The Atmospheric Oxidation Degradation Mechanism Of Dimethyl Benzenes

The aromatic compounds are important components of volatile organic compounds in the atmosphere, and are considered as key precursors of ozone and secondary organic aerosols in the atmosphere. Understanding their atmospheric oxidation mechanism is significant to assess their impact on the environment and pollution prevention and control. However, the atmospheric degradation mechanism of the aromatic compounds is extremely complicated. It is extremely difficult to study the oxidation mechanism of the aromatic compounds in the atmosphere due to a few experimental difficulties, i.e., the difficulty in identifying and detecting the highly reactive radical intermediates, in identifying and quantifying the unstable products, etc. Therefore, uncertainty and ambiguity remain in the mechanism proposed so far, despite of extensive studies. Alternatively, theoretical study can overcome some of the difficulties from the experimental studies and is becoming more and more important in the studies of reaction kinetics and mechanism.In this paper, the atmospheric oxidation mechanism of m-xylene and o-xylene initiated by OH radical is investigated by using quantum chemistry, transition state theory and RRKM-ME theory. Molecular structures of reactants, transition states and products are optimized at M06-2X/6-311++G(2df,2p), and the electronic energies are further calculated at the ROCBS-QB3 level. Based on reaction energies and Gibbs energies at the ROCBS-QB3 level, rate constants are estimated and reaction paths and the branching ratios are predicted using transition state theory and unimolecular reaction theory. Atmospheric oxidation mechanism of m-xylene and o-xylene are proposed by combining current theoretical calculations and previous experimental results.The main results are summarized as followed:1. The reaction between OH and m X is dominated by OH addition to the C2 and C4 positions, forming adducts m X-2-OH(R2) and m X-4-OH(R4). In the atmosphere, R2 and R4 reacts with O2 by irreversible H-abstraction to dimethylphenols, or by reversible additions to bicyclic radical intermediates, which would recombine again with O2 to form bicyclic peroxyl radicals, to bicyclic alkoxyl radicals by reacting with NO or HO2, and eventually to final products such as glyoxal, methyl glyoxal, and their coproducts. The effects of reaction pressure and temperature are also explored by RRKM-ME calculations. A mechanism at 298 K is proposed on the basis of current predictions and previous experimental and modeling results. The predicted product yields support the values in SAPRC mechanism, even though the predicted yield of 1.0% for glyoxal is lower than the value of ~11% from the experimental measurements.2. The atmospheric oxidation mechanism of o-xylene is similar to m-xylene, Firstly,the reaction between OH and o X is dominated by OH addition to the C1 and C3 positions, forming adducts o X-1-OH(R1) and o X-3-OH(R3). The fate of R3 is similar to R2 and R4, it reacts with O2 by irreversible H-abstraction to dimethylphenols, or by reversible additions to bicyclic radical intermediates;R1 only reacts with O2 by reversible additions to bicyclic radical intermediates which would recombine again with O2 to form bicyclic peroxyl radicals, to bicyclic alkoxyl radicals by reacting with NO or HO2, and eventually to final products such as biacetyl, butenedial, methylglyoxal, 4-oxo-2-pentenal, epoxy-2,3-butenedial and minor glyoxal.