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首页> 外文期刊>Chemistry: A European journal >The Interplay between Steric and Electronic Effects in S(N)2 Reactions
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The Interplay between Steric and Electronic Effects in S(N)2 Reactions

机译：S（N）2反应中立体效应和电子效应之间的相互作用

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Quantum chemical calculations for S(N)2 reactions of H3EX/X- systems, in which E = C or Si and X = F or Cl, are reported. In the case of the carbon system we also report on bulkier species in which the hydrogen atoms are substituted by methyl groups. It is shown how the variation in the individual energy terms of the Morokuma/Ziegler energy decomposition analysis (EDA) scheme along the reaction coordinate from reactants to products provides valuable insight into the essential changes that occur in the bond-breaking/bond-forming process during S(N)2 reactions. The EDA results for the totypical S(N)2 reaction of the systems [X center dot center dot center dot R3E center dot center dot center dot X](-), in which the interacting fragments are [X center dot center dot center dot X](2-) and [R3E](+), have given rise to a new interprelation of the factors governing the reaction course. The EDA results for the carbon system (E=C) show that there is less steric repulsion and stronger electrostatic attraction in the transition structure than in the precursor complex and that the energy increase comes mainly from weaker orbital interactions. The larger barriers for systems in which R-3 is bulkier also do not arise from increased steric repulsion, which is actually released in the transition structure. It is rather the weakening of the electrostatic attraction, and in particular the loss of attractive orbital interactions, that are responsible for the activation barrier. The D-3/l, energy minima of the silicon homologues [XH3SiX](-) is driven by the large increase in the electrostatic attractions and also of stronger orbital interactions, while the steric interactions is destabilizing.
机译：报道了H3EX / X-系统的S（N）2反应的量子化学计算，其中E = C或Si，X = F或Cl。对于碳系统，我们还报道了氢原子被甲基取代的大分子物质。它显示了Morokuma / Ziegler能量分解分析（EDA）方案的各个能量项沿反应坐标从反应物到产物的变化如何提供有价值的洞察力，以了解键断裂/键形成过程中发生的基本变化在S（N）2反应中。 EDA结果是系统[X中心点中心点中心点R3E中心点中心点中心点X]（-）的典型S（N）2反应，其中相互作用的片段为[X中心点中心点中心点X X]（2-）和[R3E]（+）引起了对控制反应过程的因素的新解释。碳系统的EDA结果（E = C）表明，与前驱体配合物相比，过渡结构中的空间排斥力较小，静电吸引力更强，并且能量增加主要来自较弱的轨道相互作用。对于R-3体积较大的系统，较大的障碍也不是由增加的空间排斥力引起的，而实际上在过渡结构中释放出来的空间排斥力却很大。确切地说，是静电吸引力的减弱，尤其是吸引力轨道相互作用的丧失，是造成活化屏障的原因。硅同系物[XH3SiX]（-）的D-3 / l能量最小值是由静电引力的大幅增加以及更强的轨道相互作用所驱动的，而空间相互作用却不稳定。

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