The calculations have been revised completely. A combined fit of rotational and rovibrational data pertaining to four vibrational states was carried out by
(1) H. S. P. Müller, 2021, unpublished.
Individual data sets and sets of spectroscopic parameters were evaluated in an iterative way. Uncertainties were assigned such that each data set fits on average within the uncertainties; the rms error of the global fit is 0.935, with values of indiviual data sets not differing greatly from this value.
Ground state rotational data up to 48 GHz were taken from
(1) R. A. H. Butler, F. C. De Lucia, D. T. Petkie, K. Møllendal, A. Horn, and E. Herbst, 2001, Astrophys. J. Suppl. Ser. 134, 319.
Additional data between 72 and 120 GHz were taken from
(2) S. L. Widicus Weaver, R. A. H. Butler, B. J. Drouin, D. T. Petkie, K. A. Dyl, F. C. De Lucia, and G. A. Blake, 2005, Astrophys. J. Suppl. Ser. 158, 188.
Further data between 522 and 1197 GHz were taken from
(3) P. B. Carrol, B. J. Drouin, and S. L. Widicus Weaver, 2010, Astrophys. J. 723, 845.
The ground state transition frequencies were taken from this work, but the uncertainties were slightly conservative. Rotational data pertaining to excited vibrational states were taken from
(4) K.-M. Marstokk and K. Møllendal, 1970, J. Mol. Struct. 5, 205;
and from (2).
Extensive ν₁₈, ν₁₂, and ν₁₇ rovibrational data were provided by
(5) B. Collier, K. Krueger, I. Miller, J. Zhao, B. E. Billinghurst, and P. L. Raston, 2021, Astrophys. J. Suppl. Ser. 253, Art. No. 40.
Excited state rotational data from (2) omitted in (5) were not considered here. In addition, 8 further v₁₈ = 1 transition frequencies were omitted because they displayed large residuals or were isolated lines at the edge of the data set and difficult to fit. Transition frequencies from (4) display almost exclusively positive residuals which are on average close to the rms value. It may thus be advisable to omit these data in future studies.
The excited state calculations should be sufficiently accurate for observational purposes. Frequencies with calculated uncertainties exceeding 0.2 MHz should be viewed with considerable caution.
Please note that the partition function values below refer to the ground vibrational state only. Vibrational correction factors have been derived for the main isotopolog in the harmonic approximation.
The ground state dipole moment was measured by
(6) K.-M. Marstokk and K. Møllendal, 1973, J. Mol. Struct. 16, 259.
These values were also used for the excited states.