The Origin of the Cation/π Interaction:  The Significant Importance of the Induction in Li⁺and Na⁺Complexes

Abstract

The interaction energies of the cation/π complexes (cation = Li+, Na+, and K+, π system = benzene, toluene, ethylbenzene, and tert-butylbenzene) were calculated at the MP2/6-311G** level. The electrostatic (Ees) and induction (Eind) energies were calculated with distributed multipoles and distributed polarizabilities model. Induction and electrostatic interactions are the major source of the attraction. The Eind values of the Li+/π complexes are 2.5−2.8 times larger than the Ees. The Eind values of the Na+/π complexes are 40−80% larger than the Ees. The induction energy is approximately proportional to R-4. The thin structure of the benzene, which enables the cation to have the short contact with carbon atoms of benzene, is essential for the large Eind. More polarizable cyclohexane is not a better cation binder than benzene. The Eind value of the Li+/cyclohexane complex is considerably smaller than that of the Li+/benzene complex. The Li+/cyclohexane complex has larger intermolecular separation, and therefore has the smaller Eind. The small Eind and negligible Ees of the Li+/cyclohexane complex are the causes of the smaller binding energy of the Li+/cyclohexane complex. The tert-butylbenzene complexes have larger binding energies than the benzene complexes. The larger Eind in the tert-butylbenzene complexes are the cause of the larger binding energy.