Nuclear hyperfine interactions, electron paramagnetic resonance

Electron-nuclear double resonance (ENDOR) spectroscopy A magnetic resonance spectroscopic technique for the determination of hyperfine interactions between electrons and nuclear spins. There are two principal techniques. In continuous-wave ENDOR the intensity of an electron paramagnetic resonance signal, partially saturated with microwave power, is measured as radio frequency is applied. In pulsed ENDOR the radio frequency is applied as pulses and the EPR signal is detected as a spin-echo. In each case an enhancement of the EPR signal is observed when the radiofrequency is in resonance with the coupled nuclei. 

Electron nuclear double resonance (ENDOR) and electron spin-echo envelope modulation (ESEEM) are two of a variety of pulsed EPR techniques that are used to study paramagnetic metal centers in metalloenzymes. The techniques are discussed in Chapter 4 of reference la and will not be discussed in any detail here. The techniques can define electron-nuclear hyperfine interactions too small to be resolved within the natural width of the EPR line. For instance, as a paramagnetic transition metal center in a metalloprotein interacts with magnetic nuclei such as H, H, P, or these... 

The nuclear hyperfine interaction splits the paramagnetic states of an electron when it is close to a nucleus with a magnetic moment. For a random orientation of spins and nuclei, the tensor quantities in Eq. (4.11) are replaced by scalar distributions, and the resonance magnetic field is shifted from the Zeeman field // by... 

DPPH = 2,2-diphenyl-1-picrylhydrazyl ENDOR= electron-nuclear double resonance EPR = electron paramagnetic resonance ESE = electron spin echoes ESEEM = electron spin echo envelope modulation EFT = fast fourier transformations FWHM = fidl width at half maximum HYSCORE = hyperfine sublevel correlation nqi = nuclear quadrupole interaction TauD = taurme/aKG dioxygenase TWTA = traveling wave tube amphfier ZFS = zero field sphtting. 

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...

A paramagnetic atom with Td symmetry should give only one resonance line, but when this atom has a nuclear spin, the electron and nuclear spins can couple by hyperfine interaction, and for a nuclear spin I, each electronic spin component splits into 21+1 components giving the same number of Am/ = 0 resonances. For instance, the ESR spectrum of tetrahedral interstitial Al I = 5/2) produced by electron irradiation of AZ-doped silicon is an isotropic sextuplet due to transitions between the six nuclear sublevels of each electronic-spin component ([54], and references therein). The electron spin of a centre can also interact with the nuclear spins of neighbouring atoms to give additional structures and this is clearly shown for 29 Si atoms (I = 1/2) in Fig. 4 of [54]. The ESR spectrum can thus also determine the atomic structure of the centre. This can also occur for non-cubic centres and the hyperfine structure is superimposed on the orientational structure. 

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