Alternating Linewidth Effect in the ESR Spectra of Anion Radicals of Dinitromesitylenes and -durenes

Abstract

Accurate measurements of line positions in the ESR spectra of the aminodinitromesitylene, dinitroisodurene, dinitromesitylene, and dinitrodurene anion radicals have permitted calculation of the dynamic frequency shifts arising from modulation of the isotropic nitrogen hyperfine splittings. The results are in good agreement with the predictions of the ``two-jump'' model proposed by Freed and Fraenkel to explain the phenomenon of alternating linewidths observed in the spectra of such radicals. The effects of solvent composition on the ESR splitting constants and linewidths of the dinitrodurene and dinitroisodurene anions were studied in DMF—water mixtures. The observed increase in nitrogen splittings and decrease in methyl-proton splittings with increased water concentration is in agreement with previous theories, and the data may be fitted to a simple model assuming equilibrium between 1:1 solvent—solute complexes. The MI=±1 lines for both the dinitrodurene and dinitroisodurene anions were found to rapidly increase in width with increasing water concentration. The two-jump model was combined with the assumption of 1:1 solvent—solute complexes to develop a four-jump model for dinitro radical anions in mixed solvents. The four-jump model was found, however, to be incapable of explaining the rapid increase in linewidth observed. Possible sources of difficulty are discussed.