Molecular orbital calculations of rotational strengths: A study of skewed diketones

A skewed glyoxal molecule is considered as a model for the study of the optical activity of α-diketones. The rotational strengths are calculated for various angles of twist, between trans (θ=0‡) and cis (θ = 180‡). The wavefunctions are computed both by the extended Hückel method and by the SCF-CNDO method, considering all 22 valence electrons. The effect of configuration interaction is studied. All two-center terms are included in the calculation of the electric and magnetic transition moments. The results predict the rotational strength of the lowest transition to be negative for 0‡ < θ < 90‡ when the molecule is twisted in a right-handed way and to be positive for 90‡ < θ < 180‡. In general, the lowest transition is followed by a transition with opposite rotational strength, in analogy to the predictions of a simple exciton model.