Rotational and Rovibrational Spectroscopy of CH3NC of the Ground and ν4 = 1 Vibrational States

The parallel vibration−rotation band ν4 of methyl isocyanide (CH3NC), with a band center at 944.9 cm−1, was studied by FTIR spectroscopy between 890 and 980 cm−1 in order to improve the ground-state rotational constants. Such improvement is essential for the scheduled studies of excited vibrational levels and their mutual anharmonic resonances occurring at higher values of the K rotational number. Ground-state combination differences generated from this band, spanning values of J/K from 0 to 85/13, were combined with rotational data from the literature and newly measured rotational transitions, extending the J/K range from 3/0 up to 31/14, and fitted simultaneously with a fully quantitative reproduction of the data. The infrared data of the ν4 band were analyzed together with rotational data of the ν4 = 1 level, spanning values of J/K from 4/0 to 14/12. The fit in the approximation of an isolated vibrational state, with the transitions perturbed by weak local resonances excluded, yields reproduction of the data within experimental uncertainties.