The potential energy surfaces for the S_N2 reactions of allyl and propyl chlorides with 21 anionic and neutral nucleophiles was studied by using ωB97X‐D/6‐311++G(3df,2pd) computations. The “allylic effect” on S_N2 barriers was observed for all reactions, and compared with propyl substrates, the energy barriers differed by −0.2 to −4.5 kcal mol⁻¹ in the gas phase. Strong correlations of the S_N2 net activation barriers with cation affinities, proton affinities, and electrostatic potentials at nuclei demonstrated the powerful influence of electrostatic interactions on these reactions. For the reactions of anionic (but not neutral) nucleophiles with allyl chloride, some of the incoming negative charge (0.2–18 %) migrated into the carbon chains, which would provide secondary stabilization of the S_N2 transition states. Activation strain analysis provided additional insight into the allylic effect by showing that the energy of geometric distortion for the reactants to reach the S_N2 transition state was smaller for each allylic reaction than for its propyl analogue. In many cases, the interaction energies between the substrate and nucleophile in this analysis were more favorable for propyl chloride reactions, but this compensation did not overcome the predominant strain energy effect.

中文翻译：

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SN 2烯丙基效应的亲核影响和起源。

通过使用ωB97X‐D / 6‐311 ++ G（3df，2pd）计算，研究了烯丙基氯和丙基氯与21种阴离子和中性亲核试剂发生S _N 2反应的势能面。在所有反应中均观察到对S _N 2势垒的“烯丙基效应” ，并且与丙基底物相比，气相中的能垒差为-0.2至-4.5 kcal mol ^-1。S _N的强相关2个具有阳离子亲和力，质子亲和力和原子核静电势的净激活势垒证明了静电相互作用对这些反应的强大影响。对于阴离子（但不是中性）亲核试剂与烯丙基氯的反应，一些传入的负电荷（0.2-18％）迁移到碳链中，这将提供S _N 2过渡态的二级稳定性。活化应变分析通过显示反应物达到S _N的几何畸变能，提供了对烯丙基效应的更多了解。每个烯丙基反应的2过渡态均比其丙基类似物小。在许多情况下，此分析中底物与亲核试剂之间的相互作用能更适合于丙基氯反应，但这种补偿不能克服主要的应变能效应。