Identification of 13- and 14-coordinated structures of first hydrated shell of [AuCl4](-) acid aqueous solution by combination of MD and XANES.

Molecular dynamics simulations combined with multiple scatting X-ray absorption spectral calculations have been employed to study the local geometry of an [AuCl4](-) cluster in acid aqueous solution. It is found that the previously assumed simple octahedral structure with two H atoms located in the axial position of the [AuCl4](-) plane fails to correctly reproduce the experimental X-ray absorption spectra. Molecular dynamics simulations have revealed a very complicated first hydrated water shell that mainly consists of 13 or 14 ligand water molecules. In general, the first hydrated water molecules are located in two orthogonal quasi elliptic surfaces around [AuCl4](-) cluster. The water molecules configuration in the first hydrated shell is further confirmed by the X-ray absorption spectral analysis, which gives a good agreement with the experiment when the 13 hydrated and 14 hydrated complexes are evenly mixed. It shows the power of the combined molecular dynamics simulations and spectra calculations in determining the local structure of molecular ligands around the metal center.