A Case of Breakdown of the Kekulé–Structure Model

Abstract

For a long time, Kekulé structures have been used to predict and rationalize the stability, geometry, and π-electron properties of polycyclic conjugated molecules, especially hydrocarbons. We now point out an example, demonstrating that the Kekulé–structure model is not generally applicable. Namely, the molecule of dibenzo[cd,mn]indeno[123, gf]pyrene is perfectly planar and strain-free. Yet, its geometry (determined by means of an unrestricted symmetry–broken UB3LYP/6-311G(d,p) DFT method) is in complete disagreement with what one would expect on the basis of its Kekulé structures.

Key Words:

• fluoranthenes
• Kekulé structure
• localized double bond
• molecular geometry