Chemical analogy, spin-polarization

For discussing the results obtained, use can be made of a simple model (Sagdeev et al, 1969), based on the assumption that the spin density propagates successively via chemical bonds in accordance with spin-polarization and hyperconjugation mechanisms. Such consideration leads (for constant on y protons) to equation (2), which in principle is analogous to the equation derived by Barfield (1964) for the long-range spin-spin interaction constant via four a bonds ... 

Nevertheless, solid-state NMR can be usefiil for active site studies, which is nowadays predominantly employed for acid catalysis, e.g., with zeolites [32]. Chemical shifts obtained are related to the electronic state of the atom H-NMR data have been, for instance, used to determine acidity of Brpnsted sites in zeolites, NMR can discriminate in various environments in supported catalysts [33]. As long as the sites are sufficiently dilute, NMR intensity is directly related to concentration. Structural information is accessible by echo techniques which allow determining distances between spin sites. By analogy with IR methodology, adsorption sites have been investigated with probe molecules also in NMR studies. While work with N bases may require isotopic enrichment of the probe, there are opportunities to detect subtle structural features by cross-polarization and doubleresonance (e.g., " N/ A1) techniques [34]. 

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