The transition frequencies were taken from astronomical observations by
(1) J. Pety, P. Gratier, V. Guzmán, E. Roueff, M. Gerin, J. R. Goicoechea, S. Bardeau, A. Sievers, F. Le Petit, J. Le Bourlot, A. Belloche, and D. Talbi,
2012, Astron. Astrophys. 548, Art. No. A68.
The estimated uncertainties appear to be too optimistic. All values have been increased essentially uniformly by a factor of 1.7 to achieve an rms error of slightly smaller than 1.0. With respect to the first entry of Nov. 2012, additional laboratory data were included which were reported by
(2) S. Brünken, L. Kluge, A. Stoffels, O. Asvany, and S. Schlemmer, 2014, Astrophys. J. 783, Art. No. L4.
With respect to the second entry of Feb. 2014, additional laboratory data were included from
(3) M. C. McCarthy, K. N. Crabtree, M.-A. Martin-Drumel, O. Martinez, Jr., B. A. McGuire, and C. A. Gottlieb, 2015, Astrophys. J. Suppl. Ser. 217, Art. No. 10;
as well as rest frequencies from astronomical observations between 200 and 300 GHz from
(4) S. Cuadrado, J. R. Goicoechea, P. Pilleri, J. Cernicharo, A. Fuente, and C. Joblin, 2015, Astron. Astrophys. 575, Art. No. A82.
In the case of multiply reported transition frequencies, weighted averages were merged if the uncertainties differed by less than a factor of about 2.
The sextic distortion parameter H is not determined with significance, smaller than the value in (1), and possibly of correct order of magnitude. Therefore, predictions beyond 300 GHz should be viewed with some caution, those beyond 450 GHz with great caution.
The ab initio dipole moment value was reported in (1).