Atom exchange reactions

Fig. 12.4 The (squared) frequency of the motion along the reaction coordinate q for a symmetric atom exchange reaction. L is the range of the chemical barrier region, and the frequency is shown in units of the mean frequency of the unperturbed solvent. The range of frequencies in the solvent is indicated as a solid bar. The negative values for the solvent correspond to unstable solvent modes.

The hydrogen atom exchange reactions between Tetralin and diphenylmethane and the reduction of the ketones were studied in the presence of a selected group of raw whole coals, demineralized whole coals, and demineralized macerals. The macerals used in this work were obtained from two hvA bituminous coals, one from the Kentucky Upper Elkhorn No. 3 Seam (PSOC-1103) and one from the Ohio Lower Kittaning No. 5 Seam (PSOC-297) two hvB bituminous coals, one from the Indiana No. 1 Block Seam (PSOC-106) and one from the Hiawatha King 6 Mine and two hvC bituminous coals, one from the Illinois No. 2 Seam and one from the Indiana Brazil Block Seam (PSOC-828). 

P. Maitre, P. C. Hiberty, G. Ohanessian, S. S. Shaik, J. Phys. Chem. 94, 4089 (1990). Quantitative Valence Bond Computations of Curve-Crossing Diagrams for Model Atom Exchange Reactions. 

The VBSSA rule can be used to select relevant VB structures and construct the semiquantitative and quantitative curve-crossing VB diagram. Combining the MO method and VB calculations, we investigated atom exchange reactions H + HLi —> H2 + Li and H + LiH—>HLi + H, and discussed the effect of energy differences between the VB structures on the activation energy and properties of the transition state [70],... 

Until recently, two major objectives of chemical kineticists were to explain overall chemical change in terms of elementary reactions and to determine the rates of these individual steps over a wide range of temperature. Within the last twenty years, the development of experimental techniques, such as flash photolysis, shock tubes, and gaseous fast-flow systems, have made it possible to observe the rapid changes that accompany many elementary reactions. The rate of reaction is defined in terms of the appearance or disappearance of a particular chemical entity. For example, for a bimolecular atom-exchange reaction like... 

Nevertheless, for bimolecular atom-exchange reactions it has been possible [49] to identify three categories of reactive dynamics, together with intermediate cases. On the other hand, very little is known about four-center reactions [50, 51], and no results have been reported for association reactions. The following four families of reactions are now considered in some detail ... 

In this article, particular attention has been paid to the results of laboratory experiments that have provided information about the molecular dynamics of electronically adiabatic atom-exchange reactions. For a reaction like A + BC - AB + C, where the possibility of crossing between different electronic states can safely be ignored, a theoretical study comprises three distinct stages [20] (a) the determination of the potential-energy hypersurface, (b) the solution of the equations that describe the motion of A, B, and C under the influence of V, and (c) the averaging of the results of these trajectory calculations so that they reflect the distribution of collisional... 

The deuterium isotope effect in the photo-induced chlorine atom exchange reaction with HCl has been investigated by Klein et over the range 30-150 °C by using the competition for chlorine atoms between D2 and DCl (or HCl). The exchange reactions were labeled with radioactive C1. Known mixtures of Cl2, D2, and DCl (or HCl) were irradiated followed by measurement of the residual D2 as well as the activity of the DCl. From this data the rate of the isotope exchange reaction could be determined with respect to the rate of chlorination of deuterium. The results are listed in Table 12 for the reactions... 

Monnerville, M., Halvick, P. and Rayez, J.C. (1992) Time-dependent calculation of the energy resolved state-to-state transition-probabilities for 3-atom exchange-reactions. Chem. Phys. 159, 227-234. 

From a conductance study of the effect of NOC1 on S03 in S02 at —20 °C, the compounds N0(S03) C1 (n = 1, 2, or 3) have been isolated. It would appear from conductivity data that N0S03C1 behaves as a weak electrolyte in SO2.1S0 The kinetics of atom-exchange reactions among C1NO, NO, and N02 have been studied by using a time-of-flight mass spectrometer, and 14N and 15N isotopically labelled species.181... 

This result is due to specific properties of the phase-transfer catalytic system. Both trihalomethyl anions as well as halide ions form, with the quaternary ammonium cation, lipophilic ion pairs which are readily soluble in the organic phase. The generation of dihalocarbenes is in fact a reversible process and, if the starting haloform contains different halogen atoms, exchange reactions afford all possible dihalocarbenes (Houben-Weyl, Vol. E19b, pp 1591-1592). 

An effective way of estimating A G and A Gg is to carry out molecular dynamics calculations for the system being studied. In this way, cumbersome quantum-mechanical calculations involving the reacting system in the solvent are avoided. From a qualitative point of view, an atom exchange reaction involving neutral reactants in a non-polar solvent is expected to be faster in the solvent than in the gas phase. This is due to the fact that two neutral species form one species in the transition state. Very few experimental studies have been carried out one example of a reaction which has been studied is... 

Exceptions apart, isotopic exchange in C-H bonds, usually effected in the presence of bases, is sufficiently rapid only if some feature of the molecular structure weakens quite considerably the bond between carbon and hydrogen, i.e.9 renders the hydrogen atom acidic (semilabile hydrogen atom). Exchange reactions of that type are often carried out in order to achieve semilabile labeling, e.g., at the -position of a ketone, the material obtained in this way being later converted into stably labeled compounds by subsequent reactions, e.g., reduction to secondary alcohols. 

The atomic exchange reaction proceeded with a similar velocity to the parahydrogen conversion, thus establishing the identity of the latter as a surface exchange reaction. 

The first molecular dynamics simulations of bimolecular reactions in solution were those of Wilson, Hynes, and co-workers on an A -I- BC —> AB -I- C atom exchange reaction in rare gas solution. - The short-range Lennard-Jones solvent-solute interactions and intrasolvent interactions simplify the interpretation of the reaction dynamics. 

Two research groups have investigated the rates of the base-catalyzed hydrogen atom exchange reactions of aliphatic fluorocarbons (3,4,28,29). The results for a selected series of compounds are presented in Table 5. 

We have evaluated the electronic isotope effect for some gas phase heavy atom exchange reactions only with minimum basis set wavehmctions,... 

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