The microwave data were provided by
(1) R. E. Penn and R. J. Olsen, 1976 J. Mol. Spectrosc. 61, 21.
The parameter set differs slightly from that in (1). Two transition frequencies with larger residuals were omitted from the line list. Even though only transitions below 36 GHz were recorded, the J range extends to 60, such that some transitions at quite high frequencies have fairly small uncertainties. Transition frequencies with uncertaities exceeding 0.2 MHz should be viewed with caution.
The dipole moment components were determined in (1).