WHY DOES BENZENE POSSESS A D-6H SYMMETRY - A QUASI-CLASSICAL STATE APPROACH FOR PROBING PI-BONDING AND DELOCALIZATION ENERGIES

In response to a recent controversy over the issue of whether the pi-e lectrons of benzene do or do not possess a distortive tendency away fr om a D-6h symmetry, we have developed a new approach based on the quas iclassical (QC) state, which is the spin-alternant state of a chemical species and which allows definition of the pi-bonding energy in a man ner which does not depend on energy partition and is free of the dilem ma of assignment of the nuclear repulsion. The QC state concept is app lied to probe bonding energies in H-2 and C2H4 and then used to quanti fy delocalization energies of H-6 and benzene. It is shown that the pi -bonding energy of benzene is stabilized by a localizing B-2u distorti on. As such, the pi-system of benzene behaves precisely like the deloc alized Hg hexagon which is a transition state more stable in a distort ed D-3h geometry. The analogy between the delocalized pi-electrons of benzene and H-6 is further highlighted by demonstrating, computational ly, that they both possess exalted diamagnetic susceptibilities associ ated with ring currents. While H-6 simply falls apart to three H-2 mol ecules, the pi-electrons of benzene are held together by the sigma-fra me. Benzene is therefore the site of two opposing driving forces. The pi driving force tends to distort the molecule while the stronger a dr iving force of the QC state acts in the opposite direction and imposes a regular geometry. As such, benzene possesses a unique delocalized p i-component which has a dual nature; at any geometry of the C6H6 struc ture, the pi-electrons are strongly stabilized by the quantum mechanic al resonance energy (QMRE), and at the same time, they possess a globa l distortive tendency toward a D-3h structure. It is demonstrated that this dual picture of benzene is in perfect agreement with the ''aroma tic'' behavior of benzene. Applications are presented to the Stanger m odel of bent benzene, tricyclobutabenzene, and naphthalene.