摘要/Abstract

温室气体七氟醚氧化降解可以产生两种反应活性截然不同的自由基，即（CF₃）₂C（·）OCH₂F与（CF₃）₂CHOC（·）HF.采用M06-2X/6-311++G（d，p）与CBS-Q理论方法研究了两种七氟醚自由基与氧气（O₂）反应的机理.与普通的烷基自由基+O₂的无垒复合反应不同，多氟取代的七氟醚自由基与O₂反应需经过1.3~1.8 kcal·mol^-1的势垒才能生成过氧自由基中间体RO₂·.虽然O₂更易于加成到富氟的自由基位点，但与贫氟自由基位点结合生成RO₂·放热更多，且随后的六中心氢迁移生成QOOH中间体需要克服更高的势垒.QOOH分解包括三种主要的OH·再生途径，即：分步解离、三体同步解离、四中心分子内S_N2反应.对于（CF₃）₂C（OC（·）HF）OOH，三条途径互为竞争；对于（CF₃）₂C（·）OC（HF）（OOH），分子内S_N2反应为OH·再生的主要途径，分步或三体同步解离则为次要机理.研究为阐明多氟取代基对大气中OH·循环的影响规律提供了理论依据.
关键词: 七氟醚, 反应机理, OH·再生, 大气降解, 量子化学
Sevoflurane is an excellent volatile anaesthetic which has been widely in clinical use. However, it was found that sevoflurane is a potent green-house gas with a significant global warming potential. Atmospheric degradation of sevoflurane is desired for its long-term application. The reaction of sevoflurane with hydroxyl radicals (OH·) produces two radical species, namely, (CF₃)₂C(·)OCH₂F and (CF₃)₂CHOC(·)HF, which have different reactivity. Under the low-NO atmospheric conditions, it was found that both radical fragments enable to initialize the regeneration of OH·radicals in the presence of molecular oxygen (O₂). Microscopic mechanisms for the reactions of the two radicals with O₂ have been investigated for the first time in this work. Geometries of various intermediates and transition states on the doublet potential energy surfaces were optimized at the M06-2X/6-311++G (d,p) level of theory. Moreover, the single-point calculations were carried out using the composite model CBS-Q to refine the reaction energetics to the chemical accuracy. It was revealed that the formation of peroxy intermediate (RO₂·) undergoes via the definitive barriers of 1.3 or 1.8 kcal·mol^-1, in contrast to the barrierless association between the alkyl radicals and O₂. Apparently, the association of the fluorinated alkyl radicals with O₂ takes place more slowly due to the substitute effect. Although the addition of O₂ to the fluorine-rich radical site is more preferable than that to the fluorine-poor site, the latter is more exothermic in view of the exothermicity of the intermediates RO₂·. The barriers for the subsequent H-migration of RO₂·to form the QOOH intermediates are 17.9 and 21.5 kcal·mol^-1, respectively. Both barriers lie well below the reactant asymptote, indicating the isomerization paths are energetically favorable. Decomposition of QOOH takes place via three competitive mechanisms, including the step-wise bond fission, the three-body concerted cleavage, and the four-center intramolecular S_N2 reaction, to produce OH·radicals predominantly. All the reaction pathways could be competitive for (CF₃)₂C(OC(·)HF)OOH because the energies of the corresponding barriers are close. In contrast, only the S_N2 displacement energetic route is dominant for (CF₃)₂C(·)OC(HF)(OOH). Neither step-wise nor three-body pathways is important because the barrier height is roughly 7 kcal·mol^-1 higher than that for the S_N2 pathway. The isomerization of QOOH to alkoxy intermediate is of little importance due to the significant barrier even though it is highly exothermic. Implication of the current theoretical findings in the OH·radicals recycling reaction in atmosphere has been illustrated.
Key words: sevoflurane, reaction mechanism, OH·, radicals regeneration, atmospheric degradation, quantum chemistry


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