The molecule is nearly T-shaped with the KN bond slightly shorter than the KC bond. Therefore, the molecule should be considered more as potassium isocyanide rather than as potassium cyanide.
The experimental measurements were taken from
(1) P. Kuijpers, T. Törring, and A. Dymanus, 1976, Chem. Phys. Lett. 42, 423;
(2) T. Törring, J. P. Bekooy, W. L. Meerts, J. Hoeft, E. Tiemann, and A. Dymanus, 1980, J. Chem. Phys. 73, 4875;
(3) J. J. van Vaals, W. L. Meerts, and A. Dymanus, 1984, J. Mol. Spectrosc. 106, 280.
The spectroscopic parameters used for the current predictions have been reported in
(4) J. H. He, D.-V. Trung, S. Kwok, H. S. P. Müller, T. Hasegawa, T. C. Peng, and Y. C. Huang, 2008, Astrophys. J. Suppl. Ser. 177, 275.
Transitions with uncertainties larger than 100 kHz have not been merged. The predictions should be fairly reliable as long as the predicted uncertainties do not exceed 1.0 MHz.
While in the laboratory b-type transitions have been observed, their intensities are liekly very small because the b dipole moment component is about 1.5 orders of magnitude smaller than the a component; hence, transitions will be about three orders of magnitude weaker.
Hyperfine structure splitting caused by the ¹⁴N nucleus or the ³⁹K nucleus are not of relevance for astronomical observations.
The ab initio dipole moment was taken from
(5) H. S. P. Müller, unpublished.