Negligible cation effect on the vibrational relaxation dynamics of water molecules in NaClO4 and LiClO4 aqueous electrolyte solutions

Abstract

In this study, the cation effects on the vibrational relaxation dynamics of water molecules in NaClO₄ and LiClO₄ aqueous solutions are investigated via polarization selective IR pump probe experiments. The distinct peak splitting of the OD stretch of HOD molecules in concentrated NaClO₄ and LiClO₄ aqueous solutions enables us to investigate the specific cation effects on the dynamics of water molecules that are hydrogen bonded to ClO₄⁻. The reorientation of ClO₄⁻-bound water molecules shows a bi-exponential decay and the slow component of the reorientation time constant is sensitive to the function used to describe the rotational anisotropy decay. We also show that the rotational dynamics of water molecules that are hydrogen bonded to anions is restricted and cannot decay to zero in the concentrated NaClO₄ and LiClO₄ aqueous solutions. Furthermore, the cation effects (Li⁺, Na⁺) on the vibrational relaxation dynamics of water molecules hydrogen bonded to ClO₄⁻ are observed to be relatively negligible.