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

摘要

Electron spin resonance spectra of themhyphen;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 secondhyphen;order frequency shifts. More detailed information about the molecular motions was obtained in this way than is otherwise possible. Themhyphen;dinitrobenzene anion, the 3,5hyphen;dinitromesitylene anion, the 3,5hyphen;dinitrophenolate dianion, and the 3,5hyphen;dinitrobenzoate dianion radicals were obtained by electrolytic generation in solvents such as tetrahydrofuran (THF), 1,2hyphen;dimethoxyethane (DME), andN,Nhyphen;dimethylformamide (DMF). Except for the benzoate dianion in DMF, the data are well represented by a twohyphen;state model with two14N splitting constants,aIandaII. 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 ofaIandaIIthat are approximately the same as those found for the single species that are obtained in the presence of alkalihyphen;metal cations, and which correspond to the static limit. The correlation times tgr;cobserved in the spectra showing the alternating linewidth effect were in the range from 0.4ndash;0.9times;10minus;9sec, while those corresponding to the static limit are greater than about 10minus;6sec. Spectra of the 3,5hyphen;dinitrobenzoate dianion radical obtained in DMF with and without added water could not be analyzed by a twohyphen;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 theghyphen;tensor and electronhyphen;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 outhyphen;ofhyphen;phase correlation of the splittings at the two nitrogen nuclei.