How Different Are Aromatic π Interactions from Aliphatic π Interactions and Non-π Stacking Interactions?

We compare aromatic π interactions with aliphatic π interactions of double- and triple-bonded π systems and non-π stacking interactions of single-bonded σ systems. The model dimer systems of acetylene (C2H2)2, ethylene (C2H4)2, ethane (C2H6)2, benzene (C6H6)2, and cyclohexane (C6H12)2 are investigated. The ethylene dimer has large dispersion energy, while the acetylene dimer has strong electrostatic energy. The aromatic π interactions are strong with particularly large dispersion and electrostatic energies, which would explain why aromatic compounds are frequently found in crystal packing and molecular self-engineering. It should be noted that the difference in binding energy between the benzene dimer (aromatic–aromatic interactions) and the cyclohexane dimer (aliphatic–aliphatic interactions) is not properly described in most density functionals.