Measurement of g Values in the Electron Spin Resonance Spectra of Free Radicals

Abstract

Measurements have been made of the g values in the electron spin resonance spectra of 20 aromatic free radicals with an accuracy which has not previously been attainable. The largest source of error for both absolute and relative measurements resulted from variations in the difference between the magnetic field at the ESR sample and at the proton NMR probe, located just outside the center of the microwave cavity, which was used to measure the field. Except for the p-benzosemiquinone radical, g values were not found to depend on temperature over the range from room temperature to −100°C. The dependence of the g value on solvent and counterion was also investigated to some extent, and the g value of the perinaphthenyl radical was found to vary markedly with solvent. Agreement with Stone's theory of the g values of aromatic radicals is excellent, except for those radicals in which molecular distortions are expected according to the Jahn—Teller theorem and for radicals in which the unpaired electron is in a nonbonding molecular orbital.