Department of Chemistry, University of Leuven, Celestijnenlaan 200F, B-3001, Heverlee-Leuven, Belgium.
Chemphyschem. 2010 Dec 17;11(18):3996-4001. doi: 10.1002/cphc.201000480.
As a sequel to our communication on a proposed new isoprene oxidation mechanism aiming to rationalize the unexpectedly high OH and HO(2) levels observed in isoprene-rich areas (J. Peeters, T. L. Nguyen, L. Vereecken, Phys. Chem. Chem. Phys. 2009, 11, 5935), we report herein the detailed quantum chemical and statistical kinetics characterization of the crucial 1,6-H shifts in the two Z-δ-hydroxy-peroxy radicals from isoprene. Geometries, energies and vibration frequencies of all conformers of the reactant radicals and transition states are computed at the B3LYP/6-31+G(d,p) level of theory and the energies of the lowest-lying conformers are then refined at various higher levels of theory, including CBS-QB3, IRCMax(CBS-QB3//B3LYP) and CBS-APNO. The rate coefficients over a wide temperature range are calculated using multi-conformer transition state theory with WKB tunneling factors evaluated for the barrier shape found by CBS-QB3//B3LYP IRC analyses. The WKB tunneling factors for these allyl-stabilisation-assisted reactions are about 25 at ambient temperatures. The rate coefficients can be represented by Arrhenius expressions over the 250-350 K range: k(T)=1.4×10(9) exp(-6380/T) s(-1) for the Z-1-OH-4-OO(·)-isoprene radical, and k(T)=0.72×10(9) exp(-5520/T) s(-1) for Z-1-OH-4-OO(·)-isoprene. With the k(1,6-H) of order 1 s(-1) at ambient temperatures, these isomerisations can compete with and even outrun the traditional peroxy reactions at low and moderate NO levels. The importance of these reactions as key processes in the newly proposed, OH-regenerating isoprene oxidation scheme is discussed.
作为我们关于拟议的新异戊二烯氧化机制的交流的后续，该机制旨在使在富含异戊二烯的区域中观察到的异常高的 OH 和 HO(2) 水平合理化（J. Peeters、T. L. Nguyen、L. Vereecken，Phys. Chem. Chem. Phys. 2009，11，5935），我们在此报告了异戊二烯中两种 Z-δ-羟基过氧自由基中关键 1,6-H 转移的详细量子化学和统计动力学特征。在 B3LYP/6-31+G(d,p) 理论水平上计算了反应物自由基和过渡态的所有构象的几何形状、能量和振动频率，然后在各种更高的理论水平上精修了最低能量构象的能量，包括 CBS-QB3、IRCMax(CBS-QB3//B3LYP) 和 CBS-APNO。在很宽的温度范围内使用多构象过渡态理论计算了速率系数，并为通过 CBS-QB3//B3LYP IRC 分析找到的势垒形状评估了 WKB 隧穿因子。这些烯丙基稳定化辅助反应的 WKB 隧穿因子在环境温度下约为 25。在 250-350 K 范围内，速率系数可以用 Arrhenius 表达式表示：Z-1-OH-4-OO(·)-isoprene 自由基的 k(T)=1.4×10(9) exp(-6380/T) s(-1)，Z-1-OH-4-OO(·)-isoprene 的 k(T)=0.72×10(9) exp(-5520/T) s(-1)。在环境温度下，k(1,6-H)约为 1 s(-1)，这些异构化反应可以与低中和适度 NO 水平下的传统过氧反应竞争甚至超过这些反应。讨论了这些反应作为新提出的 OH 再生异戊二烯氧化方案中的关键过程的重要性。
