| As surfaces are known to have profound implications for chemical transport, reactivity, and energy budgets in atmospheric environments, we have designed a nonlinear optical sum frequency generation (SFG) system that can be used to study atmospherically relevant heterogeneous chemistry. The aims of this project were two-fold: first, to develop and characterize tailor-made organically coated substrates relevant to tropospheric chemistry and second, carry out kinetic studies that model naturally occurring heterogeneous atmospheric reactions. After construction of the broadband SFG system, we studied siloxane substrates functionalized with organic adlayers. The organic adlayers were specifically chosen to contain environmentally relevant functional groups, namely, an acid-terminated alkyl chain, several ester functionalized alkyl chains, and a non-functionalized alkyl chain. Hydrolysis of methyl ester functionalized surfaces was carried out to produce carboxylic acid functionalized surfaces and monitored using SFG. In order to access more complicated atmospherically relevant substrates, we then focused on the synthesis and characterization of a derivative of limonene, a biogenically emitted compound, chemically bound to a glass surface. We employed both electrophilic, and nucleophilic linker chemistries to increase the versatility of our approach. SFG spectra indicated that while orientation of the surface-bound terpenes depended on the linker strategy we employed, the C=C double bond was accessible to gas phase ozone regardless of the strategy applied. We then used SFG to track the interaction of a terpene-linked species with ozone and calculate reaction probabilities. Exposure of the amide-linked terpene substrate to ppm levels of ozone at a total pressure of 1 atm and 300 K yielded an initial reaction probability of approximately 1 x 10 -5, which was significantly higher than the corresponding gas phase reaction involving 1-methyl-1-cyclohexene. The chemical versatility made possible through these methodologies enabled us to synthesize, characterize, and then react organic adlayers that modeled a wider range of organic molecules present in the troposphere than available through commercial sources. The general methodologies developed here can be applied in environmental, biochemical, or material surface chemistry to generate interfaces with tunable physical and chemical properties. |
