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.