Wu Runrun, Pan Shanshan, Li Yun, Wang Liming
School of Chemistry & Chemical Engineering, South China University of Technology , Guangzhou 510640, China.
J Phys Chem A. 2014 Jun 26;118(25):4533-47. doi: 10.1021/jp500077f. Epub 2014 Jun 13.
The atmospheric oxidation mechanism of toluene initiated by OH radical addition is investigated by quantum chemistry calculations at M06-2X, G3MP2-RAD, and ROCBS-QB3 levels and by kinetics calculation by using transition state theory and unimolecular reaction theory coupled with master equation (RRKM-ME). The predicted branching ratios are 0.15, 0.59, 0.05, and 0.14 for OH additions to ipso, ortho, meta, and para positions (forming R1-R4 adducts), respectively. The fate of R2, R4, and R1 is investigated in detail. In the atmosphere, R2 reacts with O2 either by irreversible H-abstraction to form o-cresol (36%), or by reversible recombination to R2-1OO-syn and R2-3OO-syn, which subsequently cyclize to bicyclic radical R2-13OO-syn (64%). Similarly, R4 reacts with O2 with branching ratios of 61% for p-cresol and 39% for R4-35OO-syn, while reaction of R1 and O2 leads to R1-26OO-syn. RRKM-ME calculations show that the reactions of R2/R4 with O2 have reached their high-pressure limits at 760 Torr and the formation of R2-16O-3O-s is only important at low pressure, i.e., 5.4% at 100 Torr. The bicyclic radicals (R2-13OO-syn, R4-35OO-syn, and R1-26OO-syn) will recombine with O2 to produce bicyclic alkoxy radicals after reacting with NO. The bicyclic alkoxy radicals would break the ring to form products methylglyoxal/glyoxal (MGLY/GLY) and their corresponding coproducts butenedial/methyl-substituted butenedial as proposed in earlier studies. However, a new reaction pathway is found for the bicyclic alkoxy radicals, leading to products MGLY/GLY and 2,3-epoxybutandial/2-methyl-2,3-epoxybutandial. A new mechanism is proposed for the atmospheric oxidation mechanism of toluene based on current theoretical and previous theoretical and experimental results. The new mechanism predicts much lower yield of GLY and much higher yield of butenedial than other atmospheric models and recent experimental measurements. The new mechanism calls for detection of proposed products 2,3-epoxybutandial and 2-methyl-2,3-epoxybutandial.
通过在M06 - 2X、G3MP2 - RAD和ROCBS - QB3水平上的量子化学计算以及使用过渡态理论和单分子反应理论结合主方程（RRKM - ME）的动力学计算，研究了由OH自由基加成引发的甲苯大气氧化机理。预测OH加成到本位、邻位、间位和对位（形成R1 - R4加合物）的分支比分别为0.15、0.59、0.05和0.14。详细研究了R2、R4和R1的反应历程。在大气中，R2与O2反应，要么通过不可逆的氢抽取形成邻甲酚（36%），要么通过可逆的重组形成R2 - 1OO - syn和R2 - 3OO - syn，随后环化形成双环自由基R2 - 13OO - syn（64%）。类似地，R4与O2反应，生成对甲酚的分支比为61%，生成R4 - 35OO - syn的分支比为39%，而R1与O2反应生成R1 - 26OO - syn。RRKM - ME计算表明，R2/R4与O2的反应在压力为760 Torr时已达到高压极限，R2 - 16O - 3O - s的形成仅在低压下（即100 Torr时为5.4%）才重要。双环自由基（R2 - 13OO - syn、R4 - 35OO - syn和R1 - 26OO - syn）与NO反应后将与O2重组生成双环烷氧基自由基。如早期研究中所提出的，双环烷氧基自由基会开环形成产物甲基乙二醛/乙二醛（MGLY/GLY）及其相应的副产物丁烯二醛/甲基取代丁烯二醛。然而，发现了双环烷氧基自由基的一条新反应途径，导致产物MGLY/GLY和2,3 - 环氧丁二醛/2 - 甲基 - 2,3 - 环氧丁二醛。基于当前的理论以及先前的理论和实验结果，提出了一种新的甲苯大气氧化机理。与其他大气模型和近期的实验测量结果相比，新机理预测乙二醛的产率要低得多，而丁烯二醛的产率要高得多。新机理要求对所提出的产物2,3 - 环氧丁二醛和2 - 甲基 - 2,3 - 环氧丁二醛进行检测。
