Environment Research Institute, Shandong University, Qingdao 266237, China.
College of Environment and Safety Engineering, Qingdao University of Science & Technology, Qingdao 266042, China.
Int J Mol Sci. 2019 May 28;20(11):2623. doi: 10.3390/ijms20112623.
Chlorothiophenols (CTPs) are known to be key and direct precursors of polychlorinated thianthrene/dibenzothiophenes (PCTA/DTs). Self/cross-coupling of the chlorothiophenoxy radicals (CTPRs), sulfydryl-substituted phenyl radicals and thiophenoxyl diradicals evolving from CTPs are initial and important steps for PCTA/DT formation. In this study, quantum chemical calculations were carried out to investigate the homogenous gas-phase formation of PCTA/DTs from self/cross-coupling of 2,4-dichlorothiophenoxy radical (R1), 2-sulfydryl-3,5-dichlorophenyl radical (R2) and 3,5-dichlorothiophenoxyl diradical (DR) at the MPWB1K/6-311+G(3df,2p)//MPWB1K/6-31+G(d,p) level. The rate constants of crucial elementary steps were deduced over 600-1200 K, using canonical variational transition state theory with a small curvature tunneling contribution. For the formation of PCTAs, the S•/σ-C• condensation with both thiophenolic sulfur in one radical and carbon in the other radical bonded to single electron is the most efficient sulfur-carbon coupling mode, and the ranking of the PCTA formation potential is DR + DR > R2 + DR > R1 + DR > R1 + R2 > R1 + R1. For the formation of PCDTs, the σ-C•/σ-C• coupling with both carbon in the two radicals bonded to single electron is the energetically favored carbon-carbon coupling mode, and the ranking of the PCDT formation potential is: R2 + DR > R2 + R2 > R1 + DR > R1 + R2 > R1 + R1. The PCTA/DTs could be produced from R1, R2 and DR much more readily than PCDD/DFs from corresponding oxygen substituted radicals.
氯代噻吩（CTPs）是多氯噻蒽/二苯并噻吩（PCTA/ DTs）的关键和直接前体。CTPRs、巯基取代的苯自由基和来自 CTPs 的噻吩氧基双自由基的自/交叉偶联是 PCTA/ DT 形成的初始和重要步骤。在这项研究中，通过量子化学计算研究了 2,4-二氯噻吩氧基自由基（R1）、2-巯基-3,5-二氯苯自由基（R2）和 3,5-二氯噻吩氧双自由基（DR）自/交叉偶联在均相气相中形成 PCTA/ DT 的过程。在 MPWB1K/6-311+G(3df,2p)//MPWB1K/6-31+G(d,p)水平上，使用小曲率隧道贡献的正则变分过渡态理论，在 600-1200 K 范围内推导了关键基元步骤的速率常数。对于 PCTA 的形成，带有一个自由基中噻酚硫和另一个自由基中碳的 S•/σ-C•缩合是最有效的硫-碳偶联模式，PCTA 形成势的排序为 DR + DR > R2 + DR > R1 + DR > R1 + R2 > R1 + R1。对于 PCDT 的形成，两个自由基中碳的σ-C•/σ-C•偶联是最有利的碳-碳偶联模式，PCDT 形成势的排序为：R2 + DR > R2 + R2 > R1 + DR > R1 + R2 > R1 + R1。与相应的氧取代基自由基相比，R1、R2 和 DR 更容易生成 PCTA/ DT，而不是 PCDD/DF。
