Magnitude and Directionality of the Interaction Energy of the Aliphatic CH/π Interaction:  Significant Difference from Hydrogen Bond

The CCSD(T) level interaction energies of CH/π complexes at the basis set limit were estimated. The estimated interaction energies of the benzene complexes with CH<sub>4</sub>, CH<sub>3</sub>CH<sub>3</sub>, CH<sub>2</sub>CH<sub>2</sub>, CHCH, CH<sub>3</sub>NH<sub>2</sub>, CH<sub>3</sub>OH, CH<sub>3</sub>OCH<sub>3</sub>, CH<sub>3</sub>F, CH<sub>3</sub>Cl, CH<sub>3</sub>ClNH<sub>2</sub>, CH<sub>3</sub>ClOH, CH<sub>2</sub>Cl<sub>2</sub>, CH<sub>2</sub>FCl, CH<sub>2</sub>F<sub>2</sub>, CHCl<sub>3</sub>, and CH<sub>3</sub>F<sub>3</sub> are −1.45, −1.82, −2.06, −2.83, −1.94, −1.98, −2.06, −2.31, −2.99, −3.57, −3.71, −4.54, −3.88, −3.22, −5.64, and −4.18 kcal/mol, respectively. Dispersion is the major source of attraction, even if substituents are attached to the carbon atom of the C−H bond. The dispersion interaction between benzene and chlorine atoms, which is not the CH/π interaction, is the cause of the very large interaction energy of the CHCl<sub>3</sub> complex. Activated CH/π interaction (acetylene and substituted methanes with two or three electron-withdrawing groups) is not very weak. The nature of the activated CH/π interaction may be similar to the hydrogen bond. On the other hand, the nature of other typical (nonactivated) CH/π interactions is completely different from that of the hydrogen bond. The typical CH/π interaction is significantly weaker than the hydrogen bond. Dispersion interaction is mainly responsible for the attraction in the CH/π interaction, whereas electrostatic interaction is the major source of attraction in the hydrogen bond. The orientation dependence of the interaction energy of the typical CH/π interaction energy is very small, whereas the hydrogen bond has strong directionality. The weak directionality suggests that the hydrogen atom of the interacting C−H bond is not essential for the attraction and that the typical CH/π interaction does not play critical roles in determining the molecular orientation in molecular assemblies.