Solvation of Au+ versus Au0 in aqueous solution: electronic structure governs solvation shell patterns

The solvation behavior of Au(+) and Au(0) in liquid water under ambient conditions has been studied using ab initio molecular dynamics. The Au(+) aqua ion forms a rigid and well-defined quasi-linear structure in the sense of ligand field theory, where two water molecules are tightly bound to the gold cation through oxygen atoms ("cationic solvation"). Yet, transient charge accumulation in the direction perpendicular to the O-Au(+)-O linear core structure leads occasionally to the formation of a short Au(+)-H contact within the distance range of the first solvation shell, which is typical of "anionic solvation". Upon adding an electron to Au(+), the resulting solvation pattern of Au(0)(aq) has nothing in common with that of Au(+)(aq). Quite surprisingly we discover that the first solvation shell of Au(0)(aq) consists of a single water molecule and features both "anionic" and "cationic" solvation patterns depending on fluctuation and polarization effects. Thus, charging/decharging of metals dissolved in water, M(0)↔ M(+) + e(-), as occurring e.g. during elementary electrochemical steps, is expected to change dramatically their solvation behavior in the sense of re-solvation processes.