The molecule tunnels between two equivalent minima. The experimental transition frequencies were summarized by
(1) Z. Kisiel, L. Kolesniková, A. Belloche, J.-C. Guillemin, L. Pszczółkowski, E. R. Alonso, R. T. Garrod, E. Białkowska-Jaworska, I. León, H. S. P. Müller, K. M. Menten, and J. L. Alonso, 2022, Astron. Astrophys. 657, Art. No. A99.
A small number of microwave transitions was derived from
(2) E. R. Alonso, L. Kolesniková, E. Białkowska-Jaworska, Z. Kisiel, I. León, J.-C. Guillemin, and J. L. Alonso, 2018, Astrophys. J. 861, Art. No. 70.
The parameters differ considerably from those in (1), in particular at higher order. The number of paramaters is smaller here than it is in (1), and the fit is somewhat better; the uncertainties of the millimeter data were improved to 35 kHz.
The calculation should be sufficiently accurate for all astronomical observations. Caution is advised for transitions with calculated uncertainties exceeding 0.1 MHz. ¹⁴N hyperfine splitting was omitted in the calculation.
The dipole moment is from a quantum-chemical study in (2). Both a- and b-type transitions occur within each tunneling state. It is assumed that the dipole moment for transitions between the states is zero. It is conceivable that this does not hold strictly.