Microwave to lower submillimeter data were summarized by
(1) M. Bogey, C. Demuynck, and J.-L. Destombes, 1988, J. Mol. Spectrosc. 132, 277.
The microwave data were contributed by
(2) L. H. Scharpen and V. W. Laurie, 1965, J. Chem. Phys. 43, 2765;
and by
(3) R. C. Benson, W. H. Flygare, 1970, J. Am. Chem. Soc. 92, 7523.
With respect to the first entry of May 2021, the millimeter data were considerably extended by
(4) L. Bonah, B. Helmstaedter, J.-C. Guillemin, S. Schlemmer, and S. Thorwirth, 2025, J. Mol. Spectrosc. 408, Art. No. 111967.
The data fit to roughly 20 kHz using an S-reduced Hamiltonian.
The calculations were truncated at J = 100 and at 1000 GHz. They should be sufficient for all observational purposes. Transition frequencies with calculated uncertainties exceeding 0.1 MHz should be viewed with caution.
Spin-statistical weight ratios of 9 : 7 for levels with K_a + K_c being even and odd, respectively, have been taken into account.
The dipole moment was determined in (2).