " are particularly useful in the study of processes in neat liquids. Statistically valid results are much more readily obtained because all the molecules where dynamics are being run can potentially undergo the relatively rare reactive event. Thus, the number of interesting events that occurs within a given simulation... 

The particular model used in the original simulation i- of this reaction was that of a Cl + CI2 like reaction as modeled by a LEPS potential energy surface. The barrier for this symmetric reaction was normally taken to be 20 kcal/mol (—33 kT at room temperature). Other simulations used 10 and 5 kcal/mol barriers. The reactants were placed in either a 50 or 100 atom solvent (Ar in the earliest simulations Ar, He, or Xe in the later work) with periodic truncated octahedron boundary conditions. To sample the rare reactive events, as described previously, this system was equilibrated with the Cl—Cl—Cl reaction coordinate constrained at its value at the transition state dividing surface (specifically, the value of the antisymmetric stretch coordinate was set equal to zero). From symmetry arguments, this constraint is the appropriate one (except in the rare case where the solvent stabilizes the transition state sufficiently such that a well is created at the top of the gas phase barrier). For each initial configuration, velocities were chosen for all coordinates from a Boltzmann distribution and molecular dynamics run for 1 ps both forward and backward in time. 

Also noteworthy are some alkylidenes that exemplify rare reactivity for metal hydrides. The first is the cyclic carbene complex 565, the formation of which is itself unusual, proceeding as it does from the interaction of Bp Rh(CO)(py) (566) and methyl iodide. This is proposed to involve the oxidative addition of Mel and subsequent migratory insertion of CO, though at what stage the B-H activation occurs remains to be determined. More significant, however, is that on heating to 45 °C, 565 irreversibly evolves into the alkyl complex 567 via a rare reverse a-hydride migration onto the alkylidene carbon (Scheme 55, Section II-D.2). 

Vanadium is a rare element, making up only about 0.015% of Earth s crust (by mass). It is a soft white metal with high ductility and malleability, which means that it can be easily drawn into narrow wires or rolled into thin sheets. Because of the high reactivity of pure vanadium, this elanent occurs naturally within compounds (not in its elemental form). However, vanadium compounds are not normally found in the rocks or soils near oil fields, so why does this rare, reactive metal occur in oil The answer lies in the biology of ancient fife-forms. Most modem animals use iron in hemoglobin to transport oxygen through their bloodstreams (see Sections 1.1 and 14.1), and a few (such as lobsters) use copper for this function. It appears that some extinct animals used vanadium. Even today. 

The simplest molecular orbital method to use, and the one involving the most drastic approximations and assumptions, is the Huckel method. One str ength of the Huckel method is that it provides a semiquantitative theoretical treatment of ground-state energies, bond orders, electron densities, and free valences that appeals to the pictorial sense of molecular structure and reactive affinity that most chemists use in their everyday work. Although one rarely sees Huckel calculations in the resear ch literature anymore, they introduce the reader to many of the concepts and much of the nomenclature used in more rigorous molecular orbital calculations. 

Lanthanum is silvery white, malleable, ductile, and soft enough to be cut with a knife. It is one of the most reactive of the rare-earth metals. It oxidizes rapidly when exposed to air. Cold water attacks lanthanum slowly, while hot water attacks it much more rapidly. 

Cerium is an iron-gray lustrous metal. It is malleable, and oxidizes very readily at room temperature, especially in moist air. Except for europium, cerium is the most reactive of the rare-earth metals. It decomposes slowly in cold water and rapidly in hot water. 

As with other rare-earth metals, except for lanthanum, europium ignites in air at about 150 to I8O0C. Europium is about as hard as lead and is quite ductile. It is the most reactive of the rare-earth metals, quickly oxidizing in air. It resembles calcium in its reaction with water. Bastnasite and monazite are the principal ores containing europium. 

The metal has a bright silvery metallic luster. Neodymium is one of the more reactive rare-earth metals and quickly tarnishes in air, forming an oxide that spalls off and exposes metal to oxidation. The metal, therefore, should be kept under light mineral oil or sealed in a plastic material. Neodymium exists in two allotropic forms, with a transformation from a double hexagonal to a body-centered cubic structure taking place at 863oC. 

The order of alkyl halide reactivity in nucleophilic substitutions is the same as their order m eliminations Iodine has the weakest bond to carbon and iodide is the best leaving group Alkyl iodides are several times more reactive than alkyl bromides and from 50 to 100 times more reactive than alkyl chlorides Fluorine has the strongest bond to car bon and fluonde is the poorest leaving group Alkyl fluorides are rarely used as sub states m nucleophilic substitution because they are several thousand times less reactive than alkyl chlorides... 

Chlorination is carried out m a manner similar to brommation and provides a ready route to chlorobenzene and related aryl chlorides Fluormation and lodmation of benzene and other arenes are rarely performed Fluorine is so reactive that its reaction with ben zene is difficult to control lodmation is very slow and has an unfavorable equilibrium constant Syntheses of aryl fluorides and aryl iodides are normally carried out by way of functional group transformations of arylammes these reactions will be described m Chapter 22... 

Following the movement of airborne pollutants requires a natural or artificial tracer (a species specific to the source of the airborne pollutants) that can be experimentally measured at sites distant from the source. Limitations placed on the tracer, therefore, governed the design of the experimental procedure. These limitations included cost, the need to detect small quantities of the tracer, and the absence of the tracer from other natural sources. In addition, aerosols are emitted from high-temperature combustion sources that produce an abundance of very reactive species. The tracer, therefore, had to be both thermally and chemically stable. On the basis of these criteria, rare earth isotopes, such as those of Nd, were selected as tracers. The choice of tracer, in turn, dictated the analytical method (thermal ionization mass spectrometry, or TIMS) for measuring the isotopic abundances of... 

Fused Salt Electrolysis. Only light RE metals (La to Nd) can be produced by molten salt electrolysis because these have a relatively low melting point compared to those of medium and heavy RE metals. Deposition of an alloy with another metal, Zn for example, is an alternative. The feed is a mixture of anhydrous RE chlorides and fluorides. The materials from which the electrolysis cell is constmcted are of great importance because of the high reactivity of the rare-earth metals. Molybdenum, tungsten, tantalum, or alternatively iron with ceramic or graphite linings are used as cmcible materials. Carbon is frequently used as an anode material. 

Improvements in separation techniques, quaHty control, and avaHabHity of rare-earth compounds in various chemical forms, ie, mixed oxides, metals, and aHoys of various purity, morphology, and reactivity, have made these materials an essential part of everyday life. 

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