The experimental data was summarized in
(1) T. Amano, K. Hashimoto, and T. Hirao, 2006, J. Mol. Struct. 795, 190.
Assuming the submillimeter lines to be accurate to 100 kHz, essentially all experimental lines could be reproduced satisfactorily. In contrast to (1), where a fairly large number of transitions were omitted from the fit, here only two lines were not included in the final fit.
Effects of the sextic centrifugal distortion constants H were estimated from comparison with CH₃CN. These parameters will have sizeable effects for transitions with even higher quantum numbers. Therefore, it is difficult to estimate how far the predictions are reliable, possibly up to 800 GHz or slightly higher, at least for low values of K.
Note: The ¹⁴N hyperfine structure may be resolvable in cold astronomical sources. No laboratory data are known at present. Hyperfine structure has not been considered in the calculation of the partition function.
At low temperatures, it may be necessary to discern between A and E states of the molecule. The A state levels are described by K = 3n, those of E state by K = 3n ± 1. The nuclear spin-weight ratio is 2 : 1 for A-CH₃CNH⁺ with K > 0 and all other states, respectively. The J_K = 1₁ level is the lowest E state level. It is 5.5398 cm^–1 above ground.
The dipole moment is from an ab initio calculation by
(2) P. Botschwina, 2000, J. Mol. Spectrosc. 203, 203.