Ab Initio Characterization of the [H ₂ ,C,N,O] <b> ⁺ </b> Isomeric System: Structures, Spectroscopy, and Electronic States

are confirmed as the two most stable forms. For all isomers, rotational spectroscopy parameters together with fundamental vibrational frequencies and infrared intensities are accurately predicted. Proton affinities of the HNCO isomers are evaluated to elucidate preferred protonation sites. In addition, the patterns of the lowest singlet and triplet electronic states of these species, which exhibit strong valence-Rydberg character, are also presented. This work is expected to help in the identification of these protonated species in laboratory and astrophysical media.