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首页> 美国卫生研究院文献>Molecules >Theoretical Analysis on the Kinetic Isotope Effects of Bimolecular Nucleophilic Substitution (SN2) Reactions and Their Temperature Dependence
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Theoretical Analysis on the Kinetic Isotope Effects of Bimolecular Nucleophilic Substitution (SN2) Reactions and Their Temperature Dependence

机译：双分子亲核取代（SN2）反应的动力学同位素效应及其温度依赖性的理论分析

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Factors affecting the kinetic isotope effects (KIEs) of the gas-phase SN2 reactions and their temperature dependence have been analyzed using the ion-molecule collision theory and the transition state theory (TST). The quantum-mechanical tunneling effects were also considered using the canonical variational theory with small curvature tunneling (CVT/SCT). We have benchmarked a few ab initio and density functional theory (DFT) methods for their performance in predicting the deuterium KIEs against eleven experimental values. The results showed that the MP2/aug-cc-pVDZ method gave the most accurate prediction overall. The slight inverse deuterium KIEs usually observed for the gas-phase SN2 reactions at room temperature were due to the balance of the normal rotational contribution and the significant inverse vibrational contribution. Since the vibrational contribution is a sensitive function of temperature while the rotation contribution is temperature independent, the KIEs are thus also temperature dependent. For SN2 reactions with appreciable barrier heights, the tunneling effects were predicted to contribute significantly both to the rate constants and to the carbon-13, and carbon-14 KIEs, which suggested important carbon atom tunneling at and below room temperature.
机译：使用离子-分子碰撞理论和过渡态理论（TST）分析了气相SN2反应的动力学同位素效应（KIEs）及其温度依赖性的影响因素。还使用具有小曲率隧穿（CVT / SCT）的规范变分理论来考虑量子力学隧穿效应。我们已经对几种从头算和密度泛函理论（DFT）的方法进行了基准测试，以针对11种实验值预测氘KIE的性能。结果表明，MP2 / aug-cc-pVDZ方法总体上提供了最准确的预测。通常在室温下对气相SN2反应观察到的轻微的氘氘KIE是由于正常旋转贡献和显着的反向振动贡献之间的平衡所致。由于振动贡献是温度的敏感函数，而旋转贡献是与温度无关的，因此KIE也与温度有关。对于具有可观的势垒高度的SN2反应，预测隧穿效应将对速率常数以及碳13和碳14 KIE产生显着贡献，这表明在室温及低于室温时重要的碳原子隧穿。

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