The J" = 0, 1 transition frequencies were taken from
(1) M. Womack, A. J. Apponi, L. M. Ziurys, 1996, Astrophys. J. 461, 897.
The J" = 2 and 3 transition frequencies were reported by
(2) V. Lattanzi, A. Walters, J. C. Pearson, and B. J. Drouin, 2008, J. Quant. Spectrosc. Radiat. Transfer 109, 580.
In addition, infrared ground state combination differences were used in the fit. These were published in
(3) C. Chackerian, Jr., and G. Guelachvili, 1980, J. Mol. Spectrosc., 84, 447;
(4) C. Chackerian, Jr., E. S. Bus, W. B. Olson, and G. Guelachvili, 1986, J. Mol. Spectrosc., 117, 255;
(5) O. N. Ulenikov, G. A. Onopenko, N. E. Tyabaeva, J. Schroderus, and S. Alanko, 1999, J. Mol. Spectrosc. 193, 249.
With respect to the Mar. 2008 entry, the J" = 4 transitions from (2) have been remeasured, and the J" = 6 transitions have been measured with high accuracy for the first time. The data have been published in
(6) B. J. Drouin, S. Yu, J. C. Pearson, and H. S. P. Müller, 2009, J. Quant. Spectrosc. Radiat. Transfer 110, 2077.
Descrepancies of the data from (3) with those in (5) and (6) were traced to a calibration error in (3); details are described in (6).
With respect to the June 2009 entry, additional extensive data were provided by
(7) C. Bray, A. Cuisset, F. Hindle, R. Bocquet, G. Mouret, and B. J. Drouin, 2017, J. Quant. Spectrosc. Radiat. Transfer 189, 198.
Also used in the fit were A₁/A₂ splitting data from
(8) M. Abe, H. Sera, H. Sasada, 2015, J. Mol. Spectrosc. 312, 90.
The fit is similar to the one in (6). The predictions are thought to be fairly reliable throughout.
At low temperatures, it may be necessary to discern between A-CH₃D and E-CH₃D. 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-CH₃D with K > 0 and all other states, respectively. The J_K = 1₁ level is the lowest E state level. It is 9.1306 cm^–1 above ground.
Updated dipole moment and distortion corrections were taken from (7). Please note that changes in intensities already at fairly small quantum numbers are much more important than changes in the calculated frequencies. As the distortion effects are rather large, the intensities should be viewed with increasing caution beyond J of about 10 or K of about 8.