Dependence of the Vertical Excitation Energy of Benzene on the Size and Force Constant of the Excited State

Griffing1 computed the vertical excitation energy for the first electronic transition of benzene as a function of the carbon-carbon distance (rn) within the Sklar valence bond (SVB) and Goeppert-Mayer and Sklar molecular orbital (G1SMO) methods to evaluate the change on excitation of the carbon-carbon equilibrium internuclear separation (rn0) and of the corresponding stretching force constant, f. While ring expansion was correctly predicted, both methods appeared to predict that f increases on excitation, contrary to experiment. We shall demonstrate a) that Griffing's paradox may be resolved if terms higher than the second power in rn are included in the energy expressions for the states and b) that a Huckel calculation leads to results qualitatively similar to those obtained from the more elaborate calculations.