VALENCE-BOND ANALYSIS OF THE LONE-PAIR BOND WEAKENING EFFECT FOR THE X-H BONDS IN THE SERIES XH(N)=CH4, NH3, OH2, FH

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 o rder 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 ser ies (H(n)X = H2N to F). It is shown to correspond to a stabilization o f 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 c haracter, to the detriment of the orbital involved in the breaking bon d, 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 va lence state of the X atom remains unchanged throughout the dissociatio n and this is the origin of the break in the curve displaying D-e vs c hi(X), in the series (X = C to F). This break disappears when calculat ed ''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.