The data set includes rotational data from
(1) A. Bauer and M. Bogey, 1970, C. R. Acad. Sci. Paris, Ser. B 271, 892.
Additional data were taken from
(2) M. Godon, 1974, PhD thesis, Lille, France.
One transition with large residual was omitted. Several higher order parameters were estimated from the main isotopic species. The calculations have been truncated at J = 20 because of the limited data set and should, in addition, be viewed with caution at higher J and K.
The dipole moment of the main isotopic species was taken from
(3) M. Römheld, 1978, PhD thesis, Univerität Ulm, Germany.