METHYL-SUBSTITUTED ALLYL CATIONS - COMPARISON OF EXPERIMENTAL STABILITY, ROTATIONAL BARRIER, AND SOLVOLYSIS DATA WITH ABINITIO CALCULATIONS
METHYL-SUBSTITUTED ALLYL CATIONS - COMPARISON OF EXPERIMENTAL STABILITY, ROTATIONAL BARRIER, AND SOLVOLYSIS DATA WITH ABINITIO CALCULATIONS
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DOI：
10.1021/ja00514a028
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发表时间：
1979-01-01
影响因子：
15
通讯作者：
SCHLEYER, PV
中科院分区：
文献类型：
--
作者：
MAYR, H;FORNER, W;SCHLEYER, PV
甲基取代烯丙基阳离子的定量数据有三个来源：气相生成热、溶液中的旋转势垒和溶剂分解速率。NMR化学位移也提供了电荷分布的指数。本文将所有这些线在一起进行比较的STO-3G从头计算的平面和垂直的甲基取代的烯丙基阳离子的分子轨道计算结果。末端位置依次被一个、两个、三个和四个甲基取代，烯丙基阳离子的电子稳定性分别为17、15、13和11 kcal/mol;中心碳上的甲基的稳定作用要小得多（~ 5 kcal/mol）。第一个内甲基基团的空间应变被确定为3 kcal/mol，而第二个内甲基经历5 kcal/mol的稍微更大的应变。烯丙基溶剂分解速率与气相烯丙基阳离子稳定性数据相关性很好，但由于溶剂化作用，其幅度降低。计算得到的转动势垒也比在超强酸溶液中测定的无活化能势垒高4-12 kcal/mol;这种差异进一步证明了微分溶剂化效应--离域程度越高的平面形式比它们的转动过渡态溶剂化程度越低。烯丙基阳离子的转动势垒在气相中为34 kcal/mol，在超强酸溶液中为23.7±2 kcal/mol。1溶剂分解研究首先证明了烯丙基阳离子作为短寿命反应中间体的存在，先于在超强酸介质中产生和直接观察到稳定的烯丙基阳离子。1近年来，人们已经获得了许多关于气相中烯丙基阳离子的热力学数据，特别是通过离子回旋共振测量。[2]以前的理论研究主要集中在母体烯丙基阳离子上。3~ 8除少数例外，93个甲基取代的烯丙基阳离子仅用半经验方法10· 11或从头算方法研究，没有进行几何优化。12由于这些计算与实验确定的能量差有相当大的偏差，我们对甲基取代的烯丙基阳离子进行了全面的从头算分子轨道研究，并进行了几何优化，以评估和增加实验数据。
Three sources of quantitative data for methyl-substitutedallyl cations are available experimentally: Gasphase heats of formation, rotational barriers in solution, and rates of solvolysis. NMR chemical shifts also provide an index of charge distri-bution. This paper draws all these lines together in comparison with results of STO-3G ab initio molecular orbital calculations performed on planar and perpendicular methyl-substituted allyl cations. Sequential substitution of the terminal positions by one, two, three, and four methyl groups stabilizes allyl cations electronically by 17, 15, 13, and 11 kcal/mol, respectively; a methyl group on the central carbon has a much smaller stabilizing effect (~ 5 kcal/mol). The steric strain for the first endo-methyl group was determined to be 3 kcal/mol, whereas a second endo-methyl experiences a somewhat greater strain of 5 kcal/mol. Allyl solvolysis rates correlate well with gas phase allyl cation stability data, but there is a reduction in magnitude due to solvation. The calculated rotational barriers also are 4-12 kcal/mol higher than the activation free enthalpies determined in superacid solution; the difference provides further evidence fordifferential solvation effects—the more highly delocalized planar forms are solvated to a lesser extent than their rotational transition states. The rotational barrier of the parent allyl cation is predicted to be 34 kcal/mol in the gas phase but to decrease to 23.7±2 kcal/mol in superacid solution.Allyl cations, the prototype delocalized carbenium ions, have been studied experimentally for many decades. 1 The solvolysis studies, which first proved the existence of allyl cations as short-lived reactive intermediates, preceded gen-eration and direct observation of stable allyl cations in superacid media. 1 In recent years, many thermodynamic data on allyl cations in the gas phase have become available, particu-larly by means of ion cyclotron resonance measurements. 2 Previous theoretical studies have concentrated on the parent allyl cation. 3~ 8 With few exceptions, 93 methyl-substitutedallyl cations have only been investigated by semiempirical meth-ods10· 11 or by ab initio calculations without geometry optimi-zation. 12 Since these calculations give considerable deviations with experimentally determined energy differences, we un-dertook a comprehensive ab initio MO study of methyl-substituted allyl cations with geometry optimization in order to assess and to augment the experimental data.