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.