Interfacial Reaction of an Olefin-Terminated Self-Assembled Monolayer Exposed to Nitrogen Dioxide: An Investigation Into the Reaction Rate and Mechanism
Davis, Gwen Marie
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Reactions of strongly oxidizing pollutants with unsaturated hydrocarbon surfaces are important to many areas of scientific interest. For example, reactions of unsaturated hydrocarbons on the surface of tropospheric aerosols could have a great effect on the oxidizing capacity of the troposphere while the reaction products could be involved in the formation of clouds and smog. These reactions are also important in understanding the toxic effect inhalation of these pollutants have on the pulmonary surfactant of the lung, the only amicable air-water interface of the body. The fatty acids of this surfactant are as much as 30% unsaturated, and exposure to oxidizing pollutant is known to alter both the composition and function of the surfactant. Understanding the reaction mechanism will further the knowledge of how this toxicity occurs. While the reactions of strongly oxidizing pollutants, such as ozone and nitrogen dioxide, with alkenes in the gas and solution phases are well known, the interfacial reaction mechanisms of these species is not fully understood. The goal of this study is to determine the reaction mechanism when an unsaturated hydrocarbon monolayer at the gas-surface interface is exposed to gas phase nitrogen dioxide. An olefin-terminated thiol was synthesized and a self-assembled monolayer on Au(111) made and characterized using Reflection-Absorption Infrared Spectroscopy (RAIRS). This unsaturated surface was then exposed to NO2 at a pressure of 1x10-4 mbar in a UHV (Ultrahigh Vacuum) chamber. Time-resolved RAIRS was preformed in situ to monitor the reaction during exposure. X-ray Photoelectron Spectroscopy and RAIRS determined the surface reaction product as an aldehyde. While the mechanism can not be precisely determined, two mechanisms involving either the hydrogen abstraction or radical addition of the NO2 to yield an aldehyde are proposed.
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