Relationships between NMR Parameters and Structure

Ah initio calculations show that the x and z axes of the efg tensor lie in the plane of the Si-O-Si bond with the x-axis bisecting the Si-O-Si bond angle. 

Ah initio calculations have also been used to examine the effect of cations coordinated to Si-O-Si bonds by considering the (0H)3Si-0-Si(0H)a unit with lithium and sodium coordinated to the Si-O-Si site. Both xq and t were found to have the same functional dependence on a irrespective of the presence of the cations, although there was an offset depending on the number of coordinating cations and their field strength (Vermillion et al. 1998). The value of Xo was found to shift systematically to lower 

On the other hand, ti shifts to much higher values when one cation is added but shows a much smaller effect when there are two coordinating cations. The value of ti was described by the function  

Currently it appears that shift correlations may have only limited application to O NMR in groups of materials with closely related structures. 

One of the most powerful applications of these correlations between NMR parameters and stmcture is to provide a better understanding of the stmcture of amorphous materials which are very difficult to study by other techniques. Silicate glasses have been studied by relating the 8iso.cs value of Si to the bond angle distribution (Dupree and Pettifer 1984, Pettifer et al. 1988). In the case of O, the parameter has been used to determine the Si-O-Si bond angle distribution (Faman et al. 1992) using the relationship  

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