Effects of induction and resonance in the calculation of ionization potentials of substituted benzenes by perturbation molecular orbital theory

Abstract

Using perturbation theory, an equation has now been derived relating the ionization potentials of disubstituted benzenes to those of monosubstituted benzenes. The theoretical equation is closely related to the empirical one and the calculated ionization potentials for a series of benzenoid compounds compare well with measured values obtained by electron-impact and photoelectron methods. In a few cases with substituents which interact strongly with the benzene ring, the poor agreement between calculated and observed ionization potentials led to a closer examination of the measured effects. The molecular orbital method used here applies strictly to the resonance effects of substituents, whereas the parameters initially used in the equations are resonance plus induction effects combined, as obtained from photoelectron spectroscopy measurements. The photoelectron spectra of the benzenes appear to allow a separation of the resonance and induction effects and, when this is done, the calculated and observed ionization potentials agree in all cases.