The nodal structure of π-orbitals is mapped in the interaction energy of π-stacked acene dimers

We explain that the repulsive contributions to intermolecular interactions have a decisive influence on the energetic preference on prototypical π-stacked structures of acene dimers. Remarkably, this results from the wave nature of the respective π- orbitals which is rationalized by a partitioning of the exchange repulsion energy, Exr, into orbital-orbital contributions. The latter was recently proposed as the Molecular Orbital-Pair Contributions to the Exchange repulsion (MOPCE) approach. It is used here to distinguish between π-π-, π-σ-, and σ-σ-contributions to Exr. For parallel displaced acene dimers with constant interplanar distance the π-π-contributions are shown to cause oscillations of the interaction energy as a function of the displacement coordinates. The oscillations can be traced back to the squared overlap of the π-orbitals, that can be rationalized with a simple particle-in-a-box model. This reproduces and rationalizes the interaction energies of benzene, naphthalene, anthracene, tetracene, and pentacene dimers obtained with Symmetry Adapted Perturbation Theory. Our results shed light into the nature of the exchange repulsion energy as a quantum mechanical property. This means, we present a generally applicable and intuitive model for the interaction of stacked π-systems that is fully consistent with state of the art quantum chemical results and reminiscent to the Woodward-Hoffmann rules.