CH/π interaction between benzene and model neutral organic molecules bearing acid CH groups

To explore the binding properties of benzene towards small molecules bearing C–H groups with different acidities, we have undertaken ab initio quantum-chemical calculations, including correlation effects through Density Functional Theory methods, on the benzene–CH3X (X = F, Cl, Br, I, CN, NO2) adducts. Benzene acts as a Lewis base and the CH3X molecule as a Lewis acid. The partial charge transferred from benzene to the Lewis acid is mainly confined on the X group and increases with the electron withdrawing character of X. The calculations performed on the various systems predict that two different stable structures for each adduct exist: one with C3v and the other with Cs symmetry, the latter being the most stable one. A simple HOMO–LUMO model suggests that the charge is transferred from the benzene HOMO to the CH3X LUMO and that this process is easier in the systems with Cs symmetry due to the better overlap between the frontier orbitals.