Variations in Rotational Barriers of Allyl and Benzyl Cations, Anions, and Radicals
Variations in Rotational Barriers of Allyl and Benzyl Cations, Anions, and Radicals
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DOI：
10.1021/acs.joc.6b01530
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发表时间：
2016-10-21
影响因子：
3.6
通讯作者：
Borden, Weston Thatcher
中科院分区：
文献类型：
--
作者：
Li, Zhe;Bally, Thomas;Borden, Weston Thatcher
高精度量子化学计算表明，烯丙基阳离子，自由基和阴离子中的CH 2基团的旋转势垒分别为33，14和21千卡/摩尔。苄基阳离子、自由基和阴离子的势垒分别为45、11和24千卡/摩尔。这些势垒高度与每个完全共轭系统的离域稳定的大小有关。本文解决的问题，为什么这些旋转障碍，这在休克尔理论水平是独立的非键电子的数量在烯丙基和苄基，实际上计算的因素，是2.4和4.1高的阳离子和1.5和1.9高的阴离子比在自由基。我们还调查了为什么旋转的障碍是较高的苄基比烯丙基的阳离子和阴离子。正如休克尔理论所预测的那样，只有在自由基中苄基的势垒低于烯丙基。电子结构理论中的这些基本问题，以前没有被解决过，与共轭和非共轭体系中电子-电子排斥的差异有关，这取决于非键电子的数量。
High accuracy quantum chemical calculations show that the barriers to rotation of a CH2 group in the allyl cation, radical, and anion are 33, 14, and 21 kcal/mol, respectively. The benzyl cation, radical, and anion have barriers of 45, 11, and 24 kcal/mol, respectively. These barrier heights are related to the magnitude of the delocalization stabilization of each fully conjugated system. This paper addresses the question of why these rotational barriers, which at the Huckel level of theory are independent of the number of nonbonding electrons in allyl and benzyl, are in fact calculated to be factors that are of 2.4 and 4.1 higher in the cations and 1.5 and 1.9 higher in the anions than in the radicals. We also investigate why the barrier to rotation is higher for benzyl than for allyl in the cations and in the anions. Only in the radicals is the barrier for benzyl lower than that for allyl, as Huckel theory predicts should be the case. These fundamental questions in electronic structure theory, which have not been addressed previously, are related to differences in electron-electron repulsions in the conjugated and nonconjugated systems, which depend on the number of nonbonding electrons.