Molecular dynamics simulations of chemical reactions at liquid/vapor and liquid/liquid interfaces

The photodissociation of ICN in water and at the water liquid/vapor interface is studied using mixed quantum-classical molecular dynamics simulations. A water-ICN potential energy function that takes into account the different ground and excited state charges and their shift as a function of the reaction coordinate is developed. The calculations include non-adiabatic transitions between electronic states. In the bulk, the calculated absorption spectrum, cage escape probability, quantum yield of ICN and INC, and the subsequent vibrational relaxation rate are in reasonable agreement with experiments. At the interface, the reduced surface density and weaker solvent-solute interactions give rise to a reduced rate of nonadiabatic transitions and the probability for cage escape is enhanced. The overall desorption probability varies from 75% to 92% for ICN initially located just below the Gibbs surface to ICN located just above the Gibbs surface, respectively. The corresponding geminate recombination probabilities are 18% and 9% respectively (compared to 85% in the bulk). The vibrational relaxation rate of the recombined ICN is slower than in the bulk by a factor of 2.3.; The thermodynamics and dynamics of a model SN1 reaction: t-BuCl → t-Bu+ + Cl- is studied at the water liquid/vapor interface, and the water/CCl 4 and water/DCE liquid/liquid interfaces using mixed quantum-classical molecular dynamics computer simulations. The empirical valence bond approach is used to couple two diabatic states, covalent and ionic, in the electronically adiabatic limit. Umbrella sampling calculations are used to calculate the potential of mean force along the reaction coordinate (defined as the t-Bu to Cl distance) in bulk water and at the interface. We find a significant increase of the dissociation barrier height and of the reaction free energy at the interface relative to the bulk. This is shown to be due to the reduced polarity of the interface which causes destabilization of the ionic state. At the water/organic liquid/liquid interface deformation to the neat interface structure in the form of water protrusions into the organic phase may provide partial stabilization of the ionic species. Reactive flux correlation function calculations show significant deviation of the rate constant from transition-state theory.