A frequency-resolved cavity model (FRCM) for treating equilibrium and non-equilibrium solvation energies

A refined continuum medium model, denoted as the 'frequency-resolved cavity model' (FRCM), for describing solvation effects of electrically charged solutes in polar solvents is considered. The principal distinction between the commonly accepted Born-Kirkwood-Onsager model and the FRCM treatment is that in the latter case the medium polarization field induced by the solute charge distribution is subdivided into inertial and inertialess components associated with different cavities. The inertialess field, arising from solvent electronic polarization modes, involves an inner cavity confined inside a larger one, which establishes the boundary for inertial polarization modes corresponding to collective orientational and translational motions of solvent molecules outside both cavities. The model is formulated so as to be applicable to complicated chemical solutes, with no symmetry limitations imposed on the shape of their cavities and charge distributions. In introducing two cavities, we find that a single extra parameter in the refined model, chosen to control the distinct sizes of the cavities, is capable of providing the necessary additional flexibility to the FRCM parametrization scheme. By this means one can redistribute inertial and inertialess contributions to equilibrium solvation energies in a way which is consistent with existing experimental data for both equilibrium solvation energy solvent reorganization energy. (C) 1998 Elsevier Science B.V. All rights reserved.