Joshi Satya P, Pekkanen Timo T, Seal Prasenjit, Timonen Raimo S, Eskola Arkke J
Molecular Science, Department of Chemistry, University of Helsinki, P.O. Box 55 (A.I. Virtasen aukio 1), FIN-00014, Helsinki, Finland.
Phys Chem Chem Phys. 2021 Sep 22;23(36):20419-20433. doi: 10.1039/d1cp02210e.
The kinetics of the reaction between resonance-stabilized (CH)CCHCH radical (R) and O has been investigated using photoionization mass spectrometry, and master equation (ME) simulations were performed to support the experimental results. The kinetic measurements of the (CH)CCHCH + O reaction (1) were carried out at low helium bath-gas pressures (0.2-5.7 Torr) and over a wide temperature range (238-660 K). Under low temperature (238-298 K) conditions, the pressure-dependent bimolecular association reaction R + O → ROO determines kinetics, until at an intermediate temperature range (325-373 K) the ROO adduct becomes thermally unstable and increasingly dissociates back to the reactants with increasing temperature. The initial association of O with (CH)CCHCH radical occurs on two distinct sites: terminal 1(t) and non-terminal 1(nt) sites on R, leading to the barrierless formation of ROO and ROO adducts, respectively. Important for autoignition modelling of olefinic compounds, bimolecular reaction channels appear to open for the R + O reaction at high temperatures ( > 500 K) and pressure-independent bimolecular rate coefficients of reaction (1) with a weak positive temperature dependence, (2.8-4.6) × 10 cm molecule s, were measured in the temperature range of 500-660 K. At a temperature of 501 K, a product signal of reaction (1) was observed at / = 68, probably originating from isoprene. To explore the reaction mechanism of reaction (1), quantum chemical calculations and ME simulations were performed. According to the ME simulations, without any adjustment to energies, the most important and second most important product channels at the high temperatures are isoprene + HO (yield > 91%) and (2/)-3-methyl-1,2-epoxybut-3-ene + OH (yield < 8%). After modest adjustments to ROO and ROO well-depths (∼0.7 kcal mol each) and barrier height for the transition state associated with the kinetically most dominant channel, R + O → isoprene + HO (∼2.2 kcal mol), the ME model was able to reproduce the experimental findings. Modified Arrhenius expressions for the kinetically important reaction channels are enclosed to facilitate the use of current results in combustion models.
利用光电离质谱研究了共振稳定的(CH)CCHCH自由基(R)与O之间反应的动力学，并进行了主方程(ME)模拟以支持实验结果。(CH)CCHCH + O反应(1)的动力学测量在低氦浴气压力(0.2 - 5.7托)和较宽温度范围(238 - 660 K)下进行。在低温(238 - 298 K)条件下，压力依赖的双分子缔合反应R + O → ROO决定了动力学，直到在中间温度范围(325 - 373 K)，ROO加合物变得热不稳定，并随着温度升高越来越多地分解回反应物。O与(CH)CCHCH自由基的初始缔合发生在两个不同的位点：R上的末端1(t)和非末端1(nt)位点，分别导致无势垒地形成ROO和ROO加合物。对于烯烃化合物的自燃建模很重要的是，在高温(> 500 K)下，双分子反应通道似乎对R + O反应打开，并且在500 - 660 K温度范围内测量了反应(1)的与压力无关的双分子速率系数，其具有较弱的正温度依赖性，为(2.8 - 4.6)×10 cm molecule s。在501 K的温度下，在/ = 68处观察到反应(1)的产物信号(可能源自异戊二烯)。为了探索反应(1)的反应机理，进行了量子化学计算和ME模拟。根据ME模拟，在不对能量进行任何调整的情况下，高温下最重要和第二重要的产物通道是异戊二烯 + HO(产率> 91%)和(2/)-3 - 甲基 - 1,2 - 环氧丁 - 3 - 烯 + OH(产率< 8%)。在对ROO和ROO阱深(各约为每摩尔0.7千卡)以及与动力学上最主要通道R + O → 异戊二烯 + HO相关的过渡态的势垒高度进行适度调整(约每摩尔2.2千卡)后，ME模型能够重现实验结果。文中附上了动力学重要反应通道的修正阿伦尼乌斯表达式，以便于在燃烧模型中使用当前结果。
