Two Isomers of Protonated Isocyanic Acid: Evidence for an Ion-Molecule Pathway for HNCO ⇆ HOCN Isomerization

Ion-molecule reactions are thought to play a crucial role in the formation of metastable isomers, but relatively few protonated intermediates beyond HNCH+ have been characterized at high spectral resolution. We present here laboratory measurements of the rotational spectra of protonated isocyanic acid in two isomeric forms, the ground state H2NCO+ with C-2v symmetry and a low-lying bent chain HNCOH+, guided by coupled cluster calculations of their molecular structure. Somewhat surprisingly, HNCOH+ is found to be more abundant than H2NCO+, even though this metastable isomer is calculated to lie approximately 15-20 kcal/mol higher in energy. In the same way that HCNH+ serves as a key intermediate in ion-molecule reactions that form HNC via dissociative electron recombination in cold dense interstellar molecular clouds, HNCOH+ may play an analogous role in the conversion of HNCO to HOCN.