ISOMERS OF C-78 - COMPETITION BETWEEN ELECTRONIC AND STERIC FACTORS

Ab initio Hartree-Fock (3-21G and 6-31G*(5d)) calculations on the structures and stabilities of the low-energy isomers of C78 are reported. The energy ordering of the isomers can be interpreted in terms of two different physical effects: an electronic factor which maximizes the resonance stabilization energy and a steric factor which minimizes the strain energy. The nature of the three most stable isomers (C2v, C2v', and D3) seen in two different experiments can be understood in terms of competition between these two opposing effects.