The only transition measured in the laboratory thus far is the a-type J = 1 – 0 transition from
(1) M. Mladenovic, M. Lewerenz, M. C. McCarthy, and P. Thaddeus, 2009, J. Chem. Phys. 131, Art.-No. 174308.
Several spectroscopic parameters were kept fixed to theoretical values from that work. Therefore, predictions were limited to a-type transitions with K_a = 0. Even these predictions should be viewed with caution, but they might be fairly reasonable up to 100 or even 200 GHz.
¹⁴N hyperfine splitting may be resolved for low values of J. Therefore, a separate hyperfine calculation is provided for J' ≤ 3. NOTE: The partition function does not take into account the spin multiplicity g_I = 3 of the ¹⁴N nucleus !
The ab initio dipole moment was mentioned in (1). The molecule has also a fairly large b-dipole moment component. However, predictions would be completely unreliable at present and have thus been omitted.