Nucleophilic substitution reactions at liquid/liquid interfaces: molecular dynamics simulation of a model S(N)1...

The ionic dissociation step of the nucleophilic substitution reaction t-BuCl --> t-Bu(+) + Cl(-) is studied at the water/carbon tetrachloride interface using molecular dynamics computer simulations. The empirical valence bond approach is used to couple two diabatic states, covalent and ionic, in the electronically adiabatic limit. The umbrella sampling technique is used to calculate the potential of mean force along the reaction coordinate (defined as the t-Bu to Cl distance) at several interface regions of varying distances from the Gibbs dividing surface. We find a significant increase of the ionic dissociation barrier height and of the reaction free energy at the interface relative to bulk water. This is shown to be due to the reduced polarity of the interface which causes a destabilization of the pure ionic state. However, deformation to the neat interface structure in the form of water protrusions into the organic phase may provide partial stabilization of the ionic species. The importance of these structural effects is examined by repeating the calculations with an artificially smooth interface. The destabilization of the ionic state at the interface also manifests itself with a rapid (picosecond time scale) recombination dynamics of the ions to form the parent molecule followed by a slow vibrational relaxation.