化学学报 ›› 2018, Vol. 76 ›› Issue (10): 793-801.DOI: 10.6023/A18080317 上一篇    下一篇

研究论文

两种七氟醚自由基[(CF3)2C(·)OCH2F,(CF3)2CHOC(·)HF]与氧气反应及OH·再生机理的理论研究

武卫荣a,b, 员晓敏a, 侯华a, 王宝山a   

  1. a 武汉大学化学与分子科学学院 武汉 430072;
    b 济宁学院化学与化工系 曲阜 273155
  • 收稿日期:2018-08-04 出版日期:2018-10-15 发布日期:2018-09-28
  • 通讯作者: 王宝山 E-mail:baoshan@whu.edu.cn
  • 基金资助:

    项目受国家重点研发计划(No.2017YFB0902500)和国家电网有限公司总部科技项目(环保型管道输电关键技术)资助.

Theoretical Investigations on the Mechanisms for the Reactions of Sevoflurane Radicals[(CF3)2C(·)OCH2F, (CF3)2CHOC(·)HF] with O2and the OH· Radicals Regeneration

Wu Weironga,b, Yuan Xiaomina, Hou Huaa, Wang Baoshana   

  1. a College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China;
    b Department of Chemistry and Chemical Engineering, Jining University, Qufu 273155, China
  • Received:2018-08-04 Online:2018-10-15 Published:2018-09-28
  • Contact: 10.6023/A18080317 E-mail:baoshan@whu.edu.cn
  • Supported by:

    Project supported by the National Key Research and Development Program of China (No. 2017YFB0902500) and Science and Technology Project of State Grid Corporation of China:The key Technology of Environment-Friendly Gas-Insulated Transmission Line (GIL).

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(CF32C(·)OCH2F与(CF32CHOC(·)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SN2反应.对于(CF32C(OC(·)HF)OOH,三条途径互为竞争;对于(CF32C(·)OC(HF)(OOH),分子内SN2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 SN2 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 SN2 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 SN2 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