Water structure, dynamics, and vibrational spectroscopy in sodium bromide solutions

Abstract

We study theoretically the steady-state and ultrafast vibrational spectroscopy, in the OD-stretch region, of dilute HOD in aqueous solutions of sodium bromide. Based on electronic-structure calculations on clusters containing salt ions and water, we develop new spectroscopic maps that enable us to undertake this study. We calculate OD-stretch absorption line shapes as a function of salt concentration, finding good agreement with experiment. We provide molecular-level understandings of the monotonic (as a function of concentration) blueshift, and nonmonotonic line width. We also calculate the frequency time-correlation function, as measured by spectral diffusion experiments. Here again we obtain good agreement with experiment, finding that at the highest salt concentration spectral diffusion slows down by a factor of 3 or 4 (compared to pure water). For longer times than can be accessed experimentally, we find that spectral diffusion is very complicated, with processes occurring on multiple time scales. We argue that from 6 to 40 ps, relaxation involves anionic solvation shell rearrangements. Finally, we consider our findings within the general context of the Hofmeister series, concluding that this series must reflect only local ordering of water molecules.