An infrared spectroscopic study on CH/π Ieractions in molecular clusters

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CH stretching vibrations of jet-cooled benzene-acetylene and several aromatics-acetylene clusters were observed for the characterization of the intermolecular interaction, so-called activated CH/π interaction between the acidic CH group and π-electrons, under the well-defined condition. The infrared-ultraviolet double resonance spectroscopic techniques were used to measure Infrared spectra of the jet-cooled clusters. The antisymmetric CH stretching vibration of the acetylene moiety exhibits substantial low-frequency shifts upon the cluster formation with the aromatic molecules, indicating such cluster structures that acetylenic CH Is bound to the π-electrons of the aromatic ring. The magnitude of the low-frequency shifts Increased with increase of 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 CN frequency shifts.