Intensities of the T←S Transitions in Oxalyl Chloride and Bromide

The matrix elements of the spin-orbit interaction and the intensities of the T←S transitions resulting from possible perturbing processes in glyoxal and its halogen substituted derivatives are examined. The calculated results show that the T←S transitions in these molecules borrow their intensities predominantly from the 1Bu(π,π*)←1Ag and 3Bg(n,π*)←3Au transitions by means of the first-order spin-orbit interaction. An experimental study of the T←S absorptions of haloderivatives of glyoxal shows that the intensity increases with an increase in the atomic number of the halogen atom. This intensification due to halosubstitution can be interpreted in terms of two factors; (1) the halosubstitutional effects on the transition energies and on the intensities of these two perturbing transitions, and also on the transition energy of the T←S transition, and (2) the halosubstitutional effect on the matrix element of the spin-orbit interaction, which comes to involve the one-center term on the halogen atom if slight delocalizations of the n-orbital as well as of the π-orbital over the halogen atom are taken into account.