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首页> 外文期刊>Chemistry: A European journal >Ensemble-averaged QM/MM kinetic isotope effects for the S _N2 reaction of cyanide anions with chloroethane in DMSO solution
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Ensemble-averaged QM/MM kinetic isotope effects for the S _N2 reaction of cyanide anions with chloroethane in DMSO solution

机译：DMSO溶液中氰化物阴离子与氯乙烷的S _N2反应的整体平均QM / MM动力学同位素效应

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The existence of solvent fluctuations leads to populations of reactant-state (RS) and transition-state (TS) configurations and implies that property calculations must include appropriate averaging over distributions of values for individual configurations. Average kinetic isotope effects 〈KIE〉 for NC -+EtCl→NCEt+Cl ~- in DMSO solution at 30°C are best obtained as the ratio 〈f _(RS)〉/ 〈f _(TS)〉 of isotopic partition function ratios separately averaged over all RS and TS configurations. In this way the hybrid AM1/OPLS-AA potential yields 〈KIE〉 values for all six isotopic substitutions (2° α- ~2H _2, 2° β- ~2H _3, α- ~(11)C/ ~(14)C, leaving group ~(37)Cl, and nucleophile ~(13)C and ~(15)N) for this reaction in the correct direction as measured experimentally. These thermally-averaged calculated KIEs may be compared meaningfully with experiment, and only one of them differs in magnitude from the experimental value by more than one standard deviation from the mean. This success contrasts with previous KIE calculations based upon traditional methods without averaging. The isotopic partition function ratios are best evaluated using all (internal) vibrational and (external) librational frequencies obtained from Hessians determined for subsets of atoms, relaxed to local minima or saddle points, within frozen solvent environments of structures sampled along molecular dynamics trajectories for RS and TS. The current method may perfectly well be implemented with other QM or QM/MM methods, and thus provides a useful tool for investigating KIEs in relation to studies of chemical reaction mechanisms in solution or catalyzed by enzymes. Better on average! Average AM1/OPLS-AA kinetic isotope effects (KIEs) computed for NC ~-+EtCl in DMSO agree with experiment for isotopic substitutions in six positions, and their distributions for individual configurations provide insight into solvent fluctuation effects. A single-molecule view of chemical change should be replaced by a picture involving averages over representative collections of molecules (see figure).
机译：溶剂波动的存在导致反应物态（RS）和过渡态（TS）构型的增加，并且意味着属性计算​​必须包括对各个构型的值分布进行适当的平均。 DMSO溶液中NC-+ EtCl→NCEt + Cl〜-在30°C时的平均动力学同位素效应最佳，因为同位素分配函数的比率 / 〈f _（TS）>在所有RS和TS配置上分别平均比率。通过这种方式，所有六个同位素取代（2°α-〜2H _2、2°β-〜2H _3，α-〜（11）C /〜（14）的杂化AM1 / OPLS-AA势产值〈KIE〉值C，在实验中测得的正确方向上留下基团〜（37）Cl和亲核试剂〜（13）C和〜（15）N）。可以将这些热平均计算出的KIE与实验进行有意义的比较，并且其中只有一个在幅度上与实验值相差超过一个标准差。此成功与以前基于传统方法的KIE计算（不求平均）形成对比。最好使用从Hessian获得的所有（内部）振动和（外部）自由振动频率对同位素分配函数比率进行评估，这些振动频率是根据在分子动力学轨迹上为RS采样的结构的冷冻溶剂环境中确定的原子子集（松弛到局部最小值或鞍点）而确定的。和TS。当前的方法可以很好地与其他QM或QM / MM方法一起实施，因此为研究与溶液中或酶催化的化学反应机理有关的KIE提供了有用的工具。平均更好！在DMSO中为NC〜-+ EtCl计算的平均AM1 / OPLS-AA动力学同位素效应（KIEs）与在六个位置进行同位素取代的实验相符，并且它们在各个构型中的分布提供了对溶剂波动效应的了解。应该用一张图片代替一张有关化学变化的单分子视图，该图片应包括代表分子集合的平均值（见图）。

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