Cation-π interactions:: A theoretical investigation of the interaction of metallic and organic cations with alkenes, arenes, and heteroarenes

The nature of the cation-pi interaction has been examined by carrying out high level ab initio calculations of both metallic (Li+, Na+, K+, and Ag+) and organic (NH4+, C(NH2)(3)(+), and N(CH3)(4)(+)) cations with different classes of pi systems, viz. alkenes (ethene), arenes (benzene), and heteroarenes (pyrrole). The calculations, which include a rigorous decomposition of the interaction energies, indicate that the interaction of these pi systems with the metal cations is characterized by contributions from both electrostatic and induction energies, with the contribution of the latter being dominant. Though the contributions of dispersion energies are negligible in the cation-pi complexes involving Li+ and Na+, they assume significant proportions in the complexes involving K+ and Ag+. In the pi complexes of the organic cations, the repulsive exchange contributions are much larger than the attractive electrostatic contributions in the pi complexes of organic cations, and thus, the contributions of both induction and dispersion energies are important. While inclusion of electron correlation is essential in obtaining accurate estimates of the dispersion energy, it also magnifies the contribution of the induction energies in the pi complexes of the organic cations. This results in significant consequences in the evaluation of geometries and energies of these cation-pi complexes. The major difference between the cation-pi and cation-H2O complexes stems from the differences in the relative contributions of electrostatic and induction energies, a foreknowledge of which is vital in the design of ion-selective ionophores and receptors. The blue shift in the highly IR active out-of-plane CH bending mode of the pi systems in these complexes is representative of the strength of the cation-g interaction.