Substituent effects in anthrasemiquinones

1 January 1988

journal article
research article
Published by Royal Society of Chemistry (RSC) in Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases

Vol. 84 (10) , 3223-3231
https://doi.org/10.1039/f19888403223

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.

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