The first entry from Aug. 2005 has been revised considerably. Additional infrared data involving states v₂ = 2 and 3 as well as v₃ = 1 and v₁ = 1 were included along with additional pure rotational data pertaining to v₂ = 2 and 3 as well as v₃ = 1. All these data were subjected to one combined fit.
Pure rotational transition frequencies for v = 0 and v₂ = 1 were taken from
(1) U. Fuchs, S. Brünken, G. W. Fuchs, S. Thorwirth, V. Ahrens, F. Lewen, S. Urban, T. Giesen, G. Winnewisser, 2004, Z. Naturforsch., 59a, 861.
Additional v = 0 frequencies were publised in
(2) G. Cazzoli, C. Puzzarini, and J. Gauss, 2005, Astrophys. J. Suppl. Ser. 159, 181.
The v₂ = 1 direct-l-type transitions were reported by
(3) M. Winnewisser and J. Vogt, 1978, Z. Naturforsch., 33a, 1323.
Higher vibrational state rotational data were taken from
(4) J. Preusser and A. G. Maki, 1993, J. Mol. Spectrosc. 162, 484.
Finally, extensive infrared data come from
(5) A. G. Maki, G. C. Mellau, S. Klee, M. Winnewisser, and W. Quapp, 2000, J. Mol. Spectrosc. 202, 67.
The main improvement occured in the partition function, which is essentially converged at 300 K and probably still good up to about 500 K. The rotational part is well converged up to 1000 K. The frequencies are also better at higher values of J. Predictions should be reliable throughout, but should be viewed with some caution above 3 THz for ΔJ = 1 transitions and above 0.2 THz for ΔJ = 0 transitions.
The dipole moment was assumed to agree with that of the main isotopic species, see e027503.cat.
The partition function takes into account all vibrational states used in the fit.