Si Chemical Shifts in Aluminosilicates

The aluminosilicates constitute an important class of inorganic compounds. The occurrence of aluminium atoms in the second coordination sphere of the silicon, to which they are bonded through oxygen atoms, produce systematic changes in the Si chemical shift in a similar way to the changes which are associated with differences in the 

Studies on crystalline gallosilicate molecular sieves with the beta structure (Occelli et al. 1999) have led to the suggestion that substitution of Al by Ga in these structures leads to a change of 3 ppm towards higher frequencies, but a systematic determination of the range of shifts in gallosilicate compounds has yet to be made. 

A series of lead silicate glasses and their crystalline equivalents has revealed a linear relationship between the number of Pb atoms in the second coordination sphere of the structural Si tetrahedra (p) and the isotropic Si chemical shift (Bessada et al. 1994), defined by  

In a Si NMR study of microporous niobium silicate catalysts, Rocha et a/. (1998) tentatively identified resonances at - 95.6 ppm as Si(2Si,2Nb) or Si(3Si,lNb), at - 105.5, - 107 and -108 ppm as Si(3Si,lNb) or Si(4Si,0Nb) and at - 111 as Si(4Si,0Nb) groups. These assignments were made by analogy with aluminosilicates, and not based on measurements of crystalline niobium silicates of known structure. 

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Relationships between Si NMR Spectra and Structure/Bonding Eour-Coordinated Si-O-Compounds Tetrahedral Si Chemical Shifts in Silicates Si Chemical Shifts in Aluminosilicates Effects of Other Nearest Neighbours on the Si Shift... 

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