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

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 Rh-8(+)(H2O)(4) and Rh-9(+)(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 Rh-9(+)(H2O)(3) possesses a hydrogen-bonded cyclic (H2O)(3) water trimer on the top of a tri-capped Rh-9(+) trigonal prism. The tri-capped Rh-9(+) 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, Rh-8(+)(H2O)(4) contains four water molecules separately attached to a bi-capped octahedron, Rh-8(+), 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.

Automated summary

This study investigates the interactions between gas-phase rhodium clusters and water, revealing a size-dependent preference of water adsorption on rhodium clusters with different structural features. Density functional theory calculations show the presence of distinct covalent molecular adsorption and electrostatic cluster-cluster interactions in rhodium-water systems.