Reactions of Halogenated Hydroperoxides and Peroxyl and Alkoxyl Radicals from Isoflurane in Aqueous Solution

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• 作者：Roman Flyunt ; Oksana Makogon ; Sergej Naumov ; Christian Sch&ouml ; neich ; Klaus-Dieter Asmus
• 刊名：Journal of Physical Chemistry A
• 出版年：2007
• 出版时间：November 15, 2007
• 年：2007
• 卷：111
• 期：45
• 页码：11618 - 11625
• 全文大小：106K
• 年卷期：v.111,no.45(November 15, 2007)
• ISSN：1520-5215

文摘

Model systems, based on aqueous solutions containing isoflurane (CHF₂OCHClCF₃) as an example, havebeen studied in the presence and absence of methionine (MetS) to evaluate reactive fates of halogenatedhydroperoxides and peroxyl and alkoxyl radicals. Primary peroxyl radicals, CHF₂OCH(OO)CF₃, generatedupon 1-e-reduction of isoflurane react quantitatively with MetS via an overall two-electron oxidation mechanismto the corresponding sulfoxide (MetSO). This reaction is accompanied by the formation of oxyl radicalsCHF₂OCH(O)CF₃ that quantitatively rearrange by a 1,2-hydrogen shift to CHF₂OC(OH)CF₃. According toquantum-chemical calculations, this reaction is exothermic (H = -5.1 kcal/mol) in contrast to other potentiallypossible pathways. These rearranged CHF₂OC(OH)CF₃ radicals react further via either of two pathways: (i)direct addition of oxygen or (ii) deprotonation followed by fluoride elimination resulting in CHF₂OC(O)CF₂. Route i yields the corresponding CHF₂OC(OO)(OH)CF₃ peroxyl radicals, which eliminate H⁺/O₂^-.The resulting ester, CHF₂OC(O)CF₃, hydrolyzes further, accounting for the formation of HF, trifluoroaceticacid, and formic acid with a contribution of 45% and 80% in air- and oxygen-saturated solutions, respectively.A competitive pathway (ii) involves the reactions of the secondary peroxyl radicals, CHF₂OC(O)CF₂OO.The two more stable of the three above mentioned peroxyl radicals can be distinguished through their reactionwith MetS. Although the primary CHF₂OCH(OO)CF₃ oxidizes MetS to MetSO in a 2-e step, the majority ofthe secondarily formed CHF₂OC(O)CF₂OO reacts with MetS via a 1-e transfer mechanism, yieldingCHF₂OC(O)CF₂OO^-, which eventually suffers a total breakup into CHF₂O^- + CO₂ + CF₂O. Quantum-chemical calculations show that this reaction is highly exothermic (H = -81 kcal/mol). In air-saturatedsolution this pathway accounts for about 35% of the overall isoflurane degradation. Minor products (10%each), namely, oxalic acid and carbon monoxide originate from oxyl radicals, CHF₂OC(O)CF₂O and CHF₂OCH(O)CF₃. An isoflurane-derived hydroperoxide CHF₂OCH(OOH)CF₃ in high yield was generated inradiolysis of air-saturated solutions containing isoflurane and formate either via a H-atom abstraction fromformate by the isoflurane-derived peroxyl radicals or by their cross-termination reaction with superoxideO₂^-. CHF₂OCH(OOH)CF₃, is an unstable intermediate whose multistep hydrolysis is giving H₂O₂ + 2HF +HC(O)OH + CF₃CH(OH)₂. In the absence of MetS, about 55% of CHF₂OCH(OO)CF₃ undergo terminationvia the Russell mechanism and 27% are involved in cross-termination with superoxide (O₂^-) and peroxylradicals derived from t-BuOH (used to scavenge OH radicals). The remaining 18% of the primary peroxylradicals undergo termination via formation of alkoxyl radicals, CHF₂OCH(O)CF₃.