Benzene⋯acetylene: a structural investigation of the prototypical CH⋯π interaction

Abstract

The structure of a prototype CH⋯π system, benzene⋯acetylene, has been determined in the gas phase using Fourier-transform microwave spectroscopy. The spectrum is consistent with an effective C_6v structure with an H⋯π distance of 2.4921(1) Å. The HCCH subunit likely tilts by ∼5° from the benzene symmetry axis. The dipole moment was determined to be 0.438(11) D from Stark effect measurements. The observed intermolecular distance is longer than in similar benzene⋯HX complexes and than the distances observed in the benzene⋯HCCH cocrystal and predicted by many high level ab initio calculations; however, the experimentally estimated binding energy of 7.1(7) kJ mol⁻¹ is similar to previously studied benzene⋯HX complexes. Several additional sets of transitions were observed in the rotational spectrum, likely corresponding to excited states arising from low energy intermolecular vibrational modes of the dimer.