An attractive intermolecular interaction between an aliphatic C-H bond and a pi-electron system (C-H/pi interaction) was characterized on the basis of infrared spectroscopy and high level ab initio calculations. Infrared spectroscopy was applied to several isolated methane clusters with benzene, toluene, p-xylene, mesitylene, and naphthalene in the gas phase, and the spectral changes of the C-H stretch bands in the methane moiety upon the cluster formation were observed. In the theoretical approach, interaction energies of the clusters were evaluated by high-level ab initio calculations. The forbidden symmetric C-H stretch transition weakly appeared in the IR spectra of the clusters, and it confirmed the small deformation of the methane moiety from the T(d)() symmetry, which was predicted by the ab initio calculations. On the other hand, the degenerated asymmetric C-H stretch band showed complicated splitting, which is qualitatively interpreted by a hindered rotor model. Low-frequency shifts upon the cluster formation were seen in the symmetric C-H stretch frequency, though the magnitude of the shifts was extremely small and no clear correlation with the interaction energy was found. On the other hand, the size of the calculated interaction energy well correlates with the polarizability of aromatics. The S(1)-S(0) electronic transition of the aromatic moiety was also observed, and it showed low-frequency shifts upon cluster formation. These results support the dominance of the dispersion interaction over the electrostatic and charge-transfer terms in the aliphatic C-H/pi interaction.
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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.