A MOLECULAR THEORY OF SOLVATION DYNAMICS

The dynamic solvation time correlation function X(t) is, within linear response, formulated in terms of the intermolecular solute-solvent in teractions, without recourse to the intrinsically macroscopic concept of a cavity carved out of a dielectric medium. For interaction site mo dels (ISM) of both the solute and the solvent, the theory relates the fluctuating polarization charge density of the solvent to the fluctuat ing vertical energy gap that controls X(t). The theory replaces the fa ctual (or bare) solute charge distribution by a surrogate expressed in terms of the solute-solvent site-site direct correlation functions. C alculations for solute ions in water and in acetonitrile lead to X(t) and the second moment of the associated spectral density in good agree ment with molecular-dynamics Simulation results in the literature. We also use the theory to calculate X(t) for model solutes in which the ' 'sudden'' change of the charge distribution involves multipoles of hig her order. The response is qualitatively similar in the various cases studied here.