Molecular dynamics of the A+BC reaction in rare gas solution

Abstract

Molecular dynamics are computed for model atom transfers A+BC→AB+C in rare gas solvents at liquid densities. We find that the reaction dynamics can be understood in terms of a simple picture which consists of three stages: (1) activation of reactants, (2) barrier crossing, and (3) deactivation of products. The effects seen in stages (1) and (3) can be largely interpreted in terms of existing models of energy and phase decay in solution, while the effects seen in stage (2) can be largely interpreted in terms of gas phase A+BC barrier crossing dynamics. We find that transition state theory is in perfect agreement with the simulations for the 20 and 10 kcal/mol barrier reactions and is a very good description for a 5 kcal/mol reaction barrier. At low barrier curvature, dynamical effects due to the solvent are shown to induce some recrossings of the transition state barrier, thus causing rate constants calculated by simple transition state theory to be slightly too high. The Grote–Hynes modification of transition state theory, which considers the effect of the time dependent friction of the solvent on the dynamics at the transition state, predicts corrections to the rate constants in good agreement with the results from the simulations.

Molecular dynamics simulation of an activated transfer reaction in zeolites

“…Therefore we do not expect large differences in the reaction rates, as long as only the structure of the surrounding medium is changed while the forces exerted by the environment on the reactants are always kept weak and short-range. We chose to study such kind of reactions on analogous grounds to those that stimulated many previous extensive studies of XϩX 2 model reactions in rare gas solvents: 15,21,23 the relative simplicity of the interactions and of the model describing them allows to capture and understand in a great detail many general features of such processes, with particular emphasis on the solvent interaction with the reagents. 16 The dynamics of more complex reactions, besides being more difficult to simulate and understand, involves many further effects that may partially obscure the general features of the ͑crystalline͒ environment activity in which we are primarily interested.…”

“…Previous theoretical studies 13 show that reactions of this kind ͑e.g. neutral atom exchange in rare gas solvents [14][15][16] ͒ are usually characterized by smaller solvent effects compared to exchange reactions in which long-range, strong interactions ͑both of Coulombic and ion-dipole nature, like bimolecular substitutions of alkyl halides by anions in polar solvents [17][18][19][20] ͒ are active. For example, the dynamics of energy flow into the reactants is much more complex for S N 2 reactions involving polar and charged species: a larger number of solvent atoms simultaneously take part in the energy transfer to the reactants compared with the ClϩCl 2 model system.…”

A classical molecular dynamics study of recombination reactions in a microporous solid

“…In this field, computer experiments [1][2][3][4] seem suitable today for such detailed investigations, although they suffer considerable limits. The difficulties in developing and then applying these schemes are due, from a theoretical point of view, to the complex features of a chemical reaction and, from a practical one, to the available computing resources which prevent completely ''ab initio'' quantum mechanical calculations.…”

Molecules in Motion: Chemical Reaction and Allied Dynamics in Solution and Elsewhere

“…But representing a solvent in interaction with a reacting solute as an extensive collection of harmonic oscillators is hardly physically accurate; most intermolecular interactions are obviously seriously anharmonic. A simple illustration of just one of the many difficulties of such oscillator pictures can be given from our work on atom transfer A + BC reactions in rare-gas solvents (44). The physics of the solvent's influence is that of independent binary collisions with the rare-gas atoms; these are rapid and short lived, and the associated time-dependent friction is approximately Gaussian in time.…”