Interactions spin richness

Solomon noted also that a correct description of even a very simple system containing two interacting spins, requires introducing the concept of cross-relaxation, or magnetization exchange between the spins. The subsequent development has been rich and the goal of this entry is to provide a flavour of relaxation theory (section bl.l3.2T experimental techniques (section b 1.13.31 and applications (section b 1.13.4). 

From a relaxation point of view methyl groups are interesting moieties as they contain a rich network of interacting spins. In particular, cross-correlated relaxation between and H- H dipoles makes large contribution into... 

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. 

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. 

The nature of the donor site D depends on the type of oxide and its pretreatment temperature for pure oxides and, additionally, on the composition in the case of mixed oxides. The radical anion formation from TCNE (electron affinity 2.89 eV) on aluminas occurs on extraordinarily coordinated hydroxide ions on hydroxyl-rich surfaces, whereas exceptionally coordinated O2" ions play the role of the donor sites on more strongly dehydroxylated surfaces (328, 330). Accordingly, as the chemical nature of the donor site changes with the degree of surface hydroxylation, the spin concentration of the anion radical passes through two maxima the first is located between 400° and 500°C (OH- donor sites), and the second (brought about by the O2 ions) is between 600° and 700°C (328, 331). Trinitrobenzene (TNB) (electron affinity 1.0 eV) is a weaker electron acceptor than TCNE and interacts only with the 02 sites (332), thus acting more selectively than TCNE. 

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