Electron Spin Resonance

Electron spin resonance (ESR) is a spectroscopic technique that detects the transitions induced by electromagnetic radiation between the energy levels of electron spins in the presence of a static magnetic field. The method can be applied to the study of species containing one or more unpaired electron spins; examples include organic and inorganic radicals, triplet states, and complexes of paramagnetic ions. Electron paramagnetic resonance (EPR) and EPR imaging (EPRI) are often used in the literature instead of ESR and ESRI. Spectral features such as resonance frequencies, splittings, line shapes, and line widths are sensitive to the electronic distribution, molecular orientations, nature of the environment, and molecular motions. Theoretical and experimental aspects of ESR have been covered in a number of books (1–8), and reviewed periodically (9–11). The great sensitivity and specificity of ESR methods have been utilized to advantage in order to investigate and clarify important questions in polymeric systems (12). The most obvious candidates for initial studies were chain-growth and depolymerization reactions; in both cases radical intermediates are the driving force for reaction and can be detected by ESR (12–15). Analyses of radicals produced by high-energy irradiation (γ , electron beams) contributed to a better