Van der Waals Interactions in Aromatic Systems: Structure and Energetics of Dimers and Trimers of Pyridine

Abstract

Full geometry optimizations at the dispersion‐corrected DFT‐BLYP level of theory were carried out for dimers and trimers of pyridine. The DFT‐D interaction energies were checked against results from single‐point SCS‐MP2/aug‐cc‐pVTZ calculations. Three stacked structures and a planar H‐bonded dimer were found to be very close in energy (interaction energies in the range from −3.4 to −4.0 kcal mol−1). Two T‐shaped geometries are higher lying, by about 1 kcal mol−1, which is explained by the more favorable electrostatic interactions in the stacked and H‐bonded arrangements. The DFT‐D approach has proved to be a reliable and efficient tool to explore the conformational space of aromatic van der Waals complexes and furthermore provides interaction energies with errors of less than 10–20 % of Δ E. Comparisons with previous results obtained by using only partially optimized model geometries strongly indicate that unconstrained optimizations are mandatory in such weakly bonded low‐symmetry systems.