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

Abstract

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_n (n = 1, 2) in the mid- and near-infrared regions based on ab initio 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.