Semenikhin A S, Savchenkova A S, Chechet I V, Matveev S G, Liu Z, Frenklach M, Mebel A M
Samara National Research University, Samara, 443086, Russia.
Phys Chem Chem Phys. 2017 Sep 27;19(37):25401-25413. doi: 10.1039/c7cp05560a.
Density functional B3LYP/6-31G(d) and ab initio G3(MP2,CC) calculations have been carried out to determine thermal rate constants of direct H abstraction reactions from four- and five-ring polycyclic aromatic hydrocarbons (PAH) chrysene and benzo[a]pyrene by various radicals abundant in combustion flames, such as H, CH, CH, and OH, using transition state theory. The results show that the H abstraction reactions with OH have the lowest barriers of ∼4 kcal mol, followed by those with H and CH with barriers of 16-17 kcal mol, and then with propargyl radicals with barriers of 24-26 kcal mol. Thus, the OH radical is predicted to be the fastest H abstractor from PAH. Even at 2500 K, the rate constant for H abstraction by H is still 34% lower than the rate constant for H abstraction by OH. The reaction with H is calculated to have rate constants 35-19 times higher than those for the reaction with CH due to a more favorable entropic factor. The reactions of H abstraction by CH are predicted to be orders of magnitude slower than the other reactions considered and their equilibrium is strongly shifted toward the reactants, making propargyl an inefficient H abstractor from the aromatics. The calculations showed strong similarity of the reaction energetics in different H abstraction positions of benzo[a]pyrene and chrysene within armchair and zigzag edges in these molecules, but clear distinction between the armchair and zigzag sites. The zigzag sites appear to be more reactive, with H abstraction rate constants by H, CH, and OH being respectively 37-42%, a factor of 2.1, and factors of 8-9 higher than the corresponding rate constants for the H abstraction reactions from armchair sites. Although the barrier heights for the two types of edges are similar, the entropic factor makes zigzag sites more favorable for H abstraction. Rate expressions have been generated for all studied reactions with the goal to rectify current combustion kinetics mechanisms.
采用过渡态理论，通过密度泛函B3LYP/6 - 31G(d)和从头算G3(MP2,CC)计算，确定了燃烧火焰中大量存在的各种自由基（如H、CH、CH和OH）从四环和五环多环芳烃（PAH）芘和苯并[a]芘直接夺取H反应的热速率常数。结果表明，与OH的夺氢反应具有最低的势垒，约为4 kcal/mol，其次是与H和CH的反应，势垒为16 - 17 kcal/mol，然后是与炔丙基自由基的反应，势垒为24 - 26 kcal/mol。因此，预计OH自由基是从PAH夺取H最快的自由基。即使在2500 K时，H夺取H的速率常数仍比OH夺取H的速率常数低34%。由于更有利的熵因子，计算得出与H反应的速率常数比与CH反应的速率常数高35 - 19倍。预计CH夺取H的反应比其他考虑的反应慢几个数量级，且其平衡强烈偏向反应物，使得炔丙基从芳烃中夺取H的效率较低。计算表明，在这些分子的扶手椅型和锯齿型边缘内，苯并[a]芘和芘不同夺氢位置的反应能量学具有很强的相似性，但扶手椅型和锯齿型位点之间有明显区别。锯齿型位点似乎更具反应性，H、CH和OH从锯齿型位点夺取H的速率常数分别比从扶手椅型位点夺取H的相应速率常数高37 - 42%、2.1倍和8 - 9倍。尽管两种边缘的势垒高度相似，但熵因子使锯齿型位点更有利于夺氢。已为所有研究的反应生成了速率表达式，目的是修正当前的燃烧动力学机制。
