INFRARED PHOTODISSOCIATION SPECTROSCOPY AND MULTIREFERENCE ANHARMONIC VIBRATIONAL STUDY OF THE HO4+ MOLECULAR CATION

\chem{HO_4^+} has been proposed as an astrochemical sink of oxygen but has never been observed.\footnote{Xavier, G. D.; Bernal-Uruchurtu, M. I.; Hernandez-Lamoneda, R., \textit{J. Chem. Phys.} \textbf{2014}, 141, 5, 081101.} Molecular cations of \chem{HO_4^+} and \chem{DO_4^+} are produced in a supersonic expansion. They are mass-selected, and their infrared photodissociation spectra are measured with the aid of argon-tagging. Previous theoretical studies have modeled these systems as proton-bound dimers of molecular oxygen.\footnote{Xavier, F. G. D.; Hernandez-Lamoneda, R., \textit{PCCP} \textbf{2015}, 17, 16023-16032.} Several conformers were located on the quintet, triplet, and singlet surfaces, differing in energy by, at most, a few thousand wavenumbers; the singlet and triplet conformers have pronounced multiconfigurational character. Our \chem{HO_4^+} is formed in a relatively hot environment, and similar experiments have been shown capable of producing multiple conformers in low-lying electronic states.\footnote{Wagner, J. P.; McDonald, D. C.; Duncan, M. A., \textit{Angewandte Chemie-International Edition} \textbf{2018}, 57, 5081-5085.} None of the predicted \chem{HO_4^+} isomers can be ruled out \textit{a priori} based on energetic arguments. We interpret our spectra with second-order vibrational perturbation theory with resonances (VPT2+K) using quartic force fields based on an economical combination of single- and multi-reference theories. The VPT2+K simulations include the effect of electrical anharmonicity; this is particularly important for transitions involving the shared-proton stretch. Previously unidentified chain structures (H-O-O-O-O) that exist on the singlet and triplet surfaces likely represent the global minima