Gas Phase Organic Functionalization of SiO2 with Propanoyl Chloride

The reaction mechanism of propanoyl chloride (C2H5COCl) with -SiOH-terminated SiO2 films was studied using in situ surface infrared spectroscopy. We show that this surface functionalization reaction is temperature dependent. At 230 degrees C, C2H5COCl reacts with isolated surface -SiOH groups to form the expected ester linkage. Surprisingly, as the temperature is lowered to 70 degrees C, the ketone groups are transformed into the enol tautomer, but if the temperature is increased back to the starting exposure temperature of 230 degrees C, the ketone tautomer is not recovered, indicating that the enol form is thermally stable over a wide range of temperatures. Further, the enol form is directly formed after exposure of a SiO2 surface to C2H5COCl at 70 degrees C. We speculate that the enol form, which is energetically unfavorable, is stabilized because of hydrogen bonding with adjacent enol groups or through hydrogen bonding with unreacted surface -SiOH groups. The surface coverage of hydrocarbon molecules is calculated as similar to 6 x 10(12) cm(-2), assuming each reacted -SiOH group contributes to one hydrocarbon linkage on the surface. At a substrate temperature of 70 degrees C, the enol form is unreactive with H2O, and H2O molecules simply physisorb on the surface. At higher temperatures, H2O converts the ketone to the enol tautomer and reacts with Si-O-Si bridges, forming more -SiOH reactive sites. The overall hydrocarbon coverage on the surface can then be further increased through cycling H2O and C2H5COCl doses.