Copper(I) and Copper(II) Coordination Structure under Hydrothermal Conditions at 325 °C:  An X-ray Absorption Fine Structure and Molecular Dynamics Study

X-ray absorption fine structure (XAFS) spectroscopy was used to measure the coordination structure about Cu 2+, Cu 1+, and Br - in water at temperatures up to 325 °C. The hexaaqua Cu 2+ species maintains its distorted octahedral structure up to 325 °C, whereas at higher temperatures, dehydration reactions occur producing CuO. Under reducing conditions, the dibromo Cu 1+ species, [CuBr 2 ] -, is predominant at 200 °C and above for systems having excess Br - . Even for a very high salt concentration of 2.0 m NaBr, only the dibromo Cu 1+ species, [CuBr 2 ] -, is observed with no evidence of higher Br - coordination. For this dibromo-species there are no tightly bound hydration waters in the first shell. In the absence of excess Br -, a monoaqua monobromo Cu 1+ species, [Cu(H 2 O)Br] is observed. For certain systems, both Cu and Br XAFS were acquired, and a global model was used to fit the two independent sets of XAFS data. Thus, the results represent a complete picture of the coordination structure about Cu 1+ including the coordination numbers, distances for the ion−ion and water-ion associations and also a high-quality measurement of the binding strength and amount of disorder (Debye−Waller factor and the anharmonicity) of the Cu 1+ /Br - association. Molecular dynamics (MD) simulations were used to further explore the structure and the binding forces for the [CuBr 2 ] - species under hydrothermal conditions. We found quantitative agreement for the Cu−Br interactions, but the simulation has difficulty predicting the experimental Cu−H 2 O interaction. In particular, the amount of scattering from the water in the dibromo Cu 1+ complex was highly over-predicted, so that it is clear that simple intermolecular potential models do not adequately capture this structural feature.