The initial experimental frequencies have been reported by
(1) R. Bocquet, J. Demaison, J. Cosléou, A. Friedrich, L. Margulès, S. Macholl, H. Mäder, M. M. Beaky, and G. Winnewisser, 1999, J. Mol. Spectrosc. 195, 345.
While most of the reported transition frequencies were based on these new measurements, additional data were taken from earlier work by
(2) D. Dangoisse, E. Willemot, and J. Bellet, 1978, J. Mol. Spectrosc. 71, 414;
by
(3) P. Glorieux and G. W. Hill, 1978, J. Mol. Spectrosc. 70, 459;
by
(4) K. D. Tucker, G. R. Tomasevich, and P. Thaddeus, 1971, Astrophys. J. 169, 429;
and by
(5) G. Winnewisser, 1977, J. Mol. Spectrosc. 64, 340.
The fairly extensive data set of the first entry from Feb. 2000 was supplemented recently by an extensive set of lines recorded betwenn 1.1 and 1.52 THz and reported by
(6) O. Zakharenko, R. A. Motiyenko, L. Margulès, and T. R. Huet, 2015, J. Mol. Spectrosc. 317, 41.
As in that work, transition frequencies with uncertainties of 200 kHz or larger measured below 1000 GHz were omitted as were the older transition frequencies in the range of the new measurements. Most of the reported uncertainties were estimated somewhat conservatively, but were not altered. The uncertainties of the Köln measurements from (1) were made smaller by a factor of 2.
The predictions of high-lying rotational levels have improved somewhat. Overall, they should be sufficiently accurate for all astronomical purposes. Predictions with uncertainties larger than 0.3 MHz should be viewed with caution.
Because of the lower symmetry of HDCO with respect to the main isotopomer, spin-statistics do not have to be considered !
The dipole moment has been measured by
(7) J. W. C. Johns and A. R. W. McKellar, 1977, J. Mol. Spectrosc. 64 327.