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Braida B.,CNRS Theoretical Chemistry Laboratory | Hiberty P.C.,University Paris - Sud
Nature Chemistry | Year: 2013

Hypervalency in XeF 2 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 XeF 2, 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 ClF 3, SF 4, PCl 5 and others, it is suggested that charge-shift bonding is necessary, in association with the Rundle-Pimentel model, for hypervalent analogues of XeF 2 to be strongly bonded. © 2013 Macmillan Publishers Limited. Source

Braida B.,CNRS Theoretical Chemistry Laboratory | Walter C.,Institute For Organische Chemie | Engels B.,Institute For Organische Chemie | Hiberty P.C.,University Paris - Sud
Journal of the American Chemical Society | Year: 2010

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). © 2010 American Chemical Society. Source

Lane J.R.,University of Waikato | Contreras-Garcia J.,CNRS Theoretical Chemistry Laboratory | Piquemal J.-P.,CNRS Theoretical Chemistry Laboratory | Miller B.J.,University of Otago | Kjaergaard H.G.,Copenhagen University
Journal of Chemical Theory and Computation | Year: 2013

Atoms in Molecules (AIM) theory is routinely used to assess hydrogen bond formation; however its stringent criteria controversially exclude some systems that otherwise appear to exhibit weak hydrogen bonds. We show that a regional analysis of the reduced density gradient, as provided by the recently introduced Non-Covalent Interactions (NCI) index, transcends AIM theory to deliver a chemically intuitive description of hydrogen bonding for a series of 1,n-alkanediols. This regional definition of interactions overcomes the known caveat of only analyzing electron density critical points. In other words, the NCI approach is a simple and elegant generalization of the bond critical point approach, which raises the title question. Namely, is it the presence of an electron density bond critical point that defines a hydrogen bond or the general topology in the region surrounding it? © 2013 American Chemical Society. Source

Liu H.-J.,University of California at Berkeley | Raynaud C.,Charles Gerhardt Institute | Raynaud C.,CNRS Theoretical Chemistry Laboratory | Eisenstein O.,Charles Gerhardt Institute | Tilley T.D.,University of California at Berkeley
Journal of the American Chemical Society | Year: 2014

The synthesis of the cyclometalated complexes CpRu(IXy-H) (2) [IXy = 1,3-bis(2,6-dimethylphenyl)imidazol-2-ylidene; IXy-H = 1-(2-CH2C 6H3-6-methyl)-3-(2,6-dimethylphenyl)imidazol-2-ylidene-1- yl (the deprotonated form of IXy); Cp* = η5-C 5Me5] and CpRu(IXy-H)(N2) (3) was achieved by dehydrochlorination of CpRu(IXy)Cl (1) with KCH2Ph. Complexes 2 and 3 activate primary silanes (RSiH3) to afford the silyl complexes Cp(IXy-H)(H)RuSiH2R [R = p-Tol (4), Mes (5), Trip (6)]. Density functional theory studies indicated that these complexes are close in energy to the corresponding isomeric silylene species Cp(IXy)(H)Ru=SiHR. Indeed, reactivity studies indicated that various reagents trap the silylene isomer of 6, Cp(IXy)(H)Ru=SiHTrip (6a). Thus, benzaldehyde reacts with 6 to give the [2 + 2] cycloaddition product 7, while 4-bromoacetophenone reacts via C-H bond cleavage and formation of the enolate Cp(IXy)(H)2RuSiH[OC(=CH 2)C6H4Br]Trip (8). Addition of the O-H bond of 2,6-dimethylphenol across the Ru=Si bond of 6a gives Cp(IXy)(H) 2RuSiH(2,6-Me2C6H3O)Trip (9). Interestingly, CuOTf and AgOTf also react with 6 to provide unusual Lewis acid-stabilized silylene complexes in which MOTf bridges the Ru-Si bond. The AgOTf complex, which was crystallographically characterized, exhibits a structure similar to that of [Cp(iPr3P)Ru(μ-H) 2SiHMes]+, with a three-center, two-electron Ru-Ag-Si interaction. Natural bond orbital analysis of the MOTf complexes supported this type of bonding and characterized the donor interaction with Ag (or Cu) as involving a delocalized interaction with contributions from the carbene, silylene, and hydride ligands of Ru. © 2014 American Chemical Society. Source

Gould T.,Griffith University | Toulouse J.,CNRS Theoretical Chemistry Laboratory
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2014

Within exact electron density-functional theory, we investigate Kohn-Sham (KS) potentials, orbital energies, and noninteracting kinetic energies of the fractional ions of Li, C, and F. We use quantum Monte Carlo densities as input, which are then fitted, interpolated at noninteger electron numbers N, and inverted to produce accurate KS potentials vsN(r). We study the dependence of the KS potential on N, and in particular we numerically reproduce the theoretically predicted spatially constant discontinuity of vsN(r) as N passes through an integer. We further show that, for all the cases considered, the inner orbital energies and the noninteracting kinetic energy are nearly piecewise linear functions of N. This leads us to propose a simple approximation of the KS potential vsN(r) at any fractional electron number N which uses only quantities of the systems with the adjacent integer electron numbers. © 2014 American Physical Society. Source

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