Electron Spin Resonance (ESR)
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Background
Electron Spin Resonance (ESR) also known as Electron Paramagnetic Resonance (EPR) spectroscopy or electron paramagnetic resonance spectroscopy is a technique for studying chemical species that have one or more unpaired electrons. It is analogous to NMR except it deals with spins of electrons rather than the spins of atomic nuclei. It is most useful in studying free radicals or organic complexes having transition metal ions since most stable molecules have all their electrons paired. Most ESR spectra are recorded and published as first derivatives of the original signal.
Significance
In ESR a sample is placed in magnetic field which forces the unpaired electrons to orient their spin with the field. A microwave field oriented perpendicular to the magnetic field temporarily aligns the electron spin off from the magnetic axis. The magnetic field is varied to seek resonance points where the electron relaxes back to the original field orientation. The first derivative of the absorption signal is displayed to clarify properties such as anisotropy. The signal may include a number of frequencies reflecting the interaction of the free radicals with adjacent nuclei in the molecule.
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