Structure of Au₁₅(SR)₁₃ and its implication for the origin of the nucleus in thiolated gold nanoclusters

Au₁₅(SR)₁₃ is the smallest stable thiolated gold nanocluster experimentally identified so far, and its elusive structure may hold the key to the origin of the nucleus in the formation of thiolated gold nanoclusters. By an extensive exploration of possible isomers by density functional theory, we arrive at a novel structure for Au₁₅(SR) ₁₃ with high stability and whose optical absorption characteristics match those of the experiment. Different from the previous structures and the prevailing working hypothesis about the construction of thiolated gold nanoclusters, the Au₁₅(SR)₁₃ model features a cyclic [Au(I)-SR] pentamer interlocked with one staple trimer motif protecting the tetrahedral Au₄ nucleus, together with another trimer motif. This structure suggests that Au₁₅(SR)₁₃ is a transitional composition from an [Au(I)-SR]_x polymer such as Au ₁₀(SR)₁₀ to larger Au_n(SR)_m (n > m) clusters that have only the staple motifs and that the nucleation process starts from the Au₄ core.