Geometrical and Electronic Characteristics of Au<i>_n</i>O₂^– (<i>n</i> = 2–7)

Most density functionals do not properly describe the characteristics of superoxide (O₂^–) (i.e., first two vertical electron detachment energies and the excitation energies of neutralized singlet state) of small even-numbered Au<i>_n</i>O₂^– clusters. However, the second-order Møller–Plesset theory (MP2) shows significant charge transfer from Au cluster anions to oxygen molecule and so provides proper electronic characteristics of superoxide of small even-numbered Au<i>_n</i>O₂^– clusters. This has allowed us to properly describe the properties of even-numbered Au<i>_n</i>O₂^– clusters. Even in the case of odd-numbered Au<i>_n</i>O₂^– clusters, we find that Au₅^– is a chemically O₂-adsorbed singlet state at 0 K, against a commonly accepted physisorbed triplet state. This is further evidenced by our extensive coupled cluster with single, double, and perturbative triple excitations [CCSD­(T)] calculations, including the relativistic effect. However, the entropy effect makes the physisorbed triplet state more stable than the chemisorbed singlet state at higher temperatures, consistent with the experiment. The weak O₂ binding by odd-numbered cluster anions (<i>n</i> = 3, 5, and 7) could be further weakened by the entropic effect, which results in van der Waals complexes at high temperatures. The present study reports the geometrical and electronic characteristics of small Au<i>_n</i>O₂^– (<i>n</i> = 2–7) clusters including isomers, which match the corresponding photoelectron spectra (PES)