Carbon nanorings: A challenge to theoretical chemistry.

High-level quantum-chemical methods show that the binding in the inclusion complex of hexamethylbenzene (HMB) in 6-cycloparaphenilacetylene (6-CPPA) cannot be explained only in terms of electrostatic interactions-caused by the polarization associated to curved pi-conjugated systems-and the inclusion of dispersion forces is definitely needed. The theoretical description of van der Waals interactions is notoriously complicated and in fact some DFT methods cannot even predict the existence of the relatively small supramolecular nanoring studied here. However, ab initio MP2 calculations agree with experimental data and show that, in the considered complex, the HMB fragment is placed at the center of the 6-CPPA ring. The binding energy, which is not available experimentally, is calculated to be around -14 kcal mol(-1) with a lower limit of -19 kcal mol(-1).