Silicates, 29Si chemical shifts

Higgins and Woessner (106) correlated 29Si chemical shifts in the framework silicates cristobalite, quartz, albite, and natrolite with the mean Si—O bond distances and arrived at the following relationship ... 

A large upfield shift of 29Si chemical shift from 28 (S 10.66) to 29 (S - 72.45) strongly supports the structure of a pentacoordinate silicate. 

By this method also the zwitterionic silicates 9-15 were obtained The geometry at silicon in these compounds is TBP, like in anionic and neutral pentacoordinate silicon complexes. A typical crystal structure is shown in Figure 5 for compound 9. This structure apparently also exists in solution (CD3CN), as the 29Si chemical shift for 9 in this solvent (—122.9 ppm) compares well with the solid state CP-MAS shift of -121.0 pm28 31. 

Similar condensation of phenyltriethoxysilane with a spirocatechol yielded the first macrocyclic tetrasiliconate, a tetraanion containing four pentavalent silicons (equation 24a)58b. NMR evidence showed that only one meso-stereomer (C2h symmetry) was formed in solution out of the four possible diastereomers. The 29Si chemical shifts of the two unique silicons in DMSO-dg solution were —86.3 and —86.9 ppm, respectively, consistent with a pentacoordinate silicate anion. 

On the basis of the pioneering work of Lippmaa et aj (3) on 29 i MASNMR, and that of Engelhardt et a l (U) on 29 i NMR of silicate solutions, it became apparent that 29si chemical shifts are sensitive to the substituents present in the second coordination sphere. Correlations between chemical shift and structure were established, and these were soon utilized (5 7) for the structural studies of both soluble and insoluble silicates and aluminosilicates. A wide variety of zeolitic solids has by now been investigated (6-18) using 27/ i and several other... 

Si CP-MAS NMR Spectroscopy. This method is very useful for characterizing silica (15-17). The main information derived from an NMR spectrum is the chemical shift, the intensity, and the line width. The 29Si chemical shift is determined by the number and type of tetrahedral framework atoms connected to tetrahedral silicon atoms. The spectrum thus allows the detection of the number of structurally inequivalent kinds of silicon atoms of various Si (0-)4- (0Si)n units in silicates and as Q4 (m Al) for Si (OSi)4-n(OAl)m units in framework alumosilicates (Figures 4 and 5) (40). 

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

The chemical shift anisotropy of the 29Si nucleus is generally small, and thus unlike in l3C solid-state NMR at high fields, no sideband problems are encountered in MAS spectra of framework silicates. 

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. 

A powerful technique for investigating these different species is 29Si-NMR (see Fig. 8.1). Different lines can be observed in the 29Si-NMR spectrum of a silicate solution corresponding to the differently positioned 29Si nuclei in the (poly)sili-cate ions. The highest values for the chemical shift (8 = -71.5 ppm with respect to tetra methyl silane) are found for the monomeric silicate units while the resonances of fully condensed silicon atoms (i.e. Si-(0-Si)4 tetrasiloxy silane) are to be found at the lowest values for the chemical shift (8 = -110 ppm). 

Kemmitt, T. and Milestone, N.B., The ring size influence on 29Si N. M. R. chemical shifts of some spirocyclic tetra- and penta-coordinate dialato silicates, Aust. J. Chem., 48, 93, 1995. 

Many of the individual lines observed in spectra such as those shown in Figure 1 can be assigned to specific silicate structures based on spectra of well defined silicate species and detailed NMR studies of the homonuclear spin-spin coupling of Si nuclei in 29si-enriched silicate solutions r6-81. Such studies have revealed that the chemical shift of a given silicon atom depends on its connectivity, the length of the Si-O bonds, and the angle of Si-O-Si bonds, as well as the pH of the medium and the cation type. 

High resolution 29si NMR spectroscopy can provide considerable insights into the structure and distribution of silicate and aluminosilicate anions present in solutions and gels from which zeolites are synthesized. The narrowness of individual lines and the sensitivity of the chemical shift to details of the local chemical environment make it possible in many instances to identify exact chemical structures. Studies using Si NMR have shown that the distribution of anionic structures is sensitive to pH and the nature of the cations in solution. Alkali metal cation NMR has demonstrated the formation of cation-anion pairs the formation of which is postulated to affect the dynamics of silicate and aluminosilicate formation and the equilibium distribution of these species. A NMR has proven useful in identifying the connectivity of A1 to Si, but because of quadrupolar broadening cannot be used to define the precise environment of A1 atoms. 

A phyllosilicate-rich clay was studied by 29Si MAS-NMR spectroscopy.520 13C CP/MAS-NMR and 29Si MAS-NMR spectra were used to characterise silylated montmorillonites, and to estimate the extent of the silylation reaction.521 Ab initio calculations have been made of 29Si NMR chemical shifts for silicate complexes with carboxylates, amino acids and multicarboxylic acids.522 Structural changes on thermal treatment of kaolinite were followed using 29Si MAS-NMR spectra.523... 

Figure 1. 29Si-NMR chemical shifts of silic(oalumin)ates.

The samples used for NMR spectroscopy were freeze-dried to prevent any reaction during the drying process. Adamantane and the trimethylsilyl ester of double four-ring octameric silicate, Q8M8 were used to optimize experimental parameters and as external secondary (relative to TMS) chemical-shift references for 13C and 29Si, respectively. Both the Ti measurements and a discussion on the use of 29Si NMR spectroscopy for quantitative measurements will be described elsewhere (47). 

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