Do N-heterocyclic aromatic rings prefer pi-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 pi 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 pi-stacked complexes in all the three cases. Additionally, a C-H center dot center dot center dot N hydrogen-bonded complex between pyridine and phenylacetylene was also observed. The present results indicate that N-heterocyclic aromatic rings favor formation of pi-stacked complexes.