A transition in the Ni2+ complex structure from six- to four-coordinate upon formation of ion pair species in supercritical water: An X-ray absorption fine structure, near-infrared, and molecular dynamics study

The coordination structure about Ni2+ in water at temperatures up to 525 de grees C was measured by the X-ray absorption fine structure (XAFS) techniqu e. Solutions containing 0.2 m NiBr2 and 0.2 m NiBr2/0.8 m NaBr were explore d at pressures up to 720 bar. For certain systems, both Ni and Br XAFS data were acquired and a global model was used to fit the two independent sets of XAFS data. These two independent measurements gave excellent agreement o n the coordination structure of the Ni2+/Br- contact-ion pairing. The resul t is a complete picture of the coordination structure of the contact-ion pa iring including the coordination numbers, distances for the water-ion and i on-ion associations, and also a high-quality measurement of the binding str ength and amount of disorder (Debye-Waller factor and the anharmonicity) of the Ni2+/Br- association. The XAFS measurements strongly indicate a transi tional change in the coordination of Ni2+ from the octahedral Ni2+(H2O)(6) species at room temperature to the 4-coordinate structures at supercritical collclitions (e.g., T > 375 degrees C). Specifically, the equilibrium stru cture at 425 degrees C is Ni2+(Br-)(3.3)(H2O)(1.0) for the aqueous solution of 0.2 m NiBr2 with 0.8 m NaBr. At 325 degrees C, the octahedral species s till exists but it is in equilibrium with new species of lower coordination . Above 425 degrees C, at moderate pressures up to 700 bar, the stable stru ctures are a family of 4-coordinated species (NiBr(H2O)(3). Br, NiBr2(H2O)( 2), NiBr3(H2O). Na), where the degree of Bradduction and replacement of H2O in the inner shell depends on the overall Br- concentration. The most like ly symmetry of these species is a distorted tetrahedron. Measurements of th e Ni preedge ls -> 3d and to ls -> 4d transitions using X-ray absorption sp ectroscopy confirm that a symmetry change occurs at high temperature, and t he results are consistent with the XAFS and molecular dynamics (MD) conclus ion of a distorted tetrahedral structure. This observation is further confi rmed by near-infrared (NIR) spectra of the same system. The MD simulations under identical conditions were used to verify the experimental findings. A lthough we found qualitative agreement between the experimental and simulat ed first-shell coordination structure, it is, clear that refinements of the intermolecular potentials are required to quantitatively capture the true high-temperature structure.