The transition frequencies were largely taken from
(1) M. Bogey, C. Demuynck, and J. L. Destombes, 1986, Chem. Phys. Lett. 125, 383.
Additional lines were published by
(2) J. M. Vrtilek, C. A. Gottlieb, and P. Thaddeus, 1987, Astrophys. J. 314, 716;
and by
(3) F. J. Lovas, R. D. Suenram, T. Ogata, and S. Yamamoto, 1992, Astrophys. J. 399, 325.
One reportedly overlapped line from (2) was treated as being due to the ortho line only as already suggested from the residuals in that work. All uncertainties were taken as reported. Those from (1) appear to be, on average, slightly conservative, whereas those from (2) are, at least, marginally too small.
With respect to first entry of Mar. 2011, additional rest frequencies were taken from
(4) S. Spezzano, F. Tamassia, S. Thorwirth, P. Thaddeus, C. A. Gottlieb, and M. C. McCarthy, 2012, Astrophys. J. Suppl. Ser. 200, Art. No. 1.
The reported uncertainties appear to be too optimistic in part; values have been increased from 5 to 20 kHz for 3 transitions. Though rather limited in number, these data improve predictions in particular for the strong R-branch transitions with high values of K_c.
Various fits have been tried, some of these can be found in the fitting spectra section; the prefered one is also available in the archive. The parameter set is based on
(5) M. Bogey, C. Demuynck, J. L. Destombes, and H. Dubus, 1987, J. Mol. Spectrosc. 122, 313.
Predictions with uncertainties larger than 0.5 MHz should be viewed with caution.
At lower temperatures, it may be useful to distinguish between ortho-c-C₃H₂ and para-c-C₃H₂. The ortho and para levels are described by K_a + K_c = odd and even, respectively, with a spin-weight ratio of 3 : 1. The J_KaKc = 1₀₁ level is the lowest ortho state. However, it is only 1.633190 cm^–1 above ground.
Separate para and ortho predictions are available up to about 200 GHz along with separate para and ortho partition function values. Note: the spin-weight of 3 for the separate ortho predictions has been eliminated. Please note also that there are matrix-elements which mix ortho and para levels. However, these matrix-elements are small. Nevertheless, larger effects may occur in cases of accidental near-degeneracies of certain levels.
The dipole moment was reported in (3).