Effect of substituent and solvent on cation-π interactions in benzene and borazine: a computational study

A DFT and ab initio quantum chemical study has been carried out at different theoretical levels to delve into the role of the cation-pi interaction within the main group metal cations (Li+, Na+ and K+), substituted benzene and borazine. The effects of electron withdrawing and electron donating groups on these non-covalent forces of interaction were also studied. The excellent correlation between Hammett constants and binding energy values indicates that the cation-pi interaction is influenced by both inductive and resonance effects. Electron donating groups (EDG) such as -CH3 and -NH2 attached to benzene at the 1, 3 and 5 position and the three boron atoms of borazine were found to strengthen these interactions, while electron withdrawing groups (EWG) such as -NO2 did the reverse. These results were further substantiated by topological analysis using the quantum theory of atoms in molecules (QTAIM). The polarized continuum model (PCM) and the discrete solvation model were used to elucidate the effect of solvation on the cation-pi interaction. The size of the cations and the nature of the substituents were found to influence the enthalpy and binding energy of the systems (or complex). In the gas phase, the cation-pi interaction was found to be exothermic, whereas in the presence of a polar solvent the interaction was highly endothermic. Thermochemical analysis predicts the presence of thermodynamic driving forces for borazine and benzene substituted with EDG. DFT based reactivity descriptors, such as global hardness (eta), chemical potential (mu) and the electrophilicity index (omega) were used to elucidate the effect of the substituent on the reactivity of the cation-pi complexes.