Studies of octameric vinylsilasesquioxane by carbon-13 and silicon-29 cross polarization magic angle spinning and inversion recovery cross polarization nuclear magnetic resonance spectroscopy

Abstract

The acidic hydrolysis of triethoxyvinylsilane Si(C₂H₃)(OEt)₃ led to octavinyloctasilasesquioxane (C₂H₃SiO_1.5)₈1 and to a transparent derived gel 2. Compound 1 was obtained as single crystals, suitable for X-ray analysis: trigonal, space group R3̄, Z = 3, a = 13.533(2), c = 14.222(2) Å, γ = 120°. Disorder of the methylene groups was observed. Compound 1 was carefully studied by IR spectroscopy and ²⁹Si/¹³C cross polarization magic angle spinning (CP MAS) NMR spectroscopy. The CP dynamics of the various ¹³C sites were analysed by inversion recovery cross polarization (IRCP). Very different behaviours were observed for CH and CH₂ signals and could be attributed to anisotropic reorientations of the vinyl groups. The IRCP sequence enabled a complete editing of the spectra and definite assignments of the observed lines. Owing to its highly resolved spectra and small intrinsic linewidths, it appears that compound 1 could act as a secondary reference for the set-up of the Hartmann–Hahn condition in ²⁹Si CP NMR spectroscopy as well as a test for the set-up of the magic angle. All these results were applied to the study of the amorphous gel 2. Dipolar coupling constants were also strongly affected by local motions.