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.
The v₃ = 3 transition frequencies have been estimated from lower v data. 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.
The predictions have been truncated at 400 GHz and at K_a = 8. All predictions should be viewed with great caution. It is conceivable that predictions involving low values of J or K_a are quite reasonable.
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.