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

High-level ab initio calculations were carried out to evaluate the interact ion between the pi face of benzene and hydrocarbon molecules (methane, etha ne, ethylene, and acetylene). Intermolecular interaction energies were calc ulated from extrapolated MP2 interaction energies at the basis set limit an d CCSD(T) correction terms. The calculated benzene-methane interaction ener gy (-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 v ery flat near the minimum, which shows that the major source of the attract ion 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 int eraction is the major source of the attraction. Although the electrostatic energy (-0.25 kcal/mol) is small, a highly orientation dependent electrosta tic interaction determines the orientation of the C-H bond. The calculated charge distributions show that the amount of charge transfer from benzene t o methane is very small. The calculated interaction energies of benzene-eth ane, benzene-ethylene, and benzene-acetylene complexes are -1.82, -2.06, an d -2.83 kcal/mol, respectively. Dispersion interaction is again the major s ource of the attraction of these complexes. The electrostatic energy (-0.17 kcal/mol) is not large in the benzene-ethane complex, while the large elec trostatic energies of benzene-ethylene and benzene-acetylene complexes (-0. 65 and -2.01 kcal/mol) show that electrostatic interaction is also importan t for the attraction between benzene and unsaturated hydrocarbon molecules.