Nature of hydrated proton vibrations revealed by nonlinear spectroscopy of acid water nanodroplets

We use polarization-resolved femtosecond pump–probe spectroscopy to investigate the vibrations of hydrated protons in anionic (AOT) and cationic (CTAB/hexanol) reverse micelles in the frequency range 2000–3500 cm−1. For small AOT micelles the dominant proton hydration structure consists of H3O+ with two OH groups donating hydrogen bonds to water molecules, and one OH group donating a weaker hydrogen bond to sulfonate. For cationic reverse micelles, we find that the absorption at frequencies >2500 cm−1 is dominated by asymmetric proton-hydration structures in which one of the OH groups of H3O+ is more weakly hydrogen-bonded to water than the other two OH groups.