B.Sc and M.Sc. Chemistry Students

1. ELECTRON SPIN RESONANCE SPECROSCOPY
(ESR)
Prof.G.M.Dongare
Dept of Chemistry
Shri Shivaji Science College, Amravati
Magnetic field (mT)
Intensity

2. Electron spin Resonance
Spectroscopy
(ESR)
 Electron Paramagnetic Resonance also
known as Electron Spin Resonance or
pulse electron spectroscopic technique
which detect species that have unpaired
electrons.
 A compunds contains unpaired electrons.
(Exam: free radicals and many transition
metal ions)
 EPR suggested the free radicals mech. has
and useful in paramagnetic species.

3. Non-destructive technique • Applications –
Oxidation and reduction processes – Reaction
kinetics –
Examining the active sites of metalloproteins
Reaction Mechanism

4. EPR spectroscopy is the measured and
interpretation of the energy differences between the
atomic and molecular states.
Absorption of Microwave region 1000 MHz -300000
MHz wavelength λ =30 cm to 1 mm electro-magnetic
radiation.
To obtained the high resolution of a spectrum, the frequency of
the electromagnetic radiation externally applied and the amount
of radiation which passes through the sample with a detector is
measured to observe the spectroscopic absorptions

5. ESR Instrumentation
Microwave based
 IMP Components.
◦ Microwave Bridge: Electromagnetic Radiation Source
◦ Microwave Cavity: Amplifies weak signals from the sample
◦ Magnet: Splits the electronic spin energy levels
◦ Console: Contains signal processing and control electronics and a
units for acquiring a spectrum
◦ Computer: Used to analyze data as well as coordinating all the units
for acquiring a spectrum.

6. WORKING OF INSTRUMENTS
 A proton contains spin, similarly electron has a spin, which gives it a
magnetic property known as a magnetic moment (u).
 Applied external magnetic field, the paramagnetic electrons orients in
a direction parallel or antiparallel to the direction of the magnetic field.
 Formation of two distinct energy levels for the unpaired electrons

7. SCHEMATIC DIAGRAM OF ESR
INSTRUMENTS

9. EPR Spectroscopy
 EPR spectroscopy is the measurement and interpretation of the
energy differences between the atomic or molecular states.
◦ These measurements are obtained because the relationship
between the energy differences and the absorption of electro-
magnetic radiation.
◦ To acquire a spectrum, the frequency of the electromagnetic
radiation is changed and the amount of radiation which passes
through the sample with a detector is measured to observe the

10. APPLICATIONS
Used in the study of free radicals.
Used in the investigation of molecules in the triple state.
Used in the study of inorganic compounds.
Used in the structure elucidation of the organic.
Used in the study of biological systems
Used in the quantitative analysis.
Used in the spin labelling of the protein structure by using
the 2,2,6,6-tetra methyl piperidinol-n-oxyl.
Used in the determination of the polarity of the substances.
Used in the determination of the reaction velocities.

11. Used in the determination of the reaction
mechanisms.
Used in the determination of the properties of the
crude oil.
Used in the determination of the radiation decay.
Used in the detection of Mn+2 in the trace
quantities.
Used in the determination of the vanadium in the
petroleum products.
Used in the determination of the gadolinium ion.
Used in the estimation of the poly nuclear
hydrocarbons.
Used in the study of the oxidation of the bacteria
chlorophyll.
Used in the study of the oxidative enzymes.

12. ESR is employed to analyse the process of sugar transport
in bacteria.
– ESR was applied to the analysis of the influence of
diabetes on the properties of erythrocytes showing the
decrease in erythrocyte deformability due to the non-
enzymatic glycation of haemoglobin. Thus, structural
investigation often reveals medical aspects.

13. Used in the detection of the DNA binding drugs.
Used in the determination effects of the chemotherapy.
Used in the photosensitising action of the milk ingredient
or riboflavine.
Used in the determination of the water diffusion.

14. ADVANTAGES
Simple
Selective
Specifcity
High resolution
Highly accurate

15. REVIEW QUESTIONS
What is ELDOR?
What is ENDOR?
What are the requirements to get ESR spectrum?
What are the different components of ESR
spectrometer?
Define hyperfine splitting?
What is gyrometric ratio?
Name the standard used for the calibration of ESR
spectra.
What is Bohr magneton?
Define the Boltzmann distribution law.
What are the solvents used for ESR? DPPH

16. Chemical Applications of EPR
List of symbols
a = hyperfine splitting constant, units are mTesla; a0 =
isotropic hyperfine coupling constant, units are
mTesla; A∥ = hyperfine coupling constant parallel to a
unique symmetry axis, units are mTesla; A⊥ = hyperfine
coupling constant perpendicular to a unique symmetry
axis, units are mTesla; Ea = rotational activation energy,
in kJ mol−1; g = g factor; g∥ = parallel to a unique
symmetry axis; g⊥ = perpendicular to a unique
symmetry axis; τ = free radical tumbling correlational
time (in seconds).
References : Encyclopedia of Spectroscopy
and Spectrometry, 1999

17. Isotropic Hyperfine Interaction
Hyperfine structure (HFS) occurs as a result of the
magnetic interaction between the electronic spin S
and the nuclear spin I. The magnitude of this
interaction is given by A called isotropic hyperfine
coupling constant. The hyperfine
interaction in ESR spectra is analogous to the fine
structure, i.e., nuclear spin–spin interaction
in NMR spectra. As a result of this interaction,
the ESR signals or peaks are further split into
several lines (HFS).

18. 1.It depends on the magnitude of magnetic moments of
nuclear and electron spins.
2.The electron spin density in the immediate vicinity
of the nucleus (Fermi contact) (see Section 4.13.1)
3.It is independent of applied magnetic field
4.It obeys (n + 1) rule for I = 1/2 i.e., (2nI + 1)
5.The intensity ratio is obtained using Pascal triangle
(analogous to NMR spin–spin coupling)

19. Benzene Anion Radical
This radical has one unpaired
electron (S = 1/2) interacting with
six equivalent protons. Hence, the
number of peaks observed is
calculated as
(2nI + 1 = 2 × 6 × 1/2 + 1 = 7).
Therefore, the ESR spectrum
(Fig. 4.11) contains a septet, i.e.,
seven equally spaced lines.

20. .
Benzene Anion Radical
This radical has one unpaired electron
(S = 1/2) interacting with six equivalent
protons. Hence, the number of peaks
observed is calculated as
(2nI + 1 = 2 × 6 × 1/2 + 1 = 7). Therefore,
the ESR spectrum (Fig. 4.11) contains a
septet, i.e., seven equally spaced lines.

21. Naphthalene anion radical
Naphthalene Anion Radical-
This radical anion consists of one unpaired electron interacting
with two sets (α and β) of four equivalent protons each as shown
in Fig. 4.12. The ESR spectrum would thus show, i.e.,
(2nI1 + 1) × (2nI2 + 1) = (4 + 1) × (4 + 1) = 25 lines. This radical
is formed when naphthalene in solution in 1,2
dimethoxyethane is reduced with potassium metal. A green
colored solution is obtained whose EPR spectrum is shown
below. By considering the pattern and coupling constants of
a1 = 4.90 G and a2 = 1.83 G of hyperfine splitting's, the species
formed is consistent with the naphthalene radical anion.

23. ENDOR (Electron-Nuclear Double
Resonance) spectroscopy

25. What is ELDOR spectroscopy?
ELDOR(electron-electron double resonance)is double
electron spin resonance in which two microwave
frequencies participate. One of these, called the
“pump” microwave source, irradiates a portion of the
ESR spectrum.

26. Difference between ESR and NMR
ESR
1.ESR is concerned with the
magnetically induced splitting of
electronic spin states
2.Microwaves in the 3 - 400 GHz
range
3.Electrons have a greater magnetic
moment than nuclei.
4.Advantage of being highly specific
5.The electron magnetic moment in
EPR is about 660 times larger than that
of the proton, and the proton's
magnetic moment is the largest of all
the nuclei used in NMR.
NMR
1.NMR describes the splitting
of nuclear spin states.
2.Radio frequency range
between 300 and 1000 MHz
3. NMR low sensitive