The dynamic solvation time correlation function 𝒵(t) is, within linear response, formulated in terms of the intermolecular solute–solvent interactions, without recourse to the intrinsically macroscopic concept of a cavity carved out of a dielectric medium. For interaction site models (ISM) of both the solute and the solvent, the theory relates the fluctuating polarization charge density of the solvent to the fluctuating vertical energy gap that controls 𝒵(t). The theory replaces the factual (or bare) solute charge distribution by a surrogate expressed in terms of the solute–solvent site–site direct correlation functions. Calculations for solute ions in water and in acetonitrile lead to 𝒵(t) and the second moment of the associated spectral density in good agreement with molecular dynamics simulation results in the literature. We also use the theory to calculate 𝒵(t) for model solutes in which the ‘‘sudden’’ change of the charge distribution involves multipoles of higher order. The response is qualitatively similar in the various cases studied here.

We apply the theories developed in the preceding paper (paper I) to calculate various energy quantities of charge transfer (CT) reactions in nine solvents that cover a wide range of polarity, and for which interaction site models (ISM’s) may be found in the literature. Besides the two surrogate Hamiltonian theories developed in paper I, the renormalized site-density theory (RST) and the renormalized dielectric theory (RDT), we also investigate a simple harmonic approximation (HXA) for the diabatic free energy profiles, whose characteristic parameters are calculated taking specific advantage of the expression given by the extended reference interaction site method (XRISM) for the free energy of solvation. For each CT process we analyze (a) the solvent reorganization energy λ, (b) the shift of the absorption transition energy due to the solvatochromic effect, and (c) the solvent contribution to the free energy change ΔA. In addition, for a few selected examples, we also report the detailed diabatic free energy profiles. The calculations reported rely on solute–solvent and solvent–solvent pair correlation functions obtained with the XRISM integral equation method applied to nonpolarizable (with fixed mean partial charges) ISM representations of the solute and solvent molecules. To rectify the omission of the solvent electronic degrees of freedom, we correct the dielectric part of the solvent reorganization energy with an additive term designed to compensate for the use of fixed charge ISM models. Contact with theories in which the solvent is represented as a dielectric continuum medium (with or without spatial dispersion) and the solute as a set of charges inside spherical cavities carved out of the dielectric is made straightforwardly within the RDT theory by considering a particularly simple form of the solute–solvent RISM site–site direct correlation functions. Using simple ISM models for several solute species, including Reichardt’s betaine-30 dye and a porphyrin-quinone (PQ) ‘‘dyad’’ recently studied by Mataga and co-workers, we examine the ability of the molecular theories to explain the dependence of charge-transfer energetics on dipolar and nondipolar solvents. We find that the solvatochromic effect on the absorption energy of betaine-30, which forms the basis of the ET(30) empirical solvent polarity scale, is reproduced reasonably well by the RST, RDT, and HXA theories for solvents ranging from carbon tetrachloride to water. In the case of the PQ dyad, we find that the calculated values of λ in dipolar and nondipolar solvents are in good agreement with experimental estimates. Our results indicate that the molecular theories of solvation discussed in this paper can explain the observation that a solvent with vanishing molecular dipole moment, like benzene, can show unmistakable ‘‘polarity,’’ as reflected by its influence on the energetics of CT reactions. We also present calculations that corroborate the suggestion (Sec. VII of paper I) that, compared with the behavior in dipolar solvents, in nondipolar solvents the dependence of λ with the donor–acceptor separation distance is practically negligible.

We present here the results of molecular dynamics simulation of solvation dynamics (SD) in benzene, acetonitrile, and their mixtures corresponding to three sets of acetonitrile mole fractions, x ac ) 0.20, 0.50, and 0.75, at temperature and densities appropriate for ambient conditions. The change in solute-solvent interactions triggered by solute electronic S 0 f S 1 excitation is represented as dipole creation in a benzenelike solute. We find that both solvent components are active participants in the SD event, with electrostatic interactions of the dipolar solute with quadrupolar benzene molecules making an important contribution to the solvation mechanism and the steady-state Stokes shift in the fluorescence spectrum. Our model solute is preferentially solvated by acetonitrile in its S 1 state and the enhancement in the local acetonitrile concentration contributes significantly to the solvation time scale, especially in the benzene-rich mixture, where this process becomes considerably slower than the solvation coordinate relaxation in either solvent, in agreement with experimental findings. We investigate the contributions to SD from concentration fluctuations by monitoring the time evolution in the solvation structure. We find that in many respects the way that these fluctuations contribute to SD in benzene-acetonitrile mixtures resembles their contributions to the SD mechanism in mixtures of dipolar molecules.