Local magnetic field dipolar coupling

The most common interactions are dipolar coupling between two spins, chemical shielding and quadrupolar interaction for spins I> 1. The chemical-shielding interaction arises from the motion of electrons around a nucleus induced by the external applied static magnetic field. This motion generates local magnetic fields that modify the total field experienced by the nucleus, and are characteristic of the local chemical environment of the nuclei. All nuclei with a spin /> 1 possess a quadrupolar moment Q that interacts with the EFG Vrs — d2 V/dXfdx at the nuclear site. The EFG tensor Vrs is a symmetric tensor with zero trace Tr( E) = A E = 0 (from Laplace equation). 

Figure 14 Dipolar coupling. The local magnetic field generated by a neighbor magnetic nucleus (H ) has different values and orientations at the position of the observed nucleus (H ) depending on the orientation of the molecule. Thus, in some cases the local field opposes (a) the external magnetic field Bo, whereas in other cases it adds to it (b).

We have already seen that the coupled relaxation transitions between the nuclei and electrons, which give rise to the Overhauser effect, are stimulated by fluctuations in the local magnetic fields at frequencies coj and co. The intensities of these fluctuations (denoted by where ft) is the appropriate frequency) depends critically on the correlation time for, and the nature of, the nuclear electron interactions, which may be either dipolar or scalar (see above). 

The broadening introduced by residual dipolar coupling can be estimated from the magnitude of the local magnetic field, H oc- This local field is... 

Before reviewing existing examples, a very brief explanation on the mechanisms of decoherence for molecular spin qubits is necessary more details are available elsewhere [67]. Broadly speaking, the three decoherence sources for these systems are spin bath decoherence, oscillator bath decoherence and pairwise dipolar decoherence, and can be regulated by a combination of temperature, magnetic field and chemical design of the system [70]. The spin bath mainly consists of nuclear spins, but in general it also includes any localized excitations that can couple to the... 

NMR imaging of solids is a technique that can be divided into two areas, depending upon the characteristics of the sample (7-6). For "soft" solids, the presence of local, rapid molecular motions reduces homonuclear dipolar coupling and averages anisotropic chemical shielding. Thus, the natural line width is narrow (long T2) and it is relatively easy to encode spatial information with a magnetic field... 

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