Magnetic electron-nucleus double

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 gy, and g ge = free electron g-factor a = hyperfrne splitting constant A = hyperftne coupling constant for a given nucleus N (nuclear spin 7>0). 

There are also pulse EPR methods that probe the chemical or rather magnetic environment. These are pulse electron nuclear double resonance (ENDOR) and hyperfine sublevel correlation (HYSCORE) spectroscopy, which allow measuring hyperfine couplings from the unpaired electron spin to surrounding magnetically active nuclei ([20] in Fig. 3 this is a P nucleus). As these experiments are performed in frozen solution (e.g., in all examples of this chapter) or in solids, from the anisotropy and orientation dependence of the hyperfine coupling one can obtain valuable information on the structure up to 1 nm. 

ENDOR, Electron Nuclear Double Resonance, is a combination of electron- and nuclear magnetic resonance developed in 1956. The method was apparently originally intended for applications in physics to achieve nuclear polarisation [6], e.g. to obtain more nuclei with wj/ = -Vi than with nij = +Vi. But its main application became to resolve hyperfine structure that is unresolved in regular ESR in liquid and solid samples. Two lines appear in the ENDOR spectmm of a radical (5 = Vi), containing a single nucleus, one corresponding to the ms = +Vi the other to the ms = -Vi electronic level. 

Pulsed ELDOR. Distances between electron spins can be measured by double electron-electron resonance (DEER) experiments such as the four-pulse experiment illustrated in Figure 6 (34). Similar to measurements of electron-nucleus distances, this technique is based on the r dependence of the magnetic dipole interaction between electron spins and can determine larger distances, in the range 1.5-5 nm. One of two spins (color-coded green, observer) is observed by a refocused primary echo with fixed interpulse delays ti and t2, so that relaxation does not induce variations in the echo amplitude during the experiment. The second spin (color-coded red, pumped) imposes a local dipole field at the site of the first spin, with a magnitude that depends on the distance. At a variable delay t with respect... 

Figure 8.2 The particular effects influences on the energy state of an atom (a) a stripped nucieus, (b) the chemical shift (refer to Section 8.2.3), (c) the fine electron-nucleus interaction (refer to Section 7.5.8), (d) and (e) an atom in an external magnetic field (in (d) influence of both quadrupole and magnetic splitting is very complicate), (e) the inflnence of the magnetic field produced by the electron shell on the nucleus s magnetic moment position (nuclear Zeeman effect, see Section 7.7). The shift of the spectral line relative cUq is presented in the lower part of the Figure. The energy transitions scale is not followed. The long arrows represent FR transitions, the double arrow in the figure represent the NMR transitions.

The shielding at a given nucleus arises from the virtually instantaneous response of the nearby electrons to the magnetic field. It therefore fluctuates rapidly as the molecule rotates, vibrates and interacts with solvent molecules. The changes of shift widi rotation can be large, particularly when double bonds are present. For... 

The electrons modify the magnetic field experienced by the nucleus. Chemical shift is caused by simultaneous interactions of a nucleus with surrounding electrons and of the electrons with the static magnetic field B0. The latter induces, via electronic polarization and circulation, a secondary local magnetic field which opposes B0 and therefore shields the nucleus under observation. Considering the nature of distribution of electrons in molecules, particularly in double bonds, it is apparent that this shielding will be spatially anisotropic. This effect is known as chemical shift anisotropy. The chemical shift interaction is described by the Hamiltonian... 

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