CH/pi 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 C-3v and the other with C-s 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 C-s symmetry due to the better overlap between the frontier orbitals.