2019ApJS..245...31K

Methyl isocyanate (CH₃NCO) is a recently identified interstellar molecule giving rise to many detected lines. Interestingly, its delayed identification was due not to weak lines but to a very complex rotational spectrum. To date, the only published laboratory transitions for this molecule are those between rotational energy levels with K <= 3. In the present work, Stark-modulation spectroscopy was used to record the room-temperature rotational spectrum of CH₃NCO in the spectral region from 32 to 90 GHz. Observation of characteristic Stark effects, measured at specific low-voltage modulation conditions, and ¹⁴N nuclear quadrupole hyperfine structure allowed unambiguous assignment of rotational transitions up to K = 10. These newly assigned transitions were subsequently followed up to 364 GHz with the aid of Loomis-Wood-type displays. Since there are no reports on astrophysical detection of ¹³C isotopic species, first laboratory measurements between 50 and 300 GHz have also been performed for CH₃N¹³CO and ¹³CH₃NCO isotopologues. A comprehensive spectral analysis undertaken in this work made it possible to extend the knowledge of the rotational spectrum of CH₃NCO to more than 2500 new transitions. Furthermore, more than 1200 lines were identified and analyzed for each of the isotopologues. The extensive line lists and sets of molecular parameters reported in this work provide the basis for further astrophysical searches of CH₃NCO.