Electron-Spin Resonance of Electrochemically Generated Free Radicals. Isomeric Dinitrobenzene Mononegative Ions

Abstract

The mononegative ions of meta-, ortho- and para-dinitrobenzene have been prepared in acetonitrile by controlled potential electrolysis within a microwave cavity and their electron spin resonance observed. The radical spectra exhibit hyperfine structure due to the isotropic magnetic interaction of the unpaired electron with the nitrogen and ring proton nuclear moments. Coupling constants were obtained with the aid of deuterium substitutions in the parent molecule. The nitrogen hyperfine coupling constants for the isomeric meta, ortho, and para anions were found to be 4.68, 3.22, and 1.74 gauss, respectively, and were identical for each of the two nitrogen nuclei of a given radical. Proton coupling constants range between 0.42 gauss and 4.19 gauss. The larger total hyperfine width of the meta-dinitrobenzene anion compared with the ortho and para isomers is ascribed to the existence of negative spin density in the π system of the first-mentioned radical and to the possibility of quinonoid resonance in the others which places considerable unpaired spin density in the oxygen nonbonding orbitals.