Bond length alternation in hydrocarbon rings

Rövid cím: 
Bond length alternation in hydrocarbon rings
2016. 03. 25. 10:15
BME Fizikai Intézet, Elméleti Fizika Tanszék, Budafoki út 8. F-épület, III lépcsőház, szemináriumi szoba
Jenő Kürti (ELTE Dept. Biol. Phys.)

Bond length alternation (BLA) is an inherent trait of linear polyenes. In finite chains this is simple the consequence of chain end effects. Although decreasing with increasing chain length, BLA retains for infinite long chains – this is the well known Peierls distortion. The question is what happens for cyclic hydrocarbons: the annulene molecules. In the antiaromatic case (nC=4n) the existence of the BLA is a triviality due to the Jahn–Teller effect. The situation is much more subtle for aromatic (nC=4n+2) rings. The best known example is benzene, where there is no BLA.
We demonstrate theoretically, using both the Longuet-Higgins–Salem (LHS) model and density functional theory that increasing the number of carbon atoms in the aromatic rings leads to the appearance of the BLA for large enough rings. The reason for this is the pseudo Jahn–Teller effect. In the infinite limit the aromatic and the antiaromatic rings (as well as the linear chains) behave similarly, showing Peierls distortion.