Dynamical polar solvent effects on solution reactions: A simple continuum model

The influence of polar solvent dynamics in solution reactions is investigated for a simple model. In this model, a charge is subject to a chemical free energy barrier, the successful crossing of which constitutes reaction. The retarding influence of the time dependent solvent polarization is described at the continuum Debye relaxation level. The reaction rate constant k is determined as a function of the barrier curvature, the charge-solvent interaction strength, and the solvent polarization relaxation time. The reduction of k with increasingly slower solvent relaxation is found to depend sensitively on the magnitude of charge-solvent interactions. When the latter are weak, the behavior of k differs qualitatively from a standard Kramers prediction. The analogous reaction problem for a dipole is discussed briefly. Directions for a more realistic treatment of polar solvent effects on reaction rates are described.