Vibrational structure of monosubstituted octahydrosilasesquioxanes

The IR and Raman spectra of the monosubstituted hydrosilasesquioxanes RH7Si8O12, R = Co(CO)(4), Ph, CH=CHPh, and CH2CH2Ph, have been analyzed by spectral correlation and a normal coordinate analysis. They were treated as a superposition of the spectra of the siloxane cage and the substituent. The molecules PhH7Si8O12, (PhCH=CH)H7Si8O12, and (PhCH2CH2)H7Si8O12 have allowed the investigation of the organic-substituted siloxane cage CH7Si8O12, contributing each a case of an Si-C-phenyl, Si-C-vinyl, and Si-C-alkyl bond, respectively. The force constants of the different Si-C bonds were related to the SI-C bond orders and extrapolated for Si-C-acetyl of (PhC=CH)H7Si8O12, which is not yet available. The inorganic-substituted hydrosilasesquioxane [Co(CO)(4)(H7Si8O12)] could be analyzed the same way as the organic-substituted compounds and showed a similar pattern for the siloxane cage vibrations. Although most spectral features of the siloxane cages could be understood assuming a local C-3 upsilon symmetry, vibrations indicating a lower symmetry occurred in all monosubstituted compounds and could be attributed to vibrational coupling with modes of the substituent. Especially the totally symmetric ring-opening vibration at 456 cm(-1) in H8Si8O12 showed a specific dependence on the different substituents.