Electron Paramagnetic Resonance (EPR, ESR) Spectroscopy
Introduction
When an atom or molecule with an unpaired electron is placed in a magnetic
field, the spin of the unpaired electron can align either in the same direction
or in the opposite direction as the field. These two electron alignments
have different energies and application of a magnetic field to an unpaired
electron lifts the degeneracy of the ±1/2 spins of the electron.
Electron-paramagnetic-resonance (EPR) or electron-spin-resonance (ESR)
spectroscopy measures the absorption of microwave
radiation by an unpaired electron when it is placed in a strong magnetic
field.
Species that contain unpaired electrons:
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Free radicals
-
Odd electron molecules
-
Transition-metal complexes
-
Lanthanide ions
-
Triplet-state molecules
Instrumentation
A klystron tube generates monochromatic microwave radiation (~9500 MHz)
in an EPR instrument. The microwave radiation travels down a waveguide
to the sample which is held between magnets.
Spectra are obtained by measuring the absorption of the microwave radiation
while scanning the magnetic-field strength. EPR spectra are usually displayed
in derivative form to improve the signal-to-noise ratio.