Tetrasilatetrahedranes structure

The highly strained tetrasilatetrahedrane structure can only be stabilized using extremely bulky substituents like the tri-i-butylsilyl (= supersilyl) group. Thus the only tetrasilatetrahedrane derivative known so far has been synthesized by coupling (t-Bu)3SiSiBr2SiBr2Si(Bu-t)3 with two equivalents of (i-Bu SiNa92 (equation 24). 

NMR spectroscopy is a powerful tool for structural analysis. The chemical shifts of polyhedral silicons range from —22 to 39 ppm. The 29Si chemical shifts of tetrasilatetrahedrane ll32, hexasilaprismane 1237 and octasilacubanes (163, 18a44b, 18b45, 2046 and 2247) are listed in Table 10. 

As pointed out in Section II.B, tetrasilatetrahedrane (Si4R4) collapses with no significant barrier to a two-bond broken isomer when R = H, the latta being 37.3 kcal mol more stable at the BLYP/6-31G level . This enagy diffaence is decreased by 8.9 kcalmol with R = Me, but the two bonds still ranain broken. However, when R = SiHs, the bond-stretch isomer is by only 10.4 kcalmol more stable than the corresponding tetrasilatetrahedrane, having only one bond stretched. As Figure 5 shows, the structural features of the two isomers, especially the close contact between the substituents... 

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