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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

••••CH2CH3

Second Derivative

Ethyl radical ESR Spectrum

••••CH2CH3

Coupling with CH2 will produce a triplet

Coupling with CH3 will produce a quartet

Case 1. aCH2 > aCH3

would produce a triplet of quartets (12 lines)

Case 2. aCH2 < aCH3

would 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


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