The anti conformer is about 58 K higher than the gauche conformer and the later is doubly degenerate with respect to the former. It has C_S symmetry. Among the 30 vibrational modes, 18 are totally symmetric (a') and 12 belong to a". The state v₂₉ = 1 is fourth lowest in energy. The transition frequencies were taken from
(1) H. S. P. Müller, A. Walters, N. Wehres, A. Belloche, O. H. Wilkins, D. Liu, R. Vicente, R. T. Garrod, K. M. Menten, F. Lewen, and S. Schlemmer, 2016, Astron. Astrophys., 595 Art. No. A87.
¹⁴N hyperfine splitting as well as CH₃ are unlikely to be resolved in astronomical observations.
The predictions may be sufficient for astronomical observations, but should be viewed with caution if the uncertainties are larger than 0.3 MHz. The prediction of b-type transitions was omitted because they can not be predicted accurately.
Note: the statistical weight of the anti and gauche conformers is 1 : 2 because of symmetry. We have estimated vibrational corrections in the harmonic oscillator approximation using fundamental vibrations of the gauche-conformer from
(2) J. R. Durig, B. R. Drew, A. Komer, and S. Bell, 2001, Phys. Chem. Chem. Phys., 3 766,
as commented in (1). Additional information on vibrational states is also available.
The dipole moment components were assumed to agree with those in the ground vibrational state of the respective conformer, see e069505.cat.