Electron spin resonance of stable free radicals

Electron spin resonance is a relatively new phenomenon in physics and chemistry while free radicals are substances long familiar to organic chemists. Electron spin resonance, discovered by the Soviet physicist Zavoiski in 1945, is the absorption of radiation in a magnetic field of certain definite strength by materials having unpaired electrons. The absorption has the characteristics of a resonance phenomenon because the differences in the energy levels of the unpaired electron or electrons due to the Zeeman splitting are sharp. This splitting is given by $\mathrm{g\beta H}$ where g is the gyromagnetic ratio (equal to 2.0023 for “spin only” case), β the value of the Bohr magneton, and H the intensity of the magnetic field. On the other hand, this energy difference is equal to $\mathrm{hv}$ where h is Planck’s constant and v is the frequency of the absorbed radiation. Equating these two conditions, we obtain the resonance condition in conventional units of $\text{F\hspace{0.17em}=\hspace{0.17em}2.80\hspace{0.17em}H,}$ with F being given in megacycles and H in oersteds. Such resonance absorption is observed in many chemical systems, transition metal ions, rare earth and actinide ions, conduction electrons, charred materials, radiation damaged materials, and the free radicals of the organic chemist. Not only can free radicals be detected with high sensitivity by electron spin resonance, but various aspects of this resonance absorption have given the chemist a powerful tool for the elucidation of the structure of organic molecules and of the mechanism of chemical reactions.