Tracking Hydronium/Water Stretches in Magic H₃O⁺(H₂O)₂₀ Clusters through High-level Quantum VSCF/VCI Calculations

The excess proton embedded in a three-dimensional cage structure of water molecules gives essential insight into its role in water and the water-air interface. Efforts in terms of the structural analysis and vibrational spectroscopy of the magic H₃O⁺(H₂O)₂₀ cage-formed cluster have been made both experimentally and theoretically. However, theoretical interpretation of the experimental spectrum remains challenging and to date with no consideration of the contributions of different isomers. Here, we report highly accurate vibrational spectra of H₃O⁺(H₂O)₂₀ considering three typical isomers using the fully quantum vibrational self-consistent field/virtual state configuration interaction (VSCF/VCI) approach and high-level, many-body ab initio-based potential and dipole moment surfaces. The calculated spectra of these isomers show similar prominent features of the surface hydronium ion in 1000-2400 cm^-1 and water in 3000-3700 cm^-1 regions. Very good agreement with the experimental spectrum has been reached for the first time, and we provide evidence for the possibility of coexistence of these three isomers below 10 K. The spectral contributions from different types of water molecules are also analyzed, which can guide future investigation to decode the heavily diffuse band in the water OH stretching region.