ESR SPECTROSCOPY

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. -1.5
-1
-0.5
0
0.5
1
1.5
2900 3000 3100 3200 3300 3400 3500 3600 3700
Gauss
dA/dB 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 radiationm
s
=—
1
2
m
s
=—
1
2
-
Energy
Quantum theory:
transitions between energy levels
induced by magnetic field
Resonance condition
h= gm
BB
0

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
1
H)
14
N)
2 identical I=1/2 nuclei
1 I=5/2 nucleus (
17
O)
Hyperfine coupling
If the electron is surrounded by nspin-
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
1
H ½ 2 99.985
Nitrogen
14
N 1 3 99.63
15
N ½ 2 0.37
Vanadium
51
V 7/2 8 99.76
Manganese
55
Mn 5/2 6 100
Iron
57
Fe ½ 2 2.19
Cobalt
59
Co 7/2 8 100
Nickel
61
Ni 3/2 4 1.134
Copper
63
Cu 3/2 4 69.1
65
Cu 3/2 4 30.9
Molybdenum
95
Mo 5/2 6 15.7
97
Mo 5/2 6 9.46

21. •Electron Paramagnetic Resonance of
Hydrogen Atom
ItcontainsanunpairedelectronwithS=1/2anda
protonwithnuclearspinI=1/2(ms=±1/2andmI=
±1/2).Intheabsenceofmagneticfield(B0=0),the
electronspinenergylevelsaredegenerate,i.e.,have
thesameenergy(FApplicationofB0liftsthe
degeneracybetweenthem(thems=−1/2sublevel
goingdownandthems=+1/2sublevelgoingup).
Eachoftheseelectronsublevelsfurtherinteractswith
thenucleus(I=1/2)givingfoursublevelsdesignated
bythevalueofmI.Thisphenomenoniscalled
hyperfinesplitting.

22. Hyperfine splittings multiply with
the number of nuclear spinsO
.
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. ItcontainsanunpairedelectronwithS=1/2and
threeequivalentprotons.Eachprotonhasaspin
I=1/2.Therefore,thetotalnuclearspinisI=3/2.
Foreachm
s=±1/2,them
Ivalues
are+3/2,+1/2,−1/2,−3/2asshowninFig.The
interpretationoftheESRspectrumissimilartothat
ofHatom.TheESRspectrumshowsfourequally
spacedlines,(n+1),i.e.,aquartetintheintensity
ratio1:3:3:1.Thespacingbetweenanytwo
successivelinesrepresentstheconstant(A)
isotropiccoupling
Methyl radical CH3radical

25. APPLICATIONS
•Thesiteofunpairedelectrons.
•Theno.oflinecompounds decideabout
theno.andtypeofnucleipresentinthe
neighbouredoftheoddelectron.
•TheESRspectrumthevalueofgcanbe
measured bycomparingthepositionof
thelinewiththatofastandard
substanceofknowngvalue.exDPPH
powder.
•Theelectricalfieldisnotspherical.