Making Sense of Heteroatom Effects in π–π Interactions

Heteroatoms can profoundly impact the strength and preferred
geometry
of parallel and T-shaped interactions between aromatic systems. Understanding
these heteroatom effects is vital for the rational design of everything
from pharmaceuticals to asymmetric catalysts and organic materials.
We first examine how the introduction of N:, NH, and CO groups
changes the shape of the relaxed interaction energy curve for the
parallel-displaced benzene dimer. These heteroatom effects are shown
to be (1) additive; (2) dependent primarily on the orientation of
the heteroatom(s) relative to the displacement axis; and (3) driven
by electrostatic effects. We then introduce a simple conceptual model
to make sense of these results based on the interaction of a local
dipole associated with each heteroatom with the electric field of
the other aromatic system. Finally, we demonstrate how this model
can be applied to complex parallel stacked dimers as well as T-shaped
interactions, providing a way to make sense of heteroatom effects
in π–π interactions without computations.