Effects of heteroatoms on aromatic pi-pi interactions: benzene-pyridine and pyridine dimer.

Heteroatoms are found in many noncovalent complexes which are of biological importance. The effect of heteroatoms on pi-pi interactions is assessed via highly accurate quantum chemical computations for the two simplest cases of interactions between aromatic molecules containing heteroatoms, namely, benzene-pyridine and pyridine dimer. Benchmark quality estimated coupled-cluster through perturbative triples [CCSD(T)] binding energies are computed near the complete basis set limit. Comparisons to the benzene dimer are made to determine the contributions from heteroatoms. The presence of a heteroatom reduces the spatial extent of the pi-electron cloud and polarizability of pyridine as compared to benzene. As a result, the magnitude of the dispersion, exchange, and induction interactions in benzene-pyridine and pyridine dimer is generally reduced as compared to those for the benzene dimer. Benzene-pyridine and pyridine dimer bind more strongly than the benzene dimer in several configurations, and in contrast to the benzene dimer, parallel-displaced configurations can be significantly preferred over T-shaped configurations. Hydrogens para to a heteroatom are more effective "pi-hydrogen bond" donors, but aromatic rings with heteroatoms are worse "pi-hydrogen bond" acceptors.