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Electron spin resonance of electrolytically generated nitro- and cyano anion radicals. Fischer, Peter Hans Herman


The radical anions of 1,2-dinitrobenzene, 1,3-dinitro-benzene, 1,3-dinitrobenzene-d₄, 1,4-dinitronaphthalene, 1,5-dinitronaphthalene, 1,8-dinitronaphthalene, 1,3,5-trinitrobenzene, 2,2', 4,4',6,6'-hexanitrobiphenyl, 1,2-dicyanobenzene, 1,4-dicyanobenzene, 1,2,4,5-tetracyanobenzene, and 7,7',8,8'-tetracyanoquinodi-methane have been generated by electrochemical means, and the electron spin resonance spectra recorded. The spectra have been analyzed completely in terms of the various coupling constants consistent with the symmetry of the paramagnetic species. In the isomeric dinitrobenzenes, evidence is presented which indicates that the spin density at the N¹⁴ nuclei is positive. A very large anomaly in the ESR spectrum of 1,3-dinitrobenzene radical anion is explained by assumption of rotational isomerism. A relation is presented, expressing the nitrogen coupling constants of the nitro group N¹⁴ nuclei in terms of the spin density distribution of the radical anion, calculated by the simple Huckel M.O. method. Anomalous behaviour of the 1,3,5-trinitrobenzene system is explained in terms of an electron-donor-acceptor complex between radical anion and neutral molecule. The results obtained for the cyanoaromatic anions have been compared with those of Fraenkel et al. with the exception of 7,7',8,8'-tetracyanoquinodimethane anion radical, for which a more detailed discussion is presented. C¹³ splittings have been calculated theoretically and found to agree reasonably well with experimentally determined values. For the TCNQ¯ species, evidence is also present which indicates a positive spin density at the N¹⁴ nuclei.

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