The Aqueous Ca2+ System, in Comparison with Zn2+, Fe3+, and Al3+: An Ab Initio Molecular Dynamics Study

Herein, we report on the structure and dynamics of the aqueous Ca2+ system studied by using abinitio molecular dynamics (AIMD) simulations. Our detailed study revealed the formation of well-formed hydration shells with characteristics that were significantly different to those of bulk water. To facilitate a robust comparison with state-of-the-art X-ray absorption fine structure (XAFS) data, we employ a 1st principles MD-XAFS procedure and directly compare simulated and experimental XAFS spectra. A comparison of the data for the aqueous Ca2+ system with those of the recently reported Zn2+, Fe3+, and Al3+ species showed that many of their structural characteristics correlated well with charge density on the cation. Some very important exceptions were found, which indicated a strong sensitivity of the solvent structure towards the cations valence electronic structure. Average dipole moments for the 2nd shell of all cations were suppressed relative to bulk water.