Valence Bond Analysis of the Lone Pair Bond Weakening Effect for the X−H Bonds in the Series XHn = CH₄, NH₃, OH₂, FH

The dissociations of a series of H n X−H bonds (H n X = H 3 C, H 2 N, 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 (χ(X)). The weakening effect is found to exist and to be significant, gradually increasing in the series (H n X = H 2 N 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 CH 4, 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 χ(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.