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The dissociations of a series of HnX−H bonds (HnX = 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 HnX−H bond energy (De) as a function of the X electronegativity (χ(X)). The weakening effect is found to exist and to be significant, gradually increasing in the series (HnX = H2N to F). It is shown to correspond to a stabilization of the dissociated products, due to an electronic reorganization of the HnX 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 ...

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 RCCl 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 R3Si+Cl- approaches the corresponding covalent curve to a near-degeneracy. The ensuing VB mixing renders the Si−Cl bond a true charge-sh...

Utilisation d'un modele de diagramme a croisement de courbes pour la conception des stabilites dans les agregats delocalises X n z (n=3, 4, 6; z=0, −1; X=H, Li, Na, K, F, Cl, Br, I)