The lines were taken from
(1) R. D. Brown, P. D. Godfrey, P. S. Elmes, M. Rodler, and L. M. Tack, 1985, J. Am. Chem. Soc. 107, 4112.
The number of transitions was insuffient to determine centrifugal distortion terms reliably. Therefore, these were estimated from the main species and kept fixed in the analysis. Predictions should be reliable up to about 120 GHz only because of the very small data set.
The dipole moment was assumed to agree with that of the main isotopic species.