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Electron Spin Resonance Spectroscopy

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Electron Spin Resonance Spectroscopy. or It’s fun to flip electrons!. E lectron P aramagnetic R esonance spectroscopy. E lectron S pin R esonance spectroscopy. Classical theory: Electron spin moment interacts with applied electromagnetic radiation. D. E. B. 0. n. h. - PowerPoint PPT Presentation
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Electron Spin Resonance Spectroscopy or It’s fun to flip electrons!
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Page 1: Electron Spin Resonance Spectroscopy

-1.5

-1

-0.5

0

0.5

1

1.5

2900 3000 3100 3200 3300 3400 3500 3600 3700

Gauss

dA/d

B

Electron Spin Resonance Spectroscopy

orIt’s fun to flip electrons!

Page 2: Electron Spin Resonance Spectroscopy

Electron Paramagnetic Resonance spectroscopy

Electron Spin Resonance spectroscopy

Page 3: Electron Spin Resonance Spectroscopy

Principles of EMR spectroscopy

B 0E

h

Classical theory:Electron spin moment interacts with applied electromagnetic radiation

m s = —1

2

m s = —1

2-

Ene

rgy

Quantum theory:transitions between energy levelsinduced by magnetic field

Resonance conditionh = gBB0

Page 4: Electron Spin Resonance Spectroscopy
Page 5: Electron Spin Resonance Spectroscopy

The EPR experiment

• Put sample into experimental magnetic field (B)

• Irradiate (microwave frequencies)

• Measure absorbance of radiation as f(B)

Weil, Bolton, and Wertz, 1994, “Electron Paramagnetic Resonance”

Page 6: Electron Spin Resonance Spectroscopy

The hyperfine effect• The magnetic field experienced by the unpaired electron

is affected by nearby nuclei with non-zero nuclear spin

Weil, Bolton, and Wertz, 1994, “Electron Paramagnetic Resonance”, New York: Wiley Interscience.

Page 7: Electron Spin Resonance Spectroscopy

Hyperfine splitting of EPR spectra

• The magnitude of the splitting and the number of lines depend upon:– The nuclear spin of the interacting nucleus

• # of lines = 2n(I + ½) so I = ½ gives 2 lines, etc.– The nuclear gyromagnetic ratio– The magnitude of the interaction between the

electronic spin and the nuclear spin• Magnitude of the splitting typically decreases

greatly with increasing numbers of bonds between the nucleus and unpaired electron

Page 8: Electron Spin Resonance Spectroscopy

10 Gauss

No hyperfine

1H)

14N)

2 identical I=1/2 nuclei

1 I=5/2 nucleus (17O)

Hyperfine coupling

If the electron is surrounded by n spin-active nuclei with a spin quantum

number of I, then a (2nI+1) line pattern will be observed in a similar way to

NMR.

In the case of the hydrogen atom (I= ½), this would be 2(1)(½) + 1 = 2 lines.

Page 9: Electron Spin Resonance Spectroscopy

Some nuclei with spins

Element Isotope Nuclear No of % spin lines abundance

Hydrogen 1H ½ 2 99.985 Nitrogen 14N 1 3 99.63

15N ½ 2 0.37 Vanadium 51V 7/2 8 99.76 Manganese 55Mn 5/2 6 100 Iron 57Fe ½ 2 2.19 Cobalt 59Co 7/2 8 100 Nickel 61Ni 3/2 4 1.134 Copper 63Cu 3/2 4 69.1

65Cu 3/2 4 30.9Molybdenum 95Mo 5/2 6 15.7

97Mo 5/2 6 9.46 

Page 10: Electron Spin Resonance Spectroscopy

Hyperfine splittings multiply with the number of nuclear spins

O.

O-

H

H

H

HBenzoquinone anion radical:

1 proton – splits into 2 lines 1:12 protons split into 3 lines 1:2:13 protons split into 4 lines 1:3:3:14 protons split into 5 lines 1:4:6:4:1

-60 C

20 CAt higher temperature:faster motion - sharper linesshorter lifetime - smaller signal

Page 11: Electron Spin Resonance Spectroscopy
Page 12: Electron Spin Resonance Spectroscopy

0

0.5

1

1.5

2

2.5

2900 3000 3100 3200 3300 3400 3500 3600 3700

Gauss

A

-1.5

-1

-0.5

0

0.5

1

1.5

2900 3000 3100 3200 3300 3400 3500 3600 3700

Gauss

dA/d

B

Page 13: Electron Spin Resonance Spectroscopy

Prushan Example

SS

N N

OOB

FF

Cu

[Cu(Thyclops)]+

+

77 K Cryogenic ESR Spectrum of [Cu(Thyclops)]ClO4 in MeOH

Prushan, M. J.; Addison, A. W.; Butcher, R. J.; Thompson, L. K. “Copper(II) Complex Tetradentate Thioether-Oxime Ligands” Inorganica Chimica Acta, 358, 3449-3456 (2005).

Page 14: Electron Spin Resonance Spectroscopy

2nI+1

2x2x1+1

Page 15: Electron Spin Resonance Spectroscopy

N S

KlystronMicrowave source

Detector

Cavity

cryostat

Circulator

Diagram of an ESR spectrometer

Spectrophotometer

Light source

Detector

Page 16: Electron Spin Resonance Spectroscopy

If the odd, unpaired electron is associated with a nucleus with nuclear spin, can get coupling between the two spins and observe 2I+1 (I = nuclear spin) “peaks” or “valleys”.

Examples: di-t-butyl nitroxide radical; I(N) = 1;

Hyperfine Splitting

Page 17: Electron Spin Resonance Spectroscopy

vanadyl [V=O]2+ complex; I (V) = 7/2; 2(7/2) + 1 = 8 peaks

Hyperfine Splitting

Page 18: Electron Spin Resonance Spectroscopy

Signal Intensities

Follow Pascal's triangle

Page 19: Electron Spin Resonance Spectroscopy

superhyperfine splitting

carbon compound; I(C) = 0; 2(0) + 1 = 1 peak…. But:

If the odd, unpaired electron spends time around multiple sets of equivalent nuclei, additional splitting is observed: 2nI + 1; this is called “superhyperfine splitting.”

Examples:

Triplet Quartet Pentet

Page 20: Electron Spin Resonance Spectroscopy

Superhyperfine SplittingExamples:

Sextet

Septet

Octet

Superhyperfine splitting is direct evidence for COVALENCY!

Page 21: Electron Spin Resonance Spectroscopy

It is possible for the unpaired electron to spend differing amounts of time on different nuclei.

The greater the covalency, the greater is the hyperfine splitting.

Triplet: hyperfine splitting.Doublet: superhyperfine splitting. Interpretation: electron is spending most of its time on CH2 protons, but spending some time on –OH.

Pentet: hyperfine splitting.Pentet: superhyperfine splitting. Interpretation: electron is spending most of its time on one set of protons, but spending some time on other set.

Page 22: Electron Spin Resonance Spectroscopy

Septet: hyperfine splitting. IF= ½, so 2(6)(1/2) + 1 =7Triplet: superhyperfine splitting.IN= 1, so 2(1)(1) + 1 = 3So, spending most time on F’s, less on N.

Nonet: hyperfine splitting. IN= 1, so 2(4)(1) + 1 =9Pentet: superhyperfine splitting.IH= 1/2, so 2(4)(1/2) + 1 = 5So, spending most time on N’s, less on H.

Page 23: Electron Spin Resonance Spectroscopy

Superhyperfine coupling

overlapping pentet of pentets.

Page 24: Electron Spin Resonance Spectroscopy

High-field high-frequency EPR

X-band Q-band W-band D-band

0.33 1.25 3.5 4.9 Tesla

Bo

Microwave frequency

Superhyperfine interactions become more pronounced!

Page 25: Electron Spin Resonance Spectroscopy

Anisotropic Interactions: The g-tensorThe free electron has a g-value of ge=2.0023There may be spin-orbit coupling which will effect the ge

lets look at the simple case of Boron, 2p1.

If all the orbitals have same energy then the spin orbit coupling energy averages to zero over the x,y, and z coordinate.

However, if the atom is placed in a crystal which removes the degeneracy then the spin orbit coupling becomes asymmetric, px = py but do not equal to pz

Now the observed g-value will depend upon orientation of the crystal in the magnetic field.

Page 26: Electron Spin Resonance Spectroscopy

Axial symmetryg|| = gz and g = gx = gy

The g value tells you how strong the electron magnetic tensor is in a given direction.

Therefore if you orientate the crystal in a different direction the energy to resonate changes and thus the absorption will shift.

This effect is similar to shielding in the NMR experiment.

The spin-orbit coupling gives a g < g || = ge

B

gz

gy

gx B B BBB B BB

B B

BB

B B

BB

g ||

g ||

g

g

|||| Hgh

|||| H

hg

Hhg

What happens if the crystal is ground into a powder?All orientations are present however there are more chances that the g will be aligned with the field than g ||.

Bo

Bo

z

z

Page 27: Electron Spin Resonance Spectroscopy

ESR spectra of [Cu(MeTtoxBF2)]BF4 in 1:10 BuOH–DMF.(a) Room temperature (295 K) fluid spectrum (9.464 GHz). (b) 77 K cryogenic glass spectrum (9.147 GHz).

Prushan, M. J.; Addison, A. W.*; Butcher, R. J.; "Pentadentate Thioether Oxime Macrocyclic and Quasi-Macrocyclic Complexes of Copper(II) and Nickel(II)" Inorganica Chimica Acta, 300-302, 992-1003 (2000).


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