Correlation between Electronic Shell Structure and Inertness of Cu _n⁺ toward O₂ Adsorption at n = 15, 21, 41, and 49

The inertness of metal clusters in air is important for their application to novel materials and catalysts. The adsorption reactivity of copper clusters with O₂ has been discussed in connection with the electronic structure of clusters because of its importance in electron transfer from the cluster to O₂. Mass spectrometry was used to observe the reaction of Cu _n⁺ + O₂ ( n = 13-60) in the gas phase. For O₂ adsorption on Cu _n⁺, the relative rate constants of the n = 15, 21, 41, and 49 clusters were clearly lower than those with other n. Theoretical calculations indicated that the inertness of Cu₁₅⁺ with 14 valence electrons was related to the large HOMO-LUMO gap predicted for the oblate Cu₁₅⁺ structure. The Clemenger-Nilsson model was used to predict that the electronic subshell of oblate Cu₄₉⁺ with 48 electrons was closed. This electronic shell closing of Cu₄₉⁺ corresponds to the inertness for O₂ adsorption.