electron spin resonance(esr) or electron paramagnetic resononce (epr)
DESCRIPTION
Electron Spin Resonance(ESR) or Electron Paramagnetic Resononce (EPR). Applied Quantum Chemistry 20131028 Hochan Jeong. 1944 - discovery of the EPR effect E.K. Zavoisky. Electron Paramagnetic Resonance Spectroscopy. - PowerPoint PPT PresentationTRANSCRIPT
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Electron Spin Resonance(ESR) or Electron Paramagnetic
Resononce(EPR)
Applied Quantum Chemistry20131028 Hochan Jeong
1944 - discovery of the EPR effect E.K. Zavoisky
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Electron Paramagnetic Resonance Spectroscopy
• this technique can only be applied to samples having one or more un-paired electrons.
– Free radicals– Transition metal compounds
• unpaired electrons have spin and charge and hence magnetic moment
Two spin states are degenerate. -> However, if external field exists
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• ESR measures the transition between the electron spin en-ergy levels– Transition induced by the appropriate frequency radiation
• Required frequency of radiation dependent upon strength of magnetic field– Common field strength 0.34 and 1.24 T– 9.5 and 35 GHz– Microwave region
Electron Paramagnetic Resonance Spectroscopy
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Spectrometer
microwave ~ 9.5 GHz, which corresponds to about 32 mm.
The cavity is located in the middle of an electro-magnet and helps to amplify the weak signals from the sample.
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Energy level, Energy transition
When an electron is placed within an applied magnetic field, Bo, the two possible spin states of the electron have different energies.
parallel
AntiparallelE = g μB Bo Ms
g = proportionality constant
hn = g μB Bo
Ms = (+1/2 or –1/2)
Bo = Magnetic Field
μB = Bohr Magneton
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Proportionality Factor(g)
hn = g μB Bo
For a free electron 2.00232
For organic radicals1.99-2.01
For transition metal compoundsLarge variations due to spin-orbit coupling and zero-field splitting : 1.4-3.0
an EPR spectrum is obtained by holding the fre-quency of radiation constant and varying the mag-netic field.
MoO(SCN)52- 1.935
VO(acac)2 1.968
e- 2.0023
CH3 2.0026
C14H10 (anthracene) cation 2.0028
C14H10 (anthracene) anion 2.0029
Cu(acac)2 2.13
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Hyperfine Interactions
the nuclei of the atoms in a molecule or complex have a magnetic mo-ment, which produces a local magnetic field at the electron.
(interaction between the electron and the nuclei)
E = gmBB0MS + aMsmIa = hyperfine coupling constantmI= nuclear spin quantum number
A single nucleus with S =1/2 will split each electron energy level into 2 more levelsThe energy distance between levels is the hyperfine coupling constant
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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 num-ber 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.
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Relative Intensities for I = ½
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Relative Intensities for I = 1
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• Example:– Radical anion of benzene [C6H6]-
– Electron is delocalized over all six carbon atoms• Exhibits coupling to six equivalent hydrogen atoms
– So,2NI + 1 = 2(6)(1/2) + 1 = 7
– So spectrum should be seven lines with relative intensities 1:6:15:20:15:6:1
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• Example:– Pyrazine anion– Electron delocalized over ring
• Exhibits coupling to two equivalent N (I = 1)2NI + 1 = 2(2)(1) + 1 = 5
• Then couples to four equivalent H (I = ½)2NI + 1 = 2(4)(1/2) + 1 = 5
– So spectrum should be a quintet with intensities 1:2:3:2:1 and each of those lines should be split into quintets with intensities 1:4:6:4:1
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• Analysis of paramagnetic compounds– Compliment to NMR
• Examination of proportionality factors– Indicate location of unpaired electron
• Examination of hyperfine interactions– Provides information on number and type
of nuclei coupled to the electrons– Indicates the extent to which the un-
paired electrons are delocalized
Conclusions