Electron spin resonance (ESR) spectroscopy detects transitions between spin energy levels of unpaired electrons using microwave radiation. When an unpaired electron is near a nucleus with non-zero spin, the electron experiences a magnetic field from the nucleus that splits the ESR signal into multiple lines based on the nuclear spin. This splitting is called hyperfine coupling and provides information about electronic structure. Superhyperfine splitting occurs when the electron interacts with multiple equivalent nuclei and results in even finer splitting patterns. Anisotropic interactions like the g-tensor can also be observed in ESR and provide information about electronic environments.

1. Electron Spin Resonance
Spectroscopy
or
It’s fun to flip electrons!

2. Electron Paramagnetic Resonance spectroscopy
Electron Spin Resonance spectroscopy

3. Principles of EMR spectroscopy
B 0
∆E
hν
Classical theory:
Electron spin moment interacts with
applied electromagnetic radiation
m s = —
1
2
m s
= —
1
2
-
Energy
Quantum theory:
transitions between energy levels
induced by magnetic field
Resonance condition
hν = gµBB0

5. 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”

6. 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.

7. 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

8. 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.

9. Some nuclei with spins
Element Isotope Nuclear No of %
spin lines abundance
Hydrogen 1
H ½ 2 99.985
Nitrogen 14
N 1 3 99.63
15
N ½ 2 0.37
Vanadium 51
V 7/2 8 99.76
Manganese 55
Mn 5/2 6 100
Iron 57
Fe ½ 2 2.19
Cobalt 59
Co 7/2 8 100
Nickel 61
Ni 3/2 4 1.134
Copper 63
Cu 3/2 4 69.1
65
Cu 3/2 4 30.9
Molybdenum 95
Mo 5/2 6 15.7
97
Mo 5/2 6 9.46

10. Hyperfine splittings multiply with
the number of nuclear spins
O
.
O-
H
H
H
H
Benzoquinone anion radical:
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
-60 °C
20 °C
At higher temperature:
faster motion - sharper lines
shorter lifetime - smaller signal

12. 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/dB

13. Prushan Example
SS
N N
OO
B
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).

14. 2nI+1
2x2x1+1

15. N S
Klystron
Microwave source
Detector
Cavity
cryostat
Circulator
Diagram of an ESR spectrometer
Spectrophotometer
Light source
Detector

16. 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

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

18. Signal Intensities
Follow Pascal's triangle

19. 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

20. Superhyperfine Splitting
Examples:
Sextet
Septet
Octet
Superhyperfine splitting is direct
evidence for COVALENCY!

21. 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.

22. 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.

23. Superhyperfine coupling
overlapping pentet
of pentets.

24. 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!

25. 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.

26. 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.
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 BB
B B BB
BBBB
BBBB
g ||
g ||
g ⊥
g ⊥
|||| Hgh βν =
||
||
H
h
g
β
ν
=
⊥
⊥ =
H
h
g
β
ν
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

27. 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).