Knowledge Bank

Simplified complete LCAO MO theory, as outlined in the previous paper, is applied to benzene, pyridine, the diazines, the triazines, and ethylene, and the electronic spectra of these molecules discussed in the light of the calculations. Detailed examination is made of the basic assumptions in the original Goeppert-Mayer and Sklar treatment of $benzene,1$ and it is shown that a more realistic and consistent theory would result if the underlying core were allowed to adjust itself to the instantaneous positions of the $\pi$-electrons. This is approximately done in simplified complete LCAO MO theory by assignment of a value somewhat smaller than usual to the coulomb repulsion between a $\pi$-electron on a carbon atom and another $\pi$-electron on the same carbon atom. When such a value is employed in molecular orbital calculations, singlet-triplet splittings come out well and configuration interaction effects are reduced. From the valence-bond point of view, ionic structures are enhanced.$\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$ M. Goeppert-Mayer and A. L. Sklar, J. Chem. Phys. 6: 645 (1938).