The first version of this entry was created in April 2000. It was based on data summarized in
(1) H. S. P. Müller, T. Klaus, and G. Winnewisser, 2000, Astron. Astrophys. 357, L65.
The second entry from Aug. 2009 was based on new rest frequencies for the N = 1 – 0 and 2 – 1 transitions obtained from astronomical observations described in
(2) M. Padovani, C. M. Walmsley, M. Tafalla, D. Galli, and H. S. P. Müller, 2009, Astron. Astrophys. 505, 1199.
Note: Previously used, seemingly more accurate data show considerable off-sets; see (2) for further details.
Besides these data and the submillimeter data from (1), frequencies for the N = 3 – 2 transition were taken from
(3) K. V. L. N. Sastry, P. Helminger, A. Charo, E. Herbst, and F. C. DeLucia, 1981, Astrophys. J. 251, L119.
The third, present entry combines rotational and rovibrational data of several low-lying states. Ground state transitions were taken from the original data in (1–3). The effect of the additional data on the ground state data is negligible.
The v₂ = 1 rotational lines were taken from
(4) D. R. Woodward, J. C. Pearson, C. A. Gottlieb, M. Guélin, and P. Thaddeus, 1987, Astron. Astrophys. 186, L14.
Data for v₂ = 2, v₃ = 1, and for v₂ = v₃ = 1 were published in
(5) T. C. Killian, C. A. Gottlieb, and P. Thaddeus, 2007, J. Chem. Phys. 127, Art. No. 114320.
ν₃ infrared transitions were measured by
(6) H. Kanamori, K. Seki, and E. Hirota, 1987, J. Chem. Phys. 87, 73.
Data for the ν₂ + ν₃ band were given in
(7) K. Kawaguchi, T. Amano, and E. Hirota, 1988, J. Mol. Spectrosc. 131, 58.
Transitions for 5ν₂ and the hot bands 5ν₂ – ν₂ and ν₂ + ν₃ – ν₂ were published by
(8) H. Kanamori and E. Hirota, 1988, J. Chem. Phys. 89, 3962.
Additional spectroscopic parameters of states up to 2200 cm^–1 were taken from
(9) Y.-C. Hsu, J.-M. Lin, D. Papoušek, and J.-J. Tsai, 1993, J. Chem. Phys. 98, 6690;
(10) Y.-C. Hsu, Y.-J. Shiu, and C.-M. Lin, 1995, J. Chem. Phys. 103, 5919;
(11) W.-Y. Chiang, Y.-C. Hsu, 1999, J. Chem. Phys. 111, 1454;
or have been estimated from these data. NOTE: The anharmonic interaction between v₂ = v₃ = 1 and v₂ = 5 can only be accounted for approximately using simple assumptions and has thus been ignored as in (8). As hyperfine structure data are available for some states it has been ignored in the energy file. This leads to small differences for the partition function values which can be neglected. Different l-components of a given vibrational state have been treated separately but will appear in the same entry as far as appropriate. Partition function values for only the ground vibrational state are given in parentheses. The partition function values are reliable up to 300 K, quite reasonable up to 500 K, and should be viewed with great caution at higher temperatures.
Predictions should be viewed with increasing caution above 1.5 THz or N > 17.
The dipole moment is from an ab initio calculation by
(12) D. E. Woon, 1995, Chem. Phys. Lett. 244, 45.
It is assumed that vibrational changes are negligible; this may be incorrect. Transition dipole moments were estimated from calculated infrared intensities published by
(13) R. Tarroni and S. Carter, 2004, Mol. Phys. 102, 2167.