Electron Spin Resonance Spectroscopy (ESR):
The basic principle of electron spin resonance spectroscopy is that, electrons always have a
spin and thus have a magnetic moment. Thus, the magnetic resonance theory applies to
electrons too like that of nuclei, as in NMR. Especially this technique is of high value when it
comes to the compounds which contain odd electrons, i.e. those substances which have
paramagnetic behaviour (if electrons are paired as in bonded orbital then their mutual
spinning will cancel each other and there will be no response for the applied magnetic field,
whereas, if it is unpaired then it can align with the applied magnetic field and the feasibility
of getting ESR spectra is higher). Thus, the principle and the instrumentation are much
similar to that of NMR technique. It is also referred as, Electron Magnetic Resonance
(EMR) or Electron Paramagnetic Resonance (EPR) spectroscopy.
It is mostly used as a potential technique to study the formation and lifetime of free radicals,
which are the major intermediates in most of the organic reactions. Another important
application is in the estimation of trace amounts of paramagnetic ions, particularly in
biological works like, Mn2+, Mg2+ etc.
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