Zhao Qian, Møller Kristian H, Chen Jing, Kjaergaard Henrik G
State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Shanxi, Xi'an710049, China.
Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100, Copenhagen Ø, Denmark.
J Phys Chem A. 2022 Sep 22;126(37):6483-6494. doi: 10.1021/acs.jpca.2c04328. Epub 2022 Sep 2.
Alkoxy radicals are important intermediates in the gas-phase oxidation of volatile organic compounds (VOCs) determining the nature of the first-generation products. An accurate description of their chemistry under atmospheric conditions is essential for understanding the atmospheric oxidation of VOCs. Unfortunately, experimental measurements of the rate coefficients of unimolecular alkoxy radical reactions are scarce, especially for larger systems. As has previously been done for peroxy radical hydrogen shift reactions, we present a cost-effective approach to the practical implementation of multiconformer transition state theory (MC-TST) for alkoxy radical unimolecular (H-shift and decomposition) reactions. Specifically, we test the optimal approach for the conformational sampling as well as the best value for a cutoff of high-energy conformers. In order to obtain accurate rate coefficients at a reduced computational cost, an energy cutoff is employed to reduce the required number of high-level calculations. The rate coefficients obtained with the developed theoretical approach are compared to available experimental rate coefficients for both 1,5 H-shifts and decomposition reactions. For all but one of the reactions tested, the calculated MC-TST rate coefficients agree with experimental results to within a factor of 7. The discrepancy for the final reaction is about a factor of 15, but part of the discrepancy is caused by pressure effects, which are not included in MC-TST. Thus, for the fastest alkoxy reactions, deviation from the high-pressure limit even at 1 bar should be considered.
烷氧基自由基是挥发性有机化合物（VOCs）气相氧化过程中的重要中间体，决定了第一代产物的性质。准确描述它们在大气条件下的化学反应对于理解VOCs的大气氧化至关重要。不幸的是，关于单分子烷氧基自由基反应速率系数的实验测量很少，尤其是对于较大的体系。正如之前对过氧自由基氢转移反应所做的那样，我们提出了一种经济高效的方法来实际应用多构象体过渡态理论（MC-TST）于烷氧基自由基单分子（氢转移和分解）反应。具体而言，我们测试了构象采样的最佳方法以及高能构象体截止值的最佳取值。为了以降低的计算成本获得准确的速率系数，采用能量截止来减少所需的高水平计算数量。将用所开发的理论方法获得的速率系数与1,5氢转移和分解反应的现有实验速率系数进行比较。对于除一个测试反应外的所有反应，计算得到的MC-TST速率系数与实验结果的吻合度在7倍以内。最后一个反应的差异约为15倍，但部分差异是由压力效应引起的，而MC-TST中未包含压力效应。因此，对于最快的烷氧基反应，即使在1巴下也应考虑偏离高压极限的情况。
