A first-principles theoretical study of hydrogen-bond dynamics and vibrational spectral diffusion in aqueous ionic solution: Water in the hydration shell of a fluoride ion*

We present a first-principles simulation study of vibrational spectral diffusion and hy drogen-bond dynamics in solution of a fluoride ion in deuterated water. The present calculations are based on ab initio molecular dynamics simulation for trajectory genera tion and wavelet analysis for calculations of frequency fluctuations. The O–D bonds of deuterated water in the anion hydration shell are found to have lower stretching frequency than the bulk water. The dynamical calculations of vibrational spectral diffusion for hy dration shell water molecules reveal three time scales: a short-time relaxation (~100 fs) corresponding to the dynamics of intact ion-water hydrogen bonds, a slower relaxation (~7.5 ps) corresponding to the lifetimes of fluoride ion-water hydrogen bonds, and an even longer time scale (~26 ps) associated with the escape dynamics of water from the anion hydration shell. However, the slowest time scale is not observed when the vibrational spectral diffusion is calculated over O–D bonds of all water molecules, including those in the bulk.