29Si chemical shifts , patterns

FIGURE 45. 29Si chemical shift powder patterns collected under cross-polarization conditions at 39.8 MHz on X-substituted silatranes 63 with X indicated. The fluorosilatrane chemical shift powder pattern was collected under high-power H and 19F decoupling conditions at 200 MHz. Reprinted with permission from Reference 150. Copyright 1993 American Chemical Society... 

The 29Si NMR chemical shift tensors accessible from solid state NMR spectra also provide important insights into the nature of the chemical bonds in disilenes. After the disilene 962 and some other disilenes63 had been investigated by this method in earlier studies, West and coworkers recently reported the solid state NMR spectra of seven disilenes with widely differing substitution patterns as well as of two solvates of compound 9 (Table 3). These results in combination with MO calculations on model substances support a classical jr-bonding model for the Si=Si bonds64. 

Detailed information about the electronic structure of the silicon-silicon double bond was obtained from the 29Si solid state powder pattern spectrum80 of tetramesityldisilene. The anisotropy of the chemical shift of Si=Si was found to be comparable to that of C=C in ethylene. In contrast, the anisotropy of Si-Si in the analogous tetramesityldisilane was found to be small which is characteristic of alkanes as well. Based on the similarities of the chemical shift anisotropies it was concluded that the electronic structure of Si=Si closely resembles that of C=C. 

---