Bond-Distorted Orbitals and Effects of Hybridization and Resonance on C−C Bond Lengths

A localized one-electron orbital base, called bond-distorted orbital, is introduced to study hypothetically localized structures in the framework of valence bond theory. The use of valence bond method with bond-distorted orbitals allows us to evaluate the effects of hybridization and resonance on carbon−carbon bond lengths at the ab initio level. Valence bond self-consistent field studies on the delocalized and hypothetically localized structures of 1,3-butadiene and 1,3-butadiyne show that the theoretical C(sp 2 )−C(sp 2 ) and C(sp)−C(sp) single bond lengths are 1.508 and 1.446 Å, respectively, and that the theoretical resonance energies of 1,3-butadiene and 1,3-butadiyne are −7.9 and −15.8 kcal/mol, respectively.