Probing the electronic structure and chemical bonding of gold oxides and sulfides in AuOn- and AuSn- (n=1, 2)

The Au-O and Au-S interactions are essential in nanogold catalysis and nanotechnology, for which monogold oxide and sulfide clusters serve as the simplest molecular models. We report a combined photoelectron spectroscopy and ab initio study on AuO- and AuO2- and their valent isoelectronic AuS- and AuS2- species to probe their electronic structure and to elucidate the Au-O and Au-S chemical bonding. Vibrationally resolved spectra were obtained at different photon energies, providing a wealth of electronic structure information for each species. Similar spectra were observed for AuO- and AuS- and for the linear OAuO- and SAuS- species. A bent isomer was also observed as Au(S-2)(-) in the AuS2- spectra, whereas a similar Au(O-2)(-) complex was not observed in the case of AuO2-. High-level ab initio calculations were conducted to aid spectral assignments and provide insight into the chemical bonding in the AuX- and AuX2- molecules. Excellent agreement is achieved between the calculated electronic excitations and the observed spectra. Configuration interactions and spin-orbit couplings were shown to be important and were necessary to achieve good agreement between theory and experiment. Strong covalent bonding was found in both the AuX- and the XAuX- species with multiple bonding characters. While Au(S2)- was found to be a low-lying isomer with a significant binding energy, Au(O-2)(-) was shown to be unbound consistent with the experimental observation. The latter is understood in the context of the size-dependent reactivity of Au-n(-) clusters with O-2.