Geometrical and Electronic Characteristics of AunO2- (n=2-7)

Abstract

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 AunO2- 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 AunO2- clusters. This has allowed us to properly describe the properties of even-numbered AunO2- clusters. Even in the case of odd-numbered AunO2- 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 (n = 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 AunO2- (n = 2-7) clusters including isomers, which match the corresponding photoelectron spectra (PES). 2015 American Chemical Society.