EPR spectroscopy.

1. EPR
Electron Paramagnetic Resonance Spectroscopic(EPR).
by : Masresha Amare
PhD candidate in AAU.
1EPR

2. Outline
Introduction
Principle of EPR
The Zeeman effect
The Hyperfine EffectThe Hyperfine Effect
Super-hyperfine splitting
2EPR

3. Introduction
ElectronSpinResonanceSpectroscopy
AlsocalledEPRSpectroscopy
ElectronParamagneticResonanceSpectroscopy
Non-destructivetechnique
3EPR

4. What compounds can you analyze?
Applicableforspecieswithoneormoreunpairedelectrons
Freeradicals
Transitionmetalcompounds
Usefulforunstableparamagneticcompoundsgeneratedin
situsitu
Electrochemicaloxidationorreduction
4EPR

5. Energy of Transitions
ESRmeasuresthetransitionbetweentheelectron
spinenergylevels
Transitioninducedbytheappropriatefrequency
radiationradiation
Requiredfrequencyofradiationdependentupon
strengthofmagneticfield
Commonfieldstrength0.34and1.24T
9.5and35GHz
Microwaveregion
5EPR

6. Energy of Transitions
Theabsorptionofenergycausesatransitionofan
electronfromalowerenergystatetoahigherenergy
state.
InEPRspectroscopytheradiationusedisintheInEPRspectroscopytheradiationusedisinthe
gigahertzrange.
Unlikemosttraditionalspectroscopytechniques,in
EPRspectroscopythefrequencyoftheradiationis
heldconstantwhilethemagneticfieldisvariedin
ordertoobtainanabsorptionspectrum.
6EPR

7. How does the spectrometer work?
7EPR

8. How does the spectrometer work?
Theradiationsourceusuallyusediscalledaklystron
Theyarehighpowermicrowavesourceswhichhave
low-noisecharacteristicsandthusgivehigh
sensitivity
AmajorityofEPRspectrometersoperateat
approximately9.5GHz,whichcorrespondstoabout
32mm(X-band)
Theradiationmaybeincidentonthesample
continuouslyorpulsed
8EPR

9. How does the spectrometer work?
Thesampleisplacedinaresonantcavitywhich
admitsmicrowavesthroughaniris.
Thecavityislocatedinthemiddleofan
electromagnetandhelpstoamplifytheweaksignals
fromthesample.fromthesample.
Numeroustypesofsolid-statediodesaresensitiveto
microwaveenergy
Absorptionlinesaredetectedwhentheseparationof
theenergylevelsisequaltotheenergyofthe
incidentmicrowave.
9EPR

10. What causes the energy levels?
Resulting energy levels of an electron in a magnetic
field
10EPR

11. What causes the energy levels?
Whenanelectronisplacedwithinanapplied
magneticfield,B
o,thetwopossiblespinstatesofthe
electronhavedifferentenergies(Zeemaneffect)
Thelowerenergystateoccurswhenthemagnetic
momentoftheelectronisalignedwiththemagneticmomentoftheelectronisalignedwiththemagnetic
field.
Thetwostatesarelabeledbytheprojectionofthe
electronspin,M
S,onthedirectionofthemagnetic
field,whereM
S=-1/2isparallelandM
S=+1/2isanti
parallelstate
11EPR

12. Describing the energy levels
Baseduponthespinofanelectronanditsassociated
magneticmoment
Foramoleculewithoneunpairedelectron
Inthepresenceofamagneticfield,thetwo
electronspinenergylevelsareelectronspinenergylevelsare
E=g
BB
0M
S
g=proportionalityfactor
B=Bohrmagneton
M
S=electronspin B
0=Magneticfield
quantumnumber
(+½or-½)
12EPR

13. The Zeeman Effect
EPRspectroscopyareduetotheinteractionofunpairedelectronsin
thesamplewithamagneticfieldproducedbytheexternalmagnetfield
B
0.ThiseffectiscalledtheZeemanEffect.
the energies for an electron with ms = +½ and ms = -½ are
E
1/2 = 1/2geB
0 E
1/2 = -1/2geB
0
13EPR

14. The Zeeman Effect
14EPR

15. Proportionality Factor
Measured from the center
of the signal
For a free electron
2.00232
For organic radicalsFor organic radicals
Typically close to free-
electron value
1.99-2.01
For transition metal compounds
Large variations due to spin-orbit coupling and zero-
field splitting
1.4-3.0
15EPR

16. Spectra
To get finer informaon ∂A/∂H is ploed
against H to get the first derivative curve.
When phase-sensitive detection is used, the
signal is the first derivative of the absorption signal is the first derivative of the absorption
intensity
EPR 16

17. Hyperfine Interactions
EPR signal is ‘split’ by neighboring nuclei
Called hyperfine interactions
Can be used to provide informationCan be used to provide information
Number and identity of nuclei
Distance from unpaired electron
Interactions with neighboring nuclei
E = gm
BB
0M
S+ aM
sm
I
a = hyperfine coupling constant
m
I= nuclear spin quantum number
17EPR

18. Which nuclei will interact?
Every isotope of every element has a ground
state nuclear spin quantum number, I
has value of n/2, nis an integer
Isotopes with even atomic number and even
mass number have I= 0,and have no EPR spectramass number have I= 0,and have no EPR spectra

12
C,
28
Si,
56
Fe, …
Isotopes with odd atomic number and even mass
number have neven

2
H,
10
B,
14
N, …
Isotopes with odd mass number have nodd

1
H,
13
C,
19
F,
55
Mn, …
18EPR

19. Hyperfine Interaction
Interaction with a single nucleus of spin ½
19EPR

20. Hyperfine Interactions
Coupling patterns same as in NMR
More common to see coupling to nuclei with
spins greater than ½
The number of lines:The number of lines:
2NI+ 1
N= number of equivalent nuclei
I= spin
Only determines the number of lines--not the
intensities
20EPR

21. Hyperfine Interactions
Relative intensities determined by the number
of interacting nuclei
If only one nucleus interacting
All lines have equal intensityAll lines have equal intensity
If multiple nuclei interacting
Distributions derived based upon spin
For spin ½ (most common), intensities follow
binomial distribution
21EPR

22. Relative Intensities for I=1/2
N Relative Intensities
0 1
1 1 : 1
2 1 : 2 : 12 1 : 2 : 1
3 1 : 3 : 3 : 1
4 1 : 4 : 6 : 4 : 1
5 1 : 5 : 10 : 10 : 5 : 1
6 1 : 6 : 15 : 20 : 15 : 6 : 1
22EPR

23. Relative Intensities for I= ½
23EPR

24. Relative Intensities for I= 1
Example:
VO(acac)
2
Interaction with vanadium nucleus
For vanadium, I= 7/2For vanadium, I= 7/2
So
2NI+ 1 = 2(1)(7/2) + 1 = 8
You would expect to see 8 lines of equal intensity
24EPR

25. Intensities
EPR spectrum of vanadylacetylacetonate
25EPR

26. Hyperfine Interactions
Example:
Radical anion of benzene [C
6H
6]
-
Electron is delocalized over all six carbon atoms
•Exhibits coupling to six equivalent hydrogen atoms•Exhibits coupling to six equivalent hydrogen atoms
So
2NI+ 1 = 2(6)(1/2) + 1 = 7
So spectrum should be seven lines with relative
intensities 1:6:15:20:15:6:1
26EPR

27. Hyperfine Interactions
EPR spectrum of benzene radical anion
27EPR

28. Superhyperfinesplitting
If the odd, unpaired electron spends time around
multiple sets of equivalent nuclei, additional
splitting is observed: 2nI + 1; this is called
“superhyperfinesplitting.”
ESR spectrum of CH
2OH Radical
H.Fischeretal.
28EPR

29. superhyperfinesplitting
Example:
Pyrazineanion
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
29EPR

30. superhyperfinesplitting
EPR spectrum of pyrazine radical anion
30EPR

31. superhyperfinesplitting
ESR Spectrum of (CF
3)
2NO
Chem.Soc88: 371 (1966)
ESR Spectrum of tetra-azanaphthaleneanion
Chem.acta. 48:112 (1965)
31EPR

32. superhyperfinesplitting
ESR spectrum of the cyclopentadienylradical
Chem.Phys.42.3931(1965)
32EPR

33. Conclusions
Analysis of paramagnetic compounds
Compliment to NMR
Examination of hyperfine interactions
Provides information on number and type of Provides information on number and type of
nuclei coupled to the electrons
Indicates the extent to which the unpaired
electrons are delocalized
EPR 33

34. THANK YOUTHANK YOU
EPR 34