文档介绍:Introduction to Electron Spin Resonance and Spin Trapping
Michael R. Gunther
West Virginia University School of Medicine
Free Radicals and EPR
Molecules with one or more unpaired electron
Quantum mechanics: unpaired electrons have spin and charge and hence magnetic moment
Electronic spin can be in either of two directions (formally up or down)
The two spin states under normal conditions are energetically degenerate
Energetic degeneracy lost when exposed to an external magnetic field
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 EPR spectrometer
Electromagnet
Microwave source and detector (typically X band, ~ GHz)
Modulation of magnetic field and phase-sensitive detection
Spectrum 1st derivative
Weil, Bolton, and Wertz, 1994, “Electron Paramagnetic Resonance”
The EPR spectrum
A 1st derivative spectrum is obtained from the unpaired electron
hn = gBb0
g is a characteristic of the chemical environment of the unpaired electron; for free radicals it is near ; can vary widely for transition metal centers
Complicated/enhanced by hyperfine interactions with nuclei with non-zero spin
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
Hyperfine splittings are additive