ESR Study of Hindered Internal Rotation in the 3,5-Dinitrobenzamide Radical Anion and Related Compounds

The radical anions of 3,5-dinitrobenzamide, 3-nitrobenzamide, and 4-nitrobenzamide in N,N-dimethylformamide and the sodium salt of 3,5-dinitrobenzamide radical anion in tetrahydrofuran have been studied by ESR spectroscopy at different temperatures. The ESR spectra of 3,5-dinitrobenzamide radical anion exhibit line-width alternation due to the modulation of hyperfine splitting constants by the restricted rotation of the whole amido group about the benzene-to-carbonyl bond (Ea = 3.31 ± 0.04 kcal/mol). The remaining compounds experience free rotation, on the ESR time scale. Extensive extended Huckel MO calculations were performed to get information on the intramolecular torsional potentials and the equilibrium conformations of the radical anions. The spin distribution in the radical anions was calculated by the McLachlan method. Three different models were used to analyze hindrance to internal rotation of the amido group. Our results indicate that the internal rotation in the 3,5-dinitrobenzamide radical anion is controlled by the solvent interactions, whereas in the remaining radical anions it is controlled by the intramolecular torsional potential.