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.

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The molecular orientation and structure of adsorbates at oxide interfaces is driven by surface-molecule and molecule-molecule interactions and is useful for predicting reactivity and product selectivity in heterogeneous chemical reactions, including those important in catalytic processes. Using broadband vibrational sum-frequency generation spectroscopy, we probed cyclic and acyclic alkanes and olefins at buried vapor/solid and liquid/ solid interfaces of the R-alumina (0001) surface under ambient conditions. The spectroscopic signatures of the adsorbates were measured and compared with bulk phase spectra and spectra of model surfaces containing hydrocarbons covalently linked to glass slides using silane chemistry. By utilizing appropriate polarization combinations, the orientations of hydrocarbons adsorbed from the vapor were evaluated and compared to the hydrocarbon orientations at the liquid/solid interface. The data support a proposed orientation for n-alkanes and cycloalkanes at the liquid/solid interface in which the hydrocarbons lie with the planes of their carbon backbones parallel to the surface, whereas at the vapor/solid interface, the adsorbates are oriented with their carbon backbones in an orientation neither parallel nor perpendicular to the surface. The surface vibrational spectra of olefins at both types of interfaces indicate that their orientations differ from saturated counterparts, and their unique spectral signatures allow differentiation of adsorbed olefins from alkanes.

Mixtures of 1-hexanol in cyclohexane over the (0001) α-Al(2)O(3) surface were probed in the CH stretching region using vibrational broadband sum frequency generation (SFG). Below 10 mol % 1-hexanol, the alcohol adsorbs to the surface through interactions that result in observed free adsorption energies of around 14-15 kJ/mol obtained from the Langmuir adsorption model. Polarization-resolved SFG spectra indicate ordering of the alkyl tails with increasing surface coverage. Highly correlated with the ordering of 1-hexanol is an orientational change of the cyclohexane solvent from flat to most likely tilted, suggesting that cyclohexane mediates the adsorption of 1-hexanol via intercalation. Above 10 mol %, the SFG signals for 1-hexanol and cyclohexane decrease with increasing concentration of 1-hexanol, consistent with the notion that cyclohexane is excluded from the interfacial region while 1-hexanol becomes increasingly disordered. At the interface, the alcohol solute becomes the solvent at a mole fraction of only 10%, i.e., five times below what is considered the solute-to-solvent mole fraction transition in the bulk. These results provide an important benchmark for theory, inform reaction design, and demonstrate that bulk thermodynamic properties of binary mixtures are not directly transferable to interfacial environments.

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.