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 27.2 cm^–1 above the lowest rotational state (N = 1, J = 0.5, F = 0 of the ²Π_1/2 ladder). The energy difference is only 9.812 cm^–1 for C₃D ! 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.
With respect to the Aug. 2004 entry, low-N transitions in the ²Π_1/2 spin-component have been reported by
(2) E. Kim and S. Yamamoto, 2005, J. Mol. Spectrosc. 233, 93.
The D hyperfine structure may be resolvable for transitions with low values of N. Therefore, a separate hyperfine calculation is provided up to N = 5. These predictions should be viewed with some caution for the higher lying ²Π_3/2 spin component as well as for the excited ²Σ^μ. Note: The partition function given below includes the D hyperfine splitting !
Some caution is advised for transitions with predicted uncertainties 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 was assumed to be the same as for the main isotopomer, see e037501.cat.
A substantial transition dipole moment between v = 0 and v₄ = 1 can be expected – a value of 0.5 D was assumed in (1). Since no rovibrational transitions have been observed thus far for C₃D, predictions of this type of transitions should be viewed with caution.