Electron paramagnetic resonance (EPR) or electron spin resonance (ESR) spectroscopy is a method for studying materials that have unpaired electrons. The basic concepts of EPR are analogous to those of nuclear magnetic resonance (NMR), but the spins excited are those of the electrons instead of the atomic nuclei. EPR spectroscopy is useful for analyzing metal ions and organic radicals (compounds with unpaired electrons). The technique reveals some structural information but often simply provides a characteristic "finger print". The measurement requires a large magnet into which the sample is placed. Signals are detected using microwaves. In contrast to NMR and infrared (IR) spectroscopy, EPR spectroscopy is less common. For a given sample, some of the parameters of interest are g-values (analogous to chemical shift), anisotropy (asymmetry), hyperfine coupling constants (analogous to coupling constant J), and relaxation times.
History EPR was first observed in Kazan State University by Soviet physicist Yevgeny Zavoisky in 1944, and was developed independently at the same time by Brebis Bleaney at the University of Oxford.
Theory Every electron has a magnetic moment and spin quantum number s = 1 2 {\displaystyle s={\tfrac {1}{2}}} , with magnetic components m s = + 1 2 {\displaystyle m_{\mathrm {s} }=+{\tfrac {1}{2}}} or m s = − 1 2 {\displaystyle m_{\mathrm {s} }=-{\tfrac {1}{2}}} . In the presence of an external magnetic field with strength B 0 {\displaystyle B_{\mathrm {0} }} , the electron's magnetic moment aligns itself either antiparallel ( m s = − 1 2 {\displaystyle m_{\mathrm {s} }=-{\tfrac {1}{2}}} ) or parallel ( m s = + 1 2 {\displaystyle m_{\mathrm {s} }=+{\tfrac {1}{2}}} ) to the field, each alignment having a specific energy due to the Zeeman effect:
E = m s g e μ B B 0 , {\displaystyle E=m_{s}g_{e}\mu _{\text{B}}B_{0},}
where
g e {\displaystyle g_{e}} is the electron's so-called g-factor (see also the Landé g-factor), g e = − 2.0023 {\displaystyle g_{\mathrm {e} }=-2.0023} for the free electron,
μ B {\displaystyle \mu _{\text{B}}} is the Bohr magneton. Therefore, the separation between the lower and the upper state is Δ E = g e μ B B 0 {\displaystyle \Delta E=g_{e}\mu _{\text{B}}B_{0}} for unpaired free electrons. This equation implies (since both g e {\displaystyle g_{e}} and μ B {\displaystyle \mu _{\text{B}}} are constant) that the splitting of the energy levels is directly proportional to the magnetic field's strength, as shown in the diagram below.
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