ENERGY-FLOW IN AN ATOM EXCHANGE CHEMICAL-REACTION IN SOLUTION

The object of this paper is to help understand the role of energy fluctuation and transfer in thermally activated chemical reactions in solution. We examine the path of energy flow through the different modes of the solvent and the reagents via molecular dynamics simulation of a model atom exchange reaction in rare gas solution. We follow the energy ultimately needed to surmount the activation barrier from its origin in the heat bath of the solvent through the sequence of (i) an initial fluctuation involving the high translational excitation of a few solvent atoms adjacent to the reactants, (ii) hard solvent-reactant collisions producing potential energy spikes, (iii) the appearance of kinetic (translational and some rotational) energy in the reactants, and finally (iv) an “internal collision” between the reactants converting their excess kinetic energy into potential energy as the barrier is surmounted. The time scale of the entire energy flow process is ~250 fs. The role of momentum and kinetic energy is seen to be fundamental in understanding the molecular dynamic mechanism of this solution reaction, in contrast to the usual view of reaction mechanism which concentrates on potential energy and atomic positions. © 1990, American Chemical Society. All rights reserved.