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

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

Influence of Water on the Oxidation of Dimethyl Sulfide by the ·OH Radical

International audienceOxidative stress of sulfur-containing biological molecules in aqueous environments may lead to the formation of adduct intermediates that are too short-lived to be experimentally detectable. In this study we have modeled the simplest of such oxidative reactions: the attack of dimethyl sulfide (DMS) by a hydroxyl radical (·OH) to form a radical adduct, whose subsequent heterolytic dissociation leads to a radical cation (DMS+) that is important for further reactions. We have modeled the aqueous environment with a limited number of discrete water molecules, selected after an original multistep procedure, and further embedded in a polarizable continuum model, to observe the impact of the water configuration on the heterolytic dissociation of the radical adduct. Molecular dynamics and quantum chemical methods (DFT, MP2, and CCSD) were used to elucidate the lowest energy structures resulting from the ·OH attack on DMS. Subsequent high level ab initio valence bond (BOVB) calculations revealed the possibility for the occurrence of subsequent heterolytic dissociation