Multiscale Modeling of Interfacial Oxidation Mechanism at Air/Organic Interface: Reactions of CH2=CH-Terminated Self-Assembled Monolayer with OH center dot, O-3, and HO2 center dot

The first few elementary steps of the reactions of a structurally characterized CH2=CH-terminated self-assembled monolayer (SAM) with reactive oxygen species, such as OH center dot, HO2 center dot, and O-3 were investigated using the multilayer first principle technique (ONIOM[QCISD(T)/cc-pVTZ:MP2/6-311G(d,p):BHandHLYP/6-31G(d)]//ONIOM [BHandHLYP/6-311G(d,p):BHandHLYP/6-31G-(d)]) as a proxy for the interfacial oxidation mechanism on organic aerosol surfaces. The energetics of the reactions is compared with the analogous QCISD(T)/cc-pVTZ//BHandHLYP/6-311G(d,p) gas-phase results, to measure the energetic consequences of the presence of the surrounding alkene chains. All of the reactions studied are affected in such a way that the relative energies of the interfacial reactions became lower by the average value of 11.5 +/- 6.1 kJ/mol than the corresponding gas-phase values. Because of this effect, interfacial H-abstractions by OH center dot on both allylic and vinylic positions have submerged barriers (Delta E-0(double dagger) = -5.4 kJ/mol and Delta E-0(double dagger)= -5.8 kJ/mol, respectively). Also, the relative energy of the transition state of the C-C bond scission reaction started from the nonterminal adduct becoming negative (Delta E-0 = -0.8 kJ/mol). The latter channel can result in a release of vinyl alcohol and formation of alkyl radical-bearing SAM. Such lowering of the energy barriers might enhance the overall oxidation rate and change the branching ratio of the atmospheric oxidation of the unsaturated compound by OH center dot at the interface compared to the gas-phase analogues. Because of the importance of prereaction complexes, the possible orientations of the adsorbed reactive oxygen species on the surface are also investigated, and O-H band shifts significantly toward lower wavenumbers in the IR spectra for the OH center dot and HO2 center dot complexes compared to the gas-phase band, which might be used later for the detection of these species at the interface if their lifetime is accessible for such spectroscopy.