科研之友
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摘要
High-resolution EXAFS (k = 18 angstrom(-1)) and MXAN XAS analyses show that axially elongated square pyramidal [Cu(H2O)(5)](2+) dominates the structure of Cu(II) in aqueous solution, rather than 6-coordinate JT-octahedral [Cu(H2O)(6)](2+). Freezing produced a shoulder at 8989.6 eV on the rising XAS edge and an altered EXAFS spectrum, while 1s -> 3d transitions remained invariant in energy position and intensity. Core square pyramidal [Cu(H2O)(5)](2+) also dominates frozen solution. Solvation shells were found at similar to 3.6 angstrom (EXAFS) or similar to 3.8 angstrom (MXAN) in both liquid and frozen phases. However, MXAN analysis revealed that about half the time in liquid solution, [Cu(H2O)(5)](2+) associates with an axially non-bonding 2.9 angstrom water molecule. This distant water apparently organizes the solvation shell. When the 2.9 angstrom water molecule is absent, the second shell is undetectable to MXAN. The two structural arrangements may represent energetic minima of fluxional dissolved aqueous [Cu(H2O)(5)](2+). The 2.9 angstrom trans-axial water resolves an apparent conflict of the [Cu(H2O)(5)](2+) core model with a dissociational exchange mechanism. In frozen solution, [Cu(H2O)(5)](2+) is associated with either a 3.0 angstrom axial non-bonded water molecule or an axial ClO4- at 3.2 angstrom. Both structures are again of approximately equal presence. When the axial ClO4- is present, Cu(II) is similar to 0.5 angstrom above the mean O-4 plane. This study establishes [Cu(H2O)(5)](2+) as the dominant core structure for Cu(II) in water solution, and is the first to both empirically resolve multiple extended solution structures for fluxional [Cu(H2O)(5)](2+) and to provide direct evidence for second shell dynamics.
- 出版日期2015-2-28
