Ground state data were summarized by
(1) P. Pracna, J. Urban, O. Votava, Z. Meltzerová, Š Urban, V.-M. Horneman, and B. J. Drouin, 2011, J. Phys. Chem. A. 115, 1063.
The data set includes rotational data from
(2) A. Bauer and M. Bogey, 1970, C. R. Acad. Sci. Paris, Ser. B 271, 892.
Ground state combination data from (1) were also used. Our fit includes in addition the J = 1 – 0 transition frequency reported by
(3) S. G. Kukolich, 1972, J. Chem. Phys. 57, 869.
The purely K-dependent terms A and D_K were taken from
(4) J. Pliva, L. D. Le, J. W. C. Johns, Z. Lu, and R. A. Bernheim, 1995, J. Mol. Spectrosc. 170, 423.
Predictions should be reliable throughout. Some caution is advised for K > 9. This should not be a serious restriction.
At low temperatures, it may be necessary to discern between A-CH₃NC and E-CH₃NC. The A state levels are described by K = 3n, those of E state by K = 3n ± 1. The nuclear spin-weight ratio is 2 : 1 for A-CH₃NC with K > 0 and all other states, respectively. The J_K = 1₁ level is the lowest E state level. It is about 5.58 cm^–1 above ground.
The dipole moment was taken from
(5) M. Römheld, 1978, PhD thesis, Univerität Ulm, Germany.