Greenwald Erin E, North Simon W, Georgievskii Yuri, Klippenstein Stephen J
Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, TX 77842, USA.
J Phys Chem A. 2007 Jun 28;111(25):5582-92. doi: 10.1021/jp071412y. Epub 2007 Jun 1.
A two transition state model is applied to the prediction of the isomeric branching in the addition of hydroxyl radical to isoprene. The outer transition state is treated with phase space theory fitted to long-range transition state theory calculations on an electrostatic potential energy surface. High-level quantum chemical estimates are applied to the treatment of the inner transition state. A one-dimensional master equation based on an analytic reduction from two-dimensions for a particular statistical assumption about the rotational part of the energy transfer kernel is employed in the calculation of the pressure dependence of the addition process. We find that an accurate treatment of the two separate transition state regions, at the energy and angular momentum resolved level, is essential to the prediction of the temperature dependence of the addition rate. The transition from a dominant outer transition state to a dominant inner transition state is shown to occur at about 275 K, with significant effects from both transition states over the 30-500 K temperature range. Modest adjustments in the ab initio predicted inner saddle point energies yield predictions that are in quantitative agreement with the available high-pressure limit experimental observations and qualitative agreement with those in the falloff regime. The theoretically predicted capture rate is reproduced to within 10% by the expression [1.71 x 10(-10)(T/298)(-2.58) exp(-608.6/RT) + 5.47 x 10(-11)(T/298)-1.78 exp(-97.3/RT); with R = 1.987 and T in K] cm3 molecule(-1) s(-1) over the 30-500 K range. A 300 K branching ratio of 0.67:0.02:0.02:0.29 was determined for formation of the four possible OH-isoprene adduct isomers 1, 2, 3, and 4, respectively, and was found to be relatively insensitive to temperature. An Arrhenius activation energy of -0.77 kcal/mol was determined for the high-pressure addition rate constants around 300 K.
一种双过渡态模型被应用于预测羟基自由基加成到异戊二烯过程中的异构分支。外层过渡态采用相空间理论处理，该理论与基于静电势能面的长程过渡态理论计算相拟合。采用高水平量子化学估计来处理内层过渡态。在计算加成过程的压力依赖性时，基于对能量转移核旋转部分的特定统计假设从二维进行解析简化得到的一维主方程被采用。我们发现，在能量和角动量分辨水平上对两个独立过渡态区域进行精确处理，对于预测加成速率的温度依赖性至关重要。从占主导的外层过渡态到占主导的内层过渡态的转变显示在约275 K时发生，在30 - 500 K温度范围内，两个过渡态都有显著影响。对从头算预测的内层鞍点能量进行适度调整后得到的预测结果，在定量上与现有的高压极限实验观测结果一致，在定性上与衰减区的结果一致。理论预测的捕获速率通过表达式[1.71×10⁻¹⁰(T/298)⁻²·⁵⁸ exp(-608.6/RT) + 5.47×10⁻¹¹(T/298)⁻¹·⁷⁸ exp(-97.3/RT); 其中R = 1.987且T以K为单位] cm³ 分子⁻¹ s⁻¹ 在30 - 500 K范围内被再现到10%以内。对于形成四种可能的OH - 异戊二烯加合物异构体1、2、3和4，分别确定了300 K时的分支比为0.67:0.02:0.02:0.29，并且发现其对温度相对不敏感。对于300 K附近的高压加成速率常数，确定了阿仑尼乌斯活化能为 - 0.77 kcal/mol。
