Manipulation of Spin Interactions

The rich information content of solid-state spectra makes them difficult to evaluate, in particular if more than one of the interactions discussed previously has to be taken into account. The evaluation of the spectra would often be impossible but NMR methodology provides the possibility to decouple and recouple spin interactions nearly as desired (see [11] for a comprehensive introduction). Moreover, these different information sources can be separated and correlated using the multidimensional techniques discussed subsequently. 

Often it is necessary to manipulate the spin interactions also by pulse techniques [1,9,11]. These act on the spin operators in the corresponding Hamiltonians (see for example Equation 14.2) rather than on the geometric part. Depending on the applied pulse 

Undesired homonuclear spin interactions can be also suppressed using suitable multiple-pulse sequences while still exploiting the information content provided by interactions that are not affected. Using a combination of MAS and pulse decoupling it is even possible to reintroduce parts of an interaction that would be averaged out by one of the manipulation techniques alone ( recoupling ) [11]. This high flexibility of solid-state NMR enables one to fully exploit the rich information content provided by the spin interactions. It becomes particularly powerful if such experiments are combined to multidimensional NMR techniques as discussed in Section 14.3. 

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Originally designed with the aim of ehminating internuclear interactions, MAS spectroscopy was concerned with a systematic study of cross-polarization between abundant and rare spin systems. Subsequent developments have introduced many sophisticated manipulations of spin interactions to unravel the structure and dynamics of sohd materials. The latest advances described in this contribution illustrate the new methods of tackhng the problems of selectivity, sensitivity and spectral resolution. 

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