A model chemistry for the evaluation of intermolecular interaction between aromatic molecules (AIMI Model) has been developed. The CCSD(T) interaction energy at the basis set limit has been estimated from the MP2 interaction energy near the basis set limit and the CCSD(T) correction term obtained by using a medium size basis set. The calculated interaction energies of the parallel, T-shaped,and slipped-parallel benzene dimers are -1.48, -2.46, and -2.48 kcal/mol, respectively. The substantial attractive interaction in benzene dimer, even where the molecules are well separated, shows that the major source of attraction is not short-range interactions such as charge-transfer but long-range interactions such as electrostatic and dispersion. The inclusion of electron correlation increases attraction significantly. The dispersion interaction is found to be the major source of attraction in the benzene dimer. The orientation dependence of the dimer interaction is mainly controlled by long-range interactions. Although electrostatic interaction is considerably weaker than dispersion interaction, it is highly orientation dependent. Dispersion and electrostatic interactions are both important for the directionality of the benzene dimer interaction.

High-level ab initio calculations were carried out to evaluate the interaction between the π 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.

High-level ab initio calculations were carried out to evaluate the interaction between the π face of benzene and ammonia as a model of NH/π interaction. The intermolecular interaction energy was calculated from the extrapolated MP2 interaction energy at the basis set limit and a CCSD(T) correction term. The calculated interaction energy (-2.22 kcal/mol) is considerably smaller than that of the hydrogen bond between waters. The monodentate complex is slightly more stable than the bidentate and tridentate complexes. The potential energy surface is very flat near the minimum, which shows that the major source of the attraction is a longrange interaction. The HF interaction energy of the monodentate complex (0.13 kcal/mol) is repulsive. The large gain in the attraction by electron correlation correction (-2.36 kcal/mol) indicates that the dispersion interaction is significantly important for the attraction. The electrostatic energy (-1.01 kcal/mol) is also important for the attraction. The benzene-water (OH/π) interaction energy (-3.17 kcal/mol) is larger than the benzeneammonia (NH/π) interaction. The dispersion interaction is again important for the attraction in the benzenewater complex. The attraction in the benzene-ammonia complex is stronger than that in the benzene-methane (CH/π) complex (-1.45 kcal/mol). The amount of electrostatic energy is mainly responsible for the magnitude of the attractions in these three complexes. The directionality for the NH/π and OH/π interactions is mainly controlled by the electrostatic interaction.

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The interaction energies of the cation/π complexes (cation = Li+, Na+, and K+, π system = benzene, toluene, ethylbenzene, and tert-butylbenzene) were calculated at the MP2/6-311G** level. The electrostatic (E es) and induction (E ind) energies were calculated with distributed multipoles and distributed polarizabilities model. Induction and electrostatic interactions are the major source of the attraction. The E ind values of the Li+/π complexes are 2.5−2.8 times larger than the E es. The E ind values of the Na+/π complexes are 40−80% larger than the E es. The induction energy is approximately proportional to R-4. The thin structure of the benzene, which enables the cation to have the short contact with carbon atoms of benzene, is essential for the large E ind. More polarizable cyclohexane is not a better cation binder than benzene. The E ind value of the Li+/cyclohexane complex is considerably smaller than that of the Li+/benzene complex. The Li+/cyclohexane complex has larger intermolecular separation, and therefore has the smaller E ind. The small E ind and negligible E es of the Li+/cyclohexane complex are the causes of the smaller binding energy of the Li+/cyclohexane complex. The tert-butylbenzene complexes have larger binding energies than the benzene complexes. The larger E ind in the tert-butylbenzene complexes are the cause of the larger binding energy.