Influence of rare-gas tagging on the structure and vibrational properties of the hydroxymethyl cation [CH₂OH]⁺

Non-covalent interactions between the hydroxymethyl cation [CH₂OH]⁺ and three rare-gas messenger atoms (helium, neon, and argon) are investigated to understand their effects on the structure and vibrational properties. The calculations reveal that three stable binding motifs (the O-bound, H-bound, and face-on isomers) are predicted; the O-bound isomer is the global minimum in each rare-gas series. The calculations indicated that the interaction strength increases with polarizability. Vibrational analysis demonstrates a clear mode- and site-dependent behavior, where the O-H bond exhibits a significant red shift in the Ar-tagged complex, while the C-H stretching modes exhibit both red and blue shifts depending on the binding motifs. The calculations confirmed that rare-gas atoms exhibit measurable perturbation, with He acting as the least intrusive messenger atom. The study provides important insights into the interpretation of infrared photodissociation spectra of molecular cations.

Ab initio calculations were performed at second-order Møller-Plesset perturbation theory (MP2) with the aug-cc-pVTZ basis set. All of the optimized structures, vibrational frequencies, natural bond orbital analyses, and interaction energies are calculated using the same theory and basis sets, while CCSD(T) is used only for the single-point energy at the same basis sets.