Unraveling the Hydration Properties of the Ba²⁺ Aqua Ion: the Interplay of Quantum Mechanics, Molecular Dynamics, and EXAFS Spectroscopy

The structural and dynamic properties of the Ba²⁺ cation in water have been studied by combining quantum mechanical (QM) calculations, molecular dynamics (MD) simulations, and extended X-ray absorption fine structure (EXAFS) spectroscopy. An effective Ba²⁺-water interaction potential, to be used in the MD simulation of a Ba²⁺ aqueous solution, has been developed by means of QM methods, and the validity of the whole procedure has been assessed by comparing the theoretical structural results with the EXAFS experimental data. By combining distance and angular distribution functions it was possible to unambiguously identify the geometry adopted by the water molecules surrounding the ion in the solution. The Ba²⁺ ion was found to preferentially form an 8-fold first shell complex with a bicapped trigonal prism (BTP) geometry. The 8-fold complex is in equilibrium with a 9-fold structure having a tricapped trigonal prism (TTP) geometry, and the hydration shell is very diffuse and flexible, being characterized by a very fast solvent exchange process on the picosecond time scale.