Interactions between water and rhodium clusters: molecular adsorption versus cluster adsorption

Abstract

Understanding metal–water interactions and hydrogen-bonding in water droplets is important but highly challenging. Various transition metals may serve as effective coordination centers to water; however, not in all cases is water bonded to a metal center as single molecules. We report here the observations of gas-phase rhodium clusters and their interactions with water. A series of rhodium–water clusters, Rh_n^±,0(H₂O)_m (n = 3–30, m = 1–5), with isotope labels were detected by mass spectrometry after exposure to different water concentrations, among which Rh₈⁺(H₂O)₄ and Rh₉⁺(H₂O)₃ were prominent in the mass distributions, showing a size-dependent preference of water adsorption on rhodium clusters. Comprehensive density functional theory calculations reveal that the lowest energy structure of Rh₉⁺(H₂O)₃ possesses a hydrogen-bonded cyclic (H₂O)₃ water trimer on the top of a tri-capped Rh₉⁺ trigonal prism. The tri-capped Rh₉⁺ trigonal prism and the cyclic (H₂O)₃ water trimer match in sizes, charge distributions, and orbital symmetries to form effective electrostatic cluster–cluster interactions. In contrast, Rh₈⁺(H₂O)₄ contains four water molecules separately attached to a bi-capped octahedron, Rh₈⁺, at four corners via single-molecule adsorption. The difference between covalent molecular adsorption and electrostatic cluster–cluster interaction in these two proto-typical rhodium hydrates is further demonstrated by detailed natural bonding orbital, electrostatic surface potential, and charge decomposition analyses.