HCCN is an asymmetric top molecule with a low barrier to linearity, i.e., it is quasilinear, similar to the parent species methylene, CH₂, which is somewhat closer to an asymmetric top rotor.
The experimental lines were reported by
(1) F. X. Brown, S. Saito, and S. Yamamoto, 1990, J. Mol. Spectrosc. 143, 203.
Uncertainties of 20 kHz were used in the fit.
The predictions should be viewed with some caution because of the small data set. Nevertheless, the strong fine structure components may be well predicted down to about 100 GHz and up to about 900 GHz.
Hyperfine splitting due to the ¹H and ¹⁵N nuclei may broaden the lines below about 120 GHz and may be resolvable at the lowest frequencies.
The dipole moment was assumed to agree with that of the main species, see e039501.cat.