Ab initio molecular dynamics simulation of supercritical aqueous ionic solutions: Spectral diffusion of water in the vicinity of Br− and I− ions

Abstract

Supercritical aqueous ionic solutions containing bromide (Br−) and iodide (I−) ions have been studied using Car-Parrinello molecular dynamics simulation at two different concentrations and at two different densities. The time dependent trajectory has been used for calculating vibrational spectral diffusion, structural properties and, for other dynamical properties such as hydrogen bond dynamics, orientational relaxation and residence dynamics. The vibrational spectral diffusion of fluctuating OD stretch mode inside the solvation shell of the ions and in the bulk has been studied using frequency-frequency time correlation function. Hydrogen bond dynamics, orientational relaxation and residence dynamics of Sc-water have been investigated using population correlation function approach. The water molecules inside the solvation shell of Br− and I− ions are found to show complex molecular dynamics in comparison to the water-water dynamics. The concentration dependent spectral diffusion has been observed and it occurs in multiple time scales in the solvation shell of Br− and I− ions. At supercritical condition, the ion-water hydrogen bond shows faster dynamics with respect to the same at ambient condition and slower dynamics with respect to the water-water hydrogen bond dynamics at supercritical condition.