SiC₂ is a cyclic molecule with a fairly low barrier to linearity which is probably responsible for the large centrifugal distortion effects, especially in excited vibrational states involving excitation of v₃. The v₃ mode can be considered as asymmetric SiCC bending motion or as the C₂ internal rotation motion. K_a odd are permitted only in contrast to K_a even for the ground vibrational state. This is because of the different vibrational symmetry.
With respect to the first entry from Aug. 2007/Jan. 2008, the parameters have been reconsidered, mainly to estimate transition frequencies for v₃ = 3 and 4. The combined fit is described in
(1) R. C. Fortenberry, T. J. Lee, and H. S. P. Müller, 2015, Mol. Astrophys., 1, 13.
The recently extended set of ground state transition frequencies was summarized in
(2) H. S. P. Müller, J. Cernicharo, M. Agúndez, L. Decin, P. Encrenaz, J. C. Pearson, D. Teyssier, and L. B. F. M. Waters, 2012, J. Mol. Spectrosc., 271 50;
further details can be found there or in the ground state documentation e052527.cat. Transitions for v₃ = 1 and 2 come mainly from
(3) M. Izuha, S. Yamamoto, and S. Saito, 1994, Spectrochim. Acta A, 50, 1371.
Additional v₃ = 1 data were taken from
(4) M. Bogey, C. Demuynck, J. L. Destombes, and A. D. Walters, 1991, Astron. Astrophys., 247, L13.
Since the data set is fairly small and because of the possible onset of large amplitude character of the v₃ mode predictions above 500 GHz or with predicted larger than 0.3 MHz should be viewed with great caution. Transitions with predicted uncertainties larger than 100 MHz have been omitted.
The dipole moment was assumed to agree with that in the ground vibrational state, see e052527.cat; it is possible that this assumption is not as good as usually !
NOTE: The partition function takes into account the ground vibrational state only. However, contributions of excited vibrational states to the partitionfunction are available. For corrections much higher than 200 K, a list of vibrational energies is also available.