Cation-π- anion interaction: A theoretical investigation of the role of induction energies

Cation-π and the corresponding anion-π interactions have in general been investigated as binary complexes despite their association with counterions. However, a recent study of the ammonia channel highlights the important but overlooked role of anions in cation-π interactions. In an effort to examine the structural and energetic consequences of the presence of counterions, we have carried out detailed ab initio calculations on some model cation-π-anion ternary complexes and evaluated the nonpair potential terms, three-body contributions, and attractive and repulsive energy components of the interaction energy. The presence of the anion in the vicinity of the π system leads to a large redistribution of electron density and hence leads to an inductive stabilization. The resulting electronic and geometrical changes have important consequences in both chemical and biological systems. Compared to cation-π-anion ternary complexes, the magnitude of the cation-π interaction in π-cation-anion ternary complexes is markedly lower because of charge transfer from the anion to the cation.