The Nodal Structure of π-Orbitals Is Mapped in the Interaction Energy of π-Stacked Acene Dimers.

We show and explain that repulsive contributions to intermolecular interactions have a decisive influence on the energetic landscape of prototypical π-stacked systems. Remarkably, this results from the wave nature of the respective π-orbitals, which is reflected in the exchange repulsion energy, Exr. This is rationalized by partitioning Exr into orbital-orbital contributions. The latter were recently proposed as the Molecular Orbital-Pair Contributions to the Exchange repulsion (MOPCE) approach, which allows to distinguish between σ-σ-, π-σ-, and π-π-contributions to Exr. For parallel displaced acene dimers with constant inter planar distance the π-π-contributions are shown to cause oscillations in the interaction energy as a function of monomer displacements. The MOPCE partitioning also allows to relate these features to the squared overlap of the π-orbitals, which can be reproduced using a simple particle-in-a-box model. Our approach rationalizes the interaction energies of benzene, naphthalene, anthracene, tetracene, and pentacene dimers obtained with Symmetry Adapted Perturbation Theory (SAPT). The presented results shed light on the nature of the exchange repulsion energy as a quantum mechanical property by showing that it can be represented in terms of orbital-pair contributions that are intuitively accessible by considering the nodal structure of orbitals.