ABSTRACT
Electron spin resonance (ESR) or electron magnetic resonance is a type of magnetic resonance spectroscopy that deals with transitions between magnetic energy levels associated with different orientations of an electron spin in an atom or molecule, generally in an external magnetic field. Measurement of the allowed transitions between the electron magnetic energy levels produces a spectrum of an atomic or molecular system with net electron spin angular momentum. Generally such systems are defined as those having one or more unpaired electrons. Analysis of the ESR spectrum can give information about the identification of the species, the geometrical structure, the electronic structure, and the internal or overall rotational or translational motion of the species. The most common types of systems studied are free radicals, which can be regarded as atoms or molecules containing one unpaired electron, and transition metal ion and rare-earth ions. The specificity of ESR spectroscopy for species containing unpaired electrons is particularly valuable for the study of chemical reaction intermediates.
