Ligand and Solvation Effects on the Electronic Properties of Au55

Jul 7, 2009 - The presence of Cl atoms in the ligand shell favors a nonmetallic behavior .... Nature of Ligand-Protected Gold Nanoparticle Au102(MBA)4...
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NANO LETTERS

Ligand and Solvation Effects on the Electronic Properties of Au55 Clusters: A Density Functional Theory Study

2009 Vol. 9, No. 8 3007-3011

Ganga Periyasamy and F. Remacle*,† Department of Chemistry, B6c, UniVersity of Lie`ge, B-4000 Lie`ge, Belgium Received May 5, 2009; Revised Manuscript Received June 30, 2009

ABSTRACT The electronic properties of the neutral, positively and negatively charged bare Au55, passivated Au55(PH3)12, Au55(PH3)12Cl6, and solvated Au55(PH3)12Cl6 54 H2O clusters are studied using density functional theory. The presence of Cl atoms in the ligand shell favors a nonmetallic behavior while a more metallic behavior is induced by explicit solvation of Au55(PH3)12Cl6 with water molecules. The trends observed in the electronic properties upon ligation and solvation are in agreement with experimental studies.

Ligand and solvation effects on the electronic properties of gold clusters are crucial for the design and the experimental realization of logic nano devices and intelligent sensors. Therefore, bare and ligand protected gold nanoclusters have been the subject of intense research activities,1-20 for recent reviews see ref 21. These studies show that the electronic properties and the geometry of the complex are strongly affected by ligand and solvent environments. On the theory side, highly accurate ab initio quantum computations have been carried out2,14,22-29 (for recent reviews on electronic structure of gold clusters, see refs 30 and 31 and refs therein) and it was pointed out early on that the electronic structure of gold clusters is particular because of strong relativistic effects in the electronic structure of the gold atom.32,33 Among small gold clusters, Au55 possesses an ideal size (∼1.4 nm) for catalytic activity in bare and various ligand protected forms such as [Au55(PPh3)12Cl6], [Au55(BSH)12]2+ and so forth,1,34 as well as for building chemical sensors and organic electronic components.13,20 In addition, the specific coordination of Au55 clusters with DNA suggests that it could exhibit an antitumor activity because of its oxidation-resistant properties.16 Au55 is characterized by a full shell geometric structure.34 The ligand-protected Au55 clusters are of well-defined size and ligand composition and they typically do not coagulate with one another.35 Consequently, they can self-assemble into two and three-dimensional arrays with promising electrical properties.20,35 Several experimental studies were dedicated to the Au55(PPh3)12Cl6 nanoclusters, first synthesized and character* To whom correspondence should be addressed. E-mail: fremacle@ ulg.ac.be. † Director of Research, FNRS, Belgium. 10.1021/nl901430k CCC: $40.75 Published on Web 07/07/2009

 2009 American Chemical Society

ized by Schmid and collaborators in the early 1990s.1,34,35 However the contribution of the ligand shell to their electronic properties, in particular to the observed metallic and nonmetallic character, is still debated.15 Mo¨ssbauer spectroscopy studies9 suggest a nonmetallic behavior of Au55(PPh3)12Cl6 on the basis of the modified electron densities of the gold cluster core as compared to bulk material. Evidence for a nonmetallic behavior is also provided by the absence of linear contribution to the specific heat,4 which indicates a vanishing density of electronic states at the Fermi level. On the other hand, the density of states at Fermi level in XPS2,15,36 and ac- and dc- conductivity measurements37 indicates either a metallic behavior for the bare cluster or a more insulating behavior when there is a strong interaction between the ligand shell and Au55.15 Optical measurements6 on ligated clusters point to an intermediate between a metallic and a nonmetallic character as well. We computationally investigate ligand and solvation effects on the Au55 core for three charge states (positive, neutral, and negative) of the ligated Au55(PH3)12, Au55(PH3)12Cl6 and solvated Au55(PH3)12Cl6 54 H2O clusters using density functional theory (DFT). We show by analyzing the bonding patterns of the ligands and computing the ionization potentials (IP), electro-affinities (EA), and charging energies that the presence of Cl atoms in the ligand shell favors a nonmetallic behavior while a more metallic behavior is induced by explicit solvation with water molecules. All the structures reported for the bare Au55, ligated Au55(PH3)12 and Au55(PH3)12Cl6 and solvated Au55(PH3)12Cl6 54 H2O clusters were optimized at the B3LYP level with relativistic LANL2MB pseudopotentials and basis sets for the 55 gold atoms and 6-31+G(d) Gaussian basis set for the ligands, as implemented in the GAUSSIAN03 quantum

Figure 1. Optimized geometry of the stable isomers of Au55 (a), Au55(PH3)12 (b), Au55(PH3)12Cl6 (c), and Au55(PH3)12Cl6 54 H2O (d). All clusters exhibit a distorted icosahedral geometry where a central Au (red) is surrounded by two shells, a first shell of 12 Au (green) in a sphere of ∼0.29 nm and a second shell of 42 Au, the 12 face edge atoms (brown) being at ∼0.58 nm and the 30 facecentered ones (yellow) at ∼0.51 nm from the central gold atom, respectively. The diameters of the Au55 core are reported on Table S1 of Supporting Information. The four types of Au atoms carry significantly different Mulliken charges (in |e|). The central Au of Au55 is positively charged in bare Au55 (a) (qM ) 0.059) but negatively charged in all ligated forms (qM ) 0.084 for (b), -0.084 (c), and -0.070 in (d)). The average qMav of the atoms of the first shell is positive and does not vary significantly upon ligation, 0.178 (a), 0.180 (b), 0.167 (c), and 0.160 (d). In the outer shell, the 12 face edge atoms are negatively charged but less so for the ligated forms (qMav ) -0.159 (a), -0.101 (b), -0.085 (c), and -0.116 (d)). qMav on the face-centered atoms is smaller (-0.009 (a), -0.101 (b), -0.085 (c), and -0.116 (d)). For the ligated forms, the PH3 groups linked to the 12 face edge atoms are positively charged (0.084 in (b), 0.084 in (c), and 0.0690 in (d)) while the Cl atoms are strongly negatively charged (-0.359 in (c) and -0.290 in (d)).

chemistry suite of programs.38 The bare and ligated neutral complexes are open-shell species and were computed at the unrestricted DFT level. More computational details are provided in the Supporting Information. The modeling of the PPh3 ligand groups by PH3 groups reduces the computational cost and, as discussed below, is expected to be adequate for isolated clusters. All equilibrium geometries were obtained with the tight keyword for convergence except for larger Au55(PH3)12Cl6 54 H2O for which the loose convergence criterion was used. More details on the geometry optimization of Au55(PH3)12Cl6 54 H2O can be found in Supporting Information. The 6-31+G(d) basis set used for the water molecules is expected to be adequate for describing the H-bonding. The geometry optimizations were started from two different geometries, (i) a perfect icosahedral geometry with Au(110) faces and (ii) a cubohedral geometry with Au(111) faces. For all the charged states of the bare Au55 and ligated (Au55(PH3)12, Au55(PH3)12Cl6, Au55(PH3)12Cl6 54H2O) clusters, geometry optimization leads to a distorted icosahedral geometry (DIh) with an hexagonal outer layer shape (Figure 1) and a diameter of the Au55 core varying between 1.15 to 1.20 nm (see Table S1 in Supporting Information). Clusters 3008

with a distorted cubohedral geometry (DOh) are less stable than the icosahedral ones by 30-32 kcal mol-1 depending on the charge state for the bare clusters and revert to the DIh geometry for the ligated ones. These results show that anchoring ligands on a Au(110) face (as is the case in the DIh geometry) is more stable than on a Au(111) one (DOh). The observed hexagonal shape and the diameter values are agreement with the X-ray photoelectron spectroscopy (XPS),2,15,36 scanning transmisstion microscopy,5,39 transmission electron microscopy,7,35 extended X-ray absorption fine structure,3,12 electron-nuclear double resonance,40 and impedance spectroscopic experimental data.34 In all the ligated clusters (Au55(PH3)12, Figure 1b, Au55(PH3)12Cl6, Figure 1c, Au55(PH3)12Cl6 54 H2O, Figure 1d) the twelve PH3 ligands coordinate to the face edge (corner) atoms of the outer layer of the Au55 core (Au-P: ∼0.24 nm in Au55+, ∼0.26 nm in Au550 and ∼0.25 nm in Au55-) and adopt a one coordination site. On the other hand, in Au55(PH3)12Cl6 (Figure 1c) the six Cl ligands are asymmetrically coordinated to the three face-centered gold atoms (in Au55+: Au1-Cl, ∼0.23; Au2-Cl, ∼0.29; Au3-Cl, ∼0.28 nm. In Au550: Au1-Cl, ∼0.25; Au2-Cl, ∼0.27; Au3-Cl, ∼0.28 nm. In Au55-: Au1-Cl, ∼0.26; Au2-Cl, ∼0.28; Au3-Cl, ∼0.28 nm; for labeling of the Cl coordinated Au atoms, see Figure S1 in Supporting Information). As discussed in detail below, in the solvated cluster (Figure 1d), the H-bonding between the Cl and the water molecules leads to a change in the coordination of the Cl atoms that binds to a single face-centered gold atom. Though both bare and the ligated forms of Au55 exhibit similar geometries with an outer hexagonal layer for the Au55 core, their electronic properties (IP, EA, charging energies and Mulliken charge distributions) are highly affected by the ligand shell. The computed Mulliken charges (see legend of Figure 1 for their values) show that there are four types of charged gold atoms (i) 1-center, (ii) 12-first shell, (iii) 12-face edge second shell gold atoms, and (iv) 30-facecentered second shell gold atoms (Figure 1) in agreement with the Mo¨ssbauer spectra of the bare and ligated clusters.9 Comparing the bare and ligated cluster Mulliken charges shows that upon passivation, there is a significant charge transfer from the gold core to the ligand shell. This charge transfer is however reduced by the presence of water molecules. The energy differences between the frontier orbitals (∆ ) ELUMO - EHOMO) together with the atomic orbital contributions to the highest occupied molecular orbital (HOMO) (see Table 1 and Figure 2 for isocontours of the singly occupied molecualr orbital (SOMO) (highest occupied R MO) and lowest unoccupied molecular orbital (LUMO) (lowest R unoccupied MO) of neutral clusters) can be used to judge of the chemical reactivity of the clusters and the strength of the ligand effects. For the neutral bare and all investigated (Au55(PH3)12Cl6, Au55(PH3)12, Au55(PH3)12Cl6 54H2O) ligated clusters, the value of ∆ for the R MOs (∆ ) 1.3 eV) of the neutral is about equal to the value of ∆ for the anion (1.4 eV) and that of the β MO (0.98 eV) to that of the cation (0.98 eV). In addition, spin density plots (see Figure S5 in Supporting Nano Lett., Vol. 9, No. 8, 2009

Table 1. Calculated Electronic Properties (eV) for Bare and Various Ligated Clustersa IP Au55 Au55(PH3)12Cl6 Au55(PH3)12 Au55(PH3)12Cl6 54 H2O

EA

Uc

6.36 -4.34 2.01 neutral: (R) cat: 0.98 anion: 1.40 5.33 -1.91 3.43 neutral: (R) cat: 0.47 anion: 0.91 4.03 -2.33 1.69 neutral: (R) cat: 0.46 anion: 0.77 2.89 -1.90 0.99 neutral: (R)

∆ 1.30 and (β) 0.98 0.91 and (β) 0.74 0.88 and (β) 0.53 0.51 and (β) 0.32

cat: 0.24 anion: 0.42 a The adiabatic IP is computed as E(N-1)-E(N), the adiabatic EA as E(N + 1) - E(N) and the charging energy Uc as EA + IP ) E(N + 1) -2E(N) + E(N - 1). ∆ ) EHOMO - ELUMO.

Figure 2. Frontier molecular orbitals (SOMO and LUMO) of Au550 in the bare (a,b) and ligated forms Au55(PH3)12Cl6 (c,d) and Au55(PH3)12 (e,f).

Information) show that the unpaired electron of all the neutral clusters is localized on the SOMO of the cluster (see Figure 2). These two observations indicate that there is no extensive electronic relaxation upon adding or removing an electron to the neutral Au55. They are corroborated by the fact that the density of the SOMO-1 of the neutral (HOMO-1 in R MO, see Figure 2) is very similar to the density of the HOMO of the cation (shown in Figure S2 of Supporting Information for the relaxed geometry) and that the SOMO (highest occupied R MO) is similar to the HOMO of the anion (Figure S2 in Supporting Information). As a consequence, the hexagonal shape Au55 core is preserved as can be seen from Figure 1. The negatively charged nanoclusters, bare and ligated, are more stable than the other charged forms as indicated by their higher ∆ values. As discussed below, structurally, the Nano Lett., Vol. 9, No. 8, 2009

higher stability of the negatively charged clusters is reflected by less distorted equilibrium geometries, closer to a perfect icosahedral shape. The atomic contributions to the HOMO are given in Table S1 of Supporting Information. The degree of hybridization between the AO orbitals centered on the face edge and facecentered Au atoms governs the amount of distortion of the hexagonal shape of the outer layer of the clusters. A good 6s (face edge)-5d (face-centered) overlap favors a less distorted geometry. The density of the SOMO of the neutral bare radical shown in Figure 2a exhibits a dominant 33% 6s character on the face edge atoms and a dominant 50% 5d character on the face center ones. The 6s-5d hybridization is sufficient to impose the icosahedral shape but the geometry of the neutral presents some distortion and strain. The HOMO of the anion Au55- (see Figure S2 of Supporting Information) exhibits a dominant 53% 6s character on the face edge atoms that now extents to the face-centered atoms and can better hybridize with the 5d orbitals (29%) on the face center ones. The better electronic delocalization leads in a less distorted geometry for the anion (Figures S2, S3 and S4 of Supporting Information). On the other hand, in the case of the cation, the HOMO exhibits a smaller (21%) 6s character on the edge atoms and a 41% 5d character on the face-centered atoms. The lack of delocalization induces considerable strain and a highly distorted geometry. The difference in hybridization of the frontier orbitals for the different charge states explains the large computed value of the ionization energy (6.36 eV) and the low value of the electron affinity () -4.34 eV) of the bare cluster. The resulting charging energy, Uc, is 2.02 eV (see Table 1). This value is in agreement with the estimation provided by the spherical model for a vacuum medium. Uc(eV) ) e/C and C ) 4πεε0R, which leads to a value of 2.4 eV for a radius of ∼0.6 nm, as obtained from the geometry optimizations for the neutral and charged states of bare Au55. The 2.02 eV value computed for the relaxed geometries of the bare clusters in the gas phase is however much larger than the measured charging energy (0.3-0.5 eV) for passivated clusters. As discussed below, the lowering of the charging energy of the passivated clusters is essentially due to the solvent shell. In Au55(PH3)12Cl6, the addition of the ligand shell decreases the HOMO-LUMO gap by 0.5 eV. In the frontier orbitals (Figure 2c,d), we note a significant contribution (37%) of the 3p Cl orbitals indicating that the Cl atoms strongly interact with the Au55 core. The MOs responsible for the Au-P bonds are lower in energy and there is no atomic contribution of the P atoms to the frontier orbitals. The bonding to the PH3 ligands is accompanied by a small charge transfer, as reflected by the computed Mulliken charges, which are slightly negative on the face edge Au atoms and slightly positive on the PH3 groups (see legend of Figure 1). The replacement of the model PH3 ligands by PPh3 ones is not expected to significantly modify the characteristics of Au-P bonding. The PPh3 ligand is more electronegative than the PH3 one but considerably less electronegative than that of the Cl atom. Previous studies indicate that the replacement 3009

of PH3 groups by PPh3 do not significantly modify the electronic properties of Au-P bond.41 On the other hand, there is a significant charge transfer from the Au55 core to the Cl atoms that are more electronegative than the PH3 groups and induce a globally positive charge on the Au55 core. As a result, the value of the ionization potential is 5.33 eV (decreased by 1.03 eV compared to the bare cluster). The electron affinity of Au55(PH3)12Cl6 is significantly increased by 2.53 eV, which leads to a higher value of the charging energy ()3.43 eV). This increase reflects the strong effect of the ligand layer on the electronic properties and is in agreement with the ligand-induced insulating behavior reported by Boyen et al. using XPS-DOS spectroscopy.15 The fact that our results on Au55(PH3)Cl6 indicate that the electronegative Cl atoms are responsible for a significant charge transfer to the ligand shell is further confirmed by the analysis of the electronic properties of the Au55(PH3)12 for which the Cl atoms have been removed. As Au55(PH3)12Cl6, Au55(PH3)12 adopts a distorted icosahedral geometry and the phosphine groups are coordinated on the face edge atoms of the cluster with a Au-P bond of about 0.25 nm. The SOMO and LUMOs for the neutral and of the two charged states exhibit a contribution of 6s AO of the face edge Au atoms and 5d AO of the face center ones that are similar to that of the bare form and larger than that of Au55(PH3)12Cl6. As in Au55(PH3)12Cl6, the Au-P bond is due to MOs that are deeper in energy than the frontier orbitals. Though the frontier orbitals look similar to those of the bare cluster, the IP and EA are both decreased, more for the IP (2.33 eV) than for the EA (2.01 eV), resulting in a lower charging energy of 1.69 eV. In addition, the significant lowering of the charging energy when compared to Au55(PH3)12Cl6 shows that the insulating behavior of Au55(PH3)12Cl6 is induced by the Cl atoms and not by the PH3 ligands. This is in agreement with XPS-DOS study of Boyen et al.,15 which shows that the addition of Cl atoms induces a transition to a more insulating behavior of the cluster. Our computed values for the ligated clusters (1.69 for Au55(PH3)12 and 3.43 eV for Au55(PH3)12Cl6) nevertheless remain too high compared to the reported value of 0.3-0.5 eV. In order to account for such a low value, one needs to include the effect of the water medium. We therefore carried out electronic structure computations on the neutral and charged Au55(PH3)12Cl6 clusters with one layer (54 molecules) of external water molecules. The geometry of the central Au13 gold cluster (central atom and first shell of 12 atoms) was frozen during geometry optimization. The geometry relaxation preserves the distorted icosahedral geometries for all three charged states. In the minimized structures, each Cl ligand is H-bonded to four or five water molecules and the hydrogen of each PH3 group is H-bonded to one water molecule. This can be seen in Figure 1d where the Au55 core has been rotated to make the H-bond clearer. This dilution is reflected by longer Au-P (∼0.26 nm in Au55+, ∼0.28 nm in Au550 and ∼0.29 nm in Au55-) and Au-Cl (∼0.25 nm in Au55+, ∼0.28 nm in Au550 and ∼0.29 nm in Au55-) bond distances. In particular, because 3010

of the dilution effect, the Cl are bonded to a single Au atom, instead of three in the unsolvated Au55(PH3)12Cl6 cluster (Figure 1d). Note that the structural changes induced by strong H-bond with water molecules can only be accounted for by an explicit solvation computation. The solvation decreases the HOMO-LUMO gap (by 0.4 eV) and the Cl contributions to the frontier MOs by 22% (15% only in Au55(PH3)12Cl6 54 H2O compared to 37% in Au55(PH3)12Cl6). The change in structure and in the Cl contributions significantly decreases the ionization potential (by ∼4.5 eV) while the electroaffinity is basically left unchanged. This again reflects the higher stability of the anion. The calculated charging energy in the presence of solvation shell is 0.99 eV, which is much closer to the experimental value of 0.3-0.5 eV.34 Computational aspects that are expected to further decrease the value of the charging energy are the use of a basis set larger than the 6-31+G(d) basis used here to better describe the H-bonding and the addition of a second shell of water molecules. Both improvements lead to serious convergence problems for the geometry relaxation and are computationally prohibitive at this stage. In addition, the value of the charging energy will be further decreased by the interaction with neighboring clusters. Our study provides understanding of how ligand and solvation effects govern the metallic or insulating character of a nanosize gold cluster. In Au55(PH3)12Cl6, the strong coordination of the Cl ligands with the outer shell of Au atoms leads to a value of 3.5 eV for the charging energy, that is, an increase of 1.5 eV compared to the bare cluster, making the passivated cluster more insulating than Au55 in the gas phase. On the other hand, the explicit solvation of Au55(PH3)12Cl6 with 54 H2O decreases the value of the charging energy to ∼1 eV, by diluting the ligand layer through H-bonds with the water molecules. Since the dilution effect by H-bonding primarily affects the Cl coordination to the Au55 cluster, we expect that the same effect will be present in the solvated Au55(PPh3)12Cl6. The properties of ligand-protected isolated clusters and the understanding of ligand effects are essential for the control of charge transfer and redox properties in small arrays in view of nanoelectronics applications. Acknowledgment. This work is supported by the EC FP7 MOLOC project and the FRFC 2.4506 project. The work of G.P. is supported by an Inter University Attraction Pole (IAP) project “Cluster and Nanowires” of the Belgian federal government. We thank R. D. Levine (Hebrew University of Jerusalem), H.-G. Boyen (university of Hasselt, Belgium) and E. S. Kryachko (ULg, Belgium and Bogolyubov Institute for Theoretical Physics, Kiev) for several insightful discussions. Supporting Information Available: The Supporting Information contains the computation details, the labeling scheme for the Cl-Au coordination (Figure S1) and the frontier orbital (HOMO and LUMO) isocontour figures for three charged states of bare Au55 (Figure S2) and ligated Au55(PH3)12 (Figure S3) and Au55(PH3)12Cl6 (Figure S4) clusters. Figure S5 shows the spin density of the neutral Nano Lett., Vol. 9, No. 8, 2009

clusters. The atomic contributions to the frontier MOs are given in Table S1. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Schmid, G.; Klein, N.; Korste, L.; Kreibig, U.; Schonauer, D. Polyhedron 1988, 7, 605–608. (2) Marcus, M. A.; Andrews, M. P.; Zegenhagen, J.; Bommannavar, A. S.; Montano, P. Phys. ReV. B 1990, 42, 3312–3316. (3) Fairbanks, M. C.; Benfield, R. E.; Newport, R. J.; Schmid, G. Solid State Commun. 1990, 73, 431–436. (4) Goll, G.; Vonlohneysen, H.; Kreibig, U.; Schmid, G. Z. Phys. D: At. Mol. Clusters 1991, 20, 329–331. (5) Becker, C.; Fries, T.; Wandelt, K.; Kreibig, U.; Schmid, G. J. Vac. Sci. Technol., B 1991, 9, 810–813. (6) Fauth, K.; Kreibig, U.; Schmid, G. Z. Phys. D: At. Mol. Clusters 1991, 20, 297–300. (7) Vogel, W.; Rosner, B.; Tesche, B. J. Phys. Chem. 1993, 97, 11611– 11616. (8) Simon, U.; Schon, G.; Schmid, G. Angew. Chem., Int. Ed. 1993, 32, 250–254. (9) Mulder, F. M.; Vanderzeeuw, E. A.; Thiel, R. C.; Schmid, G. Solid State Commun. 1993, 85, 93–97. (10) Hermann, M.; Kreibig, U.; Schmid, G. Z. Phys. D: At. Mol. Clusters 1993, 26, S1-S3. (11) Haberlen, O. D.; Chung, S. C.; Rosch, N. Ber. Bunsen-Ges. Phys. Chem. 1994, 98, 882–885. (12) Benfield, R. E.; Filipponi, A.; Bowron, D. T.; Newport, R. J.; Gurman, S. J. J. Phys.: Condens. Matter 1994, 6, 8429–8448. (13) Schon, G.; Simon, U. Colloid Polym. Sci. 1995, 273, 101–117. (14) Hakkinen, H.; Barnett, R. N.; Landman, U. Phys. ReV. Lett. 1999, 82, 3264–3267. (15) Boyen, H. G.; Kastle, G.; Weigl, F.; Ziemann, P.; Schmid, G.; Garnier, M. G.; Oelhafen, P. Phys. ReV. Lett. 2001, 87, 276401/1–4. (16) Boyen, H. G.; Kastle, G.; Weigl, F.; Koslowski, B.; Dietrich, C.; Ziemann, P.; Spatz, J. P.; Riethmuller, S.; Hartmann, C.; Moller, M.; Schmid, G.; Garnier, M. G.; Oelhafen, P. Science 2002, 297, 1533– 1536. (17) Balasubramanian, R.; Guo, R.; Mills, A. J.; Murray, R. W. J. Am. Chem. Soc. 2005, 127, 8126–8132. (18) Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Bushnell, D. A.; Kornberg, R. D. Science 2007, 318, 430–433. (19) Lopez-Acevedo, O.; Akola, J.; Whetten, R. L.; Gronbeck, H.; Hakkinen, H. J. Phys. Chem. C 2009, 113, 5035–5038. (20) Murray, R. W. Chem. ReV. 2008, 108, 2688–2720. (21) Gold: Chemistry, Materials and Catalysis; Hutchings, G. J., Brust, M., Schmidbaur, H., Eds.; Chem. Soc. ReV. 2008, 37. (22) Thiel, R. C.; Benfield, R. E.; Zanoni, R.; Smit, H. H. A.; Dirken, M. W. Struct. Bonding (Berlin) 1993, 81, 1–39.

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(23) Garzon, I. L.; PosadaAmarillas, A. Phys. ReV. B 1996, 54, 11796–11802. (24) Hakkinen, H.; Moseler, M.; Kostko, O.; Morgner, N.; Hoffmann, M. A.; von Issendorff, B. Phys. ReV. Lett. 2004, 93. (25) Xiao, L.; Tollberg, B.; Hu, X. K.; Wang, L. C. J. Chem. Phys. 2006, 124. (26) Parker, J. F.; Choi, J. P.; Wang, W.; Murray, R. W. J. Phys. Chem. C 2008, 112, 13976–13981. (27) Li, Y.; Galli, G.; Gygi, F. ACS Nano 2008, 2, 1896–1902. (28) Zhu, M.; Aikens, C. M.; Hollander, F. J.; Schatz, G. C.; Jin, R. J. Am. Chem. Soc. 2008, 130, 5883. (29) Walter, M.; Akola, J.; Lopez-Acevedo, O.; Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Whetten, R. L.; Gronbeck, H.; Hakkinen, H. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 9157–9162. (30) Akola, J.; Walter, M.; Whetten, R. L.; Hakkinen, H.; Gronbeck, H. J. Am. Chem. Soc. 2008, 130, 3756–3757. (31) Pyykko, P. Chem. Soc. ReV. 2008, 37, 1967–1997. (32) Hakkinen, H.; Moseler, M.; Landman, U. Phys. ReV. Lett. 2002, 89. (33) Pyykko, P. Angew. Chem., Int. Ed. 2004, 43, 4412–4456. (34) Schmid, G. Chem. Soc. ReV. 2008, 37, 1909–1930. (35) Schmid, G.; Simon, U. Chem. Commun. 2005, 697–710. (36) Vanderputten, D.; Zanoni, R. Phys. Lett. A 1995, 208, 345–350. (37) Brom, H. B.; Vanstaveren, M. P. J.; Dejongh, L. J. Z. Phys. D: At. Mol. Clusters 1991, 20, 281–287. (38) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, J. T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, rev, C.02; Gaussian, Inc.: Wallingford, CT, 2004. (39) Wierenga, H. A.; Soethout, L.; Gerritsen, J. W.; Vandeleemput, B. E. C.; Vankempen, H.; Schmid, G. AdV. Mater. 1990, 2, 482–484. (40) Brom, H. B.; Baak, J.; Dejongh, I. J.; Mulder, F. M.; Thiel, R. C.; Schmid, G. Z. Phys. D: At. Mol. Clusters 1993, 26, S27-S29. (41) Qiu, Y. Q.; Qin, C. S.; Su, Z. M.; Yang, G. C.; Pan, X. M.; Wang, R. S. Synth. Met. 2005, 152, 273–276.

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