On the nature of the chemical bond in valence bond theory

Abstract

This perspective outlines a panoramic description of the nature of the chemical bond according to valence bond theory. It describes single bonds, and 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 CSBs are homonuclear bonds. Hypervalent molecules are CSBs. Then we describe multiply bonded molecules with emphasis on C2 and 3O2. The perspective outlines an effective methodology of peeling the electronic structure to the necessary minimum: a structure with a quadruple bond, and two minor structures with double bonds, which stabilize the quadruple bond by resonance. 3O2 is chosen because it is a persistent diradical. The persistence of 3O2 is due to the large CSB resonance interaction of the π-3-electron bonds. Subsequently, we describe the roles of π vs. σ in the geometric preferences in unsaturated molecules, and their Si-based analogs. Then, the perspective discusses bonding in clusters of univalent metal-atoms, which possess only parallel spins, and are nevertheless bonded due to multiple resonance interactions. The bond energy reaches ~40 kcal/mol for a pair of atoms (in n+1Cun; n~10-12). The final subsection discusses singlet excited states in ethene, ozone and SO2. It demonstrates the capability of the breathing-orbital VB method to yield an accurate description of a variety of excited states using 10 or less VB structures. Furthermore, the method underscores covalent structures which play a key role in the correct description and bonding of these excited states.

Valence Bond Theory Allows a Generalized Description of Hydrogen Bonding

“…Thus, structure 4 / 4 a is the covalent Heitler–London structure of the B----H bond, whereas structures 3 / 3 a and 5 / 5 a are the corresponding ionic structures. As such, the VB mixing of these three structures at the optimized HB distance (B----H) gives rise to the total charge-shift resonance energy (RE CS ) , of the HB.…”

My Vision of Electric-Field-Aided Chemistry in 2050

“…During the initial years in Jerusalem, I focused on applications of VB diagrams with my first Ph.D. student, Avital Shurki, and on chemical bonding with my postdoc Johnny Galbraith . Simultaneously, I have gotten engaged in intense collaborations on chemical bonding, , with Philippe Hiberty and the members of the “VB community”, which included among others, my research assistant David Danovich, and Wei Wu (from Xiamen University) who originated the friendly VB software (XMVB), and came for one year to Jerusalem to apply VB to chemical problems. The VB activity made me realize the importance of ionic structures for bonding and reactivity, especially in the presence of electric fields .…”

Strong-field molecular ionization beyond the single active electron approximation

“…Note that the interpretation of the physics of the system using directly these orbitals' properties is possible only near the equilibrium geometry. Near the dissociative limit, the physics is more appropriately discussed in Valence-Bond theoretic language 41 , using Heitler-London orbitals, which asymptotically, (as R −→ ∞), become atomic orbitals. We will occasionally evoke these in the discussion of the ionization dynamics at elongated geometries and in the dissociative limit.…”

“…6. We first note that only the CSFs |1 ≡ |σ 2 g and |3 ≡ |σ 2 u can give an asymptotically covalent configuration 1s A 1s B , where 1s A(B) denotes a Heitler-London, or rather Coulson-Fischer orbital of Valence-Bond theory 41 that becomes the 1s atomic orbital centered on proton H A or H B . A purely covalent state is attained when the oscillating populations of these two CSFs are equal, i.e.…”