THE NONLINEAR TENDENCIES IN HOMONUCLEAR X-X BONDS (X=LI TO F) AND THELONE-PAIR BOND WEAKENING EFFECT - AN AB-INITIO THEORETICAL-ANALYSIS

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 decomposi ng the interaction between the two XH(n) radicals into two terms: (i) the direct coupling between the bonding electrons, and (ii) the side i nteractions that gather the effects of lone pairs or adjacent X-H bond s 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 en ergies 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 geometr y and the overall bonding energy at a given distance. The former effec t 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 elimi nated, by correcting both the bonding energy itself and the interatomi c distance, the unweakened bonding energy becomes a quasi-linear funct ion 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.