Dynamic Frequency Shifts and the Effects of Molecular Motions in the Electron Spin Resonance Spectra of Dinitrobenzene Anion Radicals

Abstract

Electron spin resonance spectra of the m‐dinitrobenzene anion radical and several of its derivatives have been examined under a variety of conditions in order to study the alternating linewidth effect and, for the first time, the associated dynamic second‐order frequency shifts. More detailed information about the molecular motions was obtained in this way than is otherwise possible. The m‐dinitrobenzene anion, the 3,5‐dinitromesitylene anion, the 3,5‐dinitrophenolate dianion, and the 3,5‐dinitrobenzoate dianion radicals were obtained by electrolytic generation in solvents such as tetrahydrofuran (THF), 1,2‐dimethoxyethane (DME), and N,N‐dimethylformamide (DMF). Except for the benzoate dianion in DMF, the data are well represented by a two‐state model with two ¹⁴N splitting constants, a_I and a_II. The two different splittings probably arise because the nitro groups are complexed with the solvent or with cations. Even the spectra showing a rapid exchange between the two states have values of a_I and a_II that are approximately the same as those found for the single species that are obtained in the presence of alkali‐metal cations, and which correspond to the static limit. The correlation times τ_c observed in the spectra showing the alternating linewidth effect were in the range from 0.4–0.9×10⁻⁹ sec, while those corresponding to the static limit are greater than about 10⁻⁶ sec. Spectra of the 3,5‐dinitrobenzoate dianion radical obtained in DMF with and without added water could not be analyzed by a two‐state model; a more appropriate model is probably one in which the carboxylate group in addition to the two nitro groups can interact with the solvent or the cations. A few spectra were carefully studied to obtain data on the g‐tensor and electron‐nuclear anisotropic dipolar interactions as well as those arising from modulations of the isotropic splittings, and this complete analysis made it possible to estimate values of the spectral densities for the latter interaction. In most cases studied in this way, it was found that there was complete out‐of‐phase correlation of the splittings at the two nitrogen nuclei.