Cation−π and π–π Interactions in Aqueous Solution Studied Using Polarizable Potential Models

Polarizable potential models for the interaction of Li(+), Na(+), K(+), and NH4(+) ions with benzene are parametrized based on ab initio quantum mechanical calculations. The models reproduce the ab initio complexation energies and potential energy surfaces of the cation-π dimers. They also reproduce the cooperative behavior of "stacked", cation-π-π trimers and the anticooperative behavior of "sandwiched", π-cation-π trimers. The NH4(+) model is calibrated to reproduce the energy of the NH4(+)-H2O dimer and yields correct free energy of hydration and hydration structure without further adjustments. The models are used to investigate cation-π interactions in aqueous solution by calculating the potential of mean force between each of the four cations and a benzene molecule and by analyzing the organization of the solvent as a function of the cation-benzene separation. The results show that Li(+) and Na(+) ions are preferentially solvated by water and do not associate with benzene, while K(+) and NH4(+) ions bind benzene with 1.2 and 1.4 kcal/mol affinities, respectively. Molecular dynamics simulations of NH4(+) and of K(+) in presence of two benzene molecules in water show that cation-π and π-π affinities are mutually enhanced compared to the pairwise affinities, confirming that the cooperativity of cation-π and π-π interactions persists in aqueous solution.