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.
Rotational transition frequencies as well as ground state combination differences (GSCDs) from the ν₁ band have been determined by
(1) P. Jusko, A. Stoffels, S. Thorwirth, S. Brünken, S. Schlemmer, O. Asvany, 2017, J. Mol. Spectrosc. 332, 59.
The rotational data were reproduced on average to slightly better than twice the uncertainties, as reported in (1). Therefore, the reported uncertainties were doubled. In addition, GSCDs with large residuals were omitted from the line list.
The measurements likely cover more transitions than may be observed astronomically. Care is advised for transitions with calculated uncertainties larger than 0.25 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-CHD₂⁺ and para-CHD₂⁺. The para states are described by K_a + K_c odd, the ortho states by K_a + K_c even. The nuclear spin-weights are 2 and 1 for ortho-CHD₂⁺ and para-CHD₂⁺, respectively. The J_KaKc = 1₀₁ is the lowest para state. It is 7.5056 cm^–1 above ground.
Separate para and ortho predictions are available up to J = 4 along with separate para and ortho partition function values.