Qu Xiaohui, Wang Hui, Zhang Qingzhu, Shi Xiangyan, Xu Fei, Wang Wenxing
Environment Research Institute, Shandong University, Jinan 250100, P. R. China.
Environ Sci Technol. 2009 Jun 1;43(11):4068-75. doi: 10.1021/es802835e.
An understanding of the reaction mechanism of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) formation is crucial for any attempt to prevent PCDD/Fs formation. Among the polychlorophenols, 2,4,5-trichlorophenol (2,4,5-TCP) has the minimum number of CI atoms needed to form 2,3,7,8-tetrachlorinated dibenzo-p-dioxin (2,3,7,8-TeCDD), which is the most toxic among all 210 PCDD/F isomers. Experiments on the formation of PCDD/Fs from the 2,4,5-TCP precursor have been hindered by the strong toxicity of 2,3,7,8-TeCDD. In this work, we carried out molecular orbital theory calculations for the homogeneous gas-phase formation of PCDD/Fs from the 2,4,5-TCP precursor. Several energetically favorable formation pathways were revealed for the first time. The rate constants of crucial elementary steps were deduced over a wide temperature range of 600-1200 K, using canonical variational transition state theory with small curvature tunneling contribution. The rate temperature formulas were fitted. This study shows that the formation of polychlorinated dibenzo-p-dioxins (PCDDs) from the 2,4,5-TCP precursor is preferred over the formation of polychlorinated dibenzofurans (PCDFs). The chlorine substitution pattern has a significant effect on the dimerization of chlorophenoxy radicals.
了解多氯代二苯并 - 对二噁英和二苯并呋喃（PCDD/Fs）的形成反应机制对于任何预防PCDD/Fs形成的尝试都至关重要。在多氯苯酚中，2,4,5 - 三氯苯酚（2,4,5 - TCP）具有形成2,3,7,8 - 四氯代二苯并 - 对二噁英（2,3,7,8 - TeCDD）所需的最少氯原子数，而2,3,7,8 - TeCDD是所有210种PCDD/F异构体中毒性最强的。由2,4,5 - TCP前体形成PCDD/Fs的实验因2,3,7,8 - TeCDD的强毒性而受到阻碍。在这项工作中，我们对由2,4,5 - TCP前体在均相气相中形成PCDD/Fs进行了分子轨道理论计算。首次揭示了几种能量上有利的形成途径。使用具有小曲率隧道效应贡献的正则变分过渡态理论，在600 - 1200 K的宽温度范围内推导了关键基元步骤的速率常数。拟合了速率 - 温度公式。该研究表明，由2,4,5 - TCP前体形成多氯代二苯并 - 对二噁英（PCDDs）比形成多氯代二苯并呋喃（PCDFs）更有利。氯取代模式对氯苯氧基自由基的二聚化有显著影响。
