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. An RF field oriented perpendicular to the magnetic field temporarily aligns the electron spin off from the magnetic axis. When the RF field is stopped the electron relaxes relatively slowly back to the original field orientation. The relaxation causes a RF signal which can be detected with a pickup coil that is orthogonal to the excitation coil. The signal includes a number of frequencies reflecting the various locations unpaired electrons in the molecule. These frequencies can be deconvolved with fourier transform to show distinct peaks.
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