Hydrogen bond fluctuations of the hydration shell of the bromide anion

We study the hydrogen bond dynamics of solutions of LiBr and NaBr in isotopically diluted water (2% HDO:D₂O) with femtosecond spectral hole-burning spectroscopy. We study the frequency fluctuations of the O−H stretch vibrations of the HDO molecules and observe spectral dynamics with time constants of 0.8 ± 0.1 ps and 4.3 ± 0.3 ps. The slow process we assign to the hydrogen bond fluctuations of the O−H···Br⁻ hydrogen bonds of the hydration shell of the Br⁻ anion. We find that the time scale of the hydrogen bond fluctuations of the hydration shell of Br⁻ is independent of the nature of the cation and the concentration.