Additions of acids to 1,3-dienes are conventionally understood as involving discrete intermediates that undergo an ordinary competition between subsequent pathways to form the observed products. The combined experimental, computational, and dynamic trajectory study here suggests that this view is incorrect, and that solvation dynamics plays a critical role in the mechanism. While implicit solvent models were inadequate, QM/QM' trajectories in explicit solvent provide an accurate prediction of the experimental selectivity in the addition of HCl to 1,3-pentadiene. Trajectories initiated from a protonation saddle point on the potential of mean force surface are predominantly unproductive due to a gating effect of solvation that allows diene protonation only when the incipient ion pair is neither too solvent-stabilized nor too little. Protonation then leads to relatively unsolvated ion pairs, and a majority of these collapse rapidly to the 1,2-product, without barrier and without achieving equilibrium solvation as intermediates. The remainder decay slowly, at a rate consistent with equilibrium solvation as true intermediates, affording a mixture of addition products. Overall, an accurate description of the nature and pathway selectivity of the ion pair intermediates in carbocation reactions must allow for species lacking equilibrium solvation. Potential reinterpretations of a series of historically notable observations in carbocation reactions are discussed.

中文翻译：

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碳正离子反应中的溶剂化动力学和反应势垒和离子对中间体的性质

向 1,3-二烯中添加酸通常被理解为涉及离散中间体，这些中间体在后续途径之间进行普通竞争以形成观察到的产物。这里结合实验、计算和动态轨迹研究表明这种观点是不正确的，溶剂化动力学在该机制中起着关键作用。虽然隐式溶剂模型不足，但显式溶剂中的 QM/QM' 轨迹提供了对 HCl 添加到 1,3-戊二烯的实验选择性的准确预测。由于溶剂化的门控效应，仅当初始离子对既不太溶剂稳定也不太稳定时，允许二烯质子化，因此从平均力表面电位上的质子化鞍点开始的轨迹主要是非生产性的。然后质子化会导致相对未溶剂化的离子对，其中大部分会迅速坍塌为 1,2-产物，没有障碍，也没有实现平衡溶剂化作为中间体。其余部分缓慢衰减，其衰减速度与作为真正中间体的平衡溶剂化一致，提供加成产物的混合物。总体而言，碳正离子反应中离子对中间体的性质和途径选择性的准确描述必须考虑到缺乏平衡溶剂化的物质。讨论了对碳正离子反应中一系列历史上值得注意的观察结果的潜在重新解释。以与平衡溶剂化一致的速率作为真正的中间体，提供加成产物的混合物。总体而言，碳正离子反应中离子对中间体的性质和途径选择性的准确描述必须考虑到缺乏平衡溶剂化的物质。讨论了对碳正离子反应中一系列历史上值得注意的观察结果的潜在重新解释。以与平衡溶剂化一致的速率作为真正的中间体，提供加成产物的混合物。总体而言，碳正离子反应中离子对中间体的性质和途径选择性的准确描述必须考虑到缺乏平衡溶剂化的物质。讨论了对碳正离子反应中一系列历史上值得注意的观察结果的潜在重新解释。