Semenikhin Alexander S, Savchenkova Anna S, Chechet Ivan V, Matveev Sergey G, Frenklach Michael, Mebel Alexander M
Samara National Research University, Samara 443086, Russia.
Department of Mechanical Engineering, University of California at Berkeley, Berkeley, California 94720-1740, United States.
J Phys Chem A. 2021 Apr 29;125(16):3341-3354. doi: 10.1021/acs.jpca.1c00900. Epub 2021 Apr 20.
Five-membered rings are constituents of many polycyclic aromatic hydrocarbons (PAHs), and their presence on the edges of large PAHs has been repeatedly observed experimentally. However, modern kinetic combustion models often do not consider the growth of PAHs through the transformation of the five-membered rings. In connection with the above, we carried out a theoretical study of the mechanism of hydrogen-abstraction-acetylene-addition (HACA) transformation of an embedded five-membered ring on the armchair PAH edge to a six-membered ring, considering cyclopenta[,,]phenanthrene (4,5-methylenephenanthrene) as a prototype system for this process. The potential energy surface for the reactions of cyclopenta[,,]phenanthrenyl radicals produced by direct H abstractions from cyclopenta[,,]phenanthrene with acetylene has been compiled at the G3(MP2,CC)//B3LYP/6-311G(d,p) level of theory including zero-point vibrational energy corrections. The computed energies and molecular parameters were then used to solve the Rice-Ramsperger-Kassel-Marcus master equation in order to calculate the reaction rate at various pressures and temperatures, which were fitted to the modified Arrhenius equation for further kinetic modeling. The results show that the HACA transformation of the embedded five-membered ring to a six-membered ring is possible, albeit slow. The most viable reaction mechanism involves the R2 + CH reaction, where the acetylene molecules add to a σ-radical in the six-membered ring adjacent to the five-membered ring via a low entrance barrier. The predominant product of R2 + CH is predicted to be 3-ethynyl-4-cyclopenta[def]phenanthrene Pr5 via immediate H elimination from the initial addition complex. Next, Pr5 undergoes H-assisted isomerization to 4aH-pentaleno[4,3,2,1-cdef]phenanthrene Pr4, and the latter adds a H-atom eventually forming the 1-pyrenylmethyl radical Pr3: R2 + CH ⇆ 3-ethynyl-4-cyclopenta[def]phenanthrene (Pr5) + H or 4aH-pentaleno[4,3,2,1-cdef]phenanthrene (Pr4) + H; Pr5 + H ⇆ Pr4 + H; Pr4 + H → 1-pyrenylmethyl (Pr3). This HACA sequence may be competitive with the methyl radical addition to the R1 radical formed by H abstraction from the CH group in the five-membered ring of cyclopenta[,,]phenanthrene, which provides a pathway to pyrene following two H-atom losses. Relative contributions of the two mechanisms of the five- to six-membered ring transformation would strongly depend on the branching ratios of the R1 and R2 radicals produced by the H abstractions and the available concentration of CH versus CH and hence differ in different flames.
五元环是许多多环芳烃（PAHs）的组成部分，并且在大型PAHs的边缘上其存在已被多次实验观察到。然而，现代动力学燃烧模型通常不考虑通过五元环的转化来实现PAHs的生长。基于上述情况，我们进行了一项理论研究，以环戊[,,]菲（4,5-亚甲基菲）作为此过程的原型系统，研究扶手椅型PAH边缘上嵌入的五元环通过氢提取-乙炔加成（HACA）转化为六元环的机理。在G3(MP2,CC)//B3LYP/6-311G(d,p)理论水平下，包括零点振动能校正，编制了由环戊[,,]菲直接氢提取产生的环戊[,,]菲基自由基与乙炔反应的势能面。然后，使用计算得到的能量和分子参数来求解赖斯-拉姆齐格-卡塞尔-马库斯主方程，以计算在各种压力和温度下的反应速率，并将其拟合到修正的阿伦尼乌斯方程以进行进一步的动力学建模。结果表明，嵌入的五元环向六元环的HACA转化是可能的，尽管速度较慢。最可行的反应机理涉及R2 + CH反应，其中乙炔分子通过低入口势垒加成到与五元环相邻的六元环中的一个σ-自由基上。预测R2 + CH的主要产物是3-乙炔基-4-环戊[def]菲Pr5，它是通过从初始加成复合物中立即消除氢而形成的。接下来，Pr5经历氢辅助异构化生成4aH-戊搭烯并[4,3,2,1-cdef]菲Pr4，后者最终添加一个氢原子形成1-芘甲基自由基Pr3：R2 + CH ⇆ 3-乙炔基-4-环戊[def]菲（Pr5）+ H或4aH-戊搭烯并[4,3,2,1-cdef]菲（Pr4）+ H；Pr5 + H ⇆ Pr4 + H；Pr4 + H → 1-芘甲基（Pr3）。这种HACA序列可能与甲基自由基加成到由环戊[,,]菲的五元环中的CH基团氢提取形成的R1自由基相竞争，这提供了一条经过两次氢原子损失生成芘的途径。五元环到六元环转化的两种机理的相对贡献将强烈取决于氢提取产生的R1和R2自由基的分支比以及CH与CH的可用浓度，因此在不同火焰中会有所不同。
