EPR and Radical Processes

Abstract

Free radicals and radical ions are paramagnetic species whose magnetic properties originate from the presence of an odd number of electrons in their molecules. The former species are normally obtained by homolytic cleavage of a chemical bond, induced either photolytically [e.g., the generation of tert-butoxy radicals via ultraviolet (UV) irradiation of di-tert-butyl peroxide or the formation of alkyl radicals via hydrogen abstraction by alkoxy radicals or by excited carbonyl compounds, via halogen abstraction by trialkylstannyl radicals, or by direct photolysis of weak halogen–carbon bonds] or thermally [e.g., the decomposition of azo compounds such as azoisobutyronitrile (AIBN) to give alkyl radicals or that of alkylhyponitrites to give alkoxy radicals]. On the other hand, radical ions are obtained through single-electron-transfer processes resulting in the uptake of an electron by a given substrate to form its radical anion (reduction) or in the loss of an electron to give the corresponding radical cation (oxidation). Both the reduction and oxidation processes are spontaneous when reactants with appropriate redox potentials are involved; in some less favorable cases, the electron transfer may still be achieved thermally or by photostimulation, sometimes with the aid of sensitizers. Radical anions and cations of particular substrates can also be obtained through electrochemical reduction or oxidation. Examples and discussion of all these processes can be found in any good textbook of physical organic chemistry, and their detailed description is outside the scope of this introduction. Electron paramagnetic resonance (EPR) spectroscopy is the technique of choice for the study of paramagnetic species. It can, in many cases, provide a detailed picture of the electronic distribution within the molecule of the examined free radical or radical ion through the knowledge of the spin density distribution, which in a broad sense reflects the distribution of the unpaired electron within the