Investigations by 29Si cross-polarization magic angle spinning NMR spectroscopy of reaction pathways of silica gel polyfunctional modification

Abstract

Di- and trifunctionally modified silica gels (C 18) were investigated by 29Si cross-polarization magic angle spinning NMR spectroscopy and the relative amounts of surface species were determined by peak deconvolution. On the basis of structure assignment and change in species decomposition with different reaction conditions, it is possible to construct a scheme for reaction pathways in polyfunctional modifications. Thus, distinct species composition can be correlated with reactions occurring at the surface. Synthesis with and without an argon atmosphere and the use of the stationary phases in high-performance liquid chromatography (HPLC) cause dramatic changes in species composition. Derivatization in a dry argon atmosphere results in the formation of monodentate species (D 1, D 2) (difunctionally modified) or bidentate species (T 2) (trifunctionally modified), whereas without an argon atmosphere mainly condensed species (D 4 + D′ 4 and T 4 + T′ 4) are formed. Disregarding the reaction procedures, modification with dichlorosilanes preferentially forms monodentate species, and trifunctional modification favours the formation of bidentates. Condensation reactions seem to play a minor role during modification. However, they are most important in the ageing process of polyfunctional modified phases, both in the dry form and during their use in HPLC.