Heterogeneous ozonolysis of tropospherically relevant chiral and achiral olefins studied by vibrational sum frequency generation
Tropospherically relevant oxidation reactions between ozone and olefin-functionalized fused silica surfaces are studied in real time using the nonlinear optical spectroscopy vibrational sum frequency generation. Using gas-phase ozone levels that range from 30 ppb to 3 ppm, atmospheric conditions ranging from pristine remote continental regions to highly polluted urban centers are simulated. Important mechanistic differences regarding C=C double bond oxidation processes under ozone-limited and ozone-rich reaction conditions for cyclohexene-functionalized fused silica substrates are evaluated. At tropospherically relevant ozone levels, the heterogeneous reaction rates follow a Langmuir-Hinshelwood type mechanism. The product formation rates, which are determined as a function of ozone concentration, are found to be half of the olefin reaction rates. This ratio is consistent with a proposed reaction pathway involving the breaking of one C=C double bond containing two olefinic CH groups to form one methyl group and one polar carbonyl moiety as product species.
Reaction rates and reactive uptake coefficients (gamma values) are determined for cyclohexene, cyclopentene, 1-pentene, and olefin-containing diastereomers. Heterogeneous ozonolysis rates are found to strongly depend on the interfacial orientation of the C=C double bonds, which prompted the pursuit of a new, stereochemically specific kinetic parameter for heterogeneous oxidation reactions. We found that chirality-driven heterogeneous ozonolysis may impact the chemical composition of the atmosphere, and provide insight into cloud-nucleating biological materials, the potential development of atmospheric markers for distinguishing between anthropogenic and natural sources of organic compounds, and investigations into the origin of homochirality in prebiotic environments.
Polarization-resolved vibrational sum frequency generation is also used to study silica surfaces functionalized with single-strand (T15) and duplex (T15:A15) DNA oligonucleotides. These vibrational spectra yield detailed interfacial structure information, including the chirality of the stereogenic centers and the duplex structure. Broadband sum frequency spectra are also used to determine the hybridization state of carbon atoms in highly conjugated organic molecules bound to Si(111) with the goal to better develop promising candidates for organosilicon-based molecular electronic devices and sensors.
0494: Physical chemistry