Elucidation of Electron-Transfer/S_N2 Borderline Mechanism : Toward the Development of a New Reaction Theory

阐明电子转移/S_N2 边界机制：迈向新反应理论的发展

• 批准号: 12640517
• 负责人: YAMATAKA Hiroshi
• 金额: $ 2.24万
• 依托单位: Osaka University
• 依托单位国家: 日本
• 项目类别: Grant-in-Aid for Scientific Research (C)
• 财政年份: 2000
• 资助国家: 日本
• 起止时间: 2000 至 2001
• 项目状态: 已结题
• 来源: https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-12640517/
• 关键词: Electron Transfer; S_N2 Reaction; Molecular Dynamics; Borderline Reaction; Organic Reaction Mechanism; Transition State

Anomalous relation between rates and equilibria for the proton-transfer reactions of nitroalkanes is known as nitroalkane anomaly. In a typical example, the pKa value of RCH_2NO_2 decreases in the order CH_3NO_2>CH_3CH_2NO_2>(CH_3)_2CHNO_2 in water, whereas the rate of proton abstraction by hydroxide ion decreases in the same order (reaction 1). The reactions of XC_6H_4CHRNO_2 with a base are another well-known abnormal case with an abnormal Bronsted coefficient (α > 1.0) (reaction 2). Questions arise in several ways. Does the nitroalkane anomaly arise from the inherent nature of the substrates, or does the anomaly exist in the gas phase? In what way solvent molecules play the role in the anomaly? What is the origin of experimentally observed anomaly? Ab initio calculations were carried out at HF/6-31+G^*, B3LYP/6-31+G^*, MP2/6-31+G^*, and MP2/6-311+G^<**> for reactions 1 and 2. In the gas phase the reaction with OH^- gave no transition state. In contrast, the proton transfer to CN^- gave well-characterized TSs for both reactions. These reactions, however, showed no anomaly at all ; EtNO_2 is more acidic and the barrier to the proton transfer for EtNO_2 is lower than for MeNO_2. Thus, the experimentally observed anomaly was not reproduced in the gas phase. Reactions in solution were examined in two steps, first with a supermolecule model and then incorporating bulk solvent effect by using a continuum model. Transition states of the deprotonations of nitroalkanes were observed when (H_2O)_2OH^- was used as a base instead of OH^-. Activation energy-reaction energy correlation revealed that the experimental anomaly was reproduced for reaction 1. The origin of the anomaly was considered to be due to transition state imbalance.

硝基烷烃质子转移反应的速率和平衡之间的反常关系称为硝基烷烃反常。在一个典型的例子中，RCH_2NO_2在水中的pKa值按CH_3NO_2&gt；CH_3CH_2NO_2&gt；(CH_3)_2CHNO_2的顺序递减，而氢离子的质子吸收速率按相同的顺序递减(反应1)。XC_6H_4CHRNO_2与碱的反应是另一个众所周知的反常情况，具有反常的布朗斯特系数(α&gt1.0)(反应2)。问题出现在几个方面。硝化烷的异常是由底物的固有性质引起的，还是存在于气相中？溶剂分子以何种方式在异常中发挥作用？实验观测到的异常的来源是什么？反应1和反应2在HF/6-31+G^*，B3LYP/6-31+G^*，MP2/6-31+G^*和MP2/6-311+G^&lt；**&gt；**&gt；相反，质子转移到CN^-为这两个反应提供了很好的表征TSS。然而，这些反应没有表现出任何异常，ETNO_2的酸性更强，质子转移的势垒比MENO_2低。因此，实验观察到的异常没有在气相中重现。溶液中的反应分两步进行，首先用超分子模型，然后用连续介质模型考虑本体溶剂效应。用(H_2O)_2OH^-代替OH^-作为碱，观察到了硝基烷烃去质子化反应的过渡态。活化能-反应能关联表明，反应1重现了实验反常现象。反常现象的起因被认为是由于过渡态的不平衡。

项目成果

H.Yamataka: "Analysis of Borderline Substitution/Electron Transfer Pathways from Direct Ab Initio MD Simulations"Chemical Physics Letters. (印刷中). (2002)

H. Yamataka：“从直接从头开始 MD 模拟分析边界取代/电子转移路径”化学物理快报（2002 年出版）。

H. Yamataka: "Analysis Of Borderline Substitution / Electron Transfer Pathways from Direct Ab Initio MD Simulation"Chemical Physics Letters.

H. Yamataka：“从直接从头开始 MD 模拟分析边界取代/电子转移路径”化学物理快报。