The effects of electronic excitation on heterolytic side-chain reactions of substituted benzenes

Abstract

A previously described quantitative theoretical treatment of the reactions mentioned in the title is extended so that it can be used to examine the effects of electronic excitations on the rate constants and equilibrium constants. The electronic excitations considered correspond to the ¹A_1g→¹B_2u and ¹A_1g→³B_1u excitations of benzene. A new explanation emerges of why excitation to the triplet state has little effect on the pK_a value for phenol whereas excitation to the singlet state has a substantial effect. Also, it is concluded that the types of linear free energy relationship which are found among ground state reactions are not, in general, to be expected among excited state reactions. However, the relative effects of para- and meta-ring substituents are predicted to be simply related to σ_p⁰–σ_m⁰. This relationship is shown to be consistent with available experimental data.