Refined Antisymmetric Molecular-Orbital Calculations of the Energy Levels of Benzene and Hexamethylbenzene

Abstract

The calculation by Parr, Craig, and Ross (1950) of the lower π-electron energy levels of benzene has been repeated, with specific inclusion this time of (i) the contribution of the hydrogen atoms to the core potential, and (ii) exchange interactions between the π-electrons and the core. As a result, the lower energy levels (in the range 4-10 eV)are all raised by amounts averaging about 1 eV. A similar result was obtained by Niira (1953), but by a significantly different procedure. If the hydrogen atoms are replaced by methyl groups, the changed core potential reduces the energy level spacing slightly. This effect (including an allowance for the polarities of ring-radical a-bonds) can account for about 35 per cent. of the difference in energy (estimated at 3800 cm^-1) between the ¹A_gu transitions in benzene and hexamethylbenzene ; it is thus almost as important in this context as hyperconjugation and like effects.