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

The structure of a prototype CH. . .pi 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 CH. . .pi distance of 2.4921(1) A. The HCCH subunit likely tilts by similar to 5 degrees 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.