Dynamics of barrier crossing in solution: Simulations and a hard-sphere model

Molecular-dynamics simulations for the Cl+Cl₂ activated exchange reaction are reported for all the rare-gas solvents. For all except Xe, the dynamics of barrier crossing are rather similar to that in the gas phase. The role of the solvent being confined to providing the necessary activation at the foothills of the barrier and to deactivating the separating products. In Xe another mechanism becomes operative: a rescaling of the barrier (either by nonsuccessful reactants or by departing products). This caging of the reactants does not, however, lead to successful recrossings of the barrier to reaction. A simple collinear hard-sphere model is able to provide a qualitative and even semiquantitative account of this caging phenomenon including the role of the solvent mass, the number of collisions between the reactants within the cage, and also of the temperature. The use of "dressed variables," in which the solvent motion is implicitly taken into account, proves very convenient in developing simple models for reactions in solution.