Cyanic acid is a quasi-linear molecule close to the non-linear limit. The lines were taken from
(1) S. Brünken, C. A. Gottlieb, M. C. McCarthy, and P. Thaddeus, 2009, Astrophys. J. 697, 880.
Predictions with uncertainties exceeding 0.5 MHz should be viewed with caution. Predictions of transitions with K_a = 3 and 4 should be viewed with some caution, all higher K_a transitions with great caution throughout.
¹⁴N hyperfine splitting may be resolved for low values of J. Therefore, a separate hyperfine calculation is provided for J' ≤ 8 and K_a ≤ 1. The K_a = 1 hfs splitting should be viewed with caution as the corresponding parameter is from an ab initio calculation. 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.