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Chem 325
Electron Spin Resonance
Spectroscopy
(a.k.a. Electron Paramagnetic
Resonance Spectroscopy)
• Electrons: spinning, charged particles
• ‘stable’ atoms, molecules: all e- are spin-paired.
• Net spin S = 0
• Some chemical species have UNPAIRED electrons
• Atoms H, Na, transition metals
• Recall Stern-Gerlach experiment (s = ±±±± ½)
• Molecules: O2, NO
• Reactive intermediates: radicals (‘free radicals’)
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Formation of radicals
– Oxidation
M →→→→ M••••+ + e
– Reduction
M + e →→→→ M••••-
– Homolytic cleavage of bonds
R-H →→→→ R•••• + ••••H
R-R′′′′ →→→→ R•••• + ••••R′′′′
thermally, chemically, photolytically
Radicals
– Short-lived (“reactive intermediates”)
– Low concentrations
– Contain at least one unpaired electron
– Can be studied by ESR spectroscopy to give
structural information
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EPR or ESR Spectroscopy
Electron spins behave in a similar way to nuclear spins
when in a magnetic field
The electron spin magnetization (if there is any S>0) can be ‘flipped’ using EM radiation
∆E = ge µBB0
ge = 2.002319 for a free electron,
kind of chemical shift forthe electron
B0 = applied magnetic field
µB = Bohr magneton
= 9.273 ×××× 10-24 J T-1
Typical ESR magnetic field strength of 0.3400 T or 3400 G
∆E = ge µBB0 = 9.5 ×××× 109 Hz or 9.5 GHz (MW region)
Recall for 1H NMR, 60 – 900 MHz
Typical frequencies/energies of ESR transitions are much higher than those for NMR transitions
Boltzmann: = 0.9985 (about 200X more than NMR)
lower
upper
N
N
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- much bigger population difference between states
- much faster relaxation rates
- much larger transition energies/frequencies
Thus ESR is much more SENSITIVE than NMR
- Good! We are trying to observe reactive intermediates, very
low, transient, concentrations!
- ESR: down to 10-10 to 10-14 M
However, due to very short lifetimes of excited states
(ττττ < 10-6 s), by Heisenberg linewidths are 106 to 107 Hz
(cf. 1H NMR linewidths: ca. 0.1 Hz!)
Thus very broad lines, tend to overlap.
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ESR Spectra
Derivative curve
ESR Spectra
Normal ESR mode: peak position as a function
of magnetic field strength
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ESR Instrumentation
CW but now all FT types
Radicals
Steady-state flow cells or trapping in inert matrix
Increased sensitivity means less
need of big magnet ($$$) cf. NMR
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H atom ESR Spectrum
2 absorptions!
e spin coupled to H
nuclear spin (I = ½)
hyperfine coupling
constant aH
aH = 500 G or 50 mT
Magnitude of aH proportional to electron density at nucleus
H••••
Methyl radical ESR Spectrum
••••CH3
Four peaks
e coupled to 3 equivalent H nuclei
quartet structure
aH = 23 G or 2.3 mT
aH much smaller for methyl than for H radical!
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Ethyl radical ESR Spectrum
••••CH2CH3Second Derivative
Ethyl radical ESR Spectrum
••••CH2CH3
Coupling with CH2 will produce a triplet
Coupling with CH3 will produce a quartet
Case 1. aCH2 > aCH3would produce a triplet of quartets (12 lines)
Case 2. aCH2 < aCH3would produce a quartet of triplets (12 lines)
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Ethyl radical ESR Spectrum
••••CH2CH3
12 lines
•
•
•
•
•
•
•
•
•
••
•
Overlapping quartet of triplets
aCH3 = 2.69 mT aCH2 = 2.24 mT
Ethyl radical ESR Spectrum
Overlapping quartet of triplets
aCH3 = 2.69 mT aCH2 = 2.24 mT
••••CH2CH3
Magnitude of a proportional to unpaired e density on coupled nucleus
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Tree or branching diagram
Coupling with 4
equivalent nuclei of I = ½
quintet
Benzene radical anion
CH
CH
CH
CH
CH
CH
•-
Septet aH = 3.75 G = 375 mT
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1-hydroxyethyl radical
CH3
C OH
H
•
quartet of doublets aCH3 = 22.0 G aCH = 15.0 G
Acetamide radical
CC
O
N
H
H
H
H
•
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Benzyl radical
CH2
•
Benzyl radical
CH2
CH2
CH2
CH2
• •
• •
aCH2 > aCH(p) > aCH(o) > aCH(m)
Triplet of doublets of triplets of triplets
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Benzyl radical
CH2
•
Naphthalene radical anion
- •
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ESR/EPR Spectroscopy
Uses
- detection of radicals
- structure of radicals
- stability of radicals (reactive intermediates)
- rates of appearance and disappearance
- chemical kinetics
- chemistry, biology, food science, medicine, polymers
