Identification and Characterization of the Hydrolysis Products in TMOS and MTMS Monomers Using29Si NMR and Polarization Transfer Techniques

Abstract

High-resolution 29Si NMR was used to identify and characterize the hydrolysis products of tetramethoxysilane (TMOS) and methyltrimethoxysilane (MTMS). The identification of the different hydrolysis species was accomplished using the spectral editing properties of the DEPT and INEPT heteronuclear polarization transfer experiments. Previous resonances assignments for the hydrolyzed monomers in TMOS were quickly confirmed using these techniques. Unambiguous assignment of the MTMS hydrolysis products is complicated by a small spectral dispersion and the presence of two different hydrogen–silicon J couplings in these species. Analytical expressions for the INEPT signal response under multiple heteronuclear J couplings allowed these assignments. It is shown that the optimal polarization transfer efficiency in TMOS, MTMS and corresponding hydrolyzed products can only be obtained if the pulse sequence parameters are optimized for each species. The temperature dependences of the 29Si NMR resonances for the hydrolysis products in TMOS and MTMS were investigated and found to be very similar, with the magnitude and sign of the temperature response being dependent on the number of hydroxyl groups attached to the silicon. For MTMS hydrolysis products the small 29Si chemical shift dispersion along with the variation of chemical shift with temperature and solvent requires that resonance assignments be made for each experimental condition.