Reinterpreting the infrared spectrum of H + HCN: Methylene amidogen radical and its coproducts

The methylene amidogen radical (H₂CN) plays a role in high-energy material combustion and extraterresterial atmospheres. Recent theoretical work has struggled to match experimental assignments for its CN and antisymmetric CH₂ stretching frequencies (ν₂ and ν₅), which were reported to occur at 1725 and 3103 cm^-1. Herein, we compute the vibrational energy levels of this molecule by extrapolating quadruples-level coupled-cluster theory to the complete basis limit and adding corrections for vibrational anharmonicity. This level of theory predicts that ν₂ and ν₅ should occur at 1646 and 2892 cm^-1, at odds with the experimental assignments. To investigate the possibility of defects in our theoretical treatment, we analyze the sensitivity of our approach to each of its contributing approximations. Our analysis suggests that the observed deviation from experiment is too large to be explained as an accumulation of errors, leading us to conclude that these transitions were misassigned. To help resolve this discrepancy, we investigate possible byproducts of the H + HCN reaction, which was the source of H₂CN in the original experiment. In particular, we predict vibrational spectra for cis-HCNH, trans-HCNH, and H₂CNH using high-level coupled-cluster computations. Based on these results, we reassign the transition at 1725 cm^-1 to ν₃ of trans-HCNH, yielding excellent agreement. Supporting this identification, we assign a known contaminant peak at 886 cm^-1 to ν₅ of the same conformer. Our computations suggest that the peak observed at 3103 cm^-1, however, does not belong to any of the aforementioned species. To facilitate further investigation, we use structure and bonding arguments to narrow the range of possible candidates. These arguments lead us to tentatively put forth formaldazine [(H₂CN)₂] as a suggestion for further study, which we support with additional computations.