Divide and Protect: Capping Gold Nanoclusters with Molecular Gold-Thiolate Rings Hannu Ha 1 kkinen,* ,² Michael Walter, ² and Henrik Gro 1 nbeck ‡ Nanoscience Center and Department of Physics, UniVersity of JyVa ¨skyla ¨, FI-40014 JyVa ¨skyla ¨, Finland, and Competence Center for Catalysis and Department of Applied Physics, Chalmers UniVersity of Technology, SE-41296 Go ¨teborg, Sweden ReceiVed: March 30, 2006 Density functional theory calculations are used to explore phosphine- and thiolate-protected gold nanoclusters, namely, Au 39 (PH 3 ) 14 Cl 6 and Au 38 (SCH 3 ) 24 . For Au 38 (SCH 3 ) 24 , a novel structural motif is predicted, consisting of ringlike (AuSCH 3 ) 4 units protecting a central Au 14 core. The calculated optical spectrum of this species features a large optical gap (about 1.5 eV) and a prominently peaked structure, correlating with experimental findings of “molecular-like spectra” of thiolate-protected 1.1 nm gold nanoparticles. Ligand-ligand interactions and steric effects in the ligand shell are suggested as possible driving forces toward an ordered gold core structure. A novel mechanism for ligand-exchange reactions on gold clusters is proposed. 1. Introduction Synthesis, characterization, and functionalization of size- controlled, ligand-stabilized gold nanoparticles are long-standing issues in the chemistry of nanomaterials. 1,2 Ligand-protected gold nanoparticles offer an intriguing possibility to economically fabricate building blocks for potential applications in catalysis, sensing, photonics, biolabeling, and molecular electronics. These building blocks are expected to have unique size-dependent physical and chemical properties that have been documented in studies of size-selected gold clusters in the gas phase. 3,4 Collective efforts by several groups have established a series of Au core sizes in 1-3 nm range that are predominantly formed in the process of reducing gold from metal salt in the presence of phosphines (P-R 3 ) or thiols (HS-R). 5 A profound under- standing of the growth mechanism or of the physical reasons for the observed “magic” core sizes is, however, still lacking due to the complexity of the growth process and the heavy numerical burden of modeling the ligated gold clusters by state- of-the-art electronic structure calculations. Additional complica- tion is brought by the fact that ligand-exchange reactions involving phosphines and thiolates (S-R) have been shown to modify the growth pattern and to lead to stabilization of core sizes different from that of the original compound. 6-9 Moreover, high sulfur-to-gold ratios result in etching of the gold core. 10 These experimental facts point to the drastically different nature of the gold/phosphine and gold/thiolate interfaces. Here we present large-scale density functional theory (DFT) calculations for Au 39 (PH 3 ) 14 Cl 6 (1) and Au 38 (SCH 3 ) 24 (2) (Figure 1). The explored systems have core sizes of about 1.1 nm and are chemical homologues to experimentally isolated cluster compounds. The calculations reveal a novel form of gold core protection, namely, by gold-thiolate tetraunits (AuSCH 3 ) 4 ; thus, compound 2 can be written as Au 14 [(AuSCH 3 ) 4 ] 6 (2a). The interaction between (AuSCH 3 ) 4 and the remaining Au 14 core in 2a is comparable to the phosphine-Au bond strength in 1, which is suggested to have important consequences for the understanding of ligand-exchange reactions involving phos- phines and thiolates. We also discuss the implications of dif- ferent structure motifs regarding optical spectra of these species. * Corresponding author. Fax: +358 14 260 4756. E-mail: hannu.hakkinen@phys.jyu.fi. ² University of Jyva ¨skyla ¨. ‡ Chalmers University of Technology. Figure 1. (A-C) Structures of clusters 1, 2a, and 2b. (D) Au core of 1, shown by a 90° rotation about the horizontal axis on the left. (E) Au-S framework of 2a, with 45° rotation about the vertical axis on the left. (F) Au-S framework of 2b. Au, orange-brown; S, yellow; P, red; Cl, green; C, dark gray; H, white. 9927 J. Phys. Chem. B 2006, 110, 9927-9931 10.1021/jp0619787 CCC: $33.50 © 2006 American Chemical Society Published on Web 04/29/2006