Coordination number, influence chemical shifts

Classical shielding arguments indicate an electron-rich phosphorus atom, or equally, an increase in coordination number. The silicon atom seems also to be electron-rich, while the carbon has a chemical shift in the range expected for a multiply bonded species. The coupling constant data are difficult to rationalize, as it is not possible to predict the influence of orbital, spin-dipolar, Fermi contact, or higher-order quantum mechanical contributions to the magnitude of the coupling constants. However, classical interpretation of the NMR data indicates that the (phosphino)(silyl)carbenes have a P-C multiple bond character. 

The samples have also been characterised by means of solid state NMR. 27A1 chemical shifts have been shown to be sensitive to the coordination number and 29Si chemical shifts are strongly influenced by nearest neighbour effects. 

III. FACTORS INFLUENCING 119Sn CHEMICAL SHIFTS A. Coordination number... 

NMR spectroscopy provides direct information about the structure of materials from measurements of the isotropic chemical shifts. This parameter is influenced most significantly by the coordination number of the Si, which in compounds coordinated by oxygen varies by about 50 ppm in going from Si(IV) to Si(V) to Si(VI) (Table 4.1). 

To provide a hint of how the choice of the functional can influence the value of the computed parameters in a slightly different context, Table 4.3 displays a comparison of the performance of different DFT functionals in reproducing Si chemical shifts of a number of sites in silica-based materials experimentally, NMR parameters of this kind are often employed to obtain structural information. NMR interactions in these materials are essentially dominated by bonding in the first few coordination spheres therefore, a molecular cluster approach can be adopted in the modeling, in conjunction with atomic basis sets. In particular, separate explorations showed that convergence with respect to cluster size is reached when three complete atomic shells are included around the Si center (shell-3 cluster). 

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