electron paramagnetic resonance (EPR) spectroscopy

A form of molecular spectroscopy concerned with microwave-induced transitions between magnetic energy levels of electrons having a net spin and orbital angular momentum. The spectrum is normally obtained by magnetic field scanning. Also known as electron spin resonance (ESR) spectroscopy or electron magnetic resonance (EMR) spectroscopy.

The frequency (ν) of the oscillating magnetic field to induce transitions between the magnetic energy levels of electrons is measured in gigahertz (GHz) or megahertz (MHz). The following band designations are used: L (1.1 GHz), S (3.0 GHz) , X (9.5 GHz), K (22.0 GHz) and Q (35.0 GHz). The static magnetic field at which the EPR spectrometer operates is measured by the magnetic flux density (B) and its recommended unit is the tesla (T). In the absence of nuclear hyperfine interactions, B and  are related by : h ν = g µ_BB where h is the Planck constant, µ_B is the Bohr magneton, and the dimensionless scalar g is called the g-factor. When the paramagnetic species exhibits an anisotropy, the spatial dependency of the g-factor is represented by a 3x3 matrix. The interaction energy between the electron spin and a magnetic nucleus is characterized by the hyperfine coupling constant A. When the paramagnetic species has anisotropy, the hyperfine coupling is expressed by a 3x3 matrix called a hyperfine coupling matrix. Hyperfine interaction usually results in splitting of lines in an EPR spectrum. The nuclear species giving rise to the hyperfine interaction should be explicitly stated, e.g."the hyperfine splitting due to ⁶⁵Cu". When additional hyperfine splittings due to other nuclearspecies are resolved ("superhyperfine"), the nomenclature should include the designation of the nucleus, and the isotopic number