The monophenylhydrosilasesquioxanes PhH(n-1)Si(n)O(1.5n) where n=8 or 10

The first monosubstituted decasilasesquioxane, PhH(9)Si(10)O(15), and the analogous PhH(7)Si(8)O(12) molecule have been prepared and characterized by IR and Raman spectroscopy and X-ray crystallography. Both have crystallographic C-1 symmetry, but their cages exhibit an approximate C-s, and an effective C-3, symmetry, respectively in the crystalline state. The IR and Raman spectra of the two molecules are very similar and reflect the close similarity observed for the spectra of H8Si8O12 and H10Si10O15. They have been treated as a superposition of the spectra of the siloxane cage Hn-1SinO1.5n, n = 8 or 10, the phenyl substituent and the connecting moiety O3Si-C(CH)(2) and assigned on the basis of spectral correlation and normal coordinate analysis. The siloxane cage vibrations are best understood by correlation with those of the unsubstituted cages, indicating that distortions of the Si8O12 and Si10O15 cages caused by the substituent are small. A comparison of the Si-C stretching force constants indicates that Si-C-vinyl and Si-C-phenyl are of similar strength while the Si-C-alkyl bond is weaker. The notion of ring-opening vibrations, introduced for (HSiO1.5)(2n), n = 2, 3, 4, etc., is also applicable to PhH(7)Si(8)O(12) and PhH(9)Si(10)O(15). The phenyl substituent does not influence the frequency range of four- and five-membered ring-opening vibrations, however the number of such vibrations is increased.