Changes in the Electronic Transitions of Aromatic Hydrocarbons on Chemical Substitution. II. Application of Perturbation Theory to Substituted-Benzene Spectra

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Abstract
An analysis is made of the changes that occur in the 2600, 2050, and 1850 A transitions of benzene on chemical substitution. Formulas derived by first‐ and second‐order perturbation theory, with coefficients evaluated by molecular orbital theory, are presented and applied to a large collection of intensity‐change and frequency‐shift data obtained from the literature. Most of these data pertain to the 2600 A transition. The first‐order intensity formula derived for the 2600 A transition (taking the excited state to be 1B2u) is found to hold quite well for substituents with monoshift (frequency shift on monosubstitution) less than 1500 cm‐1; the first‐ and second‐order frequency‐shift formula, for substituents with monoshift less than 2000 cm‐1. No attempt is made to analyze the intensity changes observed in the 2050 and 1850 A transitions as these changes are too small, relative to the initial intensity, to be reliably measured for weakly perturbing substituents from existing spectra. The frequency shifts, however, can be measured fairly reliably for these two transitions, and it is shown that the formulas derived on the basis of 1B1u and 1E1u excited states, respectively, hold for methyl substitution at least. Empirical values of the intensity and frequency perturbation parameters for the 2600 A transition are presented for over 30 substituents; values of the frequency perturbation parameters for the 2050 and 1850 A transitions for more than half of these are also presented. The meaning of the parametric values is discussed in the light of theoretical expectations.