Laboratory study of isocyanic acid ions: Rotational spectroscopy of $NCO−$⁠, $H2NCO+$⁠, and $HNCOH+$

We report detection of protonated isocyanic acid in two isomeric forms, $H2NCO+$ and $HNCOH+$⁠, by high-resolution spectroscopy. The two ions were first observed at centimeter wavelengths by Fourier Transform (FT) microwave spectroscopy, in a discharge through HNCO heavily diluted in hydrogen in the throat of a supersonic nozzle. Spectroscopic constants derived from the two lowest rotational transitions of both isomers agree very well with those derived from theoretical structures computed at the coupled cluster level of theory. In the same molecular beam, the fundamental rotational transition of $NCO−$ was observed with well-resolved nitrogen quadrupole hyperfine structure. Detection of $NCO−$ and $H2NCO+$ in our beam was subsequently confirmed by observation of several millimeter-wave transitions in a low pressure discharge through cyanogen and water. The spectroscopic constants of $NCO−$ obtained earlier by infrared laser spectroscopy are in good agreement with the highly accurate constants derived here. Owing to the high abundance of HNCO in many galactic molecular sources, both ions are excellent candidates for astronomical detection in the radio band.