Infrared photodissociation spectroscopy and anharmonic vibrational study of the HO₄ ⁺ molecular ion

Molecular cations of HO₄ ⁺ and DO₄ ⁺ are produced in a supersonic expansion. They are mass-selected, and infrared photodissociation spectra of these species are measured with the aid of argon-tagging. Although previous theoretical studies have modeled these systems as proton-bound dimers of molecular oxygen, infrared spectra have free OH stretching bands, suggesting other isomeric structures. As a consequence, we undertook extensive computational studies. Our conformer search used a composite method based on an economical combination of single- and multi-reference theories. Several conformers were located on the quintet, triplet, and singlet surfaces, spanning in energy of only a few thousand wavenumbers. Most of the singlet and triplet conformers have pronounced multiconfigurational character. Previously unidentified covalent-like structures (H-O-O-O-O) on the singlet and triplet surfaces likely represent the global minima. In our experiments, HO₄ ⁺ is formed in a relatively hot environment, and similar experiments have been shown capable of producing multiple conformers in low-lying electronic states. None of the predicted HO₄ ⁺ isomers can be ruled out a priori based on energetic arguments. We interpret our argon-tagged spectra with Second-Order Vibrational Perturbation Theory with Resonances (VPT2+K). The presence of one or more covalent-like isomers is the only reasonable explanation for the spectral features observed.