Reactivity of cationic silver clusters with O2: a probe of interplay between clusters’ geometric and electronic structures

Abstract

We explored the size-dependent reactivity of Ag_n⁺ (n = 2–22) with O₂ under mild conditions and found that only a few sizes of Ag_n⁺, with even values of n = 4, 6, 12, 16, 18, and 22, are reactive. Possible structures of Ag_n⁺ (n = 2–22) were determined using a genetic algorithm with incomplete local optimizations at the DFT level, and the calculated bonding strengths of O₂ on these structures are consistent with experimental observations. Analyses revealed a close relationship between the reactivity of Ag_n⁺ with O₂ and its HOMO–LUMO gap: cationic silver clusters with a small HOMO–LUMO gap are reactive, which can be rationalized by the covalent character of chemical bonds between Ag_n⁺ and O₂ involving their frontier orbitals. The peculiar size-dependent HOMO–LUMO gaps and reactivity with O₂ correlate with the subtle interplay between the electronic configurations and geometric structures of these silver cluster cations.