Electronic Structure and Vibrational Signatures of the Delocalized Radical in Hydrated Clusters of Copper("II") Hydroxide CuOH+(H2O)0-2.

The copper hydroxide ion, CuOH+, serves as the catalytic core in several recently developed water-splitting catalysts, and an understanding of its chemistry is critical to determining viable catalytic mechanisms. In spite of its importance, the electronic structure of this open-shell ion has remained ambiguous in the literature. In particular, computed values for both the thermodynamics of hydration and the vibrational signatures of the mono- and dihydrates have shown prohibitively large errors compared to values from recent experimental measurements. In this work, the source of this discrepancy is demonstrated to be the propensity of this ion to exist between traditional Cu(I) and Cu(II) oxidation-state limits. The spin density of the radical is accordingly shown to delocalize between the metal center and surrounding ligands, and increasing the hydration serves to exacerbate this behavior. Equation-of-motion coupled-cluster methods demonstrated the requisite accuracy to resolve the thermodynamic discrepancies. Such methods were also needed for spectral simulations, although the latter also required a direct simulation of the role of the deuterium "tag" molecules that are used in modern predissociation spectroscopy experiments. This nominally benign tag molecule underwent direct complexation with the open-valence metal ion, thereby forming a species akin to known metal-H2 complexes and strongly impacting the resulting spectrum. Thermal populations of this configuration and other more traditional noncovalently bound isomers led to a considerable broadening of the spectral lineshapes. Therefore, at least for the CuOH+(H2O)0-2 hydrates, these benchmark ions should be considered to be delocalized radical systems with some degree of multireference character at equilibrium. They also serve as a cautionary tale for the spectroscopy community, wherein the role of the D2 tag is far from benign.