Electron Spin Resonance Study of p-Phenylene-Diamine Positive Ion

The mono‐positive radical ion of p‐phenylene diamine has been prepared by controlled potential electrolysis and studied by electron spin resonance. This ion has been formed in acetonitrile solution with sodium perchlorate as supporting electrolyte and the controlled potential electrolysis has been carried out in the microwave cavity of the ESR spectrometer. The ion exhibits a complex hyperfine structure in dilute solution; the observed spectrum consists of about 75 hyperfine components. Isotropic hyperfine coupling constants have been assigned for the three types of interacting nuclei: the aromatic ring protons, amine protons, and the nitrogen nuclei. The large nitrogen hyperfine interaction observed is in marked contrast to Würster’s Blue cation, for which this interaction is vanishingly small. ESR spectra of deuterated derivatives have been obtained and employed in making the assignment. The magnitude of the nitrogen hyperfine interaction relative to that of the amine protons is shown to be consistent with the mechanism of a $\mathrm{\pi ‐\sigma }$ electron exchange interaction on the nitrogen atom. Electrochemical generation allows the study of the relatively unstable ion‐radical, whereas other methods of preparation have been less successful.