The magnitude of the CH/π interaction between benzene and some model hydrocarbons

High-level ab initio calculations were carried out to evaluate the interaction between the pi face of benzene and hydrocarbon molecules (methane, ethane, ethylene, and acetylene). Intermolecular interaction energies were calculated from extrapolated MP2 interaction energies at the basis set limit and CCSD(T) correction terms. The calculated benzene-methane interaction energy (-1.45 kcal/mol) is considerably smaller than that of the hydrogen bond between waters. The benzene-methane complex prefers a geometry in which the C-H bond points toward the benzene ring. The potential energy surface is very flat near the minimum, which shows that the major source of the attraction is a long-range interaction. The HF interaction energy of the complex (0.85 kcal/mol) is repulsive. The large gain of the attraction energy (-2.30 kcal/mol) by electron correlation correction indicates that dispersion interaction is the major source of the attraction. Although the electrostatic energy (-0.25 kcal/mol) is small, a highly orientation dependent electrostatic interaction determines the orientation of the C-H bond. The calculated charge distributions show that the amount of charge transfer from benzene to methane is very small. The calculated interaction energies of benzene-ethane, benzene-ethylene, and benzene-acetylene complexes are -1.82, -2.06, and -2.83 kcal/mol, respectively. Dispersion interaction is again the major source of the attraction of these complexes. The electrostatic energy (-0.17 kcal/mol) is not large in the benzene-ethane complex, while the large electrostatic energies of benzene-ethylene and benzene-acetylene complexes (-0.65 and -2.01 kcal/mol) show that electrostatic interaction is also important for the attraction between benzene and unsaturated hydrocarbon molecules.