Atmospheric Oxidation Mechanism Of Benzene And Substituted Benzene

Aromatic hydrocarbons, as an important part of volatile organic compounds in theatmosphere, contribute significantly to the formation of photochemical smog and secondorganic aerosol （SOA）. Therefore, the knowledge on the atmospheric oxidation mechanism ofthe aromatic compounds in the atmosphere is crucial in understanding the impact of thearomatic hydrocarbons on atmospheric environment and in developing possible strategies tocontrol atmospheric pollution. In the atmosphere, the degradation of aromatic hydrocarbonsare mainly initiated by their reactions with the OH radical. However, the detailed mechanismsremain vague for most of the aromatic compounds. In this thesis, the oxidation mechanisms ofseveral aromatic hydrocarbons, including benzene, phenol and monochlorinated phenols, areinvestigated theoretically by using quantum chemistry and reaction kinetics calculations.The main results are summarized as followed:1. The fates of the oxyl radicals formed in the oxidation of benzene initiated by OHaddition were studied. A new reaction route, as the ring-closure to form epoxide-like radical,is identified for the oxyl radicals. For radical R2and R3, the ring-closure is found to dominateover the ring-breakage pathways which were often assumed as the sole fate for these radicalsin previous studies. Products from this new route include glyoxal,2-butenedial, and2,3-epoxy-butanedial, of which the latter has not been observed experimentally.2. The gas-phase oxidation mechanism of phenol initiated by OH radical is studied. Theinitiation of the reaction is dominated by OH addition to ortho-position, which subsequentlycombines with O₂at the ipso-position to form phenol-OH-1-OO adduct. A concerted HO₂elimination process from adduct is found to be much faster than the common ring-closure tobicyclic intermediates. The HO₂elimination process from phenol-OH-1-OO adduct forms2-hydroxy-3,5-cyclohexadienone （HCH） as the main product, and is also responsible for theexperimental fact that the rate constants for reaction between phenol-OH and O₂are abouttwo orders of magnitude higher than those between other aromatic-OH adducts and O₂. It isspeculated that HCH would isomerize to catechol through the assistance of water or phenol orthrough some heterogeneous processes. Reaction of phenol-OH with NO₂proceeds byaddition to form phenol-OH-n-NO₂（n=1,3,5）, followed by HONO-elimination from phenol-OH-1/3-NO₂to form catechol, being consistent with the experimental observation ofcatechol in the absence of O₂. The most likely pathway for2NP is the reaction of phenoxyradical and NO₂.3. The reaction mechanism monochlorinated phenols （MCPs） have been studiedtheoretical using quantum chemistry and transition state theory calculations. The rateconstants are predicted in both gas and aqueous phases. In the gas phase, ortho-additions aredominant for m-, and p-MCPs, while fractions of³4%is found for ipso-addition for o-MCP.In the aqueous phase, p-additions become significant for OH addition to o-MCP. Thecalculations for p-MCP support the proposed reaction mechanism from previous experimentaland theoretical studies. However, it remains difficult to relate the calculation to the realexperimental results because of the various interferences and secondary reactions in theexperimental studies.