Geometric Nonuniformity Localizes Valence Orbitals in Gold Clusters: Case Study on [Au₂₃(SC₆H₁₁)₁₆]

To elucidate the effect of geometric nonuniformity on electronic structures of monolayer-protected gold clusters, the valence orbitals of [Au₂₃(SC₆H₁₁)₁₆]^- (Au₂₃ ^-), which features a nonuniform Au₁₃ core, were investigated by using anion photoelectron spectroscopy and density functional theory calculations. The energy levels of the valence orbitals of Au₂₃ ^- were more appropriately described by a model comprising 1S orbitals localized on Au₃ or Au₄ units within the Au₁₃ core, rather than by the deformed jellium model. Meanwhile, the calculated valence orbitals of a hypothetical [Au₂₃(SC₆H₁₁)₁₆]^- cluster with a uniform Au₁₃ core were better described by the jellium model, suggesting that the geometric nonuniformity of the Au₁₃ core is the direct cause of electronic localization in Au₂₃ ^-. The localized nature of the molecular orbitals involved in photoabsorption accounts for the significantly larger molar absorption coefficient of Au₂₃ ^- compared to those of typical Au₁₃ superatoms with uniform structures.