A valence bond view of isocyanides' electronic structure

International audienceHigh level Valence Bond calculations support a predominantly carbenic electronic structure for isocyanides, with a secondary zwitterionic character, despite their linear geometry. This geometry results from the significant energetic stabilization due to nitrogen π lone pair donation. Results are not changed by substitution or solvation effects

The physical origin of large covalent-ionic resonance energies in some two-electron bonds

This study uses valence bond (VB) theory to analyze in detail the previously established finding that alongside the two classical bond families of covalent and ionic bonds, which describe the electron-pair bond, there exists a distinct class of charge-shift bonds (CS-bonds) in which the fluctuation of the electron pair density plays a dominant role. Such bonds are characterized by weak binding, or even a repulsive, covalent component, and by a large covalent-ionic resonance energy RECS that is responsible for the major part, or even for the totality, of the bonding energy. In the present work, the nature of CS-bonding and its fundamental mechanisms are analyzed in detail by means of a VB study of some typical homonuclear bonds (H-H, H3C-CH3, H2N-NH2, HO-OH, F-F, and Cl-Cl), ranging from classical-covalent to fully charge-shift bonds. It is shown that CS-bonding is characterized by a covalent dissociation curve with a shallow minimum situated at long interatomic distances, or even a fully repulsive covalent curve. As the atoms that are involved in the bond are taken from left to right or from bottom to top of the periodic table, the weakening effect of the adjacent bonds or lone pairs increases, while at the same time the reduced resonance integral, that couples the covalent and ionic forms, increases. As a consequence, the weakening of the covalent interaction is gradually compensated by a strengthening of CS-bonding. The large RECS quantity of CS-bonds is shown to be an outcome of the mechanism necessary to establish equilibrium and optimum bonding during bond formation. It is shown that the shrinkage of the orbitals in the covalent structure lowers the potential energy, V, but excessively raises the kinetic energy, T, thereby tipping the virial ratio off-balance. Subsequent addition of the ionic structures lowers T while having a lesser effect on V, thus restoring the requisite virial ratio (T/-V=1/ 2). Generalizing to typically classical covalent bonds, like H-H or C-C bonds, the mechanism by which the virial ratio is obeyed during bond formation is primarily orbital shrinkage, and therefore the charge-shift resonance energy has only a small corrective effect. On the other hand, for bonds bearing adjacent lone pairs and/or involving electronegative atoms, like F-F or Cl-Cl, the formation of the bond corresponds to a large increase of kinetic energy, which must be compensated for by a large participation or covalent - ionic mixing

Etude théorique des liaisons à trois électrons dans les ions radicaux

Les semi-liaisons à trois électrons, où l'orbitale liante est doublement occupée tandis que l'anti-liante correspondante l'est simplement, apparaissent de plus en plus dans les publications aussi bien expérimentales que théoriques. Pourtant, beaucoup reste à faire l'adéquation des méthodes de structure électronique au traitement des liaisons à trois électrons. Cette thèse a tout d'abord pour but de précisément mener cette analyse. Plusieurs défauts physiques importants ont été mis en évidence concernant deux méthodes de calculs très courantes; les méthodes de la fonctionnelle de la densité, puis les méthodes de perturbations Moller-Plesset; les rendant inapplicables pour de nombreux systèmes liés par trois électrons. Les causes ont été identifiées et analysées en détail. En DFT, c'est le problème délicat de la correction de self-interaction qui pose problème. Pour la méthode Moller-Plesset, ce sont des ruptures ou pseudo-ruptures de symétrie intempestives pouvant se produire dans la fonction de référence Hartree-Fock qui sont à l'origine des difficultés rencontrées. L'exploration méthodologique se termine avec la proposition de deux remèdes: tout d'abord un indice de fiabilité permettant de prévoir les situations où les prédictions en Moller-Plesset sont dignes de confiances; et une méthode de calcul adaptée, basée sur une technique multi-référence, qui peut être utilisée dans les situations d'échec des méthodes courantes. Enfin, utilisant l'expérience accumulée, cette thèse se clôt par deux courtes études destinées à améliorer notre connaissance de la physique particulière des liaisons à trois électrons. La première est consacrée à l'étude de l'effet du substituant méthyle, qui s'avère contrasté en fonction des atomes impliqués dans la semi-liaison ; la seconde est une rationalisation des raisons de la stabilité ou de l'instabilité des anions à trois électrons.The three-electron bonds, where a bonding orbital is doubly occupied and the corresponding anti-bonding orbital is singly occupied, are increasingly ubiquitous in experimental as well as in theoretical studies. However, little is known about the performances of current theoretical methods on this kind of bond. This thesis is first aimed at filling this gap, by performing a detailed method exploration. Serious weaknesses of two of the most popular methods, i.e. density functional theory and Moller-Plesset perturbation theory methods, have been identified, thus largely limiting the range of application of these methods on three-electron bonded species. The reasons for these failures are analysed in details. The well-known self-interaction problem is in cause for DFT methods, whereas symmetry breaking or near-symmetry breaking in the Hartree-Fock reference function is responsible for the contrasted performances of Moller-Plesset methods. This method exploration end with the proposition of two remedies. First, a semi- empirical model is established to predict the validity of Moller-Plesset predictions. Second, a specially designed method, based on a multi-reference technique, is proposed to deals with the most challenging situations. Using the acquired experience, this thesis ends with two studies that help shedding light on the physical nature of the three-electron bond. The first study deals with the effect of methyl substituent, which prove to depend largely on the nature of the bonded atoms. The last study is an attempt to rationalize the reasons for the stability or the instability of three-electron bonded radical anions.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

What Makes the Trifluoride Anion F3- So Special? A Breathing-Orbital Valence Bond ab Initio Study

International audienceThe ground states of the F3- and H3- hypercoordinated anions are investigated and analyzed in terms of valence bond structures by means of the breathing-orbital valence bond method. While H3- is described reasonably well as the interplay of two major Lewis structures, H2 + H- and its mirror image, the description of F3- requires a further structure, of the type F•F-F•, which strongly stabilizes the trimer relative to the dissociation products, and endows the F3- ground state with a predominant three-electron bond character. It follows that the simple picture that is closest to the true nature of F3- is a resonating combination of F2- + F• and its mirror image. This peculiarity of the F3- electronic structure is at the origin of its preferred dissociation channel leading to F2- + F• rather than to the most stable product F2 + F-, at high collision energies. The three-electron bond character of F3- is also the root cause for the failure of the Hartree−Fock and density functional methods for this species, and for its strong tendency to artifactual symmetry-breaking. As an alternative to the Rundle−Pimentel model, the origins of the stability of F3-, as opposed to the instability of H3-, CH5-, and other SN2 transition states, are analyzed in the framework of valence bond state correlation diagrams [Shaik, S.; Shurki, A. Angew. Chem., Int. Ed. 1999, 38, 586]. It is found that a fundamental factor of stability for X3- is the presence of lone pairs on the X fragment. The explanation carries over to other trihalide anions, and to isoelectronic 22-valence electron hypercoordinated anions

Valence Bond Theory—Its Birth, Struggles with Molecular Orbital Theory, Its Present State and Future Prospects

This essay describes the successive births of valence bond (VB) theory during 1916–1931. The alternative molecular orbital (MO) theory was born in the late 1920s. The presence of two seemingly different descriptions of molecules by the two theories led to struggles between the main proponents, Linus Pauling and Robert Mulliken, and their supporters. Until the 1950s, VB theory was dominant, and then it was eclipsed by MO theory. The struggles will be discussed, as well as the new dawn of VB theory, and its future