Merostabilization in radical ions, triplets, and biradicals. 4. Substituent effects on the half-wave reduction potentials and n,π* triplet energies of aromatic ketones

Abstract

The half-wave reduction potentials, measured by cyclic voltammetry, and n,π* triplet energies, measured by phosphorescence spectroscopy, were determined for a series of eighteen symmetrically and unsymmetrically substituted benzophenones. Attempts are made to correlate the results with Hammett substituent constants. Better correlations are observed when the data are correlated with a two-parameter function consisting of Hammett substituent constants and a set of substituent parameters describing variations in free radical stability. Significant deviations from "normal" behaviour are observed for benzophenones substituted by both electron-donating and electron-withdrawing substituents. These deviations are attributed to merostabilization of the radical-like species, and an empirical approach designed to evaluate the importance of this effect is developed. Abinitio calculations of molecular orbital energies in meta- and para-substituted benzaldehydes are used to evaluate the substituent effects on E_1/2^red and E_T^n,π* in terms of the effect on the energies of the n- and π*-orbitals.