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-electrons of benzene do or do not possess a distortive tendency away from a D-6h symmetry, we have developed a new approach based on the quasiclassical (QC) state, which is the spin-alternant state of a chemical species and which allows definition of the pi-bonding energy in a manner which does not depend on energy partition and is free of the dilemma of assignment of the nuclear repulsion. The QC state concept is applied to probe bonding energies in H-2 and C2H4 and then used to quantify delocalization energies of H-6 and benzene. It is shown that the pi-bonding energy of benzene is stabilized by a localizing B-2u distortion. As such, the pi-system of benzene behaves precisely like the delocalized Hg hexagon which is a transition state more stable in a distorted D-3h geometry. The analogy between the delocalized pi-electrons of benzene and H-6 is further highlighted by demonstrating, computationally, that they both possess exalted diamagnetic susceptibilities associated with ring currents. While H-6 simply falls apart to three H-2 molecules, the pi-electrons of benzene are held together by the sigma-frame. Benzene is therefore the site of two opposing driving forces. The pi driving force tends to distort the molecule while the stronger a driving force of the QC state acts in the opposite direction and imposes a regular geometry. As such, benzene possesses a unique delocalized pi-component which has a dual nature; at any geometry of the C6H6 structure, the pi-electrons are strongly stabilized by the quantum mechanical resonance energy (QMRE), and at the same time, they possess a global distortive tendency toward a D-3h structure. It is demonstrated that this dual picture of benzene is in perfect agreement with the ''aromatic'' behavior of benzene. Applications are presented to the Stanger model of bent benzene, tricyclobutabenzene, and naphthalene.