DYNAMICS OF BIMOLECULAR REACTIONS IN SOLUTION - A NONADIABATIC ACTIVATION MODEL

A simple Hamiltonian model which accounts for the main features of the role of the solvent in activated bimolecular exchange reactions is discussed. The properties of the potential energy along the reaction coordinate of the solute enter in an essential way and explain the different roles of the solvent near the barrier and at the foothills of the potential and the corresponding separation of time scales. The activation energy necessary to surmount the barrier is provided by a localized, vibrationally nonadiabatic, energy exchange between the solvent and solute. Caging with and without recrossing of the barrier is discussed. The predictions of the model are compared with exact trajectory results for the given Hamiltonian and with full molecular dynamics simulations. The influence of the physical parameters such as masses, barrier height strength of solvent-solute coupling, etc., is well accounted for by the model and is summarized by two dimensionless coupling parameters. In particular, the efficiency of solvent solute energy exchange is governed by a vibrational adiabaticity parameter.