Electron spin resonance (ESR) spectroscopy is a technique used to study materials with unpaired electrons. It detects transitions between spin energy levels induced by a microwave source in the presence of a strong magnetic field. The three key points are: 1. ESR detects the absorption of microwaves by unpaired electrons in a material when it is exposed to a strong magnetic field, which splits the electronic energy levels. 2. The absorbed frequency is dependent on factors like the local electron environment and applied field strength, allowing structural information to be obtained. 3. Hyperfine interactions with neighboring atomic nuclei further split the energy levels and provide details like the number and identity of interacting nuclei.

1. Electron spin Resonance
Spectroscopy

2. Electron e-
J.J. Thomson's Experiments leading to the "Discovery of the Electron" in 1897:
The mass of an electron is 9.10938 x 10-31 kg.
Each electron carries one unit of negative charge (1.602 x 1019 coulomb)
The electron is a subatomic particle e− or β− a negative elementary charge

4. Spin
The electron spin is the electron’s electromagnetic field angular
momentum
spin is like a vector quantity

5. All Electron carry a charge. In some electron this charge spins on
the electron axis and this circulation of e charge generates a
magnetic dipole along the axies.
 these particle also have the properties to spin on their
own axis and each of them possesses angular momentum1/2(h/2π)
in accordance with the quantum theory. The net resultant of the
angular momentum of all nuclear particles is called electron spin.
For a electron having a spin quantum number I, these are(2I +1)
spin states.

6. SPIN QUANTUM OF VARIOUS NUCLEI
Number of
protons
Number of
neutrons
Spin
quantum
number. I
Example
Even Even 0 12C, 16O, 32S
Odd/
Even
Even/
Odd 1/2,3/2,5/2
1H, 19F, 31P,
11B, 79Br &
13C, 127I,
odd odd 1 2H, 14N
6
17
35
Cl,
17
16O

7. ESR Spectroscopy
Electron Spin Resonance Spectroscopy
@lso called as
EPR Spectroscopy
Electron Paramagnetic Resonance Spectroscopy

8. PRINCIPLE
▪ ESR spectroscopy is based on the absorption of microwave radiation by an unpaired
electron when it is exposed to a strong magnetic field.
▪ The electronic energy levels of the atom or molecule will split into different levels.
▪ The magnitude of the splitting is dependent on the strength of the applied magnetic
field.
▪ The atom or molecule can be excited from one split level to another in the presence of
an external radiation of frequency corresponding to the frequency obtained from the
difference in energy between the split levels. Such an excitation is called a magnetic
resonance absorption.
▪ The magnetic resonance frequency will hence be influenced by the local environment
of the atom or molecule.

12. • Klystron tube acts as the source of radiation.
• The frequency of the monochromatic
radiation is determined by the voltage
applied to klystron.
• It is kept a fixed frequency by an
automatic control circuit and provides
a power output of about 300 milli watts.

13. The wave meter is put in between the oscillator
and attenuator to know the frequency of
microwaves produced by klystron oscillator.
The wave meter is usually calibrated in
frequency unit (megahertz) instead of
wavelength. Wave guide is a hollow,
rectangular brass tube. It is used to convey the
wave radiation to the sample and crystal.

15. The power propagated down the wave
guide may be continuously decreased by
inserting a piece of resistive material into
the wave guide.
The piece is called variable attenuator and
used in varying the power of the sample
from the full power of klystron to one
attenuated by a force 100 or more.

17. It is a non-reciprocal device which minimizes vibrations in
the frequency of microwaves produced by klystron
oscillator.
Isolators are used to prevent the reflection of microwave
power back into the radiation source.
It is a strip of ferrite material which allows micro waves
in one direction only.
It also is being stabilizing the frequency of the klystron.

19. The heart of the ESR spectrometer is the resonant sample cavity.
In most of the ESR spectrometers, dual sample cavities are generally
used.
This is done for simultaneous observation of a sample and a
reference material.
Since magnetic field interacts with the sample to cause spin
resonance the sample is placed where the intensity of magnetic
field is greatest.
A measure of quality of the cavity is ‘Q factor’ which is defined as The
sensitivity of the spectrometer is directly proportional to this value of
Q.

21. The various components of the micro wave
assembly to be coupled together by making use of
irises or slots of various sizes.

22. A Silicon crystal detectors, which converts
the radiation in D.C., has widely been used as
a detector of microwave radiation.
Microwave Bridge such as magic T and hybrid
ring variety are most common.

24. The resonant cavity is placed between the poles pieces of
an electromagnet.
An electro magnet capable of producing magnetic field of
at least 5000 gauss is required for ESR.
The field should be stable and uniform over the sample
volume.
The stability of field is achieved by energizing the magnet
with a highly regulated power supply.

25. The ESR spectrum is recorded by slowly varying the
magnetic field through the resonance condense by
sweeping the current supplied to the magnet by the
power supply.
This sweep is usually accomplished by with a
variable speed motor drive.
Both the magnet as well as the power supply may
require water cooling.

26.  The modulation of the signal at a frequency
consistent with good signal noise ratio in the crystal
detector is accomplished by a small alternating
variation of the magnetic field.
 The variation is produced by supplying an A.C.
signal to modulation coil oriented with respect the
sample in the same direction as the magnetic field.

27. If the modulation is of low frequency (400
cycles/sec or less), the coils can be mounted
outside the cavity and even on the magnet pole
pieces.
For higher modulation frequencies, modulation
coils must be mounted inside the resonant cavity
or cavities constructed of a non-metallic
material
e.g., Quartz with a tin silvered plating, because
metallic penetration is not very effective in case
of higher modulation frequencies.

29.  In order to adjust the spectrometer and to observe
the signal, a cathode ray oscilloscope has been
employed. A strip chart or X-Y recorder is used for
recording the signal.
 EPR spectra are usually displayed in derivative
form to improve the signal-to-noise ratio.

31. SCHEMATIC DIAGRAM OF AN ESR SPECTROMETER

32. ▪ The sample is placed in a resonant cavity which admits microwaves
through an iris.
▪ The cavity is located in the middle of an electromagnet and helps to
amplify the weak signals from the sample.
▪ Numerous types of solid-state diodes are sensitive to microwave energy
▪ Absorption lines are detected when the separation of the energy levels
is equal to the energy of the incident microwave.

33. What causes the energy levels?
Resulting energy levels of an electron in a magnetic field

34. Describing the energy levels
▪ Based upon the spin of an electron and its associated magnetic moment
▪ For a molecule with one unpaired electron
– In the presence of a magnetic field, the two electron spin energy levels are
E = gmBB0MS
g = proportionality factor mB = Bohr magneton
MS = electron spin B0 = Magnetic field
quantum number
(+½ or -½)

35. Hyperfine Interactions
▪ EPR signal is ‘split’ by neighboring nuclei
– Called hyperfine interactions
▪ Can be used to provide information
– Number and identity of nuclei
– Distance from unpaired electron
▪ Interactions with neighboring nuclei
E = gmBB0MS + aMsmI
a = hyperfine coupling constant
mI = nuclear spin quantum number

36. Hyperfine Interactions
Interaction with a single nucleus of spin ½

37. Hyperfine Interactions
▪ Coupling patterns same as in NMR
▪ More common to see coupling to nuclei with spins greater than ½
▪ The number of lines:
2NI + 1
N = number of equivalent nuclei
I = spin
▪ Only determines the number of lines--not the intensities

38. Hyperfine Interactions
▪ Relative intensities determined by the number of interacting
nuclei
▪ If only one nucleus interacting
– All lines have equal intensity
▪ If multiple nuclei interacting
– Distributions derived based upon spin
– For spin ½ (most common), intensities follow binomial distribution

39. Hyperfine Interactions
▪ Example:
– VO(acac)2
– Interaction with vanadium nucleus
– For vanadium, I = 7/2
– So,
2NI + 1 = 2(1)(7/2) + 1 = 8
– You would expect to see 8 lines of equal intensity
vanadyl acetylacetonate

40. Hyperfine Interactions
EPR spectrum of vanadyl acetylacetonate

41. Hyperfine Interactions
Pyrazine anion Electron delocalized over ring
Exhibits coupling to two equivalent N (I = 1)
2NI + 1 = 2(2)(1) + 1 = 5
Then couples to four equivalent
H (I = ½)
2NI + 1 = 2(4)(1/2) + 1 = 5
So spectrum should be a quintet with intensities 1:2:3:2:1
and each of those lines should be split into quintets with
intensities 1:4:6:4:1

42. Hyperfine Interactions
EPR spectrum of pyrazine radical anion

45. • Electron Spin Resonance is a powerful
non-destructive & non-intrusive analytical method.
• ESR yields meaningful structural information even
from ongoing chemical or physical processes,
without influencing the process itself.
• It is the ideal technique to complement other
analytical methods in a wide range of application
areas.

46.  Free Radicals
 Odd-electron Molecules
 Transition Metal Complexes
 Molecular Motion
 Rare Earth Ions
 Crystal / Ligand Fields
 Electron Transport
 Reaction Kinetics
etc.. Can be detected by ESR.

48. 1. Clearly mention state the model of ESR Spectormeter for your sample analysis.
2. Analysis charges would vary depending on model selection. Relevent charges are
given under each model
3. Clearly state the type of analysis required. Powder/Solution and also RT/77K For
solution sample suggest name of solvent.
4. Experiments can be carried out at ambient temperature, liquid nitrogen
temperature and most of the temperatures between liquid nitrogen temperature and 200o
C.
5. Experiments can be carried out with samples in liquid, solid or gaseous form, which
includes single crystals, metal alloys, liquid crystals, organic radicals, polymers, glasses,
powders, triplets, polyradicals, conduction electrons, drugs, rare-earths, enzymes,
proteins and gases.
6. Provide minimum 25-50 mg of solid sample in powder form.

49. References
http://web.mit.edu/speclab/www/PDF/DCIF-EPR-training-n03.pdf
http://www.rsic.iitb.ac.in/esr.html
http://www.pharmatutor.org/articles/instrumentation-electron-spin-resonance-spectroscopy

50. THANK YOU
By
ARJUN KUMAR B
M.Sc. Nano science & technology
I year