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Electron paramagnetic resonance (EPR) or electron spin resonance (ESR) spectroscopy is a method for studying materials with unpaired electrons.
The basic concepts of EPR are analogous to those of nuclear magnetic resonance (NMR), but it is electron spins that are excited instead of the spins of atomic nuclei.
Therefore, transitions from the lower to the higher level are more probable than the reverse, which is why there is a net absorption of energy.
The sensitivity of the EPR method (i.e., the minimal number of detectable spins , the implication is that the ratio of the unpaired electron's spin magnetic moment to its angular momentum differs from the free-electron value.
Spin polarization is a third mechanism for interactions between an unpaired electron and a nuclear spin, being especially important for -electron organic radicals, such as the benzene radical anion.
The symbols "a" or "A" are used for isotropic hyperfine coupling constants, while "B" is usually employed for anisotropic hyperfine coupling constants.
Since there typically are more electrons in the lower state, due to the Maxwell–Boltzmann distribution (see below), there is a net absorption of energy, and it is this absorption that is monitored and converted into a spectrum.
The upper spectrum below is the simulated absorption for a system of free electrons in a varying magnetic field.
The former applies largely to the case of isotropic interactions (independent of sample orientation in a magnetic field) and the latter to the case of anisotropic interactions (spectra dependent on sample orientation in a magnetic field).Such situation is commonly observed in powders, and the spectra are therefore called "powder-pattern spectra".In crystals, the number of EPR lines is determined by the number of crystallographically equivalent orientations of the EPR spin (called "EPR center").In practice, it is usually the frequency that is kept fixed.
A collection of paramagnetic centers, such as free radicals, is exposed to microwaves at a fixed frequency.
EPR spectroscopy is particularly useful for studying metal complexes or organic radicals.