MICROWAVE-SPECTRA, DIPOLE-MOMENTS, AND TORSIONAL POTENTIAL FUNCTION OF CIS-GLYOXAL AND CIS-GLYOXAL-D(1)

Rotational transitions of cis-glyoxal, C2H2O2, and its monodeuterated isotopomer have been detected by microwave spectroscopy, both in their vibrational ground states and in their first and second excited torsi onal v(7), as well as in their first excited v(5) bending states. Assi gnments have been extended to high J and K-a quantum numbers which lea ds to higher accuracy of the spectroscopic parameters than hitherto kn own in the literature. Tunneling splittings of the torsion-vibrational levels of 335 kHz for v(7) = 1 and of 42.6 MHz for v(7) = 2 have been observed in the glyoxal spectra. These splittings are absent in glyox al-d(1) as is expected for symmetry reasons, while local perturbations are occasionally seen in all spectra. The electric dipole moment comp onents have been determined for both molecules in several vibrational states where a value of mu(b) close to 3.40 D was found in all cases. The cis-trans enthalpy difference, Delta H = (18.60 +/- 0.57) kJ mol(- 1) ((1555 +/- 48) cm(-1)), was determined from absorption intensities of glyoxal and glyoxal-di relative to those of the OCS molecule. A tor sional potential function is given using Delta H, the cis tunneling sp littings, and literature values for the frequencies of the trans and c is torsional levels as input data. (C) 1997 Academic Press.