Analysis of the Electron Spin Resonance Spectra of Aromatic Nitrosubstituted Anion Radicals

Abstract

Molecular orbital calculations using the Hückel LCAO theory and the approximate configuration interaction treatment suggested by McLachlan have been performed on a large series of nitrosubstituted aromatic anion radicals. By properly adjusting the Coulomb and resonance integrals for the nitro group, a good quantitative correlation is obtained between the calculated spin densities and the experimental proton hyperfine splittings for most of the compounds studied. Procedures are developed to account for the variations in hyperfine splittings with different solvents, and an equation is formulated which gives excellent predictions of the splitting from N14 nuclei in the nitro groups. In addition, the calculations provide a basis for the qualitative discussion of the effects of sterically hindered nitro groups on the hyperfine splittings, and of the apparently anomalous spin densities and linewidth variations in some of the meta-dinitrobenzene anions. Polarographic half-wave potentials are also correlated with the calculated pi-electron energies. Experimental data are given for the ESR spectra of several substituted nitrobenzene anions not previously reported.