Philippe C. Hiberty

Charge-shift bonding and its manifestations in chemistry

Electron-pair bonding is a central chemical paradigm. Here, we show that alongside the two classical covalent and ionic bond families, there exists a class of charge-shift (CS) bonds wherein the electron-pair fluctuation has the dominant role. Charge-shift bonding shows large covalent-ionic resonance interaction energy, and depleted charge densities, and features typical to repulsive interactions, albeit the bond itself may well be strong. This bonding type is rooted in a mechanism whereby the bond achieves equilibrium defined by the virial ratio. The CS bonding territory involves, for example, homopolar bonds of compact electronegative and/or lone-pair-rich elements, heteropolar bonds of these elements among themselves and with other atoms (for example, the metalloids, such as silicon and germanium), hypercoordinated molecules, and bonds whose covalent components are weakened by exchange-repulsion strain (as in [1.1.1]propellane). Here, we discuss experimental manifestations of CS bonding in chemistry, and outline new directions demonstrating the portability of the new concept.

Classical Valence Bond Approach by Modern Methods

National Natural Science Foundation of China[20873106]; Ministry of Science and Technology[2011CB808504]; Israel Science Foundation[ISF 53/09]

Compact and accurate valence bond functions with different orbitals for different configurations: application to the two-configuration description of F2

Abstract A way of computing reliable and very compact ab initio classical valence bond wavefunctions is presented. The method deals with a minimal number of configuration-state functions (CSFs), with only one per Lewis structure necessary to describe the active part of a chemical system since each CSF is allowed to have its specific set of orbitals, different from one CSF to the other. The coefficients of the CSFs and their orbitals, which remain purely local without any delocalization tails, are simultaneously optimized through a non-orthogonal MCSCF technique. The method is applied to the electronic structure of F 2 . The wavefunction involves only two configurations, and yields a dissociation energy of 28.6 kcal/mol, which compares very well with the estimated full-CI result of 29–30 kcal/mol in an analogous basis set.

Compact valence bond functions with breathing orbitals: Application to the bond dissociation energies of F2 and FH

An original computational method of ab initio valence bond type is proposed, aiming at yielding accurate dissociation energy curves, while dealing with wave functions being very compact and clearly interpretable in terms of Lewis structures. The basic principle is that the wave function is allowed to have different orbitals for different valence bond structures. Thus, throughout the dissociation process, the so‐called ‘‘breathing orbitals’’ follow the instantaneous charge fluctuations of the bond being broken by undergoing changes in size, hybridization, and polarization. The method is applied to the dissociation of F2 and FH. For each molecule, a wave function involving only three valence bond configurations yields equilibrium bond lengths within 0.01 A, and dissociation energies within about 2 kcal/mol of the results of estimated or true full configuration interaction in the same basis sets. The effect of dynamical electron correlation on calculated dissociation energies is analyzed. It is shown that re...

A clear correlation between the diradical character of 1,3-dipoles and their reactivity toward ethylene or acetylene.

A series of nine 1,3-dipoles, belonging to the families of diazonium betaines, nitrilium betaines, and azomethine betaines, has been studied by means of the breathing-orbital valence bond ab initio method. Each 1,3-dipole is described as a linear combination of three valence bond structures, two zwitterions and one diradical, for which the weights in the total wave function can be quantitatively estimated. In agreement with an early proposition of Harcourt, the diradical character of 1,3-dipoles is shown to be a critical feature to favor 1,3-dipolar cycloaddition. Within each family, a linear relationship is evidenced between the weight of the diradical structure in the 1,3-dipole and the barrier to cycloaddition to ethylene or acetylene, with correlation coefficients of 0.98-1.00. The barrier heights also correlate very well with the transition energies from ground state to pure diradical states of the 1,3-dipoles at equilibrium geometry. Moreover, the weight of the diradical structure is shown to increase significantly in all 1,3-dipoles from their equilibrium geometries to their distorted geometries in the transition states. A mechanism for 1,3-dipolar cycloaddition is proposed, in which the 1,3-dipole first distorts so as to reach a reactive state that possesses some critical diradical character and then adds to the dipolarophile with little or no barrier. This mechanism is in line with the recently proposed distortion/interaction energy model of Ess and Houk and their finding that the barrier heights for the cycloaddition of a given 1,3-dipole to ethylene and acetylene are nearly the same, despite the exothermicity difference (Ess, D. H. and Houk, K. N. J. Am. Chem. Soc. 2008, 130, 10187).

A survey of recent developments in ab initio valence bond theory

Starting from the 1980s and onwards, Valence Bond theory has been enjoying renaissance that is characterized by the development of a growing number of ab initio methods, and by many applications to chemical reactivity and to the central paradigms of chemistry. Owing the increase of computational power of modern computers and to significant advances in the methodology, valence bond theory begins to offer a sound and attractive alternative to Molecular Orbital theory. This review aims at summarizing the most important developments of ab initio valence bond methods during the last two or three decades, and is primarily devoted to a description of what the various methods can actually achieve within their specific scopes and limitations. Key available softwares are surveyed.

Topology of Electron Charge Density for Chemical Bonds from Valence Bond Theory: A Probe of Bonding Types

To characterize the nature of bonding we derive the topological properties of the electron charge density of a variety of bonds based on ab initio valence bond methods. The electron density and its associated Laplacian are partitioned into covalent, ionic, and resonance components in the valence bond spirit. The analysis provides a density-based signature of bonding types and reveals, along with the classical covalent and ionic bonds, the existence of two-electron bonds in which most of the bonding arises from the covalent-ionic resonance energy, so-called charge-shift bonds. As expected, the covalent component of the Laplacian at the bond critical point is found to be largely negative for classical covalent bonds. In contrast, for charge-shift bonds, the covalent part of the Laplacian is small or positive, in agreement with the weakly attractive or repulsive character of the covalent interaction in these bonds. On the other hand, the resonance component of the Laplacian is always negative or nearly zero, and it increases in absolute value with the charge-shift character of the bond, in agreement with the decrease of kinetic energy associated with covalent-ionic mixing. A new interpretation of the topology of the total density at the bond critical point is proposed to characterize covalent, ionic, and charge-shift bonding from the density point of view.

COVALENT, IONIC AND RESONATING SINGLE BONDS

Abstract Three bond types of electron-pair bonding emerge from multi-structure valence bond (VB) computations of 10 different single bonds. The first bond type is observed in HH, LiLi, CH and SiH. These are all covalent bond types whose major bonding comes from the covalent Heitler-London (HL) configuration, with a minor perturbation from the resonance interaction between the covalent and zwitterionic (Z) configurations. The second bond type is observed for NaF. This is an ionic bond type in which the major bonding is provided by the electrostatic stabilization of the ionic configuration, Na + F − , with a slight perturbation from the HLZ resonance interaction. The third bond type is observed for FF, HF, CF and SiF. These are the resonating bond types in which the major bonding event is the resonance energy stabilization due to the HLZ mixing. No special status should be attached to either the covalency or ionicity of these last bond types, even if they may appear purely “covalent”, such as FF, or “highly ionic”, as CF, by charge distribution criterion. The phenomenon of resonating bonding is shown to emerge from weakly bound or unbound covalent HL configurations which originate when the “preparation” for bonding of the fragments becomes energy demanding, as for fluorine. The mechanism of HL bond weakening is through costly promotion energy and overlap repulsion of a lone pair with a bond pair of the same symmetry. The essential requirements for a fragment A to qualify as a resonating binder are therefore: (a) to possess two AOs which maintain a very large energy gap between them, and which by virtue of overlap capability can both enter into bonding; and (b) to have three electrons in these two AOs which thereby mutually antagonize each others bonding. The propensity for resonating bonding is discussed, in the light of these qualifications, for the main elements across the Periodic Table. It is concluded that the elements with the highest propensity for resonating bonding are F, O and N. Any combination AB where either A or B or both are resonating binders is likely to lead to a resonating bond (e.g. OO, NF, CF, CO, and so on). The resonating bonds are shown to coincide with the group of “weakened” bonds in the classification of Sanderson, and with those bonds which exhibit negative or marginally positive deformation densities in electron density determinations. Negative or marginally positive deformation densities may serve as the experimental signature of the theoretical concept of resonating bonding. The LiH bond appears to possess a special status. While the computations tend to classify this bond among the covalent types, the results also show that the HL and ionic Li + H − configurations are nearly degenerate and maintain a very weak coupling. Therefore the LiH bond will have a metastable character, as far as ionicity-covalency, in the presence of medium perturbations which are at least of the magnitude of the coupling between the ionic and covalent structures.

Valence Bond Mixing and Curve Crossing Diagrams in Chemical Reactivity and Bonding

Publisher Summary This chapter discusses the reactivity paradigms; the Valence Bond (VB) mixing diagrams that have been developed to answer a fundamental question of chemical reactivity: “What are the origins of the barrier in a chemical reaction, and what is the mechanism of transition state formation?” If a mechanism of barrier formation exists, the structure and energy of the transition state can be analyzed, and relationships that may exist between the transition state and properties of its precursor and successor reactants and products can be predicted. This chapter provides an overview of the current knowledge on the qualitative applications and quantitative implementations of the VB mixing ideas. It also describes the key elements of the VB mixing diagrams and their principles of applications to chemical problems. Most of the applications are within the domain of chemical reactivity, but some problems of bonding and structure, which can be couched as chemical transformations bearing similarity to a chemical reaction, are also discussed.

The essential role of charge-shift bonding in hypervalent prototype XeF2

Hypervalency in XeF₂ and isoelectronic complexes is generally understood in terms of the Rundle-Pimentel model (which invokes a three-centre/four-electron molecular system) or its valence bond version as proposed by Coulson, which replaced the old expanded octet model of Pauling. However, the Rundle-Pimentel model is not always successful in describing such complexes and has been shown to be oversimplified. Here using ab initio valence bond theory coupled to quantum Monte Carlo methods, we show that the Rundle-Pimentel model is insufficient by itself in accounting for the great stability of XeF₂, and that charge-shift bonding, wherein the large covalent-ionic interaction energy has the dominant role, is a major stabilizing factor. The energetic contribution of the old expanded octet model is also quantified and shown to be marginal. Generalizing to isoelectronic systems such as ClF₃, SF₄, PCl₅ and others, it is suggested that charge-shift bonding is necessary, in association with the Rundle-Pimentel model, for hypervalent analogues of XeF₂ to be strongly bonded.