Rotational dielectric friction and dipole solvation: Tests of theory based on simulations of simple model solutions

Abstract

Simulations of a simplified model system are used to test analytical theories of dielectric friction and explore its connection to dipole solvation dynamics. The simulation model consists of a point dipole solute interacting with a finite collection of dipolar solvent molecules, all situated on a simple cubic lattice and undergoing rotational Brownian motion in the pure diffusion limit. An extensive set of simulations are reported in which four model properties, the solute dipole moment and charge, and the solvent polarity and relaxation time, have been systematically varied. Static and dynamic aspects of dipole solvation observed in these systems are compared to the predictions of the simple continuum and dynamical mean spherical approximation (MSA) theories. Within the linear solvation regime the MSA theory is found to yield essentially quantitative predictions for both static and dynamic solvation properties. The simple continuum model, on the other hand, provides a poor description of either the static or the dynamic behavior. Solute rotational correlation functions of various rank and the dielectric friction functions calculated from them are compared to a variety of theories of rotational dielectric friction. Since all of the analytical theories examined rely on simple continuum descriptions of dipole solvation, they all fail to yield quantitatively accurate results. However, the more sophisticated theories do generally provide useful guides for understanding the trends observed in the data. The one instance where all of the theories fail in a qualitative manner is in predicting the rotational dynamics in the slow solvent limit. Reasons for this failure are discussed and a semiempirical approach for understanding the actual behavior in this limit is presented.