The microwave data were taken from
(1) L. F. Thomas, E. I. Sherrard, and J. Sheridan, 1955, Trans. Faraday Soc. 51, 619.
Lower millimeter wave data were taken from astronomical observations by
(2) M. Agúndez, E. Roueff, C. Cabezas, J. Cernicharo, and N. Marcelino, 2021, Astron. Astrophys. 649, Art. No. A171.
Quartic distortion parameters were evaluated by scaling quantum-chemically calculated values from
(3) H. S. P. Müller, S. Thorwirth, and F. Lewen, 2020, J. Mol. Struct. 1207, Art. No. 127769.
The frequencies in (1) were too low by around 100 kHz and were not merged; the uncertainties were increased slightly. The uncertainties in (2) appear to be quite conservative (2σ) and were adjusted. The calculation should be sufficiently accurate for observations of cold sources. Transition frequencies above 200 GHz or with K_a ≥ 2 should be viewed with caution.
The dipole moment is based on a quantum-chemical calculation in (2).