Features in Vibrational Spectra Induced by Ar-Tagging for H₃O⁺Ar_<i>m</i>, <i>m</i> = 0–3

Abstract

Understanding the spectral features for solvated hydronium has been hindered due to the strong and complex vibrational couplings that lead to broad bands in the aqueous phase. In this work, utilizing <i>ab initio</i> vibrational calculations, we determine how the vibrational couplings induced by the Ar microsolvation in H₃O⁺Ar_<i>m</i> <i>m</i> = 0–3 affect the observed spectra. With theoretical peak intensities and peak positions, we assign the experimental spectra. We also show that an increase in the number of Ar atoms results in an anticooperative blue shifting in the Ar-tagged OH stretching bands. This change in peak position of the OH stretching fundamental modulates the Fermi resonance with the bending overtone. This is observed as a distinct doublet feature at 3200 cm^–1 with varying intensities for H₃O⁺Ar₂ and H₃O⁺Ar₃. The coupling between the in-plane rotation of the hydronium and the bending modes of H₃O⁺ leads to the existence of a strong association bands around 1900 cm^–1