Structural, Thermodynamic, and Spectroscopic Evolution in the Hydration of Copper(II) Ions, Cu2+(H2O)2-8

Gas-phase clusters of the hydrated Cu(II) cation with2-8water molecules were investigated using ab initio quantum chemistry.Isomer structures, energies, and vibrational spectra were computedacross this size range, yielding a qualitative picture of this ionas an intact Cu2+ hydrate that also partially oxidizesthe surrounding water network at equilibrium. At sufficient clustersizes, these ion hydrates also become thermodynamically preferredover competitive Cu(II) hydroxide hydrates. Competitive coordinationenvironments were found to exist at some cluster sizes, due to bothhydrogen-bonding and d-orbital chemical effects, and the dominantcoordination number was found in some cases to be temperature-dependent.Clear spectral signatures of the ion's coordination environmentwere computed to exist at each cluster size, which should make experimentalverification of these computational predictions straightforward. Throughcomparison to recent studies of hydrated CuOH+, the effectivecharge on the metal center was shown to converge to approximately+1.5 in both cases, despite qualitatively different behavior of theirradical spin densities. Therefore, nominally Cu(II) ions exhibit considerableelectronic, chemical, and structural flexibility. The electronic originsof this flexibility including key roles played by the waternetwork itself are investigated in this work and should providea conceptual foundation for future studies of copper-based, water-oxidationcatalysts.