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 Cu²⁺, Cu¹⁺, and Br^- in water at temperatures up to 325 °C. The hexaaqua Cu²⁺ species maintains its distorted octahedral structure up to 325 °C, whereas at higher temperatures, dehydration reactions occur producing CuO. Under reducing conditions, the dibromo Cu¹⁺ species, [CuBr₂]^-, 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 Cu¹⁺ species, [CuBr₂]^-, 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 Cu¹⁺ species, [Cu(H₂O)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 Cu¹⁺ 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 Cu¹⁺/Br^- association. Molecular dynamics (MD) simulations were used to further explore the structure and the binding forces for the [CuBr₂]^- species under hydrothermal conditions. We found quantitative agreement for the Cu-Br interactions, but the simulation has difficulty predicting the experimental Cu-H₂O interaction. In particular, the amount of scattering from the water in the dibromo Cu¹⁺ complex was highly over-predicted, so that it is clear that simple intermolecular potential models do not adequately capture this structural feature.