Reductive Outer-Sphere Single Electron Transfer Is an Exception Rather than the Rule in Natural and Engineered Chlorinated Ethene Dehalogenation.
Reductive Outer-Sphere Single Electron Transfer Is an Exception Rather than the Rule in Natural and Engineered Chlorinated Ethene Dehalogenation.
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DOI：
10.1021/acs.est.7b01447
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发表时间：
2017-08
影响因子：
11.4
通讯作者：
Benjamin Heckel;Stefan Cretnik;Sarah Kliegman;O. Shouakar‐Stash;K. McNeill;M. Elsner
中科院分区：
文献类型：
--
作者：
Benjamin Heckel;Stefan Cretnik;Sarah Kliegman;O. Shouakar‐Stash;K. McNeill;M. Elsner
氯化乙烯（CE）如四氯乙烯、三氯乙烯和二氯乙烯是众所周知的地下水污染物。虽然还原脱卤是其环境和工程降解的关键，但潜在的反应机制仍然难以捉摸。外球还原单电子转移（OS-SET）已被提出用于维生素B12依赖的生物降解和零价金属介导的脱卤等不同过程。化合物特异性同位素效应（13 C/12 C，37 Cl/35 Cl）分析提供了一个新的机会来检验这些假设。定义的OS-SET模型反应物（CO2自由基阴离子，水中的S2-掺杂氧化石墨烯）在CE中产生强碳（εC = -7.9‰至-11.9‰），但可忽略的氯同位素效应（εCl = -0.12‰至0.04‰）。在CHCl_3（εC = -7.7‰，εCl = -2.6‰）中观察到更大的氯同位素效应，当C-Cl键断裂的放热性在有机溶剂中降低时（在甘醇二甲醚中与芳烃自由基阴离子反应），在CE中观察到更大的氯同位素效应。总之，这表明与乙烯中的逐步OS-SET（SET到π* 轨道，随后是C-Cl断裂）相比，烷烃中的解离OS-SET（SET到与C-Cl断裂一致的σ* 轨道）。不存在的氯同位素效应的氯化乙烯在所有的水OS-SET实验强烈对比显着的Cl同位素分馏在所有天然和工程还原脱卤报告的日期表明，OS-SET是一个例外，而不是规则的氯化乙烯的环境转化。
Chlorinated ethenes (CEs) such as perchloroethylene, trichloroethylene and dichloroethylene are notorious groundwater contaminants. Although reductive dehalogenation is key to their environmental and engineered degradation, underlying reaction mechanisms remain elusive. Outer-sphere reductive single electron transfer (OS-SET) has been proposed for such different processes as Vitamin B12-dependent biodegradation and zerovalent metal-mediated dehalogenation. Compound-specific isotope effect (13C/12C, 37Cl/35Cl) analysis offers a new opportunity to test these hypotheses. Defined OS-SET model reactants (CO2 radical anions, S2--doped graphene oxide in water) caused strong carbon (εC = -7.9‰ to -11.9‰), but negligible chlorine isotope effects (εCl = -0.12‰ to 0.04‰) in CEs. Greater chlorine isotope effects were observed in CHCl3 (εC = -7.7‰, εCl = -2.6‰), and in CEs when the exergonicity of C-Cl bond cleavage was reduced in an organic solvent (reaction with arene radical anions in glyme). Together, this points to dissociative OS-SET (SET to a σ* orbital concerted with C-Cl breakage) in alkanes compared to stepwise OS-SET (SET to a π* orbital followed by C-Cl cleavage) in ethenes. The nonexistent chlorine isotope effects of chlorinated ethenes in all aqueous OS-SET experiments contrast strongly with pronounced Cl isotope fractionation in all natural and engineered reductive dehalogenations reported to date suggesting that OS-SET is an exception rather than the rule in environmental transformations of chlorinated ethenes.