Solute-structure dependence of solvation dynamics studied by reference interaction-site model theory

A combination of the reference interaction-site model theory and site–site Smoluchowski–Vlasov equation is applied to estimate the dynamic response function of the average-energy relaxation of the solute–solvent system, SS(t). We calculate SS(t) for 13 model solutes with different structure, from a simple ion to an octopole, in a polar solvent. The partial charges of the ions and multipoles are changed to investigate nonlinear character of SS(t). The “nonlinear character” we study here corresponds to the response of the solvent fluctuation after the sudden change of the solute charge-distribution. Our present results reveal that SS(t) depends on the molecular structure and charge distribution of the solute. SS(t) is decomposed into two parts: one corresponding to the optical mode of solvent, the other to the acoustic mode. We show that for multipoles the optical mode is responsible for the fast part of SS(t), while the acoustic mode plays an important role in the slower dynamics. The dual nature of SS(t) is essential for the nonlinearity of solvation dynamics.