The cation radius along with the microhydrated environment are the key factors for the (micro)hydrated alkali cations interacting the circumcoronene surface. It was found that balance between M⁺–π interactions, M⁺–water complexation, and the hydrogen bonding of water to the π-system govern the formation mechanism of the cation–π complexes in solution, favoring the outer-sphere solvated Li⁺ and Na⁺–πCC complexes and the inner-sphere solvated K⁺–πCC complexes.

Abstract

Cation-π interactions are theoretically investigated for alkali metal cation (M⁺)-circumcoronene (CC) complexes (M = Li, Na, K), in gas phase and in aqueous solution with consideration of micro- and global solvation models using the DFT/PBEh-3c-RI/TZVP method. The solvent effect on the M⁺–CC energy interaction regarding the cation size and the stability of inner- and outer-sphere [M(H₂O) _n ]⁺–CC complexes are calculated by means of geometry optimizations and potential energy (PE) curves. The PE curves, calculated as a function of perpendicular distance of M⁺ to the CC plane, predicted one energy minimum for each of the isolated M⁺–CC complexes. However, for microhydrated complexes, two minima assigned to two different surface complexations were obtained. Microhydrated Li⁺ and Na⁺ favored outer-sphere complexation while inner-sphere complexation was found more stable for microhydrated K⁺. These results illustrate nicely the key role, which the cation radius plays for the polarization of the water molecules and the aromatic system.