The Mechanism of Covalent Bonding (the authors reply) (original) (raw)
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Bond electron pair: Its relevance and analysis from the quantum chemistry point of view
Journal of Computational Chemistry, 2007
This paper first comments on the surprisingly poor status that Quantum Chemistry has offered to the fantastic intuition of Lewis concerning the distribution of the electrons in the molecule. Then, it advocates in favor of a hierarchical description of the molecular wave-function, distinguishing the physics taking place in the valence space (in the bond and between the bonds), and the dynamical correlation effects. It is argued that the clearest pictures of the valence electronic population combine two localized views, namely the bond (and lone pair) Molecular Orbitals and the Valence Bond decomposition of the wave-function, preferably in the orthogonal version directly accessible from the complete active space self consistent field method. Such a reading of the wave function enables one to understand the work of the nondynamical correlation as an enhancement of the weight of the low-energy VB components, i.e. as a better compromise between the electronic delocalization and the energetic preferences of the atoms. It is suggested that regarding the bond building, the leading dynamical correlation effect may be the dynamical polarization phenomenon. It is shown that most correlation effects do not destroy the bond electron pairs and remain compatible with Lewis' vision. A certain number of free epistemological considerations have been introduced in the development of the argument. q
Toward a physical understanding of electron-sharing two-center bonds. I. General aspects
Journal of Computational Chemistry, 2006
In 1916, Lewis and Kossel laid the empirical ground for the electronic theory of valence, whose quantum theoretical foundation was uncovered only slowly. We can now base the classification of the various traditional chemical bond types in a threefold manner on the one-and two-electron terms of the quantum-physical Hamiltonian (kinetic, atomic core attraction, electron repulsion). Bond formation is explained by splitting up the real process into two physical steps: (i) interaction of undeformed atoms and (ii) relaxation of this nonstationary system. We aim at a flexible bond energy partitioning scheme that can avoid cancellation of large terms of opposite sign. The driving force of covalent bonding is a lowering of the quantum kinetic energy density by sharing. The driving force of heteropolar bonding is a lowering of potential energy density by charge rearrangement in the valence shell. Although both mechanisms are quantum mechanical in nature, we can easily visualize them, since they are of one-electron type. They are however tempered by two-electron correlations. The richness of chemistry, owing to the diversity of atomic cores and valence shells, becomes intuitively understandable with the help of effective core pseudopotentials for the valence shells. Common conceptual difficulties in understanding chemical bonds arise from quantum kinematic aspects as well as from paradoxical though classical relaxation phenomena. On this conceptual basis, a dozen different bond types in diatomic molecules will be analyzed in the following article. We can therefore examine common features as well as specific differences of various bonding mechanisms.
Journal of Molecular Structure: THEOCHEM, 1996
This paper analyzes the changes experienced by the valence localized molecular orbitals (LMOs) of benzene, singlet cyclobutadiene and (E)-1,3,Shexatriene -with respect to both energy and degree of delocalization -under the effect of several geometrical distortions of the carbon-carbon frame. The analysis shows that, at the AM1 level of theory, there is a unique symmetrizing tendency of the x-LMOs of benzene, which are at the same time maximally delocalized and energetically most stable for a totally symmetric geometry. The x-LMOs of both cyclobutadiene and hexatriene show opposite behavior, increasing their energies when subjected to bond length equalizing distortions and decreasing them when the carbon-carbon bond length alternation of the system is enhanced. The distortive properties of the x-electrons, when analyzed in terms of the corresponding AMl-LMOs, thus appear to correlate well with the aromatic character of the molecule.
This chapter outlines the basic elements of Spin-Coupled Generalized Valence Bond (SCGVB) theory, including the structure of the SCGVB wavefunction as well as its optimization and analysis. The SCGVB wavefunctions accounts for non-dynamical electron correlation yet still provides a compelling orbital description of the electronic structures of the ground and excited states of molecules and of the electronic mechanisms of chemical reactions. The insights provided by SCGVB theory are illustrated using a representative set of examples: the “traditional” HCN molecule, and its fragmentation into CH and N; the formation of “hypervalent” SF4 and SF6 from SF(n–1) + F; resonance/antiresonance in benzene and tropylium cation (C7H7+ ); the excited states of benzene, CH and O3/SO2 ; and the reaction of ground state methylene with H2 . The effects of dynamical electron correlation, which must be included to make quantitative predictions of molecular properties, can be taken into account by using...
New Landscape of Electron-Pair Bonding: Covalent, Ionic, and Charge-Shift Bonds
Structure and Bonding, 2015
We discuss here the modern valence bond (VB) description of the electron-pair bond visa -vis the Lewis-Pauling model and show that along the two classical families of covalent and ionic bonds, there exists a family of charge-shift bonds (CSBs) in which the "resonance fluctuation" of the electronpair density plays a dominant role. A bridge is created between the VB description of bonding and three other approaches to the problem: the electron localization function (ELF), atoms-in-molecules (AIM), and molecular orbital (MO)-based theories. In VB theory, CSB manifests by repulsive or weakly bonded covalent state and large covalent-ionic resonance energy, RE CS. In ELF, it shows up by a depleted basin population with fluctuations and in AIM by a positive Laplacian. CSB is derivable also from MO-based theory. As such, CSB is shown to be a fundamental mechanism that satisfies the equilibrium condition of bonding, namely, the virial ratio of the kinetic and potential energy contributions to the bond energy. The chapter defines the atomic propensity for CSB and outlines its territory: Atoms
On The Nature of the Chemical Bond in Valence Bond Theory
The Journal of Chemical Physics
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 i...
Journal of Molecular Structure Theochem, 1996
This paper analyzes the changes experienced by the valence localized molecular orbitals (LMOs) of benzene, singlet cyclobutadiene and (E)-1,3,Shexatriene -with respect to both energy and degree of delocalization -under the effect of several geometrical distortions of the carbon-carbon frame. The analysis shows that, at the AM1 level of theory, there is a unique symmetrizing tendency of the x-LMOs of benzene, which are at the same time maximally delocalized and energetically most stable for a totally symmetric geometry. The x-LMOs of both cyclobutadiene and hexatriene show opposite behavior, increasing their energies when subjected to bond length equalizing distortions and decreasing them when the carbon-carbon bond length alternation of the system is enhanced. The distortive properties of the x-electrons, when analyzed in terms of the corresponding AMl-LMOs, thus appear to correlate well with the aromatic character of the molecule.