烷基自由基β位裂解反应类反应势垒与速率常数的精确计算

Accurate Calculation of the Reaction Barriers and Rate Constants of the Pyrolysis of Alkyl Radicals in the β Position Using the Isodesmic Reaction Method

提出反应类等键方法并用于高温燃烧机理中一类重要反应——烷基自由基β位裂解反应的反应势垒和速率常数的精确校正计算.通过10种不同从头算水平对类反应中5个代表反应的反应势垒的计算发现,用反应类等键反应方法和直接从头算方法获得的5个代表反应的反应势垒最大绝对偏差的平均值分别为5.32和16.16kJ·mol-1,表明反应类等键反应方法计算的反应势垒对不同水平从头算方法的依赖性小,可在较低从头算水平计算得到精确的反应势垒,解决大分子体系反应势垒的精确计算问题.此外应用反应类等键反应方法在BHandHLYP/cc-pVDZ从头算水平计算了3个代表反应的速率常数,并与文献报道的实验值进行了比较,其在500-2000K温度区间内计算速率常数与实验速率常数中较大值与较小值的比值kmax/kmin的平均值为1.67,最大值也仅有2.49.表明应用反应类等键反应方法在较低从头算水平即可对同类反应的速率常数进行精确计算.最后在BHandHLYP/cc-pVDZ从头算水平用反应类等键反应方法计算了13个烷基自由基β位裂解反应的速率常数.

The isodesmic reaction method is proposed for the accurate calculation of the reaction barriers and rate constants for an important class of reactions in the high-temperature combustion mechanism： the pyrolysis of alkyl radicals in theβ position. The reaction barriers were calculated for a representative set of five reactions by two schemes： the first scheme is to calculate the reaction barriers directly from approximate ab initio calculations; and the second scheme is to correct the reaction barriers from the first scheme using the isodesmic reaction method. Ten different levels of ab initio calculations were used, and the absolute average maximum deviations of the reaction barriers by the isodesmic reaction method and direct ab initio calculations were 5.32 and 16.16 kJ . mo1-1, respectively, indicating that the isodesmic reaction method does not significantly depend on the level of ab initio theory used. The rate constants of the three representative reactions in the temperature range of 500-2000 K were calculated by the isodesmic reaction method. The average and maximum values of k_max/k_min between the calculated and experimental values were 1.67 and 2.49, respectively. Therefore, the isodesmic reaction method is efficient and reliable for the calculation of the reaction barriers and rate constants of reactions in a class at a modest level of ab initio theory.