Observation of linear to planar structural transition in sulfur-doped gold clusters: Au_xS⁻ (x = 2–5)

We report a joint experimental and theoretical study on the structures of a series of gold clusters doped with a sulfur atom, Au_xS⁻ (x = 2−5). Well-resolved photoelectron spectra are obtained and compared with theoretical results calculated using several density functional methods to elucidate the structures and bonding of Au_xS⁻ (x = 2−5). Au₂S⁻ is found to have an asymmetric linear global minimum structure with C_∞v symmetry, while the most stable structure of neutral Au₂S is bent with C_2v symmetry, reminiscent of H₂S. Au₃S⁻ is found to have an asymmetric bent structure with an Au–S–Au–Au connectivity. Two isomers are observed experimentally to co-exist for Au₄S⁻: a symmetric bent 1D structure (C_2v) and a 2D planar low-lying isomer (C_s). The global minimum of Au₅S⁻ is found to be a highly stable planar triangular structure (C_2v). Thus, a 1D-to-2D structural transition is observed in the Au_xS⁻ clusters as a function of x at x = 4. Molecular orbital analyses are carried out to obtain insight into the nature of the chemical bonding in the S-doped gold clusters. Strong covalent bonding between S and Au is found to be responsible for the 1D structures of Au_xS⁻ (x = 2−4), whereas delocalized Au−Au interactions favor the 2D planar structure for the larger Au₅S⁻ cluster.