6.4 Pi Interactions
Aromatic rings, and conjugated systems in general, can form a type of intermolecular interaction involving their pi orbitals. Called pi stacking, it is most measurably represented by the stacking of aromatic rings like pancakes. The rings are usually slightly displaced, so that a carbon atom of one ring is aligned to a hydrogen of the next. They can also stack significantly displaced, with a hydrogen of one ring placed over the center of another, or in a T shape with a hydrogen atom of one ring pointing to the center of another. Pi stacking has binding energies of between 8 and 12 kJ/mol or 2 to 3 kcal/mol.
Aromatic rings can also interact with polar groups, acting as though the center of the ring were a weak hydrogen-bond acceptor, probably with binding energies a little less than pi stacking. These are polar-pi interactions.
Aromatic rings can sandwich metal ions between them. Metallocenes are compounds where two aromatic rings, such as cyclopentadienyl ions (C5H5-), surround a positively charged metal ion. Each ring's negative charge is spread equally throughout all of the carbon atoms via the electrically conductive pi orbitals, and causes the entire ring to attract the metal ion. The cyclopentadienyl ion is aromatic because its negative charge completes a lone pair, which is contributed to the pi orbitals just like we saw earlier with pyrrole and furan.
Ferrocene.