Effect of magnetic coupling

Chapters 5 and 6 present some applications. In the former, the electron relaxation properties of various metal ions are reviewed and the consequent nuclear relaxation properties discussed in more detail. An analysis of the shifts is presented for some cases in which a connection with structural features of the metal-ligand moiety has been established. In Chapter 6, the effects of magnetic coupling on the shifts and relaxation are presented theoretically and examples are given. 

The theoretical approach to the understanding of the NMR parameters in paramagnetic exchange coupled dimers is discussed and simplified formulas are provided. The effect of magnetic coupling on electron relaxation times in magnetic coupled heterodimetallic systems is also discussed. The H NMR spectra of two oxidized and reduced Fe2S2-fenedoxins are interpreted and structural information is obtained. 

Equation (17) lacks experimental verification. Qualitatively, it would predict the effect of magnetic coupling on Xj to be observable when J/h vxJ OjXj" ... 

When the number of atoms increases, the calculations become more complex but the general philosophy still holds. Once the energy levels are calculated with their S values and wavefunctions, the Cjj coefficients, which represent the ratio between i and i, can be evaluated and used to compute the shifts and their temperature dependence with equation 16. Analogously, the Qj coeflScients squared can be used for the analysis of nuclear relaxation. Of eourse, also the analysis of the effects of magnetic coupling on the electronic relaxation rates of the single ions become more difficult. An analysis of the NMR spectra of a C04S11 cluster is available [37],... 

Problems in the effects of magnetic coupling are reviewed in Magnetochemistry a research proposal R. L. Carlin, Coord. Chem. Rev. (1987) 79, 215. 

Since counterclockwise rotation leads from the — / axis to the +x/ axis, the x component has a plus sign. Now consider the effect of the coupling /ab. The pure xf and y magnetization will rotate into and out of the antiphase condition with angular frequency 1rJ ... 

The rephasing effect of a 180° pulse not only removes the effects of magnetic field inhomogeneities, it also rephases signals from nuclei that differ in precession frequency because of differing chemical shifts, as illustrated in Fig. 9.1. However, if the two magnetizations differ in frequency because of scalar coupling between them, the situation is more complex. 

As a second example, we look at echoes. We saw in Chapter 9 that a 180° pulse refocuses not only chemical shifts and the effects of magnetic field inhomogeneity but also spin coupling provided that the pulse does not also disturb the spin state of the coupled nucleus (see Fig. 9.2) However, in a homonuclear spin system a nonselective pulse does effect spin states. We found in Chapter 7 that dipolar interactions have the same mathematical from as indirect spin coupling, and it is known that a 180° pulse does not produce an echo in a solid because spin states are disturbed. However, it is possible to obtain a solid echo or dipolar echo by applying the pulse sequence 90, T, 90r It is very difficult to rationalize an echo from... 

Fig. 3.5. Effect of hyperfine coupling to a single nucleus with I = 1/2 in the high-field region. At low magnetic fields, divergence is no longer linear and due allowance for this must be made. The ESR active transitions occur when only the electron-spin flips, so that only two transitions are indicated, separated by the hyperfine splitting.

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