Revisiting Structural Models for Au₁₈(SR)₁₄

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Au 18 (SR) 14 (−SR being a thiolate group) is a small, stable thiolated gold nanocluster experimentally identified in 2005, but its structure remains elusive. A previously proposed model for Au 18 (SR) 14 based on density functional theory (DFT) structural optimization consists of a Au 8 core protected by two −RS–Au–SR–Au–SR– (dimer) and two −RS–Au–SR–Au–SR–Au–SR– (trimer) motifs. Here we revisit structure prediction for Au 18 (SR) 14 from extensive exploration of the possible isomers for Au 18 (SCH 3 ) 14 by applying structural hypotheses based on both “staple motifs” and “ring and staple motifs”. Three isomers based on the staple motifs are found to be more stable than the best previous model. The two lowest-energy Au 18 (SCH 3 ) 14 isomers (I and II) also have a Au 8 core protected by two dimer and two trimer motifs, but the core geometry and electronic properties are different. The third lowest-energy isomer (III) consists of a Au 8 core protected by two −RS–Au–SR– (monomer) and two −RS–Au–SR–Au–SR–Au–SR–Au–SR– (tetramer) motifs. By changing R from CH 3 to the experimentally used cyclohexanyl group (C 6 H 11 ), we found that isomer III is the most stable for Au 18 (SC 6 H 11 ) 14 . The computed X-ray diffraction (XRD) pattern and optical spectrum of isomer III are in good agreement with the experimental data. This work suggests that Au 18 (SR) 14 may have monomer and tetramer motifs in the protective layer.