The origin of the cation/π interaction:: The significant importance of the induction in Li+ and Na+ complexes

The interaction energies of the cation/pi complexes (cation = Li+, Na+, and K+, pi system = benzene, toluene, ethylbenzene, and tert-butylbenzene) were calculated at the MP2/6-311G** level. The electrostatic (E-es) and induction (E-ind) energies were calculated with distributed multipoles and distributed polarizabilities model. Induction and electrostatic interactions are the major source of the attraction. The E-ind values of the Li+/pi complexes are 2.5-2.8 times larger than the E-es. The E-ind values of the Na+/pi complexes are 40-80% larger than the E-es. 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 E-ind. More polarizable cyclohexane is not a better cation binder than benzene. The E-ind 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 E-ind. The small E-ind and negligible E-es 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 E-ind in the tert-butylbenzene complexes are the cause of the larger binding energy.