*Author to whom correspondence should be addressed. E-mail: benjamin@chemistry.ucsc.edu Israel Journal of Chemistry Vol. 47 2007 pp. 115–127 Molecular Dynamics Study of a Model S N 1 Dissociation Reaction at Liquid/Liquid Interfaces: Effect of Liquid Polarity NIcoLAS WINtER AND ILAN BENjAMIN* Department of chemistry, University of california Santa cruz, Santa cruz, california 95064, USA (Received 24 September 2006) Abstract. 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 empiri- cal 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 (deined as the t-Bu to cl distance) in several interfacial regions of varying distances from the Gibbs di- viding surface. the results at the water/DcE interface are compared to previous mo- lecular 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 ∆G rxn increase with decreasing solvent polar- ity. In contrast with the water/carbon tetrachloride interface, a well-deined transi- tion state exists at the water/DcE interface and persists even as the solute is moved 3 to 6 Å into the DCE phase. Dynamical lux correlation calculations reveal larger deviation of the rate from tSt than in bulk water due to slower vibrational relax- ation 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 coeficient 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. INTRODUCTION the study of condensed phase chemical reactions that involve substantial charge reorganization has been and still is of great interest. Examples of such reactions in- clude nucleophilic substitution reactions (S N 1 and S N 2), acid dissociation, and photoionization reactions. All of these reactions are characterized by signiicant differ- ences among the nature of the solvent interaction with the reactants, transition state species, and the products. the inhomogeneous solvent environment at liquid/liq- uid and liquid/vapor interfaces introduces important factors that may inluence the thermodynamics and dynamics of these reactions. Liquid interfaces can be characterized by a number of unique properties such as surface polarity and surface roughness, both of which can strongly inluence the rate and equilibrium constant of a reaction and cause signiicant deviations from bulk behavior, particularly for charge shift reactions. Ex- perimental studies of these types of reactions at a liq- uid/liquid interface include second harmonic generation experiments by corn and coworkers, who examined the acid–base equilibrium of an azobenzene surfactant at the water/1,2-dichloroethane interface. 1 A number of groups have studied charge shift reactions in micro- emulsions, a type of system that, on the microscopic level, is essentially the same as the liquid/liquid inter- face. these include solvolysis of substituted benzoyl chloride at the water/isooctane interface, 2 formation of carbene chloride by abstraction of a chloride ion from a carbon trichloride anion in water/cyclohexene micro- emulsions, 3 and the nucleophilic substitution reaction of decyl bromide with the sulfonate anion in various water/