The first entry from May 1998 has been updated and reevaluated. 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 present entry takes into account only v = 0 (or K_a = 0), just as the previous entry because of the low rotational temperatures and the high energy of 128.9 cm^–1 for the first excited v₅ (or K_a) level.
The experimental lines were reported by
(1) S. Saito, Y. Endo, and E. Hirota, 1984, J. Chem. Phys. 80, 1427;
by
(2) Y. Endo and Y. Oshima, 1993, J. Chem. Phys. 98, 6618;
and by
(3) M. C. McCarthy, C. A. Gottlieb, A. L. Cooksy, and P. Thaddeus, 1995, J. Chem. Phys. 103, 7779.
The uncertainties of (2) were reduced to 5 kHz; those of (3) were increased to 30 kHz. The uncertainties of (1) were increased by a factor of about 2.5 to around 30 kHz, and the highest frequency transition was omitted because of large residuals.
The strong fine structure components may be well predicted up to about 1 THz.
In contrast to the first entry, the present one does not include hyperfine splitting. However, ¹H and ¹⁴N hyperfine splitting has been resolved in the laboratory and may be resolvable during radioastronomical observations at low frequencies. Moreover, the asymmetric distribution of the strong hyperfine components may affect the apparent transition frequencies, in particular for the J = N – 1. Therefore, predictions with ¹H and ¹⁴N hyperfine splitting are available up to 123 GHz along with adjusted partition function values.
The ab initio dipole moment (component) is from
(4) N. Inostroza, X. Huang, and T. J. Lee, 2012, J. Chem. Phys. 135, Art. No. 244310.
Rovibrational (or b-type) transitions are fairly weak, and their intensities are rather uncertain because of the large amplitude HCC bending mode.