Electron Spin Resonance
Spectroscopy
or
It’s fun to flip electrons!
Electron Paramagnetic Resonance spectroscopy
Electron Spin Resonance spectroscopy
Principles of EMR spectroscopy
Classical theory:
Electron spin moment interacts with
applied electromagnetic radiation
DE
B0
hn
Quantum theory:
1
ms = —
2
Energy
transitions between energy levels
induced by magnetic field
Resonance condition
hn = gmBB0
1
ms = -—
2
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”
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.
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
No hyperfine
Hyperfine coupling
1H)
If the electron is surrounded by n spinactive nuclei with a spin quantum
number of I, then a (2nI+1) line pattern
will be observed in a similar way to
NMR.
14N)
In the case of the hydrogen atom (I= ½),
this would be 2(1)(½) + 1 = 2 lines.
2 identical I=1/2 nuclei
1 I=5/2 nucleus17
( O)
10 Gauss
Some nuclei with spins
Element
Isotope
Nuclear
spin
No of
lines
%
abundance
Hydrogen
Nitrogen
1H
½
1
2
3
99.985
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
30.9
95Mo
5/2
6
15.7
97Mo
5/2
6
9.46
Molybdenum
14N
Hyperfine splittings multiply with
the number of nuclear spins
.
O
Benzoquinone anion radical:
H
H
H
H
1 proton – splits into 2 lines 1:1
2 protons split into 3 lines 1:2:1
3 protons split into 4 lines 1:3:3:1
4 protons split into 5 lines 1:4:6:4:1
At higher temperature:
faster motion - sharper lines
shorter lifetime - smaller signal
-
O
-60 C
20 C
2.5
1.5
21
dA/dB
A
0.5
1.5
0
1
-0.5
0.5
-1
0
-1.5
2900
2900
3000
3000
3100
3100
3200
3200
3300
3300
Gauss
Gauss
3400
3400
3500
3500
3600
3600
3700
3700
Prushan Example
+
S
S
Cu
N
N
O
O
B
F
F
[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).
2nI+1
2x2x1+1
Diagram of an ESR spectrometer
Circulator
Detector
Klystron
Microwave source
Cavity
N
S
cryostat
Spectrophotometer
Detector
Light source
Hyperfine Splitting
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
vanadyl [V=O]2+ complex; I (V) = 7/2; 2(7/2) + 1 = 8 peaks
Signal Intensities
Follow Pascal's triangle
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
Superhyperfine Splitting
Septet
Examples:
Sextet
Superhyperfine splitting is direct
evidence for COVALENCY!
Octet
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.
Septet: hyperfine splitting.
IF= ½, so 2(6)(1/2) + 1 =7
Triplet: superhyperfine splitting.IN= 1, so
2(1)(1) + 1 = 3
So, spending most time on F’s, less on N.
Nonet: hyperfine splitting.
IN= 1, so 2(4)(1) + 1 =9
Pentet: superhyperfine splitting.
IH= 1/2, so 2(4)(1/2) + 1 = 5
So, spending most time on N’s, less on H.
overlapping pentet
of pentets.
Superhyperfine coupling
High-field high-frequency EPR
Microwave frequency
X-band
0.33
Q-band
W-band
1.25
3.5
D-band
4.9
Tesla
Bo
Superhyperfine interactions become more pronounced!
Anisotropic Interactions: The g-tensor
The free electron has a g-value of ge=2.0023
There 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.
Axial symmetry
g|| = 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.
The spin-orbit coupling gives a g  < g || = ge
This effect is similar to shielding in the NMR experiment.
z
g ||
gz
Bo
gx
B B BB
hn  g||  H ||
g|| 
hn
H||
B B BB
B
Bo
z
B B BB
B B BB
gy
g 
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 ||.
g
g ||
g
hn
H 
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).