Substituent effects in anthrasemiquinones

Abstract

Electron spin resonance spectra have been obtained from series of 2-substituted anthrasemiquinones and of 3-substituted 1,8-dihydroxyanthra-semiquinones. The proton splittings are consistently assigned by means of linear correlation plots between splitting constants and a substituent-dependent parameter. All lines in the plots of the two series of compounds obey the following linear equation a^R_i=A_ix^R+ a^H_i where a_i^H and a_i^R are the splitting constants at position i before and after the substituent has been added. A_i is the slope of the line for the splittings from position i and x^R is a constant characteristic of the substituent R. x^R is comparable to the Hammett σ parameter. Electron-donating substituents at C-2 are shown to increase the spin densities at the positions 1 > 6 > 8, and to decrease them at the positions 4 > 3 > 7 > 5, with the strength of the effect indicated. Electron-withdrawing substituents have the opposite effect. Preliminary Hückel molecular-orbital calculations qualitatively predict the observed correlations, solely by changing the parameter for the resonance integral.