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A Three-Dimensional Reference Interaction Site Model Self-Consistent-Field Study on the Coordination Structure and Excitation Spectra of Cu(II)-Water Complexes in Aqueous Solution.

The molecular and solvation structures of the hydrated Cu2+ ion and their excitation spectra were investigated using the Kohn-Sham density functional theory (DFT) and the three-dimensional reference interaction site model (3D-RISM) self-consistent field method. Five stable geometrical structures were found to exist in aqueous solution: the distorted octahedral [Cu(H2O)6]2+ in Ci and D2h symmetry, the square pyramidal and trigonal bipyramidal [Cu(H2O)5]2+, and the square planar [Cu(H2O)4]2+. The distorted octahedral in Ci symetry are prefereed in the [Cu(H2O)6]2+, and the squre pyramidal and trigonal bipyramidal [Cu(H2O)5]2+ show almost same stability. Among these geometries, the six-coordinate complex [Cu(H2O)6]2+ in Ci symmetry had the lowest Helmholtz energy. Both [Cu(H2O)6]2+ had a distorted octahedral structure, i.e., two long axial bonds and four short equatorial bonds. The spatial and radial distribution function analyses for [Cu(H2O)5]2+ and [Cu(H2O)4]2+ showed that [Cu(H2O)5]2+ and [Cu(H2O)4]2+ had one and two solvent water distributions that constituted a distorted octahedron with ligand waters. The coordination numbers (CNs) derived from the distribution functions were 5.2-5.4 for [Cu(H2O)5]2+ and 5.3 for [Cu(H2O)4]2+. These results indicated that the Cu2+ ion in an aqueous solution had 5-6 coordination waters in the first hydration shell and some structures with different CNs may interchange in the solution. The excitation energies and electronic configurations of low-lying d-d excited states were calculated using the time-dependent DFT with the electric field generated by 3D-RISM. The orbital energies and electronic configurations were in a similar picture with those of the classical crystal field theory because of the highly symmetrical features of all structures. In [Cu(H2O)6]2+, the degeneracies of orbitals were resolved, whereas in [Cu(H2O)5]2+ and [Cu(H2O)4]2+, weak and strong quasi-degeneracies remained. As a result, only the four-coordinate complex generated third and fourth excited states, whereas in other complexes there were no obvious characters of degeneracies. The resulting excitation energies were in good agreement with the absorption spectra.

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