CH₃⁺ and CD₃⁺ do not posess a permanent dipole moment because they are planar symmetric top rotors. In contrast, both CH₂D⁺ and CHD₂⁺ do have a sizable dipole moment because the charge is not located in the center of mass.
With respect to the third entry from October 2013, the available data have been reconsidered and refit.
20 low-lying rotational transitions have been measured by
(1) M. Töpfer, P. Jusko, S. Schlemmer, O. Asvany, 2016, Astron. Astrophys. 593, L11.
Additional infrared data had only a small impact on the parameter values and uncertainties and were omitted. Resorting to the S reduced Hamiltonian requires one less parameter.
Predictions should be sufficiently accurate for all astronomical observations. Care is advised for transitions with calculated uncertainties larger than 0.15 MHz.
An estimate for the dipole moment was reported in
(2) M. F. Jagod, M. Rösslein, C. M. Gabrys, and T. Oka, 1992 J. Mol. Spectrosc. 153, 666.
At low temperatures, it may be necessary to discern between ortho-CH₂D⁺ and para-CH₂D⁺. The ortho states are described by K_a odd, the para states by K_a even. The nuclear spin-weights are 3 and 1 for ortho-CH₂D⁺ and para-CH₂D⁺, respectively. The J_KaKc = 1₁₁ is the lowest ortho state. It is 12.89 cm^–1 above ground.
Separate para and ortho predictions are available up to J = 4 along with separate para and ortho partition function values. Note: the spin-weight of 3 for the separate ortho predictions has been eliminated.
The earlier versions can also be accessed in the Cologne Spectroscopy Data section.