Electron paramagnetic resonance (EPR) spectroscopy measures transitions between electron spin energy levels when molecules with unpaired electrons are exposed to microwave radiation in an applied magnetic field. The document discusses the principles of EPR, including the Zeeman effect where electron spin states split into distinct energy levels. Hyperfine interactions between unpaired electrons and neighboring atomic nuclei provide information on the local electronic structure. More complex splitting patterns can arise from interactions with multiple equivalent nuclei, known as superhyperfine splitting. EPR spectroscopy thus provides insights into electron distributions and neighboring atomic environments.

1. EPR
Electron Paramagnetic Resonance Spectroscopic(EPR).
by : Masresha Amare
PhD candidate in AAU.
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2. Outline
Introduction
Principle of EPR
The Zeeman effect
The Hyperfine EffectThe Hyperfine Effect
Super- hyperfine splitting
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3. Introduction
 Electron Spin Resonance Spectroscopy
 Also called EPR Spectroscopy
 Electron Paramagnetic Resonance Spectroscopy
 Non-destructive technique
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4. What compounds can you analyze?
 Applicable for species with one or more unpaired electrons
 Free radicals
 Transition metal compounds
 Useful for unstable paramagnetic compounds generated in
situsitu
 Electrochemical oxidation or reduction
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5. Energy of Transitions
ESR measures the transition between the electron
spin energy levels
 Transition induced by the appropriate frequency
radiationradiation
Required frequency of radiation dependent upon
strength of magnetic field
 Common field strength 0.34 and 1.24 T
 9.5 and 35 GHz
 Microwave region
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6. Energy of Transitions
The absorption of energy causes a transition of an
electron from a lower energy state to a higher energy
state.
 In EPR spectroscopy the radiation used is in the In EPR spectroscopy the radiation used is in the
gigahertz range.
 Unlike most traditional spectroscopy techniques, in
EPR spectroscopy the frequency of the radiation is
held constant while the magnetic field is varied in
order to obtain an absorption spectrum.
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7. How does the spectrometer work?
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8. How does the spectrometer work?
The radiation source usually used is called a klystron
 They are high power microwave sources which have
low-noise characteristics and thus give high
sensitivity
A majority of EPR spectrometers operate at
approximately 9.5 GHz, which corresponds to about
32 mm ( X-band)
The radiation may be incident on the sample
continuously or pulsed
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9. How does the spectrometer work?
The sample is placed in a resonant cavity which
admits microwaves through an iris.
 The cavity is located in the middle of an
electromagnet and helps to amplify the weak signals
from the sample.from the sample.
Numerous types of solid-state diodes are sensitive to
microwave energy
Absorption lines are detected when the separation of
the energy levels is equal to the energy of the
incident microwave.
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10. What causes the energy levels?
Resulting energy levels of an electron in a magnetic
field
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11. What causes the energy levels?
When an electron is placed within an applied
magnetic field, Bo, the two possible spin states of the
electron have different energies (Zeeman effect)
 The lower energy state occurs when the magnetic
moment of the electron is aligned with the magneticmoment of the electron is aligned with the magnetic
field.
The two states are labeled by the projection of the
electron spin, MS, on the direction of the magnetic
field, where MS = -1/2 is parallel and MS = +1/2 is anti
parallel state
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12. Describing the energy levels
Based upon the spin of an electron and its associated
magnetic moment
For a molecule with one unpaired electron
 In the presence of a magnetic field, the two
electron spin energy levels areelectron spin energy levels are
E = gBB0MS
g = proportionality factor B = Bohr magneton
MS = electron spin B0 = Magnetic field
quantum number
(+½ or -½)
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13. The Zeeman Effect
 EPR spectroscopy are due to the interaction of unpaired electrons in
the sample with a magnetic field produced by the external magnet field
B0. This effect is called the Zeeman Effect.
the energies for an electron with ms = +½ and ms = -½ are
E1/2 = 1/2geB0 E1/2 = -1/2geB0
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14. The Zeeman Effect
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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
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16. Spectra
To get finer informa on ∂A/∂H is plo ed
against H to get the first derivative curve.
When phase-sensitive detection is used, the
signal is the first derivative of the absorptionsignal is the first derivative of the absorption
intensity
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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 = gmBB0MS + aMsmI
a = hyperfine coupling constant
mI = 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, n is 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
 12C, 28Si, 56Fe, …
Isotopes with odd atomic number and even mass
number have n even
 2H, 10B, 14N, …
Isotopes with odd mass number have n odd
 1H, 13C, 19F, 55Mn, …
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19. Hyperfine Interaction
Interaction with a single nucleus of spin ½
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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
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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
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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
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23. Relative Intensities for I = ½
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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
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25. Intensities
EPR spectrum of vanadyl acetylacetonate
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26. Hyperfine Interactions
Example:
 Radical anion of benzene [C6H6]-
 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
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27. Hyperfine Interactions
EPR spectrum of benzene radical anion
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28. Superhyperfine splitting
If the odd, unpaired electron spends time around
multiple sets of equivalent nuclei, additional
splitting is observed: 2nI + 1; this is called
“superhyperfine splitting.”
ESR spectrum of CH2OH Radical
H.Fischer etal.
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29. superhyperfine splitting
Example:
 Pyrazine anion
 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
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30. superhyperfine splitting
EPR spectrum of pyrazine radical anion
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31. superhyperfine splitting
ESR Spectrum of (CF3)2NO
Chem.Soc 88: 371 (1966)
ESR Spectrum of tetra-azanaphthalene anion
Chem.acta. 48:112 (1965)
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32. superhyperfine splitting
ESR spectrum of the cyclopentadienyl radical
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
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34. THANK YOUTHANK YOU
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