Electron paramagnetic resonance (EPR), which is also called electron spin resonance (ESR), is a spectroscopic technique responsive to transitions involving unpaired electrons and therefore can be used for paramagnetic species (including organic free radicals) containing one or more unpaired electrons. When the spin of the odd electron of a free radical is placed in a magnetic field, it may have two orientations, one parallel and the other antiparellel to the field. These two orientations are characterized by different energies. If a system containing free radicals is placed in a magnetic field and irradiated with an electromagnetic radiation (at X band in the region of microwave), species with the lower energy orientation absorb radiation of a frequency corresponding to an energy just sufficient to flip the odd electron into its higher energy state. This phenomenon gives rise to an EPR spectrum consisting of a single line. In many organic free radicals, interaction of the unpaired electron with the nuclear magnetic moments of the atoms present in the molecule (generally with hydrogen nuclei) gives rise to a more complicated system of energy levels and an the EPR spectrum consists of many lines. The analysis of the spectrum permits the identification of particular free radicals and also gives information about their electronic structure.
Paramagnetic centres can be naturally found in many systems. However, they can also be introduced on purpose; because of this, EPR can be employed not only in many fields of chemistry research but also in physics, biology and medicine. More information on this technique can be found in the web site of GIRSE (Italian Group of Electron Spin Resonance)