Electron paramagnetic resonance hyperfine splitting

R.C. Bray, F.M. Pick, and D. Samuel, Oxygen-17 hyperfine splitting in the electron paramagnetic resonance spectrum of enzymically generated superoxide. Eur. J. Biochem. 15, 352-355 (1970). 

Electron paramagnetic resonance (EPR) yields the location of unpaired electron density from hyperfine splitting by metals or atoms with nuclear spin.21 The S = 0 Fe(III)—O 2 state of oxy-Mb or Hb would be indicated by the absence of an EPR signal, although other results such as the IR or resonance Raman absorption of the O2 moiety would be needed for positive confirmation. 

Electron paramagnetic resonance (EPR) spectra are also discussed by Valentine et al. in reference 21. The splitting of the gy resonance is due to hyperfine coupling between the unpaired electron on Cu(II) and the nuclear spin of the copper nucleus... 

Tanner, S. J., Bray, R. C., and Bergmann, F., 1978, C hyperfine splitting of some molybdenum electron paramagnetic resonance signals of xanthine oxidase, Biochem. Soc. Trans. 6 1328nl330. 

Kohler J, Brouwer A C J, Groenen E J J and Schmidt J 1995 Single molecule electron paramagnetic resonance spectroscopy hyperfine splitting owing to a single nucleus Science 268 1457-60... 

ENDOR = electron nuclear double resonance EPR = electron-paramagnetic resonance ESR = electron-spin resonance NMR = nuclear magnetic resonance MA = modulation amplitude SOFT = second-order perturbation theory s-o = spin-orbit zfs = zero-field splitting (for S > 1/2) D = uniaxial zfs E = rhombic zfs g =. g-factor with principal components g, gy, and g ge = free electron g-factor a = hyperfine splitting constant A = hyperfine coupling constant for a given nucleus N (nuclear spin / > 0). 

Fig. 39. EPR (electron paramagnetic resonance) spectra of above N C6o centre N Qi (COOEt)2 together with below a simulation. The triplet splitting (above) is due to the isotropic hyperfine interaction of the electron systems with the nuclear spin Z = 1 of (natural abundance 99.6 %). Since the electronic spin is S = 3/2 (three unpaired electrons), each of the lines is three-fold degenerate. The occurrence of this degeneracy implies that the fine structure, quadrupole interaction and anisotropic hyperfine interaction are zero (complete spherical symmetry of nitrogen). In the adduct N C6i(COOEt)2 the icosahedral cage symmetry and therefore the degeneracy of nitrogen p orbitals is broken giving rise to new lines (centre). The simulation (below) is performed with the hyperfine interaction and g factor of N Cgo but in addition a fine structure interaction (D =2 G and E = 0.13 G) is included. The effect of the deviation from spherical symmetry on the quadrupole or anisotropic hyperfine interaction is too small to be detected...

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