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 C2v 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.
