Department of Theoretical Chemistry, Amsterdam Institute of Molecular and Life Sciences (AIMMS), Amsterdam Center for Multiscale Modeling (ACMM), Vrije Universiteit Amsterdam, De Boelelaan 1083, 1081 HV, Amsterdam (The, Netherlands.
Departamento de Química, Instituto de Ciências Naturais, Universidade Federal de Lavras, 37200-900, Lavras-MG, Brazil.
Chemphyschem. 2021 Apr 7;22(7):641-648. doi: 10.1002/cphc.202100090. Epub 2021 Feb 25.
We have quantum chemically investigated the rotational isomerism of 1,2-dihaloethanes XCH CH X (X = F, Cl, Br, I) at ZORA-BP86-D3(BJ)/QZ4P. Our Kohn-Sham molecular orbital (KS-MO) analyses reveal that hyperconjugative orbital interactions favor the gauche conformation in all cases (X = F-I), not only for X = F as in the current model of this so-called gauche effect. We show that, instead, it is the interplay of hyperconjugation with Pauli repulsion between lone-pair-type orbitals on the halogen substituents that constitutes the causal mechanism for the gauche effect. Thus, only in the case of the relatively small fluorine atoms, steric Pauli repulsion is too weak to overrule the gauche preference of the hyperconjugative orbital interactions. For the larger halogens, X⋅⋅⋅X steric Pauli repulsion becomes sufficiently destabilizing to shift the energetic preference from gauche to anti, despite the opposite preference of hyperconjugation.
我们在 ZORA-BP86-D3(BJ)/QZ4P 水平上用量子化学方法研究了 1,2-二卤代乙烷 XCHCHX（X=F、Cl、Br、I）的旋转异构现象。我们的 Kohn-Sham 分子轨道（KS-MO）分析表明，超共轭轨道相互作用在所有情况下（X=F-I）都有利于 gauche 构象，而不仅仅是像当前所谓的 gauche 效应模型中那样 X=F。我们表明，相反，超共轭作用与卤素取代基上孤对型轨道之间的 Pauli 排斥相互作用的相互作用构成了 gauche 效应的因果机制。因此，只有在相对较小的氟原子的情况下，空间位阻的 Pauli 排斥作用太弱，无法推翻超共轭轨道相互作用的 gauche 偏好。对于较大的卤素，X⋅⋅⋅X 空间位阻的 Pauli 排斥作用变得足够不稳定，以至于尽管超共轭作用的偏好相反，但从 gauche 到 anti 的能量偏好发生了转移。
