An important role in atmospheric chemistry is that of the reactions of ozone with unsaturated organic surfaces. Potential reactions sites for the atmospheric pollutant ozone are man-made materials that contain C=C functional groups. Previous research has shown that carboxylic acid-containing products were formed when unsaturated organic monolayers on silicon react with ozone; a band at 1860cm-1 shows the formation of a strained carbonyl species; oxidation of Au - S bonds in saturated self-assembled monolayers of alkanethiols on Au. This article explores the reaction mechanisms of ozone with unsaturated organic surfaces by using C=C terminated alkanethiol SAMs (self-assembled monolayers).
This experiment uses reflection - adsorption infrared spectroscopy (RAIRS). A Bruker IFS 66v/S spectrometer with a liquid N2 - cooled MCT detector was used to record the RAIR spectra. Each spectrum was collected by using p-polarized light with an average of 100 scans. Background references were clean Au substrates with a resolution of 2cm-1.
To minimize the effects of background water and other contaminants, the experiment was conducted in a high vacuum, with a pressure of 5x10-7Torr. The peaks obtained show the presence of asymmetric and symmetric stretching modes of the methylene units within the hydrocarbon chains. After exposure of the C=C-T SAM to about 1000L of ozone, the beaks associated with the double bonds disappear. This indicates reactions at the terminal C=C bond. The change in peak position and shape of the spectra is attributed to the monolayer disordering. This disordering could include the effects of the terminal C=C bond reactions as well as oxidation of the Au - S bond. Next, exposure time is increased. This spectra obtained here suggests that the C=C-T SAM reacts with ozone to initially form a carboxylic acid intermediate that converts to a carboxylic anhydride group with more exposure.
The RAIR spectra show...