Electronic Population Analysis on LCAO-MO Molecular Wave Functions. IV. Bonding and Antibonding in LCAO and Valence-Bond Theories

It is shown that there is a practically one-to-one correspondence between the occurrence, on the one hand, of positive (bonding) and negative (antibonding) overlap populations in LCAO theory and, on the other hand, of bonded attractions and nonbonded repulsions in VB (valence-bond) theory. This correspondence is discussed in terms of examples, and is traced for the N2 molecule both for the assumed case of no s–p hybridization, and for the actual case with hybridization. It is pointed out that repulsions between nonbonded atoms in VB theory (including those which give rise to steric hindrance) have their counterpart in negative overlap populations between the same atoms in LCAO theory. The π overlap populations for the various links in 1,3-butadiene are computed by LCAO theory. It is shown how they are affected by conjugation (see Table I) and the results are compared with those of VB theory.