Solute dependence of polar solvation dynamics studied by RISM/mode-coupling theory

摘要

We apply the combination of the reference interaction-site model (RISM) theory and mode-coupling theory (MCT) for the description of solvation dynamics in acetonitrile. The RISM theory is used for the static features of the solute - solvent system, whereas MCT is employed to capture solvent dynamics. We calculate the dynamic response function of the average-energy relaxation of the system, S-S(t), a corresponding quantity which is experimentally measured through the dynamic Stokes shift of fluorescence spectra, for example. The RISM - MCT framework turns out to be very applicable for realistic estimation of S-S(t). We vary the model solutes from a simple ion, to dumbbell dipoles, to quadrupole, to octapole. As a general feature, S-S(t) begins to relax with the initial Gaussian decay, followed by the underdamped oscillation. Higher multiplicity of the pole or larger charges put on the solute-site causes S-S(t) to relax more slowly. We suggest this behavior is related to an increased complexity of solvent solvent interactions just around the charged sites on the solute. On the other hand, the amplitude of the underdamped oscillation becomes smaller as the multiplicity of the pole increases.