New J. Chem. vol 21 pp 953-960, 1997
J. Am. Chem. Soc. vol 119, pp 9478-9482, 1997
J. Phys. Chem. vol 100 pp 6463-6468, 1996
J. Phys. Chem. vol 100 pp 5715-5720, 1996
J. Mol. struct. (Theochem) vol 338 pp 283-291, 1995
J. Am. Chem. Soc. vol 117, pp 2106-2107, 1995
A. Jarid, A. Lledos, D. Lauvergnat, Y. Jean
NEW JOURNAL OF CHEMISTRY, 1997, Vol.21, No.9, pp.953-960
The rotational barriers of methylene and dihydrogen ligands were studied in the d(6) octahedral complexes Os(NH3)(5)(CH2)(2+) and Os(NH3)(5)(H-2)(2+), respectively, using the DFT-based B3LYP methodology. In both cases, the eclipsed conformation is found to be slightly more stable than the staggered conformation, but the energy barrier is very low (<1 kcal/mol). The coupling of the orientations of methylene and dihydrogen ligands attached to the same metal center was then studied in the trans-and the Cis-Os(NH3)(4)(H-2)(CH2)(2+) complexes. Geometry optimizations were performed at the B3LYP computational level and the energies recalculated at the MP2, MP4, and CCSD(T) levels. In both the cis and trans isomers the most stable conformations are those in which the pi-bonding interactions between the octahedral t(2g) set and the p(CH2) and sigma(H2)* acceptor orbitals are maximized. The strength of the coupling in the cis isomer is found to be more than twice that in the trans isomer, a result traced to stronger Os-H-2 bonding in the former. Despite this coupling, the rotational barriers of H-2 and CH2 remain low (congruent to 1 and 4 kcal/mol, respectively, at the CCSD(T)//B3LYP lever) because these processes can be achieved without going through the conformation disfavored on ii electronic grounds. Finally, the energy difference between the cis and the trans isomers is found to be very small (1.18 kcal/mol in favor of the former at the CCSD(T)//B3LYP computational level). These results are discussed with respect to the scarce experimental data available for carbene- dihydrogen d(6) octahedral complexes.
D. Lauvergnat and P. C. Hiberty
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 1997, Vol.119, No.40, pp.9478-9482
The validity of the traditional resonance model for the amide and thioamide functional groups is examined by calculating the planar and rotated conformations of formamide and thioformamide with and without allowing for the delocalization of the nitrogen's lone pair, The vertical delocalization energies are found to be quite significant, in support of the traditional view that resonance is an important feature of the electronic structure of amides and thioamides, The rotational barriers are however not solely due to conjugation; they also partly arise from the preferred orientation of the nitrogen's lone pair as perpendicular to the molecular plane, even in the absence of conjugation. The resonance stabilization of the planar conformers is responsible for about one-half of the rotational barrier of formamide and two-thirds of that of thioformamide, The larger rotational barrier of thioformamide is therefore due to a greater importance of conjugation effects relative to formamide.
D. Lauvergnat, P. Maître, P. C. Hiberty and F. Volatron
JOURNAL OF PHYSICAL CHEMISTRY, 1996, Vol.100, No.16, pp.6463-6468
The dissociations of a series of H(n)X-H bonds (H(n)X = H3C, H2N, HO, F) are investigated by means of an ab initio valence bond method, in order to probe the existence and the mechanism of the weakening effect that a lone pair on the X atom might exert on the adjacent bonds. One manifestation of the lone pair bond weakening effect is a break in the curve displaying the variations of the H(n)X-H bond energy (D-e) as a function of the X electronegativity (chi(X)). The weakening effect is found to exist and to be significant, gradually increasing in the series (H(n)X = H2N to F). It is shown to correspond to a stabilization of the dissociated products, due to an electronic reorganization of the H(n)X fragment that gradually undergoes a rehybridization throughout the dissociation. By this mechanism, the lone pair(s) acquire more s character, to the detriment of the orbital involved in the breaking bond, thus increasing the average s character in the valence state of the X atom. In contrast, when no lone pairs are present as in CH4, the valence state of the X atom remains unchanged throughout the dissociation and this is the origin of the break in the curve displaying D-e vs chi(X), in the series (X = C to F). This break disappears when calculated ''unweakened'' bond energies are plotted, thus supporting Pauling's idea of a simple relationship between H(n)X-H bond strength and the X vs H electronegativity difference.
D. Lauvergnat, P. C. Hiberty, D. Danovich and S. Shaik
JOURNAL OF PHYSICAL CHEMISTRY, 1996, Vol.100, No.14, pp.5715-5720
VB calculations with breathing orbitals (BOVB) show that the H3Si-Cl and H3C-Cl bonds are qualitatively different. The differences are rooted in the properties of the H3Si+ and H3C+ species. Thus, the H3C+ cation has an evenly distributed charge and relatively large ionic radius, and therefore the cation maintains a long distance from the counterion Cl-. Consequently, the ionic-covalent mixing remains of secondary influence and shortens slightly the R(CCl) distance in agreement with the Pauling recipe for polar bonds. On the other hand, in H3Si+ the charge is highly localized on silicon. Consequently, the cation acquires a diminished effective size along the missing coordination site. This allows a close approach of Cl- as well as a very large electrostatic interaction between the Si+ and Cl- centers in the ionic VB structure. Consequently, the ionic potential energy curve R(3)Si(+)Cl(-) approaches the corresponding covalent curve to a near-degeneracy. The ensuing VB mixing renders the Si-Cl bond a tote charge-shift bond whose major character is the charge fluctuation inherent in the resonating wave function. The effect of ionicity on the Si-Cl bond length does not follow the Pauling recipe. Indeed, by mixing of the ionic structure the R(SiCl) minimum shifts to a longer distance in comparison with the covalent minimum. The new minimum is simply an intermediate distance between the covalent and ionic minima in keeping with the charge-shift nature of the bond. The manifestations of the diminished effective size of R(3)Si(+) are its strong coordinating ability with electronegative and electron-rich ligands. Implications on the R(3)Si(+) problem are discussed.
D. Lauvergnat and P. C. Hiberty
THEOCHEM-JOURNAL OF MOLECULAR STRUCTURE, 1995, Vol.338, pp.283-291
The X-X homonuclear bonds in a series of typical H(n)X-XH(n) molecules (X = Li, Be, B, C, N, O, F; n = 0, 1, 2, 3) are analyzed by decomposing the interaction between the two XH(n) radicals into two terms: (i) the direct coupling between the bonding electrons, and (ii) the side interactions that gather the effects of lone pairs or adjacent X-H bonds with each other or with the bending electrons. The latter non-bonded interactions are shown to display strongly non- linear variations as X is taken from left to right across the periodic table, and to explain the non-linear tendencies observed in X-X bond lengths and bonding energies across the periodic table. The lone-pair bond weakening effect is clearly in evidence, and appears as a particularly strong repulsive force of rather long range, that affects both the equilibrium geometry and the overall bonding energy at a given distance. The former effect leads to comparatively much elongated O-O and F-F bond lengths, and to a damped increase of the non-bonded repulsions in the series (X = N to F). When the effect of non-bonded interactions is completely eliminated, by correcting both the bonding energy itself and the interatomic distance, the unweakened bonding energy becomes a quasi-linear function of the atomic number of the bonded atoms across the periodic table, and smoothly increases along with the electronegativity of the XH(n) dissociated fragments.
D. Lauvergnat and Y. Jean
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 1995, Vol.117, No.7, pp.2106-2107
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