Nature and Strength of M-S Bonds (M ) Au, Ag, and Cu) in Binary Alloy Gold Clusters A. H. Pakiari* ,† and Z. Jamshidi ‡ Chemistry Department College of Sciences, Shiraz UniVersity, P.O. Box 71454, Shiraz, Iran, and Chemistry and Chemical Engineering Research Center of Iran, P.O. Box 14335-186, Tehran, Iran ReceiVed: January 15, 2010; ReVised Manuscript ReceiVed: July 22, 2010 The interactions of pure (Au k , Ag k , and Cu k ; k ) 1-3) and binary alloy (Au n Ag m and Au n Cu m ; m + n ) k e 3) metal clusters with hydrogen sulfide (H 2 S) have been investigated by using density functional theory (BP86, B3LYP, and CAM-B3LYP) and ab initio methods (MP2 and CCSD(T)), with a focus on the nature of metal-sulfur bonds. Binding energy calculations indicate that for pure metal clusters, the tendency of metal to interact with H 2 S has the order of Au > Cu > Ag. In binary alloy clusters, alloying Au k with copper and silver decreases the attraction of Au toward H 2 S, while alloying Ag k and Cu k by gold increases the attraction of Ag and Cu toward H 2 S, significantly. Dissociation energy values for isolated metal clusters specify the more favorable formation of binary alloy clusters (Au n Ag m and Au n Cu m ) over pure ones. The nature of M-S bonds (M ) Au, Ag, and Cu) is also interpreted by means of the quantum theory of atoms in molecules (QTAIM), natural bond orbital (NBO), and energy decomposition analysis (EDA). According to these theories, the M-S bonds are found to be partially electrostatic and partially covalent. EDA results identify that these bonds have less than 35% covalent character and more than 65% electrostatic, and the covalent character increases in different metals in the order Au > Cu > Ag. 1. Introduction Bimetallic nanoclusters, which are often referred to as nanoalloys, have recently drawn considerable attention in basic studies and applications, 1 due to their particular and unique structural, 2-7 electronic, 2,3 optical, 8-10 and magnetic 11-13 proper- ties. Most of the interest and research has concentrated on nanoalloys of the late transition metals (TMs; groups 8-11), in particular, those formed between the group 11 metals (Cu, Ag and Au). Coinage metal nanoalloys, especially gold, have become an active research field lately because of their novel catalytic behavior 14 and their potential applications in nano- electronics and nanosensors. 15 In the past few years, atomic and molecular chemisorptions on small coinage metal clusters, have received considerable attention. 16 Although much of the experimental and theoretical work so far has been accomplished on pure coinage metal clusters, little information is available about the interactions of molecule with gold-silver and gold-copper binary clusters. 17,18 Interaction of coinage metals with molecules containing sulfur atoms is extremely important in the formation of self-assembled monolayers, 19 single-molecule devices, 20 and markers of bio- logical molecules, such as DNA and proteins. 21 Analyzing the nature of M-S bonds is important to designing new molecular devices, and getting highly stable metal-molecule junctions (in single-molecule devices). In TM chemistry, analysis of the bonding situation is frequently discussed in terms of the familiar Dewar-Chatt -Duncanson (DCD), donor-acceptor model. 22 The DCD model is based on heuristic considerations, and the analysis of the metal-ligand bond is often performed using charge partitioning schemes. 23 However, accurate theoreti- cal calculations clearly provide a good basis for the development and support of a chemical model, which is in agreement with physical origin of the chemical bond. 24 Therefore, we present here a systematic study on the structure and electronic properties of the pure and binary alloy coinage metal clusters complexed with H 2 S molecule. The nature of M-S bonds has been discussed by three quantum chemical methods, which are widely used for analyzing the chemical bonds in TM compounds: natural bond orbital (NBO), quantum theory of atoms-in- molecules (QTAIM), and energy decomposition analysis (EDA). 2. Method of Calculations The geometries of pure and binary alloy coinage metal clusters complexes with H 2 S molecule were fully optimized by second-order Moller-Plesset perturbation theory (MP2), and density functional theory (DFT). The DFT methods used are the gradient-corrected functional proposed by Becke and Perdew (BP86), 25 Becke’s three parameter hybrid functional incorporat- ing the correlation functional of Lee, Yang, and Parr (B3LYP), 26 and new hybrid exchange-correlation functional presented by Yanai et al. who combine B3LYP at short-range with an increasing amount of exact HF exchange at long-range, which results in a functional called CAM-B3LYP. 27 These calculations have been done using Gaussian 03 and 09 suite of programs. 28 For coinage metals, we used pseudopotential-based augmented correlation-consistent basis sets, aug-cc-pVDZ-PP, 29 based on the small core relativistic pseudopotentials (PPs) of Figgen et al. 30 In these basis sets, 19 outermost electrons of metal are explicitly described by the (9s, 8p, 7d, 2f)/[5s, 5p, 4d, 2f] basis. Dunning’s aug-cc-pVDZ basis sets 31 were used for hydrogen ((5s, 2p)/[3s, 2p]) and sulfur ((13s, 9p, 2d)/[5s, 4p, 2d]) atoms. The harmonic vibrational frequencies were calculated at all of the optimized geometries, and real frequencies were detected in all of the cases. The binding energy E b of the complex M k -SH 2 is defined as the absolute value of the energy difference E b ) E M k -SH 2 - (E M k + E H 2 S ), and all binding energies are corrected for the basis set superposition error (BSSE). 32 To improve the calculated binding energies, coupled-cluster CCS- * Corresponding author. E-mail: pakiari@susc.ac.ir. † Shiraz University. ‡ Chemistry and Chemical Engineering Research Center of Iran. J. Phys. Chem. A 2010, 114, 9212–9221 9212 10.1021/jp100423b  2010 American Chemical Society Published on Web 08/05/2010