Another piece of the π

The effect of heteroatoms on aromatic π–π interactions has been investigated theoretically

Credit: © 2009 ACS

Aromatic π–π interactions are important to many biological functions, including stabilizing the structure of DNA and in the binding of ligands to enzyme active sites. Although much of the previous work to understand the nature of such interactions has been directed towards benzene and substituted-benzene dimers, efforts are refocusing on identifying the effects of heteroatoms.

Now Edward Hohenstein and David Sherrill at the Georgia Institute of Technology have studied¹ the interactions between the individual molecules that make up pyridine–pyridine and pyridine–benzene dimers. They compare them to those of benzene–benzene dimers to understand how the presence of a nitrogen atom in the aromatic ring influences the π–π interactions. The dimers can take three different geometrical arrangements: sandwich, T-shaped and parallel displaced (a horizontally offset sandwich arrangement). Using high-level quantum chemical calculations, Hohenstein and Sherrill investigated the bonding interactions in the various structural configurations possible in these arrangements.

The researchers saw that the inclusion of a nitrogen atom into the aromatic ring unsurprisingly creates a dipole, which is significant in pyridine–pyridine dimers but relatively insignificant in pyridine–benzene dimers. The heteroatom also diminishes the 'reach' of the π-electron cloud, thus reducing 'exchange-repulsion', and reduces the polarizability of the monomer, which lessens the effect of dispersion and induction forces.