Effects of Electron Transfer on High-Resolution NMR Spectra

The 15.1-Mc/sec NMR spectra of isopropylquinone and 2,5-diethoxyquinone in equilibrium with the corresponding semiquinone radicals in pyridine show line positions and intensities that depend on the radical concentrations. These spectra are compared with curves calculated by the density matrix technique with the assumption that modulated isotropic hyperfine interactions provide the only relaxation mechanism. Satisfactory agreement is found for the methyl protons in isopropylquinone from which we estimate that T1>100T2 for the methine protons; however the line shapes for the methyl protons in 2,5-diethoxyquinone suggest that other relaxation mechanisms may be present.