C_(18) Is a Polyyne

摘要

Cyclic carbon clusters have stimulated much attention during the past years, both experimentally and theoretically. There is now growing evidence that cyclocarbons play a decisive role in the formation of fullerenes. Numerous theoretical efforts notwithstanding, a consensus on the bonding in mono-cyclic carbon clusters C_n (n = 10-20) is still lacking. We present theoretical arguments and computational evidence that monocyclic C_(18) has a flattened circular polyyne structure. The tendency for bond alternation in the C_n systems is greater than that in C_nH_n systems. Previous calculations on this system led to erroneous conclusions about structures. Our studies also show that density functional theory (DFT) errs consistently in the prediction of energies for isomeric cumulenic and acetylenic structures. Nevertheless, the combination of RHF and DFT calculations provides firm conclusions about structural trends in the potentially doubly-aromatic cyclocarbons, C_(10), C_(14), and C_(18).