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

• DOI: 10.1021/jp990435c
• Publication Date: 2002-07-26T05:40:56Z
• Authors: Markus M. Hoffmann; John G. Darab; Bruce J. Palmer; John L. Fulton
• Keywords: Coordination number

The coordination structure about Ni2+ in water at temperatures up to 525 °C was measured by the X-ray absorption fine (XAFS) technique. Solutions containing 0.2 m NiBr2 and NiBr2/0.8 NaBr were explored pressures 720 bar. For certain systems, both Ni Br XAFS data acquired a global model used fit two independent sets of data. These measurements gave excellent agreement on Ni2+/Br- contact-ion pairing. result is complete picture pairing including numbers, distances for water−ion ion−ion associations, also high-quality measurement binding strength amount disorder (Debye−Waller factor anharmonicity) association. strongly indicate transitional change from octahedral Ni2+(H2O)6 species room temperature 4-coordinate structures supercritical conditions (e.g., T > 375 °C). Specifically, equilibrium 425 Ni2+(Br-)3.3(H2O)1.0 aqueous solution with 0.8 NaBr. At 325 °C, still exists but it new lower coordination. Above moderate 700 bar, stable are family 4-coordinated (NiBr(H2O)3·Br, NiBr2(H2O)2, NiBr3(H2O)·Na), where degree Br- adduction replacement H2O inner shell depends overall concentration. most likely symmetry these distorted tetrahedron. Measurements preedge 1s → 3d 4d transitions using spectroscopy confirm that occurs high temperature, results consistent molecular dynamics (MD) conclusion tetrahedral structure. This observation further confirmed near-infrared (NIR) spectra same system. MD simulations under identical verify experimental findings. Although we found qualitative between simulated first-shell structure, clear refinements intermolecular potentials required quantitatively capture true high-temperature