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 CH₄, CH₃CH₃, CH ₂CH₂, CHCH, CH₃NH₂, CH ₃OH, CH₃OCH₃, CH₃F, CH ₃Cl, CH₃ClNH₂, CH₃ClOH, CH ₂Cl₂, CH₂FCl, CH₂F₂, CHCl₃, and CH₃F₃ 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 CHCl₃ 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.