What Is Special about Aromatic–Aromatic Interactions? Significant Attraction at Large Horizontal Displacement

High-level ab initio calculations show that the most stable stacking for benzene–cyclohexane is 17% stronger than that for benzene–benzene. However, as these systems are displaced horizontally the benzene–benzene attraction retains its strength. At a displacement of 5.0 Å, the benzene–benzene attraction is still ∼70% of its maximum strength, while benzene–cyclohexane attraction has fallen to ∼40% of its maximum strength. Alternatively, the radius of attraction (>2.0 kcal/mol) for benzene–benzene is 250% larger than that for benzene–cyclohexane. Thus, at relatively large distances aromatic rings can recognize each other, a phenomenon that helps explain their importance in protein folding and supramolecular structures.