Electron paramagnetic resonance (EPR) spectroscopy is also known as electron spin resonance (ESR) spectroscopy. It is the name given to the process of resonant absorption of microwave radiation by paramagnetic ions or molecules, with at least one unpaired electron spin, in the presence of a static magnetic field. EPR has a wide range of applications in chemistry, physics, biology and medicine. It may be used to probe the static structure of solid and liquid systems, and is also useful in investigating dynamic processes.
EPR spectroscopy provides an experimental route to study the magnetic interactions in paramagnetic materials. The elucidation of parameters like the electronic Zeeman interaction (g), hyperfine (A), nuclear quadrupole (Q) and zero-field or electronic quadrupole (D) tensors, which characterise the interactions, can lead to an understanding of atomic and molecular structure at magnetic sites. High sensitivity and the ability to investigate small scale order in powders, polymers, and frozen solutions are key advantages of EPR spectroscopy. Additional techniques employing both the EPR and NMR method (ENDOR-electronic nuclear double resonance) are available and extend the scope of such investigations. EPR spectroscopy can be combined with chemical techniques such as spin trapping to detect and follow free radical reactions in biological systems, etc.