Alternating Linewidths and Related Phenomena in the Electron Spin Resonance Spectra of Nitro-Substituted Benzene Anions

Abstract

Studies of linewidths in the electron spin resonance spectra of a number of nitro-substituted benzene anions are reported. The radicals were generated electrolytically in N,N-dimethylformamide solutions, and spectra were obtained over a range from above to below room temperature. Pronounced alternations in the linewidths of the nitrogen lines in many of the spectra were observed. If MN is the total z component of the nuclear spin angular momentum in compounds containing two equivalent nitrogen nuclei, the alternating linewidth phenomenon causes a broadening of the lines for which MN=±1 and a reduction in the amplitude of the central line. Large alternating linewidths were observed in the room-temperature spectra of the anions of dinitrodurene, dinitromesitylene, and 2,6-dinitro-3,5-dimethyl-4-acetyl-t-butylbenzene; and also in a spectrum which shows splittings from two equivalent nitrogen nuclei that was obtained from trinitromesitylene. Slight alternating linewidth effects at room temperature that became more marked at low temperatures were found in the spectra of the 2,6-dinitrotoluene and m-dinitrobenzene anions. No linewidth alternations were detectable even at low temperatures in the spectra of the o-dinitrobenzene, p-dinitrobenzene, and 2,6-dinitrophenolate anions. The 2,6-dinitroaniline anion, which was investigated only at room temperature, showed no evidence of an alternating linewidth effect. None of the spectra of the mononitrobenzene anions examined exhibited any anomalous linewidth phenomena. The experimental observations are in general agreement with the recently developed theory of linewidths. The theory attributes the alternation in widths in these radicals to an out-of-phase correlation of a modulation of the isotropic hyperfine splittings of the two equivalent nitrogen nuclei. This out-of-phase modulation probably arises from either fluctuating solvent complexes with the nitro groups, or from internal rotations of these groups relative to the plane of the benzene ring, and both types of motion may occur simultaneously. Steric hindrance of the nitro groups was found to enhance the magnitude of the alternations in width, as did lowering of the temperature.