Department of Theoretical Chemistry Faculty of Chemistry, Jagiellonian University in Krakow, Gronostajowa 2, 30-87, Krakow, Poland.
Department of Chemistry Faculty of Natural and Agricultural Sciences, University of Pretoria, Lynnwood Road, Hatfield, Pretoria, 0002, South Africa.
Chemphyschem. 2020 Mar 17;21(6):494-502. doi: 10.1002/cphc.202000066. Epub 2020 Feb 14.
It is shown herein that intuitive and text-book steric-clash based interpretation of the higher energy "in-in" xylene isomer (as arising solely from the repulsive CH⋅⋅⋅HC contact) with respect to the corresponding global-minimum "out-out" configuration (where the clashing C-H bonds are tilted out) is misleading. It is demonstrated that the two hydrogen atoms engaged in the CH⋅⋅⋅HC contact in "in-in" are involved in attractive interaction so they cannot explain the lower stability of this isomer. We have proven, based on the arsenal of modern bonding descriptors (EDDB, HOMA, NICS, FALDI, ETS-NOCV, DAFH, FAMSEC, IQA), that in order to understand the relative stability of "in-in" versus "out-out" xylenes isomers one must consider the changes in the electronic structure encompassing the entire molecules as arising from the cooperative action of hyperconjugation, aromaticity and unintuitive London dispersion plus charge delocalization based intra-molecular CH⋅⋅⋅HC interactions.
本文表明，基于直观和教科书上的位阻冲突解释，高能“内-内”二甲苯异构体（仅源于排斥性 CH⋅⋅⋅HC 接触）相对于相应的全局最小“外-外”构象（其中冲突的 C-H 键倾斜）是具有误导性的。我们已经证明，“内-内”二甲苯中参与 CH⋅⋅⋅HC 接触的两个氢原子之间存在吸引相互作用，因此它们不能解释这种异构体的较低稳定性。我们已经基于现代成键描述符（EDDB、HOMA、NICS、FALDI、ETSNOCV、DAFH、FAMSEC、IQA）的武器库证明，为了理解“内-内”与“外-外”二甲苯异构体的相对稳定性，必须考虑整个分子的电子结构变化，这些变化源于超共轭、芳香性和非直观的伦敦分散作用以及基于分子内 CH⋅⋅⋅HC 相互作用的电荷离域协同作用。
