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首页> 外文期刊>Chemistry: A European journal >The Thermal Decomposition of Thiirane: A Mechanistic Study by Ab Initio MO Theory
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The Thermal Decomposition of Thiirane: A Mechanistic Study by Ab Initio MO Theory

机译：硫杂环戊烷的热分解：从头算MO理论的机理研究

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摘要

Using high-level ab initio MO methods, we have identified two reaction pathways with different thermodytion pathways with different thermodynamic and kinetic properties for the thermal decomposition of the three-membered heterocycle thiirane (C_2H_4S) and related derivatives. A hemolytic ring opening, followed by attack of the generated diradical on another thiirane molecule, and subsequent elimination of ethane in a fast radical chain reaction results in the formation of disulfur molecules in their triplet ground state (~3S_2) and requires activation enthalpies of #DELTA#H_(298) = 222 kJ mol~(-1) and #DELTA#G_(298) = 212 kJ mol~(-1). This reaction mechanism would result in a first-order rate law in agreement with one reported gas-phase experiment but does neither match the experimental activation energy nor does it explain the observed retention of the stereochemical configuration in the thermal decomposition of certain substituted thiiranes. Alternatively, sulfur atoms can be transferred from one thiirane molecule to another with the intermediate formation of thiirane 1-sulfide (C_2H_4S_2). This molecule can either decompose unimolecularly to ethane and disulfur in its excited singlet state (~1S_2) or, by means of spin crossover, S_2 in its triplet ground state may be formed. On the other hand, the thiirane 1-sulfide may react with itself and transfer one sulfur atom from one molecule to another with formation of thiirane 1,1-disulfide (C_2H_4S_3), which is an analogue of thiirane sulfone; thiirane is formed as the second product. The 1,1-disulfide may then decompose to ethane and S_3. In still another bimolecular reaction, the thiirane 1-sulfide may react with itself in a strongly exothermic reaction to give S_4 and two equivalents of ethane. This series of reactions results in a second-order rate law and requires activation enthalpies of #DELTA#H_(298) = 109 kJ mol~(-1) and #DELTA#G_(298) = 144 kJ mol~(-1) for the formation of thiirane 1-sulfide, while the consecutive reactions require less activation enthalpy. Elemental sulfur (S_8) is eventually formed by oligomerization of either S_2, S_3, or S_4 in spin-allowed reactions. These findings are in agreement with most experimental data on the thermal desulfurization of thiirane and its substituted derivatives. Thiirane 1-persulfide (C_2H_4S_3) with a linear arrangement of the three sulfur atoms as well as zwitterions and radicals derived from thiirane are not likely to be intermediates in teh thermal decomposition of episulfides.
机译：我们使用高水平的从头算MO方法，确定了三元杂环硫杂环丁烷（C_2H_4S）及其相关衍生物的热分解反应的两个反应路径，这些反应路径具有不同的热敏化途径和不同的热力学和动力学性质。溶血性开环，然后将生成的双自由基攻击另一个硫杂环丁烷分子，随后在快速自由基链反应中消除乙烷，导致形成三重基态（〜3S_2）的二硫分子，并需要＃的活化焓DELTA＃H_（298）= 222 kJ mol〜（-1）和＃DELTA＃G_（298）= 212 kJ mol〜（-1）。该反应机理将导致一级速率定律与一个报道的气相实验一致，但既不匹配实验活化能，也不能解释观察到的立体化学构型在某些取代的噻喃的热分解中的保留。备选地，硫原子可以从一个硫杂环丁烷分子转移到另一硫杂环丁烷分子，中间形成硫杂环丁烷1-硫化物（C_2H_4S_2）。该分子可以在其激发单重态（〜1S_2）下单分子分解为乙烷和二硫，或者可以通过自旋交联形成三重基态的S_2。另一方面，硫杂环丁烷1-硫化物可与自身反应并将一个硫原子从一个分子转移至另一分子，形成硫杂环丁烷1,1-二硫键（C_2H_4S_3），这是硫杂环丁烷砜的类似物;硫杂环丁烷形成为第二产物。然后，1，1-二硫化物可分解为乙烷和S_3。在又一个双分子反应中，硫杂环丁烷1-硫化物可以在强烈的放热反应中与其自身反应，得到S_4和两个当量的乙烷。该系列反应导致二级速率定律，并需要＃DELTA＃H_（298）= 109 kJ mol〜（-1）和＃DELTA＃G_（298）= 144 kJ mol〜（-1）的活化焓生成硫杂环丁烷1-硫化物，而随后的反应需要较少的活化焓。元素硫（S_8）最终通过在自旋允许的反应中S_2，S_3或S_4的低聚形成。这些发现与关于硫杂环丁烷及其取代衍生物的热脱硫的大多数实验数据一致。具有三个硫原子的线性排列的硫杂环丁烷1-过硫化物（C_2H_4S_3）以及由硫杂环丁烷衍生的两性离子和自由基不太可能成为环硫化物热分解的中间体。

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