Hydrogen bonding between ethynyl aromates and triethylamine: IR spectroscopic and computational study

Ethynylpyridines (EPs) and ethynylbenzene (EB) are multifunctional systems able to participate in hydrogen-bonded complexes as both donors and acceptors of the H-atom. Their structures and stabilities are mainly a function of the hydrogen-bonding properties of the partner in the complex and the surroundings in which the complexation occurs. In this paper, IR spectroscopy and quantum chemical calculations are employed to characterize hydrogen-bonded complexes of 2- and 3-EP and EB with triethylamine (TEA) in tetrachloroethene (C₂Cl₄) solution. The formation of CH⋯N hydrogen bonds is experimentally confirmed by the appearance of TEA concentration-dependent signals in the IR spectra of the EPs and EB. Along with the signals due to unassociated CH and CC oscillators (2-EP: 3308 cm^-1 and 2120 cm^-1; 3-EP: 3308 cm^-1 and 2116 cm^-1; EB: 3313 cm^-1 and 2113 cm^-1) weak, red-shifted signals arise at ~3215 ± 5 cm^-1 and ~2105 ± 5 cm^-1 which are assigned to the stretching vibrations of hydrogen-bonded CH⋯ and CC⋯ oscillators, respectively. This result is at variance with those of previous investigations of EB and TEA in the gas phase. In the 2-EP⋯TEA complex these bands remain at the same position with increasing TEA concentration. However, in the 3-EP⋯TEA and EB⋯TEA complexes the CH⋯ stretching band demonstrates a slightly reduced red-shift as the TEA concentration increases, whereas the CC⋯ stretching band absorbs at the same wavenumber in the investigated TEA concentration range. The results of B3LYP-D3 calculations indicate that complexes with more or less linear CH⋯N intermolecular hydrogen bonds are more stable than other, dispersion-driven complexes. Complexes with the C_s symmetrical TEA conformer are predicted to have larger binding energy than those formed with the C₃ and C₁ symmetrical conformers. The predicted IR spectral shifts are slightly different for complexes with the three different TEA conformers. Association constants of hydrogen-bonded complexes at 26 °C are estimated to be ~0.1 mol^-1 dm³.