Vibrations of Monosubstituted Octasilasesquioxanes

Abstract

The vibrational spectra of monosubstituted spherosiloxanes of the type RH₇Si₈O₁₂ can be understood as a superposition of the spectral features of H₈Si₈0₁₂ and of the substituent R. We show this for the FT-Raman spectra of RH₇Si₈O₁₂ with R = −CH₂−CH₂−C₆H₅ and −CH = CH−C₆H₅. The force fields of H₈Si₈0₁₂ and of the organic substituent, ethylbenzene and styrene, respectively, were combined and normal coordinate analysis was applied. The spectra of phenethyl-H₇Si₈O₁₂ and styryl-H₇Si₈O₁₂ were correlated with the spectrum of H₈Si₈O₁₂. The Si-X stretching frequencies in the spherosiloxanes are larger than in most other siloxane compounds. We have shown that this is due to the different X-Si-O-Si conformations, which are anti and syn, respectively, and can be explained by the dependence of the bond order on the X-Si-O-Si dihedral angle.