The experimental lines were taken from
(1) M. C. McCarthy, E. S. Levine, A. J. Apponi, and P. Thaddeus, 2000, J. Mol. Spectrosc. 203, 75.
The number of transition frequencies is too small to determine D reliably. Therefore, it was estimated from the main isotopolog. The calculated frequencies should be viewed with caution above 25 GHz. Values for eQq and C were evaluated upon comparison with experimental and quantum chemically derived values of other HC_xN molecules presented in
(2) T. F. Giesen, M. E. Harding, J. Gauss, J.-U. Grabow, and H. S. P. Müller, 2020, J. Mol. Spectrosc. 371, Art. No. 111303.
The ¹⁴N hyperfine splitting may be resolved in the ISM at low values of J. Therefore, separate calculations with ¹⁴N hyperfine splitting are available for J" < 10. The partition function takes into account the hyperfine splitting, but no vibrational contributions !
The dipole moment was assumed to agree with that of the main isotopic species calculated by
(3) P. Botschwina and M. Horn, 1997, J. Mol. Spectrosc. 185, 191.