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The semi-empirical valence-shell SCF MO theory (the INDO method) by Pople et al.has been modified for the purpose of calculating the electronic excitation energies of organic compounds. All the necessary one-center parameters have been evaluated from the valence-state ionization data of Hinze and Jaffé, combined with the Slater-Condon atomic parameters. The INDO method thus modified has been applied to carbonyl compounds and conjugated dienes. The calculated excitation energies have been found to be satisfactory in many respects.

The ultraviolet absorption spectra of cisand transisomers of various β-substituted styrenes have been measured both in the gaseous state and in n-hexane solution. The excitation energies observed for cisisomers are either smaller or greater than those of the trans, depending on the type of β-substituents. The isomeric difference in the energy of the lowest allowed transition was found to be linearly correlated with Taft’s steric substituent constants. The electronic states of the various isomeric pairs were calculated by the LCAO–SCF–CI method of the Pariser-Parr-Pople type. The results were found to be entirely in line with the observed spectral features.

The ultraviolet absorption spectra of the cisand transisomers of various 1,2-disubstituted ethylenes were measured in isopentane solution. Each isomer showed a single intense band assignable to the lowest π–π*transition. For the majority of the compounds studied, both energy and oscillator strength of transition are found to be greater for transisomers than for the corresponding cisisomers, in accord with the general trend predicted by the Pariser-Parr-Pople calculations. Existence of some anomalous cases which contradict the generalization indicates a limited validity of the usual assumption of virtual orbitals.