Role of the Multipolar Electrostatic Interaction Energy Components in Strong and Weak Cation−π Interactions

Density functional and Møller–Plesset second-order perturbation (MP2) calculations have been carried out on various model cation−π complexes formed through the interactions of Mg²⁺, Ca²⁺, and NH₄⁺ cations with benzene, <i>p</i>-methylphenol, and 3-methylindole. Partial hydration of the metal cations was also considered in these model studies to monitor the effect of hydration of cations in cation−π interactions. The binding energies of these complexes were computed from the fully optimized structures using coupled cluster calculations including triple excitations (CCSD­(T)) and Gaussian-G4-MP2 (G4MP2) techniques. An analysis of the charge sharing between the donor (the π-systems) and the acceptors (the cations) together with the partitioning of total interaction energies revealed that the strong and weak cation−π interactions have similar electrostatic interaction properties. Further decomposition of such electrostatic terms into their multipolar components showed the importance of the charge–dipole, charge–quadrupole, and charge–octopole terms in shaping the electrostatic forces in such interactions. The computed vibrational spectra of the complexes were analyzed for the specific cation−π interaction modes and have been shown to contain the signature of higher order electrostatic interaction energy components (quadrupole and octopole) in such interactions