-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!
Electron Paramagnetic Resonance spectroscopy
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
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
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.
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
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
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
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).
2nI+1
2x2x1+1
N S
KlystronMicrowave source
Detector
Cavity
cryostat
Circulator
Diagram of an ESR spectrometer
Spectrophotometer
Light source
Detector
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
Hyperfine Splitting
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 SplittingExamples:
Sextet
Septet
Octet
Superhyperfine splitting is direct evidence for COVALENCY!
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 =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.
Superhyperfine coupling
overlapping pentet of pentets.
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!
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.
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
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).