A Molecular Cluster Study on Activated CH/π Interactions:  Infrared Spectroscopy of Aromatic Molecule−Acetylene Clusters

CH stretching vibrations of jet-cooled benzene−acetylene and several aromatics−acetylene clusters were observed in order to characterize the intermolecular interaction, so-called activated CH/π interaction between an acidic CH group and π-electrons. The infrared−ultraviolet double resonance spectroscopic techniques were used for measuring their vibrational spectra. The antisymmetric CH stretching vibration of the acetylene moiety exhibits a remarkable low-frequency shift upon the cluster formation with the aromatic molecules, indicating such a cluster structure that the acetylenic CH group is bound to π-electrons of the aromatic ring. The low-frequency shifts observed for the clusters show a positive correlation with the π-electron density of the ring, as expected for an ordinary π-hydrogen bond. On the other hand, the polarizability of the proton-accepting molecules showed a weakly negative correlation with the CH frequency shifts. These results show that the intermolecular interaction between the activated CH group and π-electrons is characterized as a π-hydrogen bond rather than as a van der Waals interaction.