The orbital penetration term in aromatic substituent effects

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Abstract
The interaction of a substituent with a benzene ring is analysed as that of two weakly interacting independent systems. The analysis confirms that three distinct effects of substituents should be recognized, and that in addition to inductive and mesomeric effects the influence of interpenetration of π-electrons on the substituent and the aromatic system should be separately included. The effect of orbital penetration on the aromatic π-system is represented by a repulsive potential, and calculations made of the spectral intensity of the 2600 Å transition of substituted benzenes with one and two substituents. For F, Cl and CH3 the experimental values are fairly well reproduced, one empirical parameter being needed to describe each substituent. The method of calculation is the same as that earlier used for the anilinium ion, avoiding the use of explicit forms of molecular wave functions.