Vibrational properties of small rhodium clusters: role of magnetism, charge state, and isomerization effects
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Extensive density functional theory calculations dedicated to analyze the structure, electronic properties, and vibrational behavior of small and positively charged rhodium clusters are presented. Following the experimental results of Harding et al. [D.J. Harding et al., J. Chem. Phys. 133, 214304-1 (2010)] Rh19+ , Rh11+ , Rh12+ , and Rh13+ clusters are considered and the infrared (IR) spectra for various structural isomers is simulated. The calculations reveal a complex interplay between the distribution and intensity of the IR active frequencies with the atomic structure, magnetism, and charge state of the systems, as well as the crucial role played by high-energy isomers to explain experimental data. Based on a direct comparison between theory and experiment we predict that, for Rh9+, a weighted average of simulated IR spectra corresponding to our lowest energy 9-atom cubic cluster and the closest in energy compact isomer can yield an acceptable agreement between theory and experiment. The possibility of considering mixtures of various IR spectra to explain the measured data is supported by nudged-elastic-band calculations that reveal the existence of inter-conversion processes between different isomers with relatively small energy barriers ( ~0.6 eV). In addition, the recent observation of bi-exponential decays in reactivity experiments of rhodium clusters interacting with N2O species around those sizes also supports this claim. For Rh11+ and Rh12+ clusters, we also obtain that compact high-energy structures with low spin magnetizations are the ones having an IR spectra more in agreement with experiments. Finally for the most common compact and cubic Rh13+ clusters considered in the literature, for which there are no experimental IR spectra to compare with, well defined vibrational features are predicted which could help to identify the atomic configuration of this highly relevant structure.
KeywordsClusters and Nanostructures
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