Vibrational probing of the hydrogen-bond structure and dynamics of water in aqueous NaPF6 solutions

Abstract

Direct measurements of the individual dynamics of water in bulk and ionic hydration shells in aqueous ionic solutions are quite experimentally challenging because the different subsets of water in bulk and ionic hydration shells are not spectrally well-resolved in most aqueous ionic solutions. In contrast, the different subsets of water in the bulk, cationic hydration shell, and anionic hydration shell in aqueous NaPF₆ solutions were found to be spectrally well distinct. Such spectral features allowed us to study the individual dynamics of the different subsets of water in aqueous NaPF₆ solutions. In this work, we studied the hydrogen-bond (H-bond) structure and dynamics of water in aqueous NaPF₆ solutions at different NaPF₆ concentrations by FTIR, Raman, and IR pump–probe spectroscopy. Three different subsets of water in the bulk, cationic hydration shell, and anionic hydration shell were found to have their own characteristic hydroxyl stretch peaks (eigen spectra) in FTIR and Raman spectra and have unique vibrational lifetimes independent of NaPF₆ concentration. However, the orientational relaxation dynamics, r(t), were not able to be separately measured for three different subsets of water. The overall orientational relaxation times were found to be linearly dependent on the solution viscosity and were reasonably well described by the Debye–Stokes–Einstein equation. Finally, the frequency-dependent transition dipole moments of the hydroxyl (–OD) stretch vibration obtained in neat water and aqueous 3.0 M NaPF₆ solution were compared and found to be dependent on the nature of H-bonds.