INSTRUMENTATION OF ELECTRON SPIN RESONANCE (ESR) SPECTROSCOPY-M.ScSEM-3.pdf

This PPT represent a epr spectroscopy and is instrumental part and it uses

1. Intrumentation
◎Source(Klystron, isolator, wave meter, and attenuator)
◎Circulator or Magic -T
◎Sample Cavity
◎Magnet System
◎Crystal Detector
◎Auto amplifier and Phase sensitive Detector
◎Oscilloscope and Pen Recorder
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2. Klystron
•Itisavacuumtube
whichcanproduce
microwave
oscillationscentered
onasmallrangeof
frequency
•Itisoperateat9500
MHz.
•Thefrequencyofthe
monochromatic
radiation is
determinedbythe
voltageappliedto
Klystron.
Source(Klystron, isolator, wave meter, and attenuator)
Attenuator
•Attenuator is used
to adjust the level
of the microwave
power incident
upon the sample.
•It processes an
absorption
element and
corresponds to a
neutral filter in
light absorption
measurement.
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Isolator
•Itisanonreciprocal
devicewhichminimizes
vibrationsin the
frequency of
microwavesproduced
byKlystronoscillator.
•Thevariationsoccurin
thefrequencyduetothe
backwardreflectionsin
theregionbetweenthe
Klystronandcirculator.
•Isolatorisastripof
ferritematerial.
wave meter
•It is fixed in between
the isolator and
attenuator to
know the frequency of
microwaves produced
by Klystron oscillator.
•Usually it is calibrated
in frequency units
instead of wavelength.

3. Circulator or Magic -T
◎The microwave radiations obtained
from the klystron enter the
circulator through a waveguide by a
loop of wire which is coupled with
an oscillating magnetic field. It is set
up a corresponding field in the
waveguide.
◎A waveguide is generally made up of
a hallo rectangularcopperor brass
tubing. It hassilverorgoldplating
inside the tube for the production of
a highly conducting flat surface.
◎The operation of the four-port
circulator in ESR spectroscopy is
given below in the picture :
•The microwave radiation enters arm 1.
•Arm 2 is connected to the resonant cavity and sample.
•Generally, arm 4 contains a terminating load for absorbing any
power which might be reflected from the detector arm.
•Arm 4 is attached to the detector of the electron spin
resonance spectrometer.
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4. Sample Cavity
◎Thesamplecavityisanimportant
componentoftheelectronspinresonance
(ESR)spectrometer.Thecavitysystemis
constructedinsuchawaythattheapplied
magneticfieldismaximuminthesample
dimension.
◎InmostESRspectrometers,duelsample
cavitiesareusedduetosimultaneous
observationofasampleandareference
material.Byuseofreferencematerial,the
sourcesoferrorarecompensatedby
comparingrelativesignalheights.
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5. Magnet System
◎In an EPR spectrometer, the resonant cavity
is placed in a magnetic assembly that
includes the magnet with a dedicated power
supply. It has a homogeneous magnetic
field thatcan vary from 0 to 500 gauss.
◎The magnetic field should be stableand
uniformover the sample volume. The
stability of the field can be enlarged by a
high-regulating power supply system.
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Magnet

6. ◎Themostcommonlyused
detector is
asiliconcrystaldetector.
InESRspectrometers,it
actsasamicrowave
rectifierthatconverts
microwavepowerintoa
directcurrent(DC)
output.
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Crystal Detector Auto amplifier and Phase
sensitive Detector
Afterdetectionbythecrystal
detector,thesignalundergoesa
narrow-band amplifierthat
amplifiedthesignalwithalotof
noise.Thenoisecanbereduced
bytheoperationofthephase-
sensitivedetector.

7. Oscilloscope and Pen Recorder
Finally, the signal from the phase-sensitive detector and sweep unit is recorded by the
oscilloscope or pen recorder.
◎Dual-trace oscilloscopes with two vertical inputs are extremely useful
components of electron spin resonance (ESR) instruments.
◎In an analog oscilloscope, the vertical amplifier acquires the signals to be
displayed. It provides a signal large enough to deflect the CRT’s (Cathode ray
tube) beam and better delays the signal by a fraction of a microsecond.
◎There are several inputs for voltages in most modern oscilloscopes. Therefore, it
can be used to plot one varying voltage versus another.
◎Some analog oscilloscopes have a feature of Z input. For example, if a pair of
waves of known frequency is used to generate a circular-T figure and a higher
unknown frequency is applied to the Z input.
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8. ESR
Spectrometer
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9. Working :
◎The Klystron oscillator is set to produce microwaves.
◎After passing though the isolator, wave meter and attenuator the microwaves are
entered into the circulator on magic T
◎Then it reaches the detector which acts as a rectifier, i.e. converting the
microwave power into the direct current.
◎If the magnetic field around the resonating cavity having the sample is changed to
the value required for the resonance, the recorder will show an absorption peak.
◎If the magnetic field is swept slowly over a period of several minutes, the recorder
will show the derivative of the microwave absorption spectrum against magnetic
field as shown below:
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10. Presentation of ESR Spectrum:
Gray
•The ESR spectrum is
obtained by plotting
intensity against
the strength of a magnetic
field.
•The better way is to
represent ESR spectrum
as a derivative
curve in which the first
derivative(slope) of the
absorption
curve is plotted against the
strength of the magnetic
field
Black
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The total area covered by
either the absorption or
derivative curve is
proportional to the
number of unpaired
electrons in the sample.

11. LANDE SPLITTING FACTOR (G) OR, ELECTRON G -FACTOR
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Electron g-factoris the ratio of magnetic moment of the electron to the total spin angular
momentum of the electron. For a free electron the value of g is 2.0023. In many free
radicals, the value of the odd electron is close to that of a free electron, but in metal ions g
values are often quite different from the free electron value.
Factor affecting g –values
•Ingeneral,themagnitudeofgdependsupontheorientationofthemolecule/ioncontaining
theunpairedelectronwithrespecttothemagneticfieldapplied.
•Inasolution,orinthegasphase,gisaveragedoverallorientationsbecauseofthefree
motionofthemolecules/ions.
•However,inacrystal,themovementofmolecules/ionsisrestricted.
•Iftheparamagneticradicalorionislocatedinaperfectlycubiccrystalsite,i.e.,an
octahedralortetrahedralsite,theg-valueisindependentoftheorientationofthecrystaland
issaidtobeisotropic.
•Inacrystalsiteoflowersymmetry,thevalueofgdependsupontheorientationofthe
crystalandissaidtobeanisotropic.

12. 27
The value of g for an unpaired electron in gaseous atom/molecule/ionfor which Russel
Saunders coupling (L-S coupling) is applicable, is given by the expression
•Thez-axisisdefinedcoincidentwiththehighestfoldrotationaxis,whichcanbe
determinedbyx-raycrystallography.Thegzvalueisequivalenttog
||,theg-valueobtained
whenthez-axisisparalleltotheexternalmagneticfield.
•Theg-valuesalongxandyaxesaregxandgy,whichinatetragonalsiteareequaland
referredtoasg
⊥,theg-valueobtainedwhenexternalmagneticfieldisperpendiculartothe
z-axis.
•Ifθbetheanglebetweenthemagneticfieldandthez-axis,theexperimentalg-valueis
givenby……….

13. Substitution of these values in last equation yields g = 4/3 , identical with the experimental
value.
◎No such agreement is found, however, when the unpaired electron is placed in a chemical
environment, either in a free radical, or in a transition metal ion complex crystal lattice.
◎In such a chemical environment, the orbital motion of the electron is strongly perturbed and
the orbital degeneracy is partly removed or quenched.
◎Jahn Teller distortions also serve to lift the orbital degeneracy.On the other hand, a certain
amount of orbital degeneracy tends to be sustained as the result of spin orbit coupling.
◎Thus, complete removal of orbital degeneracy is prevented by spin orbit coupling but
higher fold degeneracies are often decreased by this effect.
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14. ◎Thus, if an electron has orbital angular momentum, this is maintained by coupling to the
spin angular moment, and if it has a spin angular momentum, this tends to generate
orbital angular momentum. Consequently, because of the quenching and sustaining
competition, the orbital degeneracy is partly but not completely removed and a net orbital
magnetic moment results, giving rise to a g value different from the value 2.0023 expected
if the orbital degeneracy were completely removed.
◎In most free radicals, the orbital contributions to the magnetic moment are very small and
the g values are nearly equal to free electron value of 2.0023.
◎The small deviations (±0.05) often observed for most free radicals are accounted for by the
mixing of low lying excited states with the ground state.
◎The properties of transition metals are determined to a large extent by the relative
magnitudes of the crystal field and spin-orbit coupling. These two interactions have
opposite effects on orbital degeneracy. Here three cases may be distinguished :
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15. (i)Theeffectofspin-orbitcouplingismuchlargerthanthatofthecrystalfield:
Therareearthionsfallinthisclassbecausethef-electronsaredeepseatedandwellshieldedfrom
thecrystalfieldeffectssothatthespinorbitcouplingisnotdisturbed.So,gvaluescalculated
usingpreviousEq.forrareearthionsareingoodagreementwithexperimentalvalues.
(ii)Theeffectofcrystalfieldisstrongenoughtodestroythespin-orbitcoupling:
Thefirstrowtransitionmetalsfallinthiscategory.Theorbitaldegeneracyisnotremoved
completelybecauseoftheeffectofsomespin-orbitcoupling.Consequently,anetorbital
magneticmomentresults,givingrisetoagvaluedifferentfromthefreeelectronvalue.However,
gvalueisclosertothefreeelectronvalueof2.0023thanpredictedfromEq.Ionshavingan
orbitallynon-degenerategroundstatesuchasFe
+3
(
6
S)andMn
+2
(
6
S)havegvaluenearlyequalto
thefreeelectronvalue,sincepracticallythereisnoorbitalangularmomentum.Theslight
deviationfromthefreeelectronvalueisduetosmallspinorbitcoupling.
(iii)Theeffectofcrystalfieldisstrongenoughtodestroythespin-orbitcouplingcompletely:
Thiscorrespondstocovalentbondingandisapplicabletothecomplexesof4dand5dtransition
metalsandtothestrongfieldcomplexesof3dtransitionmetalssuchascyanides.
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16. Some solved problems………
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17. 32

18. 33

19. 34

20. 35

21. 36

22. 37

23. 38

24. Difference between NMR and ESR
NMR
•Differentenergystatesareproduced
duetothealignmentofthenuclear
magneticmomentsrelativetoapplied
magneticfieldandthetransition
betweentheseenergystatesoccurs
ontheapplicationofanappropriate
frequencyintheradiofrequency
region.
•NMR absorptionpositionsare
expressedintermsofchemicalshifts.
•Nuclearspin-spincouplingcausesthe
splittingofNMRsignals.
ESR
•Differentenergystatesareproduceddue
tothealignmentoftheelectronicmagnetic
momentsrelativetoappliedmagneticfiled
andthetransitionbetweentheseenergy
statesoccursontheapplicationofan
appropriatefrequencyinthemicrowave
region.
•ESRabsorptionpositionsareexpressed
intermsof“g”values.
•Couplingoftheelectronicspinwith
nuclearspins(hyperfinecoupling)causes
thesplittingofESRsignals.
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25. Hyperfine interaction
◎Thedetectionofelectronspinresonancesignalprovesthepresenceof
unpairedelectronsinthesample.Theintensityoftheabsorptionlineis
proportionaltothenumberofunpairedelectrons.TheESRspectroscopyhas
beenwidelyusedinstudyingvariousphotochemical,electro-chemicalreaction
proceedingviafreeradicalmechanismbecauseofthepresenceofhyperfine
structurewhichistheresultoftheinteractionbetweentheunpairedelectrons
andmagneticnucleiintheparamagneticspecies.Itfurthergivestwoimportant
information:
1.It tells about the environment of the molecule and distribution of the electron
density within the molecule.
2.It allows identification of paramagnetic substance in number of cases.
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26. The resonance frequency of an electron actually depends upon the magnetic field at
the electron and electron in fact is affected by the applied field, Ho and any local field
due to magnetic fields of nuclei, H
local
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TheeffectofmagneticmomentsofnucleionESRspectrumisknownashyperfine
interactionandisresponsibleforsplittingofESRline,givingrisetohyperfine
structure.WhenanunpairedelectroncomesinthevicinityofanuclearwithaspinI,an
interactiontakesplacewhichcausestheabsorptionsignaltobesplitinto2I+1
components,whereIisthespinquantumnumberofthenucleus.Thecauseof
splittinginanisotropicsystemisthenuclearspinelectronspincouplingarisingmainly
fromtheFermicontactterm.

27. 42
Thiseffectissimplyillustratedbyconsideringasanexampleofhydrogenatom
(I=½fortheproton).Theprotonthushasamagneticmomentandtheelectron
willbeaffectedbythemagneticfieldofthenucleus(aproton)aswellasthatof
theappliedmagneticfield.Theenergyoftheelectronwillbemodifiedbythe
orientationofthemagneticmomentoftheprotonwhichcanalsobeparallelor
anti parallel to the magnetic field.
Therelativeorientationofthenuclearmagneticmomentandtheelectron
magneticmomentcausesasplittingoforiginal2levelto4level.

28. THE SELECTION RULE FOR THE HYPERFINE TRANSITION IS:
∆m
s= ±1, ∆m
I= 0
◎This selection rule may be interpreted by saying that the nuclear motion is
much slower than the electronic motion so that during the time electron
changes its spin orientation, the nucleus, being heavier has no time to
reorient its spin so that ∆m
I= 0.
◎The EPR spectrum of a Hydrogen atom in a solid matrix consists of 2 peaks
of equal intensity centered at g=2.0023. The two-energy levels of a free
electron in a magnetic field are shown in the Figure with m =½ aligned with
the field and m = +½ aligned opposed to the field. (The spectrum of a free
electron would consist of a single peak corresponding to a transition between
these levels).
◎For each value of electron spin angular momentum quantum number m
s, the
nuclear spin angular momentum quantum number m
I, can have values of ±½,
giving rise to fourdifferent energy levels.
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29. INTERACTION OF UNPAIRED ELECTRON WITH ONE PROTON
Possible orientations of electrons and nuclear magnetic moments
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30. 45

31. 46
When the interaction between the 2 energy states and the nuclear spin due to proton is
considered, each energy state is further split up into energy levels corresponding to m
I=+½
& m
I=–½ when m is nuclear spin angular momentum quantum number. Thus two hyperfine
lines are observed (splitting of original line into a doublet results) and the energy difference
between the two values of the field at which the lines occur is called the hyperfine splitting
constant.

32. The magnitude of the splitting, expressed in
terms of coupling constant depends on following
factors :
1) The ratio of
the nuclear
magnetic
moment to the
nuclear spin.
2) The electron
spin density in
the immediate
vicinity of
nucleus (Fermi
contact
interaction).
3) An
anisotropic
effect.
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33. 48

34. 49

35. 50
Hyperfine energy level resulting from interaction of an unpaired electron with
varying numbers of equivalent protons

36. FOREX:Incaseofnaphthalenenegativeioni.e.preparedbyaddingsodiumto
naphthalenecontainsanoddelectronwhichisdelocalizedovertheentire
naphthalenering.
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Inthiscasetherearetwosetsof(α&β)of4
equivalentprotonseach.Anatalofn+1or5peaks
expectedforanelectrondelocalizedoneitherof4
equivalentprotons.
Thus the ESR spectrum would show 25 lines.

37. 52

38. Interaction of unpaired Electron with two equivalent Protons
53
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