Ground state data up to K = 6 were taken from
(1) M. Winnewisser, E. F. Pearson, J. Galica, and B. P. Winnewisser, 1982, J. Mol. Spectrosc. 91, 255.
A transition with partially resolved ¹⁴N hyperfine splitting was reported by (2) H. K. Bodenseh and K. Morgenstern, 1970, Z. Naturforsch. 25a, 150.
The purely K-dependent terms A, D_K, and H_K were assumed to agree with those of methyl cyanide. The values were taken from
(3) H. S. P. Müller, L. R. Brown, B. J. Drouin, J. C. Pearson, I. Kleiner, R. L. Sams, K. Sung, M. H. Ordu, and F. Lewen, 2015, J. Mol. Spectrosc. 312, 22.
Note: The A₁/A₂ lines are provided as separate transitions since 14 Aug. 2018 even if there is no evidence for splitting between the lines.
Predictions should be reliable up to 300 or 400 GHz and up to K = 7. This should not be a serious restriction because the related HCNO molecule is a dark cloud molecule. Perturbations occur at higher K, but their analyses appear to be not so straightforward.
¹⁴N hyperfine splitting may be resolvable at low values of J and possibly at the highest K. Therefore, predictions with hyperfine splitting have been provided up to J" = 5 (47 GHz) together with appropriate partition function values. Vibrational contributions have not been considered in the calculation of the partition function yet.
At low temperatures, it may be necessary to discern between A-CH₃CNO and E-CH₃CNO. 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-CH₃CNO with K > 0 and all other states, respectively. The J_K = 1₁ level is the lowest E state level. It is about 5.4 cm^–1 above ground.
The dipole moment was reported by
(4) P. B. Blackburn, R. D. Brown, F. R. Burden, J. G. Crofts, and I. R. Gillard, 1970, Chem. Phys. Lett. 7, 102.