The interaction of benzene with chloro- and fluoromethanes:: Effects of halogenation on CH/π interaction

High-level ab initio calculations were carried out to evaluate the interaction Of C6H6 with CH3Cl, CH2Cl2, CHCl3, and CHF3. Intermolecular interaction energies were calculated from extrapolated MP2 interaction energies at the basis set limit and CCSD(T) correction terms. The calculated interaction energies of the complexes are -3.0, -4.5, -5.6, and -4.2 kcal/mol, respectively. These values are significantly larger than the interaction energy of C6H6-CH4 complex (-1.5 kcal/mol). The interaction energy Of C6H6-CHCl3 is slightly larger than that of the hydrogen bond between waters. The calculated potentials of the complexes are very flat near the minima. Substantial attraction still exists even if the molecules are well-separated. This shows that the major source of attraction in the complexes is not short-range interactions such as charge transfer but long-range interactions such as electrostatic and dispersion. A large gain of attraction by electron correlation indicates that dispersion interaction is the major source of attraction. The size of attraction depends on the substituents of methane considerably. Substitution of hydrogen atoms of methane by chlorine and fluorine atoms increases attractive electrostatic interaction. Substitution by chlorine atoms also increases dispersion interaction significantly. The calculated potentials show that the substitution of methane does not enhance short-range interactions.