Center for Computational Quantum Chemistry, University of Georgia, Athens, Georgia 30602, USA.
J Chem Phys. 2017 May 21;146(19):194304. doi: 10.1063/1.4983128.
Alkyl combustion models depend on kinetic parameters derived from reliable experimental or theoretical energetics that are often unavailable for larger species. To this end, we have performed a comprehensive investigation of the tert-butyl radical (R in this paper) autoignition pathways. CCSD(T)/ANO0 geometries and harmonic vibrational frequencies were obtained for key stationary points for the R + O and QOOH + O mechanisms. Relative energies were computed to chemical accuracy (±1 kcal mol) via extrapolation of RCCSD(T) energies to the complete basis-set limit, or usage of RCCSD(T)-F12 methods. At 0 K, the minimum energy R + O pathway involves direct elimination of HO (30.3 kcal mol barrier) from the tert-butyl peroxy radical (ROO) to give isobutene. This pathway lies well below the competing QOOH-forming intramolecular hydrogen abstraction pathway (36.2 kcal mol barrier) and ROO dissociation (35.9 kcal mol barrier). The most favorable decomposition channel for QOOH radicals leads to isobutene oxide (12.0 kcal mol barrier) over isobutene (18.6 kcal mol barrier). For the QOOH + O pathways, we studied the transition states and initial products along three pathways: (1) α-hydrogen abstraction (42.0 kcal mol barrier), (2) γ-hydrogen abstraction (27.0 kcal mol barrier), and (3) hydrogen transfer to the peroxy moiety (24.4 kcal mol barrier). The barrier is an extensive modification to the previous 18.7 kcal mol value and warrants further study. However, it is still likely that the lowest energy QOOH + O pathway corresponds to pathway (3). We found significant spin contamination and/or multireference character in multiple stationary points, especially for transition states stemming from QOOH. Lastly, we provide evidence for an A∼-X∼ surface crossing at a C-symmetric, intramolecular hydrogen abstraction structure.
烷基燃烧模型依赖于从可靠的实验或理论热力学中得出的动力学参数，而这些参数对于较大的物种通常是不可用的。为此，我们对叔丁基自由基（本文中用 R 表示）的自动点火途径进行了全面研究。对于 R + O 和 QOOH + O 反应机制中的关键静止点，我们获得了 CCSD(T)/ANO0 几何形状和简谐振动频率。通过将 RCCSD(T)能量外推至完全基组极限，或使用 RCCSD(T)-F12 方法，计算得到了化学精度（±1 kcal mol）的相对能量。在 0 K 时，能量最低的 R + O 途径涉及从叔丁基过氧自由基（ROO）中直接消除 HO（30.3 kcal mol 势垒），生成异丁烯。这条途径远低于竞争的 QOOH 形成分子内氢提取途径（36.2 kcal mol 势垒）和 ROO 解离（35.9 kcal mol 势垒）。对于 QOOH 自由基，最有利的分解通道导致异丁烯氧化物（12.0 kcal mol 势垒）生成，而不是异丁烯（18.6 kcal mol 势垒）。对于 QOOH + O 途径，我们研究了沿着三条途径的过渡态和初始产物：（1）α-氢提取（42.0 kcal mol 势垒），（2）γ-氢提取（27.0 kcal mol 势垒），和（3）氢转移到过氧基部分（24.4 kcal mol 势垒）。该势垒对之前的 18.7 kcal mol 值进行了广泛的修正，需要进一步研究。然而，最低能量的 QOOH + O 途径很可能对应于途径（3）。我们在多个静止点，特别是来自 QOOH 的过渡态，发现了显著的自旋污染和/或多参考特征。最后，我们提供了在一个 C 对称的、分子内氢提取结构处存在 A∼-X∼ 表面交叉的证据。
