Tuning the vibrational coupling of H₃O⁺ by changing its solvation environment

This study demonstrates how the intermode coupling in the hydronium ion (H₃O⁺) is modulated by the composition of the first solvation shell. A series of rare gas solvated hydronium ions (H₃O⁺Rg₃, where Rg = Ne, Ar, Kr, and Xe) is examined via reduced-dimensional anharmonic vibrational (RDAV) ab initio calculations. We considered six key vibrational normal modes, namely: a hindered rotation, two H-O-H bends, and three O-H stretches. Between the O-H stretches and the H-O-H bends, the first is more sensitive to solvation strength. Our calculations revealed that the Fermi resonance between the first overtones of O-H bends and the fundamentals of O-H stretches led to complex spectral features from 3000 to 3500 cm^-1. Such an interaction is not only sensitive to the type of rare gas messengers surrounding the H₃O⁺ ion, it also exhibits an anomalous H → D isotope effect. Although it is accepted that visible combination tones (∼1900 cm^-1) arise from the complex coupling between the hindered rotation and the H-O-H bends, the origin of their intensities is not yet clearly understood. We found that the intensity of these combination tones could be much stronger than their fundamental H-O-H bends. Within our theoretical framework, we tracked the combination tone's intensity back to the asymmetric O-H stretches. This simple notion of intensity borrowing is confirmed by examining eight complexes (H₃O⁺·Rg₃ and D₃O⁺·Rg₃) with spectral features awaiting experimental confirmations.