CH/π interactions

Abstract

Recent gas phase measurements and high level ab initio calculations of the CH/π interactions show that the nature of the CH/π interactions is significantly different from conventional hydrogen bonds, although the CH/π interactions were often regarded as the weakest class of hydrogen bonds. A clear correlation between the binding energies of the benzene clusters with alkanes in the gas phase and the polarizabilities of alkanes was reported. The correlation suggests that the dispersion interaction is the major source of the attraction of the CH/π interactions. Ab initio calculations also show that the major source of attraction in the CH/π interactions is the dispersion interaction and the electrostatic contribution is small. The nature of the “typical CH/π interactions is close to that of van der Waals interactions, if some exceptional “activated” CH/π interactions of highly acidic C–H bonds (C–H bonds of acetylene and chloroform) are excluded. Conventional hydrogen bonds are sufficiently strong and directional due to the large contribution of the highly orientation dependent electrostatic interactions to the attraction. The hydrogen bond is important in controlling structures of molecular assemblies, since the hydrogen bond is sufficiently strong and directional. On the other hand the “typical” CH/π interactions are weak and their directionality is very weak. Although the “typical” CH/π interactions are often regarded as important interactions in controlling the structures of molecular assemblies as in the cases of conventional hydrogen bonds, the importance of the “typical” CH/π interactions is questionable.