A first principles simulation study of vibrational spectral diffusion in aqueous NaBr solutions: Dynamics of water in ion hydration shells

A theoretical study of vibrational spectral diffusion in aqueous NaBr solutions is presented by means of ab initio molecular dynamics simulations and time series analysis. The OD stretch frequencies of deuterated water hydrogen bonded to the bromide ions are found to be higher than those in the bulk which implies a somewhat weaker Br–water hydrogen bonds than those between water molecules. The dependence of OD stretch frequencies of hydration shell water on the length and angle of the associated ion–water hydrogen bonds is also investigated. The dynamics of frequency fluctuations of hydration shell and all water molecules are studied through calculations of hole dynamics and frequency time correlations. The roles of the dynamics of ion–water and water–water hydrogen bonds, escape dynamics of water from ion hydration shells and also that of other non-hydrogen-bonded dynamical modes in influencing the dynamics of vibrational spectral diffusion are discussed. 2012 Elsevier B.V. All rights reserved.