UC Santa Cruz

Using a recently developed empirical valence bond model for the nucleophilic substitution reaction S_N2 in solution, we examine microhydration effects on the benchmark Cl^- + CH₃C &rarr 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. We find that 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. A number of 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. These and other properties are a rapidly varying function of the reaction coordinate. Deviation from transition state theory due to barrier recrossing is not large and falls between the behavior in bulk water and bulk chloroform

Using the same empirical valence bond (EVB) model for the nucleophilic substitution reaction S_N2 in solution, we study the benchmark Cl^- + CH₃C &rarr CH₃Cl + Cl^- reaction at the water--chloroform liquid--liquid interface. The reaction free-energy profile is determined as a function of the reagents' location relative to the interface. We find that the activation free energy is very sensitive to the reagents' location and to the orientation of the nucleophilic attack. The barrier height at the interface is equal or slightly larger than the barrier in bulk water and approaches the value in bulk chloroform only when the solute is a few nanometers deep into the organic phase. 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 &rarr CH₃Cl + Cl^- in water clusters of different sizes is studied using the previously developed empirical valence bond model. 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 dynamical correction to the rate due to barrier recrossing is small.

The effect of a tetramethylammonium cation phase transfer catalyst on the benchmark Cl^- + CH₃C &rarr 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 (S_N2) Cl^- + CH₃C &rarr 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. The activation free energy remains almost constant relative to that in bulk water, despite the fact that the barrier is expected to significantly decrease as the reaction is carried out near the vapor phase. We show that this is due to the combined effects of a clustering of water molecules around the nucleophile and a relatively weak hydration of the transition state.

In order to investigate the factors that contribute to 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. For electronic transitions that involve a change in the permanent dipole moment of the solute, the transition from the bulk water to the liquid--vapor interface involves a spectral shift consistent with the lower polarity of the interface. The change in the spectral width relative to that in the bulk is determined by several factors, which, depending on the nature of the transition and the dipole moment of the initial state, can result in a narrowing or broadening of the spectrum. These factors include the location of the interface region (which directly correlates with local polarity), the heterogeneity of the local solvation shell, and the width of the surface region. 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.