Ab Initio Characterization of the [H₂,C,N,O]⁺ Isomeric System: Structures, Spectroscopy, and Electronic States

Protonated species play a key role in ion-molecule chemistry relevant to astrochemical environments. In this work, we present a high-level theoretical characterization of the low-lying isomers of the [H,H,C,N,O]⁺ system. The exploration of the ground-state potential energy surface, using coupled-cluster (CC) theory, led to the identification of ten protonated isomers. Equilibrium structures and relative energies have been determined using composite schemes rooted in CC theory and accounting for extrapolation to the complete basis set limit and the effects of core correlation. H₂NCO⁺ and HNCOH⁺ 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.