A theoretical study of the electronic structure of .beta.-substituted alkyl radicals
A theoretical study of the electronic structure of .beta.-substituted alkyl radicals
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β-取代烷基电子结构的理论研究
DOI：
10.1021/ja00428a009
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发表时间：
1976
影响因子：
15
通讯作者：
D. Wood
中科院分区：
文献类型：
--
作者：
A. Rossi;D. Wood
本文用分子轨道计算方法从理论上研究了β-取代烷基自由基的构象选择性。结果发现，这些物种的构象依赖于超共轭效应和非键相互作用的微妙平衡。因此，超共轭倾向于支持-Cl ^CFhX 8（其中X8是周期表第二行或后续行的取代基）型自由基的平面重叠或重叠反（对于非平面自由基中心）构象。由于空间效应，对于具有大体积X8取代基（如Br或I）的非平面自由基，获得了对交错构象的增加的偏好。还讨论了p-p和dp同共轭相对于超共轭的程度。对于电负性很强的取代基如X8= F或OR，没有发现pp同共轭，但对于X8取代基如Cl，pp和dp同共轭似乎都存在。一个可能的机制，为外消旋的光学活性-CR ^CFhX 8自由基和外消旋率被追踪到庞大的X8取代基。
The conformational preferences of/3-substituted alkyl radicals are examined theoretically by means of molecular or-bital calculations. It is found that conformations of these species depend on a delicate balance of hyperconjugative effects and nonbonded interactions. Thus, hyperconjugation tends to favor the planar eclipsed or eclipsed anti (for nonplanar radical cen-ters) conformations for radicals of the type-CI^ CFhX8 (where X8 is a substituent from the second or succeeding rows of the periodic table). An increased preference for the staggered conformation is obtained for nonplanar radicals with bulky X8 sub-stituents, such as Br or I, due to steric effects. The extent ofp-p and dp homoconjugation relative to hyperconjugation is also discussed. No pp homoconjugation is found for very electronegative substituents such as X8= F or OR, but both pp and dp homoconjugation appears to be present for X8 substituents such as Cl. A possible mechanism for the racemization of optically active-CR^ CFhX8 radicals is presented and the rate of racemization is traced to the bulkiness of the X8 substituent.