What Is Special about Aromatic-Aromatic Interactions? Significant Attraction at Large Horizontal Displacement.
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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 , 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.