Nucleophilic substitution (S_N2) and base-induced elimination (E2), two indispensable reactions in organic synthesis, are commonly assumed to proceed under stereospecific conditions. Understanding the way in which the reactants pre-orient in these reactions, that is its stereodynamics, is essential in order to achieve a detailed atomistic picture and control over such processes. Using crossed beam velocity map imaging, we study the effect of steric hindrance in reactions of Cl⁻ and CN⁻ with increasingly methylated alkyl iodides by monitoring the product ion energy and scattering angle. For both attacking anions the rebound mechanism, indicative of a direct S_N2 pathway, is found to contribute to the reaction at high relative collision energies despite being increasingly hindered. An additional forward scattering mechanism, ascribed to a direct E2 reaction, also contributes at these energies. Inspection of the product energy distributions confirms the direct and fast character of both mechanisms as opposed to an indirect reaction mechanism which leads to statistical energy redistribution in the reaction complex. This work demonstrates that nonstatistical dynamics and energetics govern S_N2 and E2 pathways even in sterically hindered exchange reaction systems.

中文翻译：

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位阻S _N 2 / E2反应 中直接动力学的守恒

亲核取代（S _Ñ 2）和碱诱导的消除（E2），在有机合成中不可缺少的两个反应中，通常假定立体特异性的条件下进行。为了获得详细的原子图并控制这种过程，了解反应物在这些反应中的预取向方式，即其立体动力学，是必不可少的。使用交叉束速度图成像，我们研究的空间位阻的影响以Cl反应^-和CN ^-与通过监控产物离子能量和散射角日益甲基化烷基碘。对于两种进攻性阴离子，其回弹机制均表明直接为S _N尽管受到越来越多的阻碍，但是发现在图2中的途径中，在较高的相对碰撞能量下仍有助于反应。归因于直接E2反应的另一种正向散射机制也有助于产生这些能量。对产物能量分布的检查证实了两种机理的直接和快速特征，与间接反应机理相反，间接反应机理导致了反应复合物中能量的统计重新分布。这项工作表明，即使在空间受阻的交换反应系统中，非统计动力学和能量学也能控制S _N 2和E2途径。