The Oblate Structure and Unexpected Resistance in Reactivity of Ag₁₅⁺ with O₂

Theoretical studies have been carried out to investigate the unusual reactivity of Ag⁺₁₅ cations with oxygen. Our previous work has shown that the reactivity of free metal clusters with oxygen entails a spin excitation that causes reduced reactivity in clusters with filled electronic shells and large HOMO-LUMO gaps. Earlier experiments on Ag⁺₁₅ have shown that the cluster exhibits remarkable resistance to reactivity with oxygen despite having a valence electron count that is not expected to result in a filled electronic shell within the spherical jellium model. It is shown that Ag⁺₁₅, Ag₁₄, and Ag⁻₁₃ clusters with 14 valence electrons, all have oblate bilayer atomic structures that lead to a splitting of the superatomic D-shell in a manner analogous to crystal field splitting of d-states in transition metals. The importance of the oblate structure is demonstrated by showing a correlation between an oblate deformation coefficient and the HOMO-LUMO gap of the different structures of Ag⁺₁₅. The oblate deformation results in the splitting of the electronic subshells and leads to an unusually large HOMO-LUMO gap and the unusual resistance of the cationic species to oxidation.