Ab initio molecular dynamics studies of hydrogen bonded structure, molecular motion, and frequency fluctuations of water in the vicinity of azide ions

First principles theoretical studies of vibrational spectral diffusion of the stretch modes of water and azide (N-3(-)) ions are presented by means of ab initio molecular dynamics simulations for two different concentrations of the ions. The vibrational spectral diffusion of hydration shell water in a dilute solution containing a single azide ion is found to occur with three time scales while two time scales are found for the spectral diffusion in the solution of higher ion concentration. The frequency time correlation of the stretching vibration of azide ion is also found to have two time scales. The vibrational spectral diffusion of the stretching mode of azide ions in the concentrated solution is found to occur at a slightly faster rate while that of the water OD modes becomes slower with increase of ion concentration. The effects of dispersion interactions are also investigated by using a dispersion corrected density functional. The time constants of frequency correlations and dynamical spectral shifts are analyzed in terms of the relaxation of azide ion-water and water-water hydrogen bonds. The results of present theoretical calculations are compared with the available experimental and other theoretical results. (C) 2015 AIP Publishing LLC.