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.
Several higher order parameters were estimated from the main isotopic species. The calculations have been truncated at J = 20 because of the very 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.