Rotational analysis of some vibronic bands in the 3A u←1A g transition of glyoxal. Spin splittings in the lowest triplet state of the isolated molecule

A rotational analysis of three vibronic bands (000, 720, and 510) in the phosphorescence excitation spectrum of glyoxal (CHO–CHO) has been performed. From this analysis we deduce the rotational, spin-rotation, and spin–spin (plus spin-orbit) constants of the lowest triplet state of the isolated molecule. 3Au glyoxal has been shown to exist in nearly the same trans-planar (C2h) configuration as the lowest excited singlet state. Both geometries are very similar to that of the ground state. The dynamic implications of these structural findings are discussed. We also compare the spin splittings determined in this work with those previously measured in the condensed phase, using optically detected magnetic resonance methods.