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

Abstract

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–H⋯N 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–H⋯π hydrogen bonded complexes. Additionally a very weak electrostatic C–H⋯N hydrogen bond enhances the stability of the complex marginally. The multidentate C–H⋯π hydrogen bonded complexes are stabilized by a substantial contribution from the dispersion energy.