The experimental millimeter and lower submillimeter lines are from
(1) J. Behrend, P. Mittler, G. Winnewisser, K. M. T. Yamada, and M. Winnewisser, 1990, J. Mol. Spectrosc. 141, 265.
Additional terahertz data were taken from
(2) S. P. Belov, M. Liedtke, T. Klaus, R. Schieder, A. H. Saleck, J. Behrend, K. M. T. Yamada, G. Winnewisser, and A. F. Krupnov 1994, J. Mol. Spectrosc. 166, 489;
and from
(3) K. M. T. Yamada, J. Behrend, S. P. Belov, and G. Winnewisser, 1996, J. Mol. Spectrosc. 176, 397.
While the data appear to be precise to 20 or 30 kHz for the most part, small discrepancies in K_a = 3 – 2 data from (1) and (2) exist. Even though these discrepancies may be unimportant for astronomical observations, predictions should be viewed with some caution throughout.
The equivalent H nuclei lead to spin-statistical weight ratios of 3 : 1 for ortho and para states which are described by K_c being odd and even, respectively.
The dipole moment is from an ab initio calculation from
(4) D. E. Woon and E. Herbst, 2009, Astrophys. J. Suppl. Ser. 185, 273.