The Renner-Teller effect splits the two Σ components of the CCH bending mode ν₄ (≈ 600 cm^–1) of C₃H such that the lower component ²Σ^μ is only 27.20 cm^–1 above the lowest rotational state (N = 1, J = 0.5, F = 0 of the ²Π_1/2 ladder). This leads to considerable Coriolis interaction, as shown by
(1) S. Yamamoto, S. Saito, H. Suzuki, S. Deguchi, N. Kaifu, S.-I. Ishikawa, and M. Ohishi, 1990, Astrophys. J. 348, 363.
Additional millimeter wave transitions have ben reported by
(2) C. A. Gottlieb, J. M. Vrtilek, E. W. Gottlieb, P. Thaddeus, and Å. Hjalmarson, 1985, Astrophys. J. 294, L55.
Further low frequency transitions were taken from astronomical observations by
(3) N. Kaifu, M. Ohishi, K. Kawaguchi, S. Saito, S. Yamamoto, T. Miyaji, K. Miyazawa, S.-I. Ishikawa, C. Noumara, S. Harasawa, M. Okuda, and H. Suzuki, 2004, Publ. Astron. Soc. Japan 56, 69.
With respect to the Aug. 2004 entry, additional submillimeter lines have been reported by
(4) M. Caris, T. F. Giesen, C. Duan, H. S. P. Müller, S. Schlemmer, and K. M. T. Yamada, 2009, J. Mol. Spectrosc. 253, 99.
These data do not only include pure rotational transitions, but also transitions between the higher lying ²Π_3/2 state and the ²Σ^μ state. Predictions for the rovibrational transitions between the lower lying ²Π_1/2 state and the ²Σ^μ state, which occur around 1 THz, should be viewed with some caution; the same applies to all transitions having predicted uncertainties much larger than 1 MHz.
The state numbers 0 and 1 refer to the ground vibrational state and the excited v₄ = 1, ²Σ^μ state, respectively.
The dipole moment is from an ab initio calculation by
(5) D. E. Woon, 1995, Chem. Phys. Lett. 244, 45.
A substantial transition dipole moment between v = 0 and v₄ = 1 can be expected – a value of 0.5 D was assumed in (1).