The syn conformer is by far the lowest one. Information is also available for two low-lying excited vibrational states and the ground vibrational state of the skew (or gauche) conformer. The transition frequencies were summarized in
(1) L. Kolesniková, A. Belloche, J. Koucký, E. R. Alonso, R. T. Garrod, K. Luková, K. M. Menten, H. S. P. Müller, P. Kania, and Š. Urban, 2022, Astron. Astrophys., 659, Art. No. A111.
By far the most data are from this work. Carefully scrutinized earlier lower frequency data of limited extent were taken from
(2) K.-M. Marstokk, H. Møllendal, and S. Samdal, 2000, J. Mol. Struct., 524, 66.
The calculation should be accurate enough for all observational purposes. The ¹⁴N hyperfine splitting was not considered.
Please note: The partition function takes into account the ground vibrational state only. Combined vibrational and conformational correction factors were basically taken from (1).
The dipole moment components in the ground vibrational state were determined in (2). The c-component may be non-zero, but it is very small and very uncertain. Therefore, it was omitted in the calculation.