This is an update of version 1 from Mar. 2000. The experimental data were summarized by
(1) H. S. P. Müller and F. Lewen, 2017, J. Mol. Spectrosc. 331, 28.
Besides new data betwen 1.37 and 1.5 THz from that study, transition frequencies were taken mostly from
(2) H. S. P. Müller, R. Gendriesch, F. Lewen, and G. Winnewisser, 2000, Z. Naturforsch. 55a, 486;
and from
(3) R. Cornet and G. Winnewisser, 1980, J. Mol. Spectrosc. 80, 438.
Additional transition frequencies with ¹H hyperfine splitting were taken from
(4) K. D. Tucker, G. R. Tomasevich, and P. Thaddeus, 1971, Astrophys. J. 169, 429;
and from
(5) K. D. Tucker, G. R. Tomasevich, and P. Thaddeus, 1972, Astrophys. J. 174, 463.
Three more transitions are from
(6) J. C. Chardon, C. Genty, D. Guichon, N. Sungur, and J. G. Théobald, 1974, Rev. Phys. Appl. 9, 961;
and one more from
(7) D. Dangoisse, E. Willemot, and J. Bellet, 1978, J. Mol. Spectrosc. 71, 414.
The predictions should be accurate enough for all astronomical observations. Predictions with calculated uncertainties exceeding 0.2 MHz should be viewed with caution.
The ¹H hyperfine splitting can be resolved for Q-branch transitions with low values of J and K_a = 1. Transitions with K_a = 2 do not show hyperfine splitting while those with K_a = 3 are deemed to be too high in energy. A separate hyperfine calculation is provided. The partition function takes into account the hyperfine splitting.
At low temperatures, it may be necessary to discern between ortho-H₂C¹⁸O and para-H₂C¹⁸O. The ortho states are described by K_a odd, the para states by K_a even. The nuclear spin-weights are 3 and 1 for ortho-H₂C¹⁸O and para-H₂C¹⁸O, respectively. The J_KaKc = 1₁₁ is the lowest ortho state. It is 10.4890 cm^–1 above ground.
The dipole moment was assumed to be the same as for the main isotopic species, see e030501.cat.