Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322-0300, United States.
J Phys Chem A. 2021 Aug 5;125(30):6514-6528. doi: 10.1021/acs.jpca.1c05431. Epub 2021 Jul 26.
Accompanying the rapidly growing list of σ-hole bonds has come the acknowledgment of parallel sorts of noncovalent bonds which owe their stability in large part to a deficiency of electron density in the area above the molecular plane, known as a π-hole. The origins of these π-holes are probed for a wide series of molecules, comprising halogen, chalcogen, pnicogen, tetrel, aerogen, and spodium bonds. Much like in the case of their σ-hole counterparts, formation of the internal covalent π-bond in the Lewis acid molecule pulls density toward the bond midpoint and away from its extremities. This depletion of density above the central atom is amplified by an electron-withdrawing substituent. At the same time, the amplitude of the π*-orbital is enhanced in the region of the density-depleted π-hole, facilitating a better overlap with the nucleophile's lone pair orbital and a stabilizing n → π* charge transfer. The presence of lone pairs on the central atom acts to attenuate the π-hole and shift its position somewhat, resulting in an overall weakening of the π-hole bond. There is a tendency for π-hole bonds to include a higher fraction of induction energy than σ-bonds with proportionately smaller electrostatic and dispersion components, but this distinction is less a product of the σ- or π-character and more a function of the overall bond strength.
伴随着 σ- -hole 键数量的快速增长，人们也开始承认存在类似的非共价键，这些键的稳定性在很大程度上归因于分子平面上方区域电子密度的缺乏，这种区域被称为 π- 空穴。对一系列包括卤素、硫属元素、磷属元素、碳族元素、氧族元素和 spodium 键在内的广泛分子进行了对这些 π- 空穴起源的探究。与 σ- 空穴类似，路易斯酸分子中内部共价 π 键的形成将密度拉向键中点并使其远离两端。这种在中心原子上方的密度耗散通过吸电子取代基得到放大。与此同时，在密度耗尽的 π- 空穴区域，π*- 轨道的幅度增强，从而促进与亲核试剂的孤对轨道更好地重叠以及稳定的 n → π* 电荷转移。中心原子上孤对电子的存在会削弱 π- 空穴并使其位置发生一定程度的偏移，从而导致 π- 空穴键整体强度减弱。π- 空穴键比 σ- 键包含更多的诱导能，而其静电和色散分量的比例较小，但这种区别与其说是 σ- 或 π- 键的特性所致，不如说是整体键强度的函数。
