Rotational and rovibrational data involving states v₂ up to 3 as well as v₃ = 1 and v₁ = 1 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.
Predictions should be reliable throughout, but should be viewed with some caution above 3 THz for strongly allowed transitions and for weakly allowed transitions if their predicted uncertainties exceed 1 MHz.
The dipole moment was assumed to agree with that of the main isotopic species, see e027508.cat.
The partition function takes into account all vibrational states used in the fit.