A Low-Frequency Infrared Study of the Reaction of Methoxymethylsilanes with Silica

A thin film infrared technique is used to observe bands due to hydrogen-bonded and chemisorbed methoxymethylsilanes on fumed silica in the low-frequency region below 1300 cm−1. The low-frequency region contains the characteristic bands due to Si–O–Si, Si–O, Si–C, Si–CH3, and SiO–C modes. Band assignments are aided by ab initio calculations and comparison to thin film experiments of adsorbed chloromethylsilanes. The spectral interpretation was expected to be more complicated than that of the corresponding chlorosilanes because the strong SiO–C alkoxy bands lie in the same region as the Si–O–Si bands. However, the SiO–C bands are weak in intensity when participating in hydrogen-bonding interactions enabling easy detection of the Si–O–Si bands due to chemisorbed species. By combining the low-frequency data with the spectral information for the hydroxyl region, a clearer picture of the nature of the bonding to the surface is obtained. When adsorbed at room temperature, all methoxy groups participate in hydrogen bonding with the surface hydroxyl groups. When the reaction is performed at 150°C, the silanes are chemisorbed via a Si–O–Si bond and the remaining methoxy groups of the chemisorbed species are hydrogen bonded to the surface hydroxyl groups. At reaction temperature of 400°C there is no evidence of hydrogen bonding but the spectra are complicated by the reaction of methanol with the surface.