Yang Mo, Wang Juan
National Key Laboratory of Science and Technology on Aero-Engine Aero-Thermodynamics, School of Energy and Power Engineering, Beihang University, Beijing 100191, PR China.
J Phys Chem A. 2024 Jun 6;128(22):4517-4531. doi: 10.1021/acs.jpca.4c00480. Epub 2024 May 28.
Cycloalkanes serve as an important class of chemical components in both fossil and alternative transportation fuels and have attracted considerable attention from the combustion community. Hydrogen abstractions from cycloalkanes by hydroxyl radicals initiate the fuel decomposition process and trigger off the subsequent chain reactions and thus play an important role in both combustion and atmospheric chemistry. The target of this study is to fill the vacancy in kinetics data toward the H-abstraction reactions by hydroxyl radical from three typical dimethylcyclohexane isomers through first-principles and direct dynamics. The rate constants involving 18 elementary reactions in total were accurately determined by the multipath canonical variational transition state theory with the multidimensional small-curvature correction for tunneling (MP-CVT/SCT), over a broad temperature range of 200-2000 K. The significant roles of multistructural torsional anharmonicity and recrossing effects were stressed per abstraction site, while the quantum tunneling effect was found to be slight at temperatures of interest in combustion. The discrepancies observed among different reaction systems at a similar abstraction site highlight the fuel molecular effects on site-specific rate constants. The comparison results of total rate constants given by different dynamics approaches prove the importance of considering the torsional anharmonicity, recrossing, and tunneling effects, and the robust feature of the simplified MS-CVT/SCT. The calculated total constants for dimethylcyclohexane isomers by OH are consistent with those measured for methylcyclohexane and 1,4-dimethylcyclohexane at low temperatures. The branching ratio analysis confirms the predominant role of the tertiary abstraction at low-to-intermediate temperatures and its growing competition with distinct secondary abstractions as temperature increases. The calculated rate constants were eventually fitted into the analytical expressions and incorporated into the kinetic models to learn about the influences on modeling performance.
环烷烃是化石燃料和替代运输燃料中的一类重要化学成分，受到了燃烧学界的广泛关注。羟基自由基从环烷烃中夺取氢原子引发了燃料分解过程，并触发了后续的链式反应，因此在燃烧和大气化学中都起着重要作用。本研究的目标是通过第一性原理和直接动力学填补羟基自由基从三种典型二甲基环己烷异构体夺取氢反应动力学数据的空白。通过多路径正则变分过渡态理论结合多维小曲率隧道效应校正（MP-CVT/SCT），在200-2000K的宽温度范围内准确测定了总共18个基元反应的速率常数。每个夺取氢位点都强调了多结构扭转非谐性和再穿越效应的重要作用，而在燃烧相关温度下量子隧道效应很微弱。在相似夺取氢位点不同反应体系间观察到的差异突出了燃料分子对位点特异性速率常数的影响。不同动力学方法给出的总速率常数比较结果证明了考虑扭转非谐性、再穿越和隧道效应的重要性，以及简化的MS-CVT/SCT的稳健性。羟基自由基与二甲基环己烷异构体反应计算得到的总速率常数与低温下甲基环己烷和1,4-二甲基环己烷的测量值一致。分支比分析证实了在低温到中温下三级夺取氢的主导作用，以及随着温度升高其与不同二级夺取氢竞争的增加。最终将计算得到的速率常数拟合为解析表达式并纳入动力学模型，以了解其对建模性能的影响。
