Kopinke Frank-Dieter, Georgi Anett
Department of Environmental Engineering, Helmholtz Centre for Environmental Research - UFZ , Permoserstrasse 15, D-04318 Leipzig, Germany.
J Phys Chem A. 2017 Oct 19;121(41):7947-7955. doi: 10.1021/acs.jpca.7b05782. Epub 2017 Oct 10.
The oxidation of three isotopologues of methylcyclohexane (MCH: CH, CD, c-CD-CH) by OH-radicals (OH) in aqueous solution was investigated. Intermolecular and intramolecular H/D kinetic isotope effects (KIE = k:k) for the abstraction of H and D atoms by OH were measured. These KIEs reflect inter- and intramolecular selectivities of hydrogen abstraction, i.e., the selection of OH attack on carbon-hydrogen bonds in different molecules and in different positions of one molecule, respectively. The intermolecular selectivity of OH attack in aqueous solution is largely discriminated against in comparison with the intramolecular selectivity. The observed extent of discrimination cannot be explained by partial diffusion control of the overall reaction rates. A cage model, where OH and hydrocarbon molecules are entrapped in a solvent cage, is more appropriate. The much higher intramolecular KIEs compared to the intermolecular KIEs of the same chemical reaction, R-H + OH → R + HO, indicate a high degree of mobility of the two reaction partners inside of the solvent cage. This mobility is sufficient to develop an intramolecular selectivity comparable to that of gas-phase reactions of OH. Furthermore, literature data on KIEs of H-abstraction by OH in aqueous and gas phases are discussed. There is a general tendency toward lower selectivities in the aqueous phase.
研究了水溶液中甲基环己烷（MCH：CH、CD、c-CD-CH）的三种同位素异构体被羟基自由基（OH）氧化的情况。测量了OH夺取H和D原子的分子间和分子内氢/氘动力学同位素效应（KIE = kH/kD）。这些KIE反映了氢夺取的分子间和分子内选择性，即分别是OH对不同分子中碳氢键以及一个分子不同位置碳氢键进攻的选择性。与分子内选择性相比，水溶液中OH进攻的分子间选择性受到很大歧视。观察到的歧视程度不能用总反应速率的部分扩散控制来解释。一个将OH和烃分子困在溶剂笼中的笼状模型更为合适。与相同化学反应R-H + OH → R + HO的分子间KIE相比，分子内KIE要高得多，这表明两个反应伙伴在溶剂笼内具有高度的迁移率。这种迁移率足以产生与OH气相反应相当的分子内选择性。此外，还讨论了关于OH在水相和气相中夺取H的KIE的文献数据。在水相中一般存在选择性较低的趋势。
