Effect of Lone Pairs on Molecular Resonance Energy

In an effort to understand the theoretical parameters of charge-shift bonding, computational experiments have been designed to elucidate the factors effecting molecular resonance energy. Valence bond theory calculations have been used to calculate resonance energies of homonuclear bonds in the series [H_nX-XH_n]^Z, where n = 0-3 and Z = 2n - 6, 2n - 4, 2n - 2, and 2n for X = C, N, O, and F, respectively. It is shown that the resonance energy decreases as the number of lone pairs increases. Calculated orbital contraction coefficients show that this unexpected result is due to the dominance of orbital size over the lone-pair effect. These results are irrespective of the HXX bond angles. It is also shown that the resonance energy increases with decreasing HXX bond angles due to the increase in p character in the bonding orbital.