Theoretical study on the structures and vibrational spectra of (H₂O–Ar_<i>n</i>)⁺, <i>n</i> = 1, 2: formation of a hemi-bond of water radical cation

The possible existence of a hemi-bond in the cationic complex of water and Ar has been actively debated in the literature. We simulated vibrational spectra of low-energy conformers of H₂O⁺-Ar_<i>n</i> (<i>n</i> = 1, 2) in the mid- and near-infrared regions based on <i>ab initio</i> anharmonic algorithms with potential energy at CCSD/aug-cc-pVTZ quality. Decent agreements between experimental data and spectra simulated with four types of normal modes, intermolecular translation, H-O-H bending, and O-H stretching, validate our computational algorithms. By cross-examination of the available experimental data and our simulations, we believe that both the hydrogen-bond and hemi-bond conformers of H₂O⁺-Ar₂ should coexist under the experimental conditions. Our simulated spectra of hemi-bond conformers of H₂O⁺-Ar₂ further suggest that spectral features in the under-explored part of the near-infrared region may provide additional spectral features to double check the existence of the hemi-bond conformer.