Recombination, Dissociation, and Transport of Ion Pairs across the Liquid/Liquid Interface. Implications for Phase Transfer Catalysis

Molecular dynamics simulations are used to calculate several free energy profiles relevant to the recombination/dissociation and transport of individual ions and ion pairs across the water/chloroform interface. Tetra methyl ammonium (TMA(+)) and tetra butyl ammonium (TBA(+)) (typically used as phase transfer catalysts) and a chloride ion (as an example of a transferred nucleophile) are considered. The free-energy profiles for the transfer of the three ions and the two ion pairs (TMA(+)Cl(-) and TBA(+)Cl(-)) across the interface, as well as the potential of mean force for the dissociation of these two ion pairs at different interface locations, are calculated and correlated with structural and energetic changes at the interface. These equilibrium calculations, together with nonequilibrium trajectory calculations, provide molecular insight into the mechanism of phase transfer catalysis. In particular, water surface fluctuations are strongly coupled to the ion-pair location along the interface normal and the ion-pair bond length.