Introduction,theory of Esr spectra,instrumentation,and application

1. Electron Spin Resonance
spectroscopy
Mr.M.Ragu M.Sc,SET
Assistant Professor
Vivekananda College
Tiruvedakam west
Madurai-625234

2. Electron Paramagnetic Resonance spectroscopy
Electron Spin Resonance spectroscopy

4. ESR is discovered by soviet
physicist zavoisky in 1945.

5. ELECTRON SPIN RESONANCE
SPECTROSCOPY
DEFINITION:
ESR is a branch of absorption
spectroscopy in which radiation
having frequency in the microwave
region is absorbed by paramagnetic
substances to induce transition
between magnetic energy levels of
electrons with unpaired spins.

6. N S
Klystron
Microwave source
Detector
Cavity
cryostat
Circulator
Diagram of an ESR spectrometer
Spectrophotometer
Light source
Detector

7. • ESR spectra observed in microwave
region of the electromagnetic
radiation.
• ESR observed primarily in systems
containing unpaired electron.
• COMPARISION BETWEEN NMR and
ESR

8. COMPARISION BETWEEN ESR and NMR

9. • Which molecules possessing
electrons with unpaired spins.
This phenomenon has been
designated by different names,
1. Electron paramagnetic
resonance(EPR)
2. Electron spin resonance(ESR)
3. Electron magnetic resonance(EMR)

10. • Now I discuss basic aspects of
ESR
i) Hyperfine splitting
ii) Applications

11. ESR theorem
• Electron spin s=1/2 spin angular
momentum quantum number
can have value of ms=±1/2.
• Presence of magnetic field this
degeneracy resolved, low
energy state ms=-1/2 in aligned
applied field high energy state
ms=+1/2 in opposed applied
field. E=h=gHo

12. How does EPR Work?
Like a proton, an electron has a spin, which gives it a
magnetic property known as a magnetic moment.
When an external magnetic field is supplied, the
paramagnetic electrons can either orient in a direction
parallel or antiparallel to the direction of the magnetic field
.
This creates two distinct energy levels for the unpaired
electrons and measurements are taken as they are driven
between the two levels.

13. The EPR experiment
• Put sample into
experimental magnetic
field (B)
• Irradiate (microwave
frequencies)
• Measure absorbance of
radiation as f(B)
Weil, Bolton, and Wertz, 1994, “Electron Paramagnetic Resonance”

14. Electron Spin Resonance Spectroscopy
or
It’s fun to flip electrons!

16. Principles of EMR spectroscopy
B 0
DE
h
Classical theory:
Electron spin moment interacts with
applied electromagnetic radiation
m s = —
1
2
m s = —
1
2
-
Energy
Quantum theory:
transitions between energy levels
induced by magnetic field
Resonance condition
h = gmBB0

17. The hyperfine effect
• The magnetic field experienced by the unpaired electron
is affected by nearby nuclei with non-zero nuclear spin
Weil, Bolton, and Wertz, 1994, “Electron Paramagnetic Resonance”, New York: Wiley Interscience.

18. Hyperfine splitting of EPR spectra
• The magnitude of the splitting and the number of lines
depend upon:
– The nuclear spin of the interacting nucleus
• # of lines = 2n(I + ½) so I = ½ gives 2 lines, etc.
– The nuclear gyromagnetic ratio
– The magnitude of the interaction between the
electronic spin and the nuclear spin
• Magnitude of the splitting typically decreases
greatly with increasing numbers of bonds between
the nucleus and unpaired electron

19. 10 Gauss
No hyperfine
1H)
14N)
2 identical I=1/2 nuclei
1 I=5/2 nucleus (17O)
Hyperfine coupling
If the electron is surrounded by n spin-
active nuclei with a spin quantum
number of I, then a (2nI+1) line pattern
will be observed in a similar way to
NMR.
In the case of the hydrogen atom (I= ½),
this would be 2(1)(½) + 1 = 2 lines.

20. Some nuclei with spins
Element Isotope Nuclear No of %
spin lines abundance
Hydrogen 1H ½ 2 99.985
Nitrogen 14N 1 3 99.63
15N ½ 2 0.37
Vanadium 51V 7/2 8 99.76
Manganese 55Mn 5/2 6 100
Iron 57Fe ½ 2 2.19
Cobalt 59Co 7/2 8 100
Nickel 61Ni 3/2 4 1.134
Copper 63Cu 3/2 4 69.1
65Cu 3/2 4 30.9
Molybdenum 95Mo 5/2 6 15.7
97Mo 5/2 6 9.46

21. • Electron Paramagnetic Resonance of
Hydrogen Atom
It contains an unpaired electron with S = 1/2 and a
proton with nuclear spin I = 1/2 (ms = ±1/2 and mI =
±1/2). In the absence of magnetic field (B0 = 0), the
electron spin energy levels are degenerate, i.e., have
the same energy (F Application of B0 lifts the
degeneracy between them (the ms = −1/2 sublevel
going down and the ms = +1/2 sublevel going up).
Each of these electron sublevels further interacts with
the nucleus (I = 1/2) giving four sublevels designated
by the value of mI. This phenomenon is called
hyperfine splitting.

22. Hyperfine splittings multiply with
the number of nuclear spins
O
.
O-
H
H
H
H
Benzoquinone anion radical:
1 proton – splits into 2 lines 1:1
2 protons split into 3 lines 1:2:1
3 protons split into 4 lines 1:3:3:1
4 protons split into 5 lines 1:4:6:4:1
-60 C
20 C
At higher temperature:
faster motion - sharper lines
shorter lifetime - smaller signal

24. It contains an unpaired electron with S = 1/2 and
three equivalent protons. Each proton has a spin
I = 1/2. Therefore, the total nuclear spin is I = 3/2.
For each ms = ±1/2, the mI values
are +3/2, +1/2, −1/2, −3/2 as shown in Fig. The
interpretation of the ESR spectrum is similar to that
of H atom. The ESR spectrum shows four equally
spaced lines, (n + 1), i.e., a quartet in the intensity
ratio 1:3:3:1. The spacing between any two
successive lines represents the constant (A)
isotropic coupling
Methyl radical CH3 radical

25. APPLICATIONS
• The site of unpaired electrons.
• The no. of line compounds decide about
the no. and type of nuclei present in the
neighboured of the odd electron.
• The ESR spectrum the value of g can be
measured by comparing the position of
the line with that of a standard
substance of known g value. ex DPPH
powder.
• The electrical field is not spherical.