Rotational data were taken from
(1) K. Takeda, S. Masuda, K. Harada, and K. Tanaka, 2016, Chem. Phys. Lett. 651, 168.
Transition frequencies with Cl hyperfine splitting were published by
(2) K. Tanaka, K. Harada, C. Cabezas, and Y. Endo, 2018, J. Mol. Spectrosc. 345, 39.
Rovibrational data were also included. These were taken from
(3) Y. Sumiyoshi, K. Tanaka, and T. Tanaka, 1993, Chem. Phys. Lett. 214, 17;
and from
(4) W. Y. Fan, Z. Liu, and P. B. Davies, 1998, J. Mol. Spectrosc. 191, 99.
The Dunham fit follows (4) with modifications related to (2). Hyperfine splitting was deemed to be unimportant for observational purposes. Transition frequencies should be reliable up to 300 GHz, possibly higher.
The dipole moment was taken from a quantum-chemical calculation in (2).
The partition function takes excited vibrational states into account.