Kinetic Pathways of Ion Pair Dissociation in Water

We have successfully applied the transition path sampling method to the deterministic dynamics of a manybody system with long-ranged interactions. The process we investigate, dissociation of a model Na+Clion pair in water, involves a wide range of transition pathways in which collective solvent motions play an important role. Transition states along these pathways encompass a broad distribution of ionic separations. Ion pairs in contact remain associated for ∼20 ps on average, a time scale considerably longer than the ∼3ps predicted by transition state theory with ionic separation as the order parameter. In contrast to theories of barrier crossing based upon frictional effects (with or without memory), we find that the discrepancy between these time scales arises from neglected solvent free energy barriers rather than stochastic buffeting of the ion pair.