An ab initio molecular dynamics study of supercritical aqueous ionic solutions: Hydrogen bonding, rotational dynamics and vibrational spectral diffusion

Abstract

The structure and dynamics of supercritical aqueous ionic solutions are investigated through ab initio molecular dynamics simulations. Explicit results are obtained for two different densities and, for the higher density, two different concentrations are considered. The structure of the solutions is investigated through ion–water correlations, hydrogen bond and coordination numbers of the ions and water molecules and also through frequency–structure correlations of water molecules in the hydration shells and bulk phases. It is found that the stretch frequencies of water in the anion hydration shell are lower than those in the bulk water contrary to the behavior of the corresponding ambient solutions. On the dynamical side, calculations are made for diffusion, orientational relaxation, dynamics of hydrogen bonds and dangling OD groups. The dynamics of vibrational spectral diffusion is also investigated and its connections to the underlying molecular dynamics of water in hydration shells and bulk phases are established.