Electron T1 measurements in short-lived free radicals by dynamic polarization recovery

A dynamic polarization recovery method for measurement of electron spin T1 relaxation times in free radicals in liquids is described, which is valid even in the presence of chemically induced dynamic electron polarization (CIDEP) and fast chemical decay of the radicals. The method is based on pulsed microwave perturbation and detection of transient magnetization following radical creation in a short pulse. Analysis of the experimental approach and a theoretical description of the method is presented together with a detailed discussion of the advantages and the limitations of the technique. Electron T1 measurements are presented for 14 short-lived free radicals generated in aqueous solution. The magnitudes of the observed relaxation times, which range from 0.1 to 4 μs, are discussed within the framework of current theories of relaxation for small radicals in liquids. It is tentatively concluded that the spin rotation mechanism is responsible for the very short T1's in this series of radicals