Molecular modeling of hydration properties of hydrophobic ions Li+@C60 and K+@C60

Abstract

The Gibbs free energies of solvation (ΔGs) and the electronic structures of endohedral metallofullerenes M+@C60 (M+= Li+, K+) were calculated within the framework of the density functional theory and the polarizable continuum model. In water environment, the equilibrium position of K+ is at the center of the fullerene cavity whereas that of Li+ is shifted by 0.14 nm toward the fullerene cage. The Li+ cation is stabilized by interactions with both the fullerene and solvent. The equilibrium structures of both endohedral metallofullerenes are characterized by very close ΔGs values. In particular, the calculated ΔGs values for K+@C60 are in the range from −124 to −149 kJ mol−1 depending on the basis set and on the type of the density functional. Molecular dynamics simulations (TIP3P H2O, OPLS force field, water sphere of radius 1.9 nm) showed that the radial distribution functions of water density around C60 and M+@C60 are very similar, whereas orientations of water dipoles around the endohedral metallofullerenes resemble the hydration pattern of isolated metal ions.