Ion Pair Interactions in Aqueous Solution: Self-Consistent Reaction Field (SCRF) Calculations with Some Explicit Water Molecules

Ion pair interactions in aqueous solution and the role of explicit water molecules have been investigated using ab initio self-consistent reaction field (SCRF) calculations with and without some explicit water molecules. Both the Polarizable Continuum Model (PCM) and the Self-Consistent Isodensity surface Polarizable Continuum Model (SCIPCM) were used for this study. The results suggest that the inclusion of explicit water molecules on hydrogen bonding sites on a solute and the use of a continuum model to complete the hydration environment is more accurate than a continuum model alone. This procedure is also computationally more economical than an explicit bulk water model to represent ion pair interactions in aqueous solution. The results also demonstrate that the SCIPCM method is superior to the PCM method for the interaction of ion pairs, and that the PCM method should not be used for oppositely charged ion pairs until further improvements are introduced.