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 Au10(SR)10 to larger Aun(SR)m (n > m) clusters that have only the staple motifs and that the nucleation process starts from the Au4 core.