Copper(I) and copper(II) coordination structure under hydrothermal conditions at 325 degrees 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 degreesC. The hexaaqua CU2+ species maintains its distorted octah edral structure up to 325 degreesC, whereas at higher temperatures, dehydra tion reactions occur producing CuO. Under reducing conditions, the dibromo Cu1+ species, [CuBr2](-), is predominant at 200 degreesC and above for syst ems having excess Br-. Even for a very high salt concentration of 2.0 m NaB r, only the dibromo Cu1+ species,[CuBr2](-), is observed with no evidence o f higher Br- coordination. For this dibromo-species there are no tightly bo und hydration waters in the first shell. In the absence of excess Br-, a mo noaqua 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 h igh-quality measurement of the binding strength and amount of disorder (Deb ye-Waller factor and the anharmonicity) of the Cu1+/Br- association. Molecu lar dynamics (MD) simulations were used to further explore the structure an d the binding forces for the [CuBr2](-) species under hydrothermal conditio ns. We found quantitative agreement for the Cu-Br interactions, but the sim ulation has difficulty predicting the experimental Cu-H2O interaction. Ln p articular, the amount of scattering from the water in the dibromo Cu1+ comp lex was highly over-predicted, so that it is clear that simple intermolecul ar potential models do not adequately capture this structural feature.