PRESENT STATUS OF NMR CALCULATIONS ON MOLECULES

Having established reliable procedures they could then calculate shieldings for candidate materials to establish what the structures of newly synthesized compounds 

For medium size gas-phase molecules the preferred method would now be GIAO-MP2 using a doubly polarized basis set like 6-31 l(2d,p). It would also be desirable to calculate the geometries incorporating some correlation, e g., using the MP2 or BLYP method (see Jensen (1999) for a discussion of density functional methods like BLYP). 

Let us briefly review some of the early experimental data, focusing on the observed correlations between shielding and structure. These correlations are observed for both silicate species in aqueous solution (e.g., Harris and Knight 1983 Kinrade and Swaddle 1988) and for crystalline silicates (Lippmaa et al. 1980 Kirkpatrick 1988) and have been reproduced by calculations on molecular clusters in the gas phase. The most important trends for the Si NMR shift are  

the shift becomes more negative as the coordination number of the Si increases 

the shift becomes more negative as we increase the degree of polymerization, compactly described using Q notation, in which n denotes the number of O atoms of the silicate tetrahedron which bridge to other tetrahedral cations 

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