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The pure rotational lines were taken from
(1) R. Hinze, H. Zerbe-Foese, J. Doose, and A. Guarnieri,
1996, J. Mol. Spectrosc. 176, 133.
Infrared data were described in
(2) L. Nemes, D. Luckhaus, M. Quack, and J. W. C. Johns,
2000, J. Mol. Struct. 517, 217;
the lines were kindly communicated by L. Nemes.
An initial fit involved the interacting states
v9 = 1, v6 = 1,
v5 = 1, v8 = 1,
and other "dark" states. The v8 = 1
data were fit somewhat poorly, such that they were
omitted, as were paramaters of higher states;
the best fit parameters of v8 = 1
were retained as fixed in the final fits.
The J values were limited to 30 because of the
limited data with microwave accuracies.
Calculations of transitions frequencies with Ka
exceeding 4 should be viewed with increasing caution as should
be those with calculated uncertainties exceeding 0.2 MHz.
Spin-statistics were considered in the calculations.
The ortho states are described by Ka even,
the para states by Ka odd.
The nuclear spin-weights are 3 and 1 for ortho-H2C2O
and para-H2C2O, respectively.
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
for a posteriori corrections of the column densities.
The dipole moment was reported in 1.
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