The very limited data set of the first entry from July 2012 was extended significantly by measurements between 158 and 1084 GHz by
(1) H. S. P. Müller, J. C. Guillemin, F. Lewen, and S. Schlemmer, 2022, J. Mol. Spectrosc. 384, Art. No. 111584.
The initial experimental data were taken from
(2) R. A. Creswell and R. H. Schwendemann, 1974, Chem. Phys. Lett. 27, 521.
Also used in the fit were microwave data from
(3) C. Hirose, 1974, Bull. Chem. Soc. Japan 47, 1311.
Uncertainties of data from (2) were increased to 20 kHz from 10 kHz because of the sizes of the average residuals. Four transitions with large residuals from (3) were omitted.
The calculation should be sufficiently accurate for all observational purposes. Calculated frequencies with uncertainties exceeding 0.1 MHz should be viewed with some caution.
At very low temperatures, it may be necessary to discern between ortho-c-C₂H₄¹⁸O and para-c-C₂H₄¹⁸O. The ortho states are described by K_a + K_c even, the para states by K_a + K_c odd. The nuclear spin-weight ratio is 5 : 3 for ortho-c-C₂H₄¹⁸O : para-c-C₂H₄¹⁸O. The J_KaKc = 1₀₁ is the lowest para state. It is 1.1925 cm^–1 above ground. Because of this very small energy difference, separate analyses are probably only required for absorption transitions originating in the 1₀₁ level.
The dipole moment was assumed to agree with that of the main isotopolog, taken from
(4) G. L. Cunningham Jr., A. W. Boyd, R. J. Myers, W. D. Gwinn, and W. I. Le Van, 1951, J. Chem. Phys. 19, 676 ;
corrected by 0.02 D due to an updated reference value for OCS, see (1).