Shaik Sason, Danovich David, Hiberty Philippe C
Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 9190401, Israel.
CNRS, Institut de Chimie Physique UMR8000, Université Paris-Saclay, 91405 Orsay, France.
J Chem Phys. 2022 Sep 7;157(9):090901. doi: 10.1063/5.0095953.
This Perspective outlines a panoramic description of the nature of the chemical bond according to valence bond theory. It describes single bonds and demonstrates the existence of a "forgotten family" of charge-shift bonds (CSBs) in which the entire/most of the bond energy arises from the resonance between the covalent and ionic structures of the bond. Many of the CSBs are homonuclear bonds. Hypervalent molecules (e.g., XeF) are CSBs. This Perspective proceeds to describe multiple bonded molecules with an emphasis on C and O. C has four electron pairs in its valence shell and, hence, 14 covalent structures and 1750 ionic structures. This Perspective outlines an effective methodology of peeling the electronic structure to the minimal and important number of structures: a dominant structure that displays a quadruple bond and two minor structures with π + σ bonds, which stabilize the quadruple bond by resonance. O is chosen because it is a diradical, which is persistent and life-sustaining. It is shown that the persistence of this diradical is due to the charge-shift bonding of the π-3-electron bonds. This section ends with a discussion of the roles of π vs σ in the geometric preferences of benzene, acetylene, ethene, and their Si-based analogs. Subsequently, this Perspective discusses bonding in clusters of univalent metal atoms, which possess only parallel spins (M), and are nevertheless bonded due to the resonance interactions that stabilize the repulsive elementary structure (all spins are up). The bond energy reaches ∼40 kcal/mol for a pair of atoms (in Cu; n ∼ 10-12). The final subsection discusses singlet excited states in ethene, ozone, and SO. It demonstrates the capability of the breathing-orbital VB method to yield an accurate description of a variety of excited states using merely 10 or few VB structures. Furthermore, the method underscores covalent structures that play a key role in the correct description and bonding of these excited states.
这篇展望根据价键理论对化学键的本质进行了全景式描述。它介绍了单键，并证明了存在一个“被遗忘的家族”——电荷转移键（CSB），其中整个/大部分键能源于键的共价结构和离子结构之间的共振。许多CSB是同核键。超价分子（如XeF）就是CSB。这篇展望接着描述了多键分子，重点是碳和氧。碳在其价层中有四对电子，因此有14种共价结构和1750种离子结构。这篇展望概述了一种有效的方法，可将电子结构简化为最少且重要的结构数量：一个显示四重键的主导结构和两个具有π + σ键的次要结构，后者通过共振使四重键稳定。选择氧是因为它是一个双自由基，具有持久性且对生命维持至关重要。结果表明，这个双自由基的持久性源于π - 3 - 电子键的电荷转移键合。本节最后讨论了π键与σ键在苯、乙炔、乙烯及其硅基类似物的几何偏好中的作用。随后，这篇展望讨论了单价金属原子簇中的键合，这些原子簇仅具有平行自旋（M），但由于共振相互作用使排斥性基本结构稳定（所有自旋向上），它们仍然形成键。对于一对原子（在铜中；n ∼ 10 - 12），键能达到约40千卡/摩尔。最后一个小节讨论了乙烯、臭氧和SO中的单重激发态。它展示了呼吸轨道价键方法仅使用10个或很少的价键结构就能准确描述各种激发态的能力。此外，该方法强调了在正确描述和键合这些激发态中起关键作用的共价结构。
