Electron Spin Resonance Spectra of <i>α</i>-Nitronylnitroxide Radicals; Solvent Effects; Nitrogen Hyperfine Tensor; g Anisotropy

Abstract

This paper describes a detailed electron spin resonance study of three α-nitronylnitroxide doublet radicals: α-methylnitronylnitroxide (α-MNN), α-phenylnitronylnitroxide (α-φNN), and α-0-tolylnitronylnitroxide (α-0TNN). The average g values and the isotropic hyperfine coupling constants for the paramagnetic nuclei have been measured. A solvent-induced variation in the isotropic coupling constants has been observed and some evidence is given for slight changes in g value with solvent. Studies of polycrystalline samples of α-MNN indicate a molecular anisotropy is associated with the g tensors for these molecules. Estimates of the diagonal elements of the g tensors have been obtained from spectra of the radicals in solid solutions of Duco Cement. The principal value of the nitrogen hyperfine tensor along the π direction has been determined from the solid solution spectra. The other principal values of the nitrogen hyperfine coupling tensors are estimated from observed linewidths. Spin densities have been calculated from the hyperfine coupling constants. The unpaired electron distribution is found to be essentially the same in all of the doublet radicals studied and is found to be primarily localized in the nitronylnitroxide ring. The nitronylnitroxide ring is indicated to be of C2V symmetry with the unpaired electron occupying a π molecular orbital that transforms as the B1 representation in the C2V point group. The nitronylnitroxides are found to be similar in many respects to the simple nitroxide radicals.