The observed transitions were taken from
(1) S. Saito and M. Goto, 1993, Astrophys. J. 410, L53; and from
(2) S. Takano, T. Klaus, and G. Winnewisser, 1998, J. Mol. Spectrosc. 192, 309.
Some of the lines omitted from the fits in (1) and (2) are included in the present calculation. Uncertainties of several lines from (1) have been raised, in particular those of closely spaced lines. Conversely, some uncertainties from (2) have been lowered. The sextic distortion constant H was estimated. Predictions to higher values of N should be viewed with caution.
The dipole moment was taken from the main isotopomer published in
(3) E. A. Scarl and F. W. Dalby, 1974, Can. J. Phys. 52, 1429.