The interplay of dielectric and mechanical relaxation in solvation dynamics

Abstract

Quantum molecular dynamics simulations are used to explore the possible coupling between dielectric solvation, the response of the solvent to a change in charge distribution of the solute, and mechanical solvation, the response of the solvent to a change in solute size or shape. The hydrated electron is chosen as a solvation probe, due to its large increase in spatial extent upon photoexcitation and significant contraction in size upon non-adiabatic relaxation. The strong displacement of translational solvent modes upon excitation hampers the effectiveness of individual solvent molecule rotations in providing relaxation, decreasing the relative amplitude of the inertial response. Following non-adiabatic relaxation, solvent molecules can freely translate and reorient, leading to rapid, effective initial solvation. These results suggest that in many situations where solutes undergo changes in both charge distribution and size, solvation can become rate-limited by the relatively slow viscoelastic solvent response.