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Electron Spin Resonance(ESR) or Electron Paramagnetic Resononce (EPR)

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.

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Electron Spin Resonance(ESR) or Electron Paramagnetic Resononce (EPR)

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  1. Electron Spin Resonance(ESR) or Electron Paramagnetic Resononce(EPR) Applied Quantum Chemistry 20131028 HochanJeong 1944 - discovery of the EPR effect E.K. Zavoisky

  2. Electron Paramagnetic Resonance Spectroscopy • this technique can only be applied to samples having one or more unpaired 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

  3. Electron Paramagnetic Resonance Spectroscopy • ESR measures the transition between the electron spin energy 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

  4. Spectrometer microwave ~ 9.5 GHz, which corresponds to about 32 mm. The cavity is located in the middle of an electromagnet and helps to amplify the weak signals from the sample.

  5. Energy level, Energy transition E = g μB BoMs Antiparallel g = proportionality constant Ms = (+1/2 or –1/2) Bo = Magnetic Field parallel μB = Bohr Magneton When an electron is placed within an applied magnetic field, Bo, the two possible spin states of the electron have different energies. hn= g μB Bo

  6. Proportionality Factor(g) hn= g μB Bo • For a free electron • 2.00232 • For organic radicals • 1.99-2.01 • For transition metal compounds • Large variations due to spin-orbit coupling and zero-field splitting : 1.4-3.0 an EPR spectrum is obtained by holding the frequency of radiation constant and varying the magnetic field.

  7. Hyperfine Interactions (interaction between the electron and the nuclei) the nuclei of the atoms in a molecule or complex have a magnetic moment, which produces a local magnetic field at the electron. E = gmBB0MS + aMsmI a = hyperfine coupling constant mI= nuclear spin quantum number A single nucleus with S =1/2 will split each electron energy level into 2 more levels The energy distance between levels is the hyperfine coupling constant

  8. No hyperfine 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. 1 H) 14 N) 2 identical I=1/2 nuclei 17 1 I=5/2 nucleus ( O)

  9. Relative Intensities for I = ½

  10. Relative Intensities for I = 1

  11. 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

  12. 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

  13. Conclusions • 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 unpaired electrons are delocalized

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