Molecular Dynamics Study of a Model SN1 Dissociation Reaction at Liquid/Liquid Interfaces: Effect of Liquid Polarity

The ionic dissociation step of the nucleophilic substitution reaction: t-BuCl → t-Bu⁺ + Cl⁻ is studied at the water/dichloroethane (DCE) interface using molecular dynamics computer simulations. The t-BuCl is modeled using an empirical valence bond method where two diabatic states, covalent and ionic, are coupled in the electronically adiabatic limit. Umbrella sampling is used to determine the potential of mean force (PMF) along the reaction coordinate R (defined as the t-Bu to Cl distance) in several interfacial regions of varying distances from the Gibbs dividing surface. The results at the water/DCE interface are compared to previous molecular dynamics calculations of t-BuCl at the water liquid/vapor and water/carbon tetrachloride interfaces. As in the other systems, the transition state shifts to larger R values, and the activation barrier and ΔG_rxn increase with decreasing solvent polarity. In contrast with the water/carbon tetrachloride interface, a well-defined transition state exists at the water/DCE interface and persists even as the solute is moved 3 to 6 Å into the DCE phase. Dynamical flux correlation calculations reveal larger deviation of the rate from TST than in bulk water due to slower vibrational relaxation of the product ions. However, the increased density at the water/DCE interface increases the rate of dissociation relative to the water liquid/vapor interface. The transmission coefficient at the water/DCE interface was found to be 25% of the TST rate prediction, or about twice the rate at the water liquid/vapor interface.