Crystalline hydrates of calix[4]arene-para-sulfonic acid with n (n=6-16) water molecules: a structure modeling

The structures of crystalline hydrates of calix[4]arene-para-sulfonic acid with n (n = 6-16) water molecules and the activation barriers to surface proton migration were calculated within the framework of the density functional theory (DFT) using the PBE gradient-corrected functional, the "hard" basis set of projector-augmented waves (PAW), a corresponding pseudopotential, periodic boundary conditions, and the VASP program package. The energies of formation of crystalline hydrates from calix[4]arene-para-sulfonic acid and n water molecules calculated per water molecule are in the range of 0.4-0.9 eV and depend on n. The adsorption energy of water on the surface is in the range of 0.5-0.7 eV. The activation barriers to proton transfer across the surface calculated for the most stable crystal (n = 8) are close to experimental data and depend on the number of superstoihiometric water molecules, being equal to similar to 0.2 eV provided three superstoihiometric water molecules per surface SO3H group.