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 nearly 50 kHz for J = 2 – 1 and around 200 kHz for J = 3 – 1. These transition frequencies were not merged, and the uncertainties were adjusted. 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).