Binary complexes of tertiary amines with phenylacetylene. Dispersion wins over electrostatics

The structures of the binary complexes between phenylacetylene and several tertiary amines viz., triethylamine, 1-ethylpiperidine, 1-ethylpiperazine, 1-azabicyclo[2.2.2]octane, and 1,4-diazabicyclo[2.2.2]octane were inferred using infrared-optical double resonance spectroscopy. The IR spectra in the acetylenic C–H stretching region clearly rule out the formation of electrostatic dominated C–HN hydrogen bonded complexes. The IR spectra also point to the fact that all the five tertiary amines interact with the extended π electron density of the phenylacetylene moiety, leading to the formation of multidentate C–Hπ hydrogen bonded complexes. Additionally a very weak electrostatic C–HN hydrogen bond enhances the stability of the complex marginally. The multidentate C–Hπ hydrogen bonded complexes are stabilized by a substantial contribution from the dispersion energy.