烷烃与氢过氧自由基氢提取反应类反应能垒与速率常数的精确计算

Accurate Calculation of the Energy Barriers and Rate Constants of Hydrogen Abstraction from Alkanes by Hydroperoxyl Radical

氢过氧自由基从烷烃分子中提取氢的反应是碳氢燃料中低温燃烧化学中非常重要的一类反应。本文用等键反应方法计算了这一类反应的动力学参数。所有反应物、过渡态、产物的几何结构均在HF/6-31+G(d)水平下优化得到。以反应中的过渡态反应中心的几何结构守恒为判据,该反应类可用等键反应处理。本文选取了乙烷和氢过氧自由基的氢提取反应为参考反应,其它反应作为目标反应,用等键反应方法对目标反应在HF/6-31+G(d)水平的近似能垒和反应速率常数进行了校正。为了验证方法的可靠性,选取C5以下的烷烃分子体系,对等键反应方法校正结果和高精度CCSD(T)/CBS直接计算结果进行了比较,最大绝对误差为5.58k J?mol^(-1),因此,采用等键反应方法只需用低水平HF从头算方法就可以再现高精度CCSD(T)/CBS计算结果,从而解决了该反应类中大分子体系的能垒的精确计算。本文的研究为碳氢化合物中低温燃烧模拟中重要的烷烃与氢过氧自由基氢提取反应提供了准确的动力学参数。

Hydrogen abstraction from alkanes by hydroperoxyl radical is an important reaction class in the combustion of hydrocarbon fuel, particularly at low and intermediate temperature regimes. In this study, kinetic parameters for this reaction class are calculated using the isodesmic reaction method based on conservation of geometric structures for the reaction center of the transition states. The geometries for all the reactants, transition states, and products are optimized at the HF/6-31＋G（d） level. Hydrogen abstraction from ethane by hydroperoxyl radical is chosen as the reference reaction; other reactions are target reactions. The isodesmic reaction method is used to correct the approximate energy barriers and rate constants of the target reactions at the HF/6-31 ＋G（d） level. To validate the reliability of the isodesmic reaction method, the energy barriers calculated by the isodesmic reaction method and at a high level of CCSD（T）/CBS for alkanes containing less than five carbon atoms are compared. The maximum absolute difference of energy barriers between the isodesmic reaction method and CCSD（T）/CBS method is 5.58 kJ. mol-＇. Therefore, after correction, using the isodesmic reaction method, the low-level HF method can reproduce the high-level CCSD（T）/CBS calculated energy barriers. Thus, we have solved the problem of accurately calculating energy barriers for large molecular systems in this reaction class. The present work provides accurate kinetic parameters for hydrogen abstraction from alkanes by hydroperoxyl radical, which are important for combustion modeling at low and intermediate temperature regimes.