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

摘要

Polarizable potential models for the interaction of Li~+, Na~+, K~+, and NH_4~+ 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—JT dimers. They also reproduce the cooperative behavior of "stacked", cation—π—π trimers and the anticooperative behavior of "sandwiched", π—cation—π trimers. The NH_4~+ model is calibrated to reproduce the energy of the NH_4~+ — 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 abenzene 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 NH_4~+ ions bind benzene with 1.2 and 1.4 kcal/mol affinities, respectively. Molecular dynamics simulations of NH_4~+ 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.