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 °C, C₂H₅COCl reacts with isolated surface -SiOH groups to form the expected ester linkage. Surprisingly, as the temperature is lowered to 70 °C, the ketone groups are transformed into the enol tautomer, but if the temperature is increased back to the starting exposure temperature of 230 °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 °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 ∼6 × 10¹² cm^-2, assuming each reacted -SiOH group contributes to one hydrocarbon linkage on the surface. At a substrate temperature of 70 °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.