Is hyperconjugation responsible for the "gauche effect" in 1-fluoropropane and other 2-substituted-1-fluoroethanes?
Is hyperconjugation responsible for the "gauche effect" in 1-fluoropropane and other 2-substituted-1-fluoroethanes?
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DOI：
10.1039/a901974j
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发表时间：
1999-08-01
期刊：
影响因子：
--
通讯作者：
Borden, WT
中科院分区：
文献类型：
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作者：
Rablen, PR;Hoffman, RW;Borden, WT
在 MP2/6-311++G**(6D)//MP2/6-31+G* 理论水平上计算了一系列 2-取代-1-氟乙烷的能量和几何形状，了解 C-C 键旋转的最大值和最小值。结果不支持超共轭模型的预测，即 1,2-二氟乙烷和 1-氯-2-氟乙烷都强烈倾向于稀疏构象，而 1-氟-2-甲硅烷基乙烷强烈倾向于反构象。氟和 C-2 取代基之间存在竞争性静电相互作用，由左旋构象异构体的详细几何形状和计算的左旋反能量差对极性溶剂存在的敏感性表明。然而，扭转势能的傅立叶分析与超共轭电子捐赠到 C-F sigma* 轨道完全一致，这有助于这些 1-氟乙烷的构象偏好。傅里叶分析还表明，超共轭导致计算出的 C-C 和 C-F 键长以及氟原子电荷的微小变化。扭转势能、几何结构和原子电荷的变化以及对溶剂的敏感性均符合碳键超共轭给电子能力的预期排序：C-Si > C-H > C-C > C-Cl > C-F。
The energies and geometries of a series of 2-substituted-1-fluoroethanes were computed at the MP2/6-311++G**(6D)//MP2/6-31+G* level of theory for both the maxima and minima of the rotation about the C-C bond. The results did not support the predictions of a hyperconjugative model, that both 1,2-difluoroethane and 1-chloro-2-fluoroethane would strongly prefer a gauche conformation, and that 1-fluoro-2-silylethane would strongly prefer an anti conformation. The existence of competing electrostatic interactions between the fluorine and the substituents at C-2 was indicated by the detailed geometries of the gauche conformers and by the calculated sensitivity of the gauche-anti energy differences to the presence of a polar solvent. However, Fourier analyses of the torsional potential energies were wholly consistent with hyperconjugative electron donation into the C-F sigma* orbital contributing to the conformational preferences of these 1-fluoroethanes. Fourier analyses also showed that hyperconjugation contributes to the small variations in C-C and C-F bond lengths and in fluorine atomic charges that were computed. The torsional potential energies, variations in geometry and atomic charge, and sensitivity to solvent were all in accord with the expected ranking of hyperconjugative electron donating ability of bonds to carbon, C-Si > C-H > C-C > C-Cl > C-F.