29Si chemical shift ranges

Figure 14. The five possible local environments of a Si atom tetrahedron together with their characteristic 29Si-chemical shift ranges (boxed areas). Si(aAl) represents Si bonded, through oxygen bridges, to nAl atoms where n ranges from 0 to 4. The chemical shifts of the five resonances of zeolite ZK-4 are shown in broken vertical lines. (Figures adapted from References 25 and 35).

NMR spectroscopy is a powerful tool for structural analysis. The chemical shifts of polyhedral silicons range from —22 to 39 ppm. The 29Si chemical shifts of tetrasilatetrahedrane ll32, hexasilaprismane 1237 and octasilacubanes (163, 18a44b, 18b45, 2046 and 2247) are listed in Table 10. 

Hydrogen. - The H nucleus has been traditionally the most important for n.m.r. investigations. It has spin-, natural abundance of practically 100%, high gyromagnetic ratio, and relatively short relaxation time, and consequently is the most sensitive of all nuclei. Its chemical shift range is however small (about lOp.p.m.), and in the field of solid-state n.m.r., where homonuclear dipolar broadening is a severe problem for 100%-abundant nuclei, the width of the lines frequently, but by no means always, tends to obscure the important chemical shift information which we have come to expect from 13 C and 29Si n.m.r. 

NMR spectroscopy is ideally suited for characterizing the silicate and aluminosilicate species present in the media from which zeolites are formed. The nuclei observable include 29si, 27 Al, and all of the alkali metal cations. The largest amount of information has come from 29gi spectra ri-31. This nucleus has a spin of 1/2, no quadrupole moment, and a chemical shift range of about 60 ppm. As a consequence, it is possible to identify silicon atoms in specific chemical structures. 27 Al, on the other hand, is a spin 3/2 nucleus and has a sizeable quadrupole moment. This results in broad lines and limits the amount of information that can be extracted from 27Al spectra. 

The silicate species discussed in the preceding section can react with aluminate anions, Al(OH)4 to produce aluminosilicate anions. Si NMR spectra of solid silicates and aluminosilicates indicate that the replacement of Si by A1 in the second coordination sphere of a give Si causes a low-field shift of about 5 ppm. Since each Si atom can have up to four metal atoms in its second coordination spere, fifteen possible Qn(mAl) structural units can be envisioned. The estimated chemical shift ranges for these units are given in Table 3. It is apparent from this table that the 29si spectrum of an aluminosilicate solution in which A1 and Si atoms were statistically distributed would be much more complex than that of an analogous solution containing only silicate species. 

Figure 4. Typical ranges of 29Si chemical shifts of Qn units in silicates. (Reproduced with permission from reference 40. Copyright 1990.)...

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