Theoretical study of HOCl-catalyzed keto-enol tautomerization of β-cyclopentanedione in an explicit water environment.
Theoretical study of HOCl-catalyzed keto-enol tautomerization of β-cyclopentanedione in an explicit water environment.
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DOI：
10.1021/jp401409y
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发表时间：
2013-09-05
期刊：
影响因子：
--
通讯作者：
Chatfield DC
中科院分区：
文献类型：
--
作者：
D'Cunha C;Morozov AN;Chatfield DC
研究了β-环戊二酮（CPD）在溶液中酸催化酮-烯醇互变异构反应的机理。反应剖面首先计算了一个有限的溶剂化环境使用从头算和密度泛函方法。包含一个外显水分子水合壳的系统的势垒高度在很大程度上取决于参与质子转移的水的数量，并且在较小但重要的程度上取决于在质子转移链中与水形成氢键的水的数量（每个这样的水平均减少4.4千卡/摩尔的势垒）。当一个完整或接近完整的水合壳存在时，得到了8-13 kcal/mol的势垒，这与相关分子非酸催化酮烯醇互变异构的计算结果一致。HOCl的存在使势垒在气相中降低了4.5 kcal/mol，这与众所周知的酮-烯醇互变异构可以由酸或碱催化的原理相一致。从CPD、HOCl和324显式水的300 K分子动力学模拟中获得的反应物构象快照开始模拟反应。在第一个水化壳中的水沿着反应坐标的每一步固定或能量最小化的情况下，在QM/MM水平上计算反应曲线。在这两种情况下，观察到屏障高度的实质性变化，主要取决于与第一水合壳水的静电相互作用（氢键），较小程度上取决于与更远的水的静电相互作用和几何扭曲效应。对于最低势垒，参与质子转移的水对势垒的还原程度与有限溶剂化结果一致，但未观察到与参与质子转移的水的氢键作用对势垒的进一步还原。据推测，这是因为高度灵活的结构，如广泛的氢键网络，最适合反应是熵不利的，因此可能不会显著贡献观察到的反应速率。
The mechanism of acid-catalyzed keto-enol tautomerization of β-cyclopentanedione (CPD) in solution is studied computationally. Reaction profiles are first calculated for a limited solvation environment using ab initio and density functional methods. Barrier heights for systems including up to one hydration shell of explicit water molecules depend strongly on the number of waters involved in proton transfer, and to a lesser but significant extent on the number of waters forming hydrogen bonds with waters in the proton transfer chain (each such water reduces the barrier by 4.4 kcal/mol on average). Barriers of 8-13 kcal/mol were obtained when a full or nearly full hydration shell was present, consistent with calculations for non-acid-catalyzed keto-enol tautomerization of related molecules. The presence of HOCl reduced the barrier by 4.5 kcal/mol viz-a-viz the gas phase, consistent with the well-known principle that keto-enol tautomerization can be acid or base catalyzed. Reaction was also modeled beginning with snapshots of reactant conformations taken from a 300 K molecular dynamics simulation of CPD, HOCl and 324 explicit waters. Reaction profiles were calculated at a QM/MM level with waters in the first hydration shell either fixed or energy minimized at each step along the reaction coordinate. A substantial variation in barrier height was observed in both cases, depending primarily on electrostatic interactions (hydrogen bonding) with first-hydration-shell waters and to a lesser extent on electrostatic interactions with more distant waters and geometric distortion effects. For the lowest barriers, the extent of barrier reduction by waters involved in proton transfer is consistent with the limited-solvation results, but further barrier reduction due to hydrogen-bonding to waters involved in proton transfer is not observed. It is postulated that this is because highly flexible structures such as extensive hydrogen bonding networks optimal for reaction are entropically disfavored and so may not contribute significantly to the observed reaction rate.
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