Nanoscale, 13, 11396-11402 (2021)

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

Yuhan Jia ^‡,a,b, Haiming Wu ^‡,a, Xiaoyun Zhao ^c, Hanyu Zhang ^a, Lijun Geng ^a, Hongchao Zhang ^a,b, Si-Dian Li ^*,c, Zhixun Luo ^*,a,b, and Klavs Hansen ^d 

^a Beijing National Laboratory for Molecular Sciences (BNLMS), State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China
E-mail: zxluo@iccas.ac.cn

^b University of Chinese Academy of Sciences, Beijing 100049, P. R. China

^c Institute of Molecular Science, Shanxi University, Taiyuan 030006, China
E-mail: lisidian@sxu.edu.cn

^d Centre for Joint Quantum Studies and Department of Physics, School of Science, Tianjin University, 92 Weijin Road, Tianjin 300072, China

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.