A Semi-Empirical Theory of the Electronic Spectra and Electronic Structure of Complex Unsaturated Molecules. II

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Abstract
The theory of electronic spectra and electronic structure, the elucidation of which was begun in the first paper of this series, is further developed and applied to ethylene, butadiene, benzene, pyridine, pyrimidine, pyrazine, and s‐triazine. A realistic and consistent LCAO‐MO π‐electron theory should allow the σ‐electrons to adjust themselves to the instantaneous positions of the mobile π‐electrons. This is accomplished in the theory by assignment of empirical values to the Coulomb electronic repulsion integrals and Coulomb penetration integrals which enter the formulas, these values being obtained in a prescribed way from valence state ionization potentials and electron affinities of atoms. Use of the empirical values in the molecular orbital theory reduces the magnitude of computed singlet‐triplet splittings and the effects of configuration interaction without complicating the mathematics. From the valence‐bond point of view, ionic structures may be said to be enhanced. The applications to hydrocarbons and heteromolecules which are considered show that the theory can correlate known π‐electron spectral wavelengths and intensities very successfully, which, together with the simple structure of the theory, signals that manifold applications of the theory are in order elsewhere.