Molecular dynamics simulations of charge-transfer reactions at liquid interfaces

Using an empirical valence bond model for the nucleophilic substitution reaction SN2 in solution, we examine microhydration effects on the benchmark Cl- + CH3C → CH3Cl + Cl- reaction in liquid chloroform. Specifically, the effect of the hydration of the reactive system by one-to-five water molecules on the reaction free-energy profile and the rate constant is examined. The activation free energy is highly sensitive to the number of water molecules hydrating the nucleophile, increasing the barrier by about 4 kcal/mol by the first water molecule. With five water molecules, the barrier height is 10 kcal/mol larger than the barrier in bulk chloroform and only 3 kcal/mol below the barrier in bulk water. Properties vary monotonically with the number of water molecules, including the rate of change in the system's electronic structure and the solvent stabilization of the transition state. Other properties are a rapidly varying function of the reaction coordinate. Using the same empirical valence bond (EVB) model for the nucleophilic substitution reaction SN2 in solution, we study the benchmark Cl- + CH3C →CH 3Cl + Cl- reaction at the water--chloroform liquid--liquid interface. We show that this is due to the ability of the nucleophile to keep part of its hydration shell. This suggests that for the catalytic effect of the nonpolar solvent to be appreciable, the nucleophile must be transferred away from the interface. The dynamical correction to the rate, the variation in the system's electronic structure and other system properties as a function of the location with respect to the interface, provide additional insight into the system's behavior. The benchmark nucleophilic substitution reaction Cl- + CH3C →CH3Cl + Cl- in water clusters of different sizes is studied. The reaction activation-free energy, the variation in the system's electronic structure and other system properties are determined as a function of cluster size from 3 to 40 water molecules. The barrier height increases monotonically with the number of water molecules and reaches 90% of the value in bulk water with about 15 water molecules. The contribution of the water is analyzed utilizing a solvent coordinate and its coupling to the electronic state of the solute. The effect of a tetramethylammonium cation phase transfer catalyst on the benchmark Cl- + CH 3C →CH3Cl + Cl- reaction at the water--chloroform liquid--liquid interface is investigated by a molecular dynamics-empirical valence bond (EVB) model. The effect of the catalyst on the reaction free-energy profile at different interface locations and in bulk chloroform is examined. We find that, because of significant water "pollution", activation of the nucleophilic attack is limited to the bulk organic region. The barrier height at the interface is equal to or slightly larger than the barrier in bulk water and is unaffected by the presence of the catalyst. In bulk chloroform, our calculations suggest that the barrier height, which is much lower than in bulk water, moderately increases when a few water molecules interact with the system and when the catalyst forms an ion pair with the nucleophile. Thus, the catalyst is most effective if its role is limited to bringing the nucleophile to the bulk organic phase. Molecular dynamics calculations of the benchmark nucleophilic substitution reaction (SN2) Cl- + CH 3C →CH3Cl+Cl- are carried out at the water liquid--vapor interface. The reaction free-energy profile and the activation free energy are determined as a function of the reactants' location normal to the surface. In order to investigate the electronic absorption line shape of a chromophore adsorbed at the water liquid--vapor interface, molecular dynamics simulations of a series of dipolar solutes undergoing various electronic transitions at various locations along the interface normal are studied. The contribution of the heterogeneity of the local solvation shell can be determined by comparing surface water with bulk methanol, whose polarity is comparable to one of the surface regions.