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

Abstract

Gas-phase clusters of the hydrated Cu­(II) cation with 2–8 water molecules were investigated using ab initio quantum chemistry. Isomer structures, energies, and vibrational spectra were computed across this size range, yielding a qualitative picture of this ion as an intact Cu2+ hydrate that also partially oxidizes the surrounding water network at equilibrium. At sufficient cluster sizes, these ion hydrates also become thermodynamically preferred over competitive Cu­(II) hydroxide hydrates. Competitive coordination environments were found to exist at some cluster sizes, due to both hydrogen-bonding and d-orbital chemical effects, and the dominant coordination number was found in some cases to be temperature-dependent. Clear spectral signatures of the ion’s coordination environment were computed to exist at each cluster size, which should make experimental verification of these computational predictions straightforward. Through comparison to recent studies of hydrated CuOH+, the effective charge on the metal center was shown to converge to approximately +1.5 in both cases, despite qualitatively different behavior of their radical spin densities. Therefore, nominally Cu­(II) ions exhibit considerable electronic, chemical, and structural flexibility. The electronic origins of this flexibilityincluding key roles played by the water network itselfare investigated in this work and should provide a conceptual foundation for future studies of copper-based, water-oxidation catalysts.

Persistence of a Delocalized Radical in Larger Clusters of Hydrated Copper(II) Hydroxide, CuOH+(H2O)3–7

“…The assignment of this lowest-energy structure differs from the assignments of ref , which found poor agreement with the free-OH stretch regions of the infrared spectra of isomers computed with MP2 and three different density functionals. This outcome is consistent with the results of our separate analysis of Cu 2+ hydrates, where EOM-IP-CCSD was found to stabilize 5- and 6-coordinate structures relative to many commonly used density functionals. The 5-coordinate isomer 6B and 6-coordinate isomer 6C sit only 0.68 and 2.01 kcal/mol, respectively, above 6A.…”

“…Finally, the thermodynamic competition between CuOH + and Cu 2+ hydrates will become a critical consideration in both the bulk hydration of these ions and in hydrated copper-based catalysts. In a separate analysis similar to the present one, the properties of Cu 2+ (H 2 O) 2–8 have been computed . The thermodynamic crossover from CuOH + to Cu 2+ was computed to occur within this size range, which is consistent with observations from previous experimental studies. , A convergent, mixed electronic structure was also observed to occur within these alternative Cu 2+ hydrates, and the radical behavior was found to be quite similar to the trends observed in the present analysis.…”