Wu Xiaoqing, Huang Can, Niu Shiyao, Zhang Feng
National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, P. R. China.
Phys Chem Chem Phys. 2020 Oct 15;22(39):22279-22288. doi: 10.1039/d0cp02984j.
Toluene's removal mechanism in the atmosphere is mainly attributed to the OH radical, which includes major OH-addition and minor H-abstraction reactions. The cresols and RO2 derived from OH-adducts reacting with O2 have significant impacts on the generation of secondary organic aerosols (SOA) and O3. However, computed branching ratios of various OH-adducts at various theoretical levels are largely inconsistent, mainly because previously reported barrier heights of the OH-addition reaction showed a strong method dependence. In the present study, we demonstrate that this reaction involves a nonnegligible anharmonic effect (during the process of movement of OH to the benzene ring), which has been overlooked by previous studies. The reaction kinetics of toluene + OH was systematically studied by a high-level quantum chemical method (CCSD(T)-F12/cc-pVQZ-F12//B2PLYP-D3/6-311++G(d,p)) combined with RRKM/master equation simulations. The particle-in-a-box approximation was used to treat the anharmonicity in this system. The final total rate coefficient is calculated to be 3.02 × 10-12 cm3 molecule-1 s-1 at 300 K and 1 atm. The main products for toluene + OH are computed as ortho-adducts (69.8%), benzyl radical + H2O (11.9%), ipso-adduct (7.3%), para-adduct (5.1%), and meta-adduct (5.1%). Our results indicate that both high level quantum chemical calculations for the crucial barrier heights and appropriate treatments for the anharmonicity determine the accuracy of the final computed total rate coefficients and branching ratios. Further analysis of the branching ratios of various reaction channels provides insight into the atmosphere-initiated oxidation of toluene.
甲苯在大气中的去除机制主要归因于羟基自由基（OH），这包括主要的OH加成反应和次要的氢提取反应。由OH加合物与O₂反应生成的甲酚和RO₂对二次有机气溶胶（SOA）和O₃的生成有重大影响。然而，在不同理论水平下计算得到的各种OH加合物的分支比存在很大差异，主要是因为先前报道的OH加成反应的势垒高度显示出强烈的方法依赖性。在本研究中，我们证明该反应涉及一个不可忽略的非谐效应（在OH向苯环移动的过程中），而这一点被先前的研究所忽视。通过高级量子化学方法（CCSD(T)-F12/cc-pVQZ-F12//B2PLYP-D3/6-311++G(d,p)）结合RRKM/主方程模拟，系统地研究了甲苯 + OH的反应动力学。采用盒中粒子近似来处理该系统中的非谐性。在300 K和1 atm条件下，最终计算得到的总速率系数为3.02×10⁻¹² cm³·分子⁻¹·s⁻¹。计算得出甲苯 + OH的主要产物为邻位加合物（69.8%）、苄基自由基 + H₂O（11.9%）、本位加合物（7.3%）、对位加合物（5.1%）和间位加合物（5.1%）。我们的结果表明，对于关键势垒高度的高水平量子化学计算以及对非谐性的适当处理，决定了最终计算得到的总速率系数和分支比的准确性。对各种反应通道分支比的进一步分析为甲苯在大气中的初始氧化提供了深入见解。
