Extended CNDO calculations upon benzonitrile and ethynylbenzene

The {CNDO}/{CI} method has been used to examine in detail the role of σ electrons in the lower electronic transitions of benzonitrile and ethynylbenzene. The calculated and observed energies and intensities of essentially π → π ^∗ transitions are in good agreement with experiment. For each molecule, the parametrization used gives two additional low-lying states of ¹A₂ symmetry. Analyses of the molecular orbitals and charge density distributions shows that the electronic transitions are more complex than simple molecular orbital and charge transfer theory predicts.