Nonlinear continuum approach to solvation in polar liquids

A novel approach which allows one to treat the nonlinear dielectric response of the solvent in a general fashion is suggested to calculate the nonequilibrium solvation free energies along the reaction coordinate. A simple continuum model of the dielectric response of the solvent is used to investigate the effects of dielectric saturation on the solvation energies and on the activation energies of electron transfer reactions. A simple expression for the solvation energy of a lone spherical solute is obtained. To calculate the solvation energy of an ion pair the noninteracting redox centers approximation is used. This approximation is valid if the donor and acceptor are far enough from each other. The obtained expressions are in good agreement with the results of numerical simulations and allow one to explain the observed deviations from the linear response theory. The activation energies of the charge separation/recombination reaction A + B reversible arrow A(+) + B- are calculated. It is shown that the dependence of the activation energy on the driving force of the reaction must deviate from Marcus' formula in the strong saturation regime. General relations for the solvation free energies are derived, which are valid for most of the molecular models used in numerical simulations. (C) 1996 American Institute of Physics.