A new insight into π-π stacking involving remarkable orbital interactions.

For more than half a century, the phenomenon of π-π stacking has attracted much attention in several research fronts including materials science, chemical synthesis, and even drug design. Despite intense theoretical and experimental exploration, no unified description of the factors contributing to π-π stacking interactions and their weak bonding process has been proposed. In this work, based on calculations of the simplest prototype of π-π stacking, namely the benzene sandwich dimer (together with benzene-phenol, toluene and benzonitrile) using the density functional theory with dispersion correction, previously rarely studied intermolecular orbital interaction is discussed in detail and shown to involve considerable hybridizations of some of the orbitals which make a large contribution to the total interaction energy. We now propose a unified model for the often nebulous π-π stacking process and its analogs: firstly when the two monomers are too far apart, the dispersion effect will play a dominant role in bringing them together, but when they are too close, Pauli repulsion will force them apart. Secondly, at the equilibrium distance, electrostatic interaction, Pauli repulsion, dispersion and intermolecular orbital interaction are all pronounced, with part of the molecular orbitals of the two monomers interacting with each other to form a weak intermolecular bond.