Line-width effects in the ESR spectra of electro-generated radical anions of nitrobenzonitriles

Abstract

The ESR spectra of electrogenerated radical anions of ortho-, meta- and paranitrobenzonitriles in dimethyl formamide have been recorded in the temperature range 20°C to -70°C. Improved resolution has been obtained in the case of metanitrobenzonitrile radical anion over that reported earlier. From a consideration of the experimental coupling constants and calculated spin densities in the case of ortho-nitrobenzonitrile radical anion, it is suggested that the nitro group is coplanar with the phenyl ring in this radical anion. Freed-Fraenkel relaxation matrix theory has been used in the analysis of line-width data. From the line-width data, using the dipolar coefficients calculated from McLachlan, UHFAA and INDO spin densities, the correlation times for molecular tumbling motions have been obtained for the case of meta- and para-nitrobenzonitrile radical anions. From the data obtained in the present work and also from the experimental data of other workers, it is suggested that INDO spin densities are preferable for calculation of dipolar coefficients of aromatic nitro radical anions. The signs of a few proton and nitrogen hyperfine coupling constants in meta- and para-nitrobenzonitrile radical anions have also been determined from the present line-width analysis.