DIRECT ASSESSMENT OF SIGMA-AROMATICITY IN 3-MEMBERED RINGS BY PERTURBATION-THEORY

The existence of sigma-aromaticity in three-membered rings has been ex amined by applying perturbation theory to localized-bond models based on orthogonal hybrid AOs from STO-3G and 3-21G MO calculations. The ST O-3G and 3-21G sigma-aromatic energies of cyclopropane are estimated t o be 323 and 330 kJ mol(-1), respectively. It is shown that the sigma- aromaticity is caused mainly by the three-orbital-six-electron interac tion among the three bonds forming the ring, and further that the stab ilization due to this interaction originates in the great stability of the lowest a(1)' MO of the ring system.