Double-bond fixation and aromatic stability in cyclic difulvalene systems

Abstract

On the basis of second-order Jahn–Teller theory and of the semiempirical Pariser–Parr–Pople type SCF-MO method, we have examined the energetically most favourable ground-state geometrical structures with respect to C–C bond length of cyclic difulvalene systems. The cyclic difulvalene systems examined are classified into two groups, one in which a molecule undergoes symmetry reduction from the fully symmetrical nuclear arrangement and the other in which a molecule suffers no symmetry reduction, suggesting that the former behaves as a polyolefin and the other as a typical aromatic hydrocarbon. This marked differences is explained in terms of a polarization stability by cyclization.