Nonequilibrium solvation dynamics in solution reactions

Abstract

We construct a theoretical framework for the description of nonequilibrium solvation and solvent participation in the reaction coordinate for solution reactions. The framework is illustrated by a model of reactive dipole isomerization. We show that a multidimensional reaction coordinate picture is equivalent to a one dimensional description in which a generalized friction characterizes and quantifies nonequilibrium solvation effects on the reaction rate. The adiabatic regime where equilibrium solvation and mean potential ideas are correct is identified. Several distinct regimes of nonequilibrium solvation are identified and described in molecular terms. In the effective mass regime, equilibrium solvation ideas give the reaction barrier curvature correctly, but solvent inertia modifies the barrier passage rate. In the nonadiabatic regime, the solvent is ‘‘frozen’’ during the barrier passage and cannot provide equilibrium solvation. In the polarization caging regime, the reacting species adjust to the moving solvent, rather than vice versa, and the solvent is heavily involved in the reaction coordinate. The rate constant in each of these regimes is related to reactive and solvent dynamics.