Substituent effects on nitroaromatic radical anions in aqueous solution

Abstract

The effect of substituents on the ESR parameters of nitroaromatic radical anions in aqueous solutions has been studied by observing the spectra of some 20 such radicals using the in situ radiolysis steady-state ESR technique. The results focus on the effect of substitution and disubstitution at positions ortho to the nitro group. Higher nitrogen splittings were found to result in lower hyperfine constants for both para and ortho protons (or methyl groups), while meta hydrogens are affected to a much smaller extent. An inverse linear correlation is in fact found between a/sub NO$sub 2$//sup N/ and a/sub para//sup H/. When a/sub NO$sub 2$//sup N/ increases to values greater than approximately 20 G the meta proton splittings become higher than those for the para and ortho protons. The effect of an ortho OH group is relatively small but the effect of its basic form (O$sup -$) is comparable to that of a methyl group. A strong synergistic effect of the second substituent in ortho disubstituted nitroaromatics is observed. It is also found that the rate of electron-transfer reaction from (CH$sub 3$)$sub 2$COH to the parent compounds can be correlated with the spin density on the ring of the resultant radical anionmore » as indicated by the inverse correlation with a/sub NO$sub 2$//sup N/.« less