Institut für Energie- und Klimaforschung IEK-8: Troposphäre, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany.
Phys Chem Chem Phys. 2012 Oct 28;14(40):13933-48. doi: 10.1039/c2cp42434g. Epub 2012 Sep 11.
The reversible gas-phase addition of OH radicals to the trimethylbenzenes was investigated in pulsed experiments utilizing VUV flash-photolysis resonance-fluorescence of H(2)O in the temperature range of 275-340 K. Triexponential OH decays were observed in the presence of the trimethylbenzenes, indicating the participation of more than one adduct species. Analytical solutions for the system of differential equations with two adduct isomers were derived, and the OH decay curves were evaluated based on this reaction model. This led to significant improvements of fit qualities and notable changes in OH rate constants compared to a previous model with a single adduct species. The detailed analysis was confined to 1,3,5-trimethylbenzene where reversible formation of two OH-aromatic ortho- and ipso-adduct isomers is feasible in accordance with the extended reaction model. Only after inclusion of additional isomerization reactions, consistent thermochemical data were obtained from the fitted rate constants. Reaction enthalpies of -83 ± 7 kJ mol(-1) and -35 ± 22 kJ mol(-1) were derived for the formation of one adduct isomer and the isomerization into the other, respectively. Based on literature data, the more and less stable adducts were assigned to ipso- and ortho-adduct isomers, respectively. The potential isomerization precluded the determination of primary yields of adduct isomers but formation of the ipso-adduct in any case is a minor process. For the rate constants of the OH + 1,3,5-trimethylbenzene reaction an Arrhenius expression k(OH) = 1.32 × 10(-11) cm(3) s(-1) exp(450 ± 50 K/T) was obtained. Based on the same approach, the rate constants of the OH reactions with 1,2,3-trimethylbenzene and 1,2,4-trimethylbenzene were derived as k(OH) = 3.61 × 10(-12) cm(3) s(-1) exp(620 ± 80 K/T) and k(OH) = 2.73 × 10(-12) cm(3) s(-1) exp(730 ± 70 K/T), respectively.
采用 VUV 光解瞬态吸收法，在 275-340 K 的温度范围内，利用 H(2)O 的光致荧光，对 OH 自由基与三甲苯的气相可逆加成反应进行了脉冲实验研究。在三甲苯存在下，观察到 OH 的三指数衰减，表明有不止一种加成产物参与。推导出了具有两种加成异构体的微分方程组的解析解，并基于该反应模型对 OH 衰减曲线进行了评估。与之前具有单一加成产物的模型相比，这一方法显著提高了拟合质量，并显著改变了 OH 速率常数。详细分析仅限于 1,3,5-三甲苯，因为根据扩展的反应模型，在此处，两种可逆形成的 OH-芳基邻位和对位加成异构体是可行的。只有在加入额外的互变异构反应后，才能从拟合的速率常数中获得一致的热化学数据。形成一个加成异构体的反应焓为-83 ± 7 kJ mol(-1)，异构化为另一个异构体的反应焓为-35 ± 22 kJ mol(-1)。根据文献数据，将更稳定和不太稳定的加成物分别分配给对位和邻位加成异构体。由于潜在的互变异构作用，无法确定加成异构体的初级产率，但加成异构体的形成无论如何都是一个次要过程。对于 OH + 1,3,5-三甲苯反应的速率常数，得到了 k(OH) = 1.32 × 10(-11) cm(3) s(-1) exp(450 ± 50 K/T)的阿伦尼乌斯表达式。基于相同的方法，推导了 OH 与 1,2,3-三甲苯和 1,2,4-三甲苯的反应速率常数，分别为 k(OH) = 3.61 × 10(-12) cm(3) s(-1) exp(620 ± 80 K/T)和 k(OH) = 2.73 × 10(-12) cm(3) s(-1) exp(730 ± 70 K/T)。
