Carbon-13 NMR Investigation of Surface Modified Silica Gels
Surface modified silica gels containing chemically bound molecules of the type ≡ Si-O-Si(OH)n(C6H5)3−n (n=1,2) and ≡ Si-O-Si(OH)2CH2C6H5 have been investigated by C NMR. At low temperature (77 K) the signals from the phenyl carbons have a broad low-symmetric chemical shift dispersion, characteristic for a “rigid” molecule. At room temperature the dispersion is different and characteristic of an axial symmetric chemical shift anisotropy caused by molecular reorientations. The shape of the spectra has been calculated for different possible molecular reorientations and compared with the experimental spectra. From this analysis the reorientation which takes place in the molecules, as well as the orientation of the molecules relative to the surface, and the geometry of the surface-adsorbate bond were obtained.
KeywordsSilica Surface Methylene Carbon Chemical Shift Anisotropy Rapid Rotation Fixed Orientation
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