Reactivity of cationic silver clusters with O 2 : a probe of interplay between clusters' geometric and electronic structures.

We explored the size-dependent reactivity of Ag n + ( n = 2-22) with O 2 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 O 2 on these structures are consistent with experimental observations. Analyses revealed a close relationship between the reactivity of Ag n + with O 2 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 O 2 involving their frontier orbitals. The peculiar size-dependent HOMO-LUMO gaps and reactivity with O 2 correlate with the subtle interplay between the electronic configurations and geometric structures of these silver cluster cations.