The lines were taken from
(1) R. D. Brown, P. D. Godfrey, D. McNaughton, A. P. Pierlot, and W. H. Taylor, 1990, J. Mol. Spectrosc. 140, 340.
Additional lines used in
(2) A. Guarnieri and A. Huckauf, 2003, Z. Naturforsch. 58a, 275,
were kindly communicated by A. Huckauf.
Some higher order centrifugal distortion constants were kept fixed to values esimated from the main isotopomer, see e042501.cat. Predictions for higher values of K_a should be viewed with caution.
At low temperatures, it may be necessary to discern between ortho-H₂C₂¹⁸O and para-H₂C₂¹⁸O. 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-H₂C₂¹⁸O and para-H₂C₂¹⁸O, respectively. The J_KaKc = 1₁₁ level is the lowest ortho level. It is 9.7240 cm^–1 above ground.
The dipole moment was assumed to be the same as for the main isotopomer, see e042501.cat.