University of Campinas, Chemistry Institute, Monteiro Lobato Street, Campinas, Sao Paulo 13083-862, Brazil.
Phys Chem Chem Phys. 2021 Aug 28;23(32):17329-17337. doi: 10.1039/d1cp02806e. Epub 2021 Aug 4.
The origin of the fluorine gauche effect has been debated for decades and recently different interpretations have been raised in the scientific community as new computational methods emerged and were applied to rationalize 1,2-difluoroethane (DFE) gauche preference. In this context, we revisited 1,2-difluoroethane (DFE) and its chlorine and bromine derivative conformational preferences through a comparative approach: the conformational behavior and hyperconjugative, steric and electrostatic contributions for the internal rotational barrier of DFE were compared with several analogue backbones, such as peroxides, disulfides and ammonia boranes. By using the Natural Bond Orbital (NBO) analysis it was found that hyperconjugation is the driving force of the conformational preference in DFE and its chlorine and bromine analogues. Electrostatics was found to be negligible and steric effects played a minor role in general, but are important in ClCHCHCl and BrCHCHBr to counterbalance gauche stabilization by hyperconjugation and for the preference of the anti conformer.
氟 gauche 效应的起源已经争论了几十年，最近随着新的计算方法的出现并被应用于合理化 1,2-二氟乙烷（DFE） gauche 偏好，科学界提出了不同的解释。在这种情况下，我们通过比较的方法重新研究了 1,2-二氟乙烷（DFE）及其氯和溴衍生物的构象偏好：通过自然键轨道（NBO）分析发现，超共轭是 DFE 及其氯和溴类似物构象偏好的驱动力。与过氧化物、二硫化物和氨硼烷等几种类似骨架相比，DFE 的内部旋转势垒的超共轭、空间位阻和静电贡献。静电作用可以忽略不计，一般来说空间位阻效应的作用较小，但在 ClCHCHCl 和 BrCHCHBr 中，超共轭的 gauche 稳定作用和反式构象的偏好需要通过静电作用来平衡。
