Role of the carboxy proton in heterogeneous electron transfer to o-nitrobenzoic acid

Abstract

Heterogeneous electron transfer (ET) to o-nitrobenzoic acid has been investigated in NN-dimethylformamide containing tetrabutylammonium perchlorate. Although primary anion radicals undergo fast homogeneous decay, voltammetric measurements allowed the ET rate constant to be determined at various temperatures. These rate constants are abnormally low for an aromatic nitro derivative, even ortho-substituted, as also shown by comparison with the ET rate constant for methyl o-nitrobenzoate, which is at least two orders of magnitude higher. However, the e.s.r. spectrum demonstrates the presence of a hydrogen bond in o-nitrobenzoic acid anion radical, which is not present in the parent molecule, showing that the ET process entails significant structural change. The latter rather than solvent reorganisation around the charge of the anion-radical, localised by the ortho effect, is believed to provide the major contribution to the charge-transfer activation free energy.