EPR spectroscopy, also known as ESR spectroscopy, is a technique used to study materials with unpaired electrons. It is analogous to NMR but excites electron spin rather than nuclear spin. EPR detects paramagnetism by applying a static magnetic field and microwave radiation, which causes the unpaired electron spins to absorb energy and transition between energy levels. This absorption is measured to produce an EPR spectrum. EPR is useful for studying metal complexes, organic radicals, and other species with one or more unpaired electrons like free radicals and some transition metal complexes.

1. Mr.Halavath Ramesh
16-MCH-001
Department of Chemistry
Loyola College-Chennai
University of Madras

2. Electron Para magnetic resonance(EPR) or Electron Spin Resonance(ESR) Spectroscopy is a
method for studying materials with unpaired electrons.EPR spectroscopy is particularly useful for
studying metal complexes or organic radicals. The basic concepts of EPR are analogous to those of
nuclear magnetic resonances (NMR),But it is electron spin that are excited instead of the spins of
atomic nuclei. It is a Non-destructive technique. It is a branch of absorption spectroscopy in which
radiation having frequency in microwave region. This is a technique for detecting paramagnetism.
Zeeman Effect: The effect of splitting of a spectral line into several components in the presences of a
static magnetic field.
ESR Phenomenon is shown by
* Atoms having odd number of electrons.
* Ions having partly filled inner electron shells.
* Free radicals having unpaired electrons.
Application for species with one or more unpaired electrons.
Free Radicals
Transition metal complexes.
Useful unstable paramagnetic compounds generated in situ.
* Electrochemical Oxidation or reduction.
Principle
1.The unpaired electrons are excited to a high energy state under the magnetic field by the absorption
of microwave radiations.

3. 2. The excited electron changes its direction of spin and relaxes in to the ground state by emitting its
energy.
3.The transition between two different energy levels takes places by absorbing a quantum of radiation
of frequency in the microwave region.
4. Microwave absorption is measured as a function of the magnetic field by ESR spectroscopy.
5.In ESR the energy levels are produced by the interaction of magnetic moment of an unpaired electron
in a molecules with an applied magnetic field.
6. The ESR spectrum results in due to the transitions between these energy levels by absorbing
radiations of microwave frequency.
Instrumentation
1. Source. 2. sample cavity. 3. Magnet System. 4. crystal Detector.
5. Auto- Amplifier and phase sensitive Detector 6. Oscilloscope.
Applications
1. ESR Spectroscopy is one of the main methods used to study metallo proteins particularly those
containing molybdenum ,copper ,iron etc.
2. Both copper and non-haem iron do not absorb radiation in visible and ultra violet range, posses
ESR absorbance peak in one of their oxidation state.
3. Hence their appearance and dis appearances of their ESR signal are used to monitor their activity in
multi enzyme system.
4. In metallo proteins the metal atom has characteristic number of ligands co ordinate to it in a
definite geometrical arrangement. Studies using ESR have shown that their geometry is frequently
distorted.
5. This techniques ESR has been extended by spin labeling.

4. Origin of an EPR Signal
Every electron has a magnetic moment and spin quantum number S= ½ with
magnetic components ms = +1/2 and ms= - ½. In the presences of an external magnetic
field with strength B0 ,the electrons magnetic moment alignment having a specific energy
due to the Zeeman effect.
Where, ge is the electron so-called g-factor.ge= 2.0023 for the free electron μB is
the Bohr magnetron.

5. Therefore the separation between the lower and the upper state is ΔЀ= ge µB
B0 for unpaired free electrons. An unpaired electron can move between the two energy
levels by either absorbing or emitting a photon of energy hυ such that resonances
condition hυ= ΔE, is obeyed .This leads to the fundamental equation of EPR
Spectroscopy.
hυ= geμB Bο

13. EPR Spectroscopy
If the compound containing unpaired electrons, such as an organic molecule
with a free radicals, is exposed to an external magnetic field, the energy levels accessible to
the unpaired electronic spin are split by that field.Since an electron features a spin of s= ½,
two energy levels are possible; Ms= -1/2,the low energy state, and Ms=+1/2 ,the high energy
state. As the field strength is increased ,the energy difference between these two states
increase linearly. This phenomenon is exploited in EPR spectroscopy. Where the compound
placed inside the magnetic field is exposed to constant frequency microwave radiation. At
magnetic field strength corresponding to an energy difference resonant with the energy of the
applied microwave frequency ,electrons are promoted from the low energy to the high energy
state. These transitions are absorbed as the absorption of a portion of the micro wave
intensity. Since these absorptions are fairly broad, an accurate measure of the spacing
between the peaks is usually obtained by examining the first derivative spectrum. Therefore
EPR spectra are conventionally recorded as the rate of change of absorption versus field
strength.
EPR is a powerful and sensitive technique for the study of macromolecular
structure and function because EPR signals are generated only by unpaired electrons.

21. New words
Hygroscope: Hygroscope is the phenomenon of attracting and holding water molecules from
the surrounding environment which is normal or room temperature.
Hyperfine splitting: In atomic physics hyperfine structure refers to small shifts and splitting
in the energy levels of atoms, Molecules, and ions due to interaction between the state of the
nucleus and the state of the electron clouds.
What is G Factor in ESR?
In atomic physics, the electron spin g-factor is often defined as the absolute values or negative
of ge. The z-Component of the magnetic moment then becomes. The value gs is roughly equal
to 2.002319 and is known to extraordinary precision.