The synthesis, characterization, and oxidation reaction of a tropospherically relevant terpene bound to a glass surface are reported. Vibrational broadband sum frequency generation (SFG) is used to characterize the various terpene-modified glass surfaces and track their interaction with ozone. SFG spectra indicate that, although orientations of the surface-bound terpenes depend on the linker strategies employed, the CdC double bond is accessible to gas-phase ozone regardless of the strategy applied. Exposure of the terpene-functionalized surface to ppm levels of ozone at 1 atm and 300 K yields an initial reaction probability of approximately 1 × 10 -5 per surface collision, which is significantly higher than the corresponding gas-phase reaction involving 1-methyl-1-cyclohexene (5 × 10 -7 from gas-phase collision theory). The interaction of ozone with a saturated octyl silane-functionalized glass surface leads to a slight molecular reorientation, or tilting, of the terminal CH 3 groups on a much slower time scale. Our work demonstrates that SFG spectroscopy can be used to determine reaction probabilities of heterogeneous atmospheric reactions and bridges the gap between atmospheric chemistry and surface functionalization.
We report vibrational sum frequency generation (SFG) spectra of glass surfaces functionalized with 1-pentene, 2-hexene, cyclopentene, cyclohexene, and a menthenol derivative. The heterogeneous reactions of ozone with hydrocarbons covalently linked to oxide surfaces serve as models for studying heterogeneous oxidation of biogenic terpenes adsorbed to mineral aerosol surfaces commonly found in the troposphere. Vibrational SFG is also used to track the C=C double bond oxidation reactions initiated by ozone in real time and to characterize the surface-bound product species. Combined with contact angle measurements carried out before and after ozonolysis, the kinetic and spectroscopic studies presented here suggest reaction pathways involving vibrationally hot Criegee intermediates that compete with pathways that involve thermalized surface species. Kinetic measurements suggest that the rate limiting step in the heterogeneous C=C double bond oxidation reactions is likely to be the formation of the primary ozonide. From the determination of the reactive uptake coefficients, we find that ozone molecules undergo between 100 and 10000 unsuccessful collisions with C=C double bonds before the reaction occurs. The magnitude of the reactive uptake coefficients for the cyclic and linear olefins studied here does not follow the corresponding gas-phase reactivities but rather correlates with the accessibility of the C=C double bonds at the surface.
While many biogenic and anthropogenic organic constituents in the atmosphere are surface-active and chiral, the role of stereochemistry in heterogeneous oxidation chemistry in the atmosphere has not yet been evaluated. Here, we present nonlinear vibrational surface spectra of fused silica substrates functionalized with quinuclidine diastereomers during exposure to 10(11) to 10(13) molecules of ozone per cm(3) in 1 atm helium to model ozone-limited and ozone-rich tropospheric conditions. Kinetic studies show that diastereomers that orient their reactive C=C double bonds toward the gas phase exhibit heterogeneous ozonolysis rate constants that are 2 times faster than diastereomers that orient their C=C double bonds away from the gas phase. Insofar as our laboratory model studies are representative of real world environments, our studies suggest that the propensity of aerosol particles coated with chiral semivolatile organic compounds to react with ozone may depend on stereochemistry. We expect that the differences in chemical accessibility will lead to the enrichment of one oxidation product stereoisomer over the other. The oxidation products could be gaseous or surface-bound, indicating that kinetic resolution could lead to the stereochemical enrichment of the gas phase or the aerosol, which may have also been important in prebiotic chemistry. Implications of these results for chiral markers that would allow for source appointments of anthropogenic versus biogenic carbon emissions are discussed.
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