The intramolecular cation–π interaction of some aryl amines and its drastic influence on the basicity of them: AIM and NBO analysis

In this study, drastic influence of the intramolecular cation–π interaction on the basicity of selected amines has been considered. The optimized minimum energy geometries of different studied amines and their protonated structures were determined by using DFT calculations at the B3LYP/6-311++G(d,p) level of theory. Geometry optimizations indicate that the most stable structures of protonated amines are stabilized by intramolecular cation–π interaction. The proton affinity (PA) of selected amines is controlled by the strength of intramolecular cation–π interaction of ammonium with aromatic ring. These cation–π interactions strongly influence the basicity of amines. Natural bond orbital (NBO) analysis was performed to calculate the second-order interaction energies (E(2)) between the donor and acceptor. Natural resonance theory (NRT) was used to calculate the natural bond order and predict bond polarity. Quantum theory of atoms in molecules (QTAIM) was also applied to determine the nature of cation–π interaction. QTAIM studies showed that the intramolecular cation–π interaction in these structures is electrostatic (closed-shell).