Do N-heterocyclic aromatic rings prefer π-stacking?

The IR-UV double resonance spectroscopy of phenylacetylene complexes with triazine, pyrazine and pyridine in the acetylene C-H group of phenylacetylene was investigated. These spectra indicate that in the complexes of triazine, pyrazine and pyridine the acetylenic group is minimally perturbed and the predominant interaction is with the π electron density of the phenyl ring of phenylacetylene. Geometries of the complexes optimized at M06-2X/aug-cc-pVDZ and MP2/aug-cc-pVDZ levels, combined with highly accurate energy calculations at the complete basis set (CBS) limit of CCSD(T), indicate the formation of π-stacked complexes in all the three cases. Additionally, a C-H...N hydrogen-bonded complex between pyridine and phenylacetylene was also observed. The present results indicate that N-heterocyclic aromatic rings favor formation of π-stacked complexes.