Detection of Gas Phase Free Radicals by Electron Spin Resonance Spin Trapping Techniques

Abstract

ELECTRON spin resonance (ESR) spectroscopy provides the best readily available method for detecting free radicals in reacting systems. Stable free radicals can be detected at concentrations as low as 10−8 M with commercially available equipment. Special techniques, however, are necessary for the direct detection of short lived free radicals (for example, flow systems). Recently an indirect ESR method of detecting short lived free radicals in solution has been developed which involves free radical addition to a nitroso function, for example, 2-nitroso-2-methylpropane1, or a nitrone function, for example, phenyl N-t-butyl nitrone2–5. The reaction produces stable nitroxides which build up to readily detectable concentrations in the presence of a free radical source. The general usefulness of the nitrone has been found to exceed that of the nitroso compound because of its greater stability in a variety of reaction conditions. Most work has been done with phenyl N-t-butyl nitrone (PBN) in solution3–5. The free radical addition reaction is called a spin trapping reaction. The nitrone or nitroso compound is called a “spin trap” and the radical addition product is called a “spin adduct”. In this report we describe our first results on the trapping of free radicals in the gas phase.