Intensely luminescent homoleptic alkynyl decanuclear gold(I) clusters and their cationic octanuclear phosphine derivatives

Treatment of Au(SC(4)H(8))Cl with a stoichiometric amount of hydroxyaliphatic alkyne in the presence of NEt(3) results in high-yield self-assembly of homoleptic clusters (AuC(2)R)(10) (R = 9-fluorenol (1), diphenylmethanol (2), 2,6-dimethyl-4-heptanol (3), 3-methyl-2-butanol (4), 4-methyl-2-pentanol (4), 1-cyclohexanol (6), 2-borneol (7)). The molecular compounds contain an unprecedented catenane metal core with two interlocked 5-membered rings. Reactions of the decanuclear clusters 1-7 with gold-diphosphine complex [Au(2)(1,4-PPh(2)-C(6)H(4)-PPh(2))(2)](2+) lead to octanuclear cationic derivatives [Au(8)(C(2)R)(6)(PPh(2)-C(6)H(4)-PPh(2))(2)](2+) (8-14), which consist of planar tetranuclear units {Au(4)(C(2)R)(4)} coupled with two fragments [AuPPh(2)-C(6)H(4)-PPh(2)(AuC(2)R)](+). The titled complexes were characterized by NMR and ESI-MS spectroscopy, and the structures of 1, 13, and 14 were determined by single-crystal X-ray diffraction analysis. The luminescence behavior of both Au(I)(10) and Au(I)(8) families has been studied, revealing efficient room-temperature phosphorescence in solution and in the solid state, with the maximum quantum yield approaching 100% (2 in solution). DFT computational studies showed that in both Au(I)(10) and Au(I)(8) clusters metal-centered Au → Au charge transfer transitions mixed with some π-alkynyl MLCT character play a dominant role in the observed phosphorescence.     

Ligand exchange reactions on Au(38) and Au(40) clusters: a combined circular dichroism and mass spectrometry study

The thiolate-for-thiolate ligand exchange reaction between the stable Au(38)(2-PET)(24) and Au(40)(2-PET)(24) (2-PET: 2-phenylethanethiol) clusters and enantiopure BINAS (BINAS: 1,1'-binaphthyl-2,2'-dithiol) was investigated by circular dichroism (CD) spectroscopy in the UV/vis and MALDI mass spectrometry (MS). The ligand exchange reaction is incomplete, although a strong optical activity is induced to the resulting clusters. The clusters are found to be relatively stable, in contrast to similar reactions on [Au(25)(2-PET)(18)](-) clusters. Maximum anisotropy factors of 6.6 × 10(-4) are found after 150 h of reaction time. During the reaction, a varying ratio between Au(38) and Au(40) clusters is found, which significantly differs from the starting material. As compared to Au(38), Au(40) is more favorable to incorporate BINAS into its ligand shell. After 150 h of reaction time, an average of 1.5 and 4.5 BINAS ligands is found for Au(38) and Au(40) clusters, respectively. This corresponds to exchange of 3 and 9 monodentate 2-PET ligands. To show that the limited exchange with BINAS is due to the bidentate nature of the ligand, exchange with thiophenol was performed. The monodentate thiophenol exchange was found to be faster, and more ligands were exchanged when compared to BINAS.     

Structural, electronic, optical, and chiroptical properties of small thiolated gold clusters: the case of Au6 and Au8 cores protected with dimer [Au2(SR)3] and trimer [Au3(SR)4)] motifs

We report results of a theoretical study, based on density functional theory (DFT), on the structural, electronic, optical, and chiroptical properties of small thiolated gold clusters, [Au(n)(SR)(m) (n = 12-15, 16-20; m = 9-12, 12-16)]. Some of these clusters correspond to those recently synthesized with the surfactant-free method. To study the cluster physical properties, we consider two cluster families with Au(6) and Au(8) cores, respectively, covered with dimer [Au(2)(SR)(3)] and trimer [Au(3)(SR)(4)] (CH(3) being the R group) motifs or their combinations. Our DFT calculations show, by comparing the relaxed structures of the [Au(6)[Au(2)(SR)(3)](3)](+), [Au(6)[Au(2)(SR)(3)](2)[Au(3)(SR)(4)]](+), [Au(6)[Au(2)(SR)(3)][Au(3)(SR)(4)](2)](+), and [Au(6)[Au(3)(SR)(4)](3)](+) cationic clusters, that there is an increasing distortion in the Au(6) core as each dimer is replaced by a longer trimer motif. For the clusters in the second family, Au(8)[Au(3)(SR)(4)](4), Au(8)[Au(2)(SR)(3)][Au(3)(SR)(4)](3), Au(8)[Au(2)(SR)(3)](2)[Au(3)(SR)(4)](2), Au(8)[Au(2)(SR)(3)](3)[Au(3)(SR)(4)], and Au(8)[Au(2)(SR)(3)](4), a smaller distortion of the Au(8) core is observed as dimer motifs are substituted by trimer ones. An interesting trend emerging from the present calculations shows that as the number of trimer motifs increases in the protecting layer of both Au(6) and Au(8) cores, the average of the interatomic Au(core)-S distances reduces. This shrinkage in the Au(core)-S distances is correlated with an increase of the cluster HOMO-LUMO (H-L) gap. From these results, it is predicted that a larger number of trimer motifs in the cluster protecting layer would induce larger H-L gaps. By analyzing the electronic transitions that characterize the optical absorption and circular dichroism spectra of the clusters under study, it is observed that the molecular orbitals involved are composed of comparable proportions of orbitals corresponding to atoms forming the cluster core and the protecting dimer and trimer motifs.     

A new 19-metal-atom cluster [(Me2PhP)10Au12Ag7(NO3)9] with a nearly staggered-staggered M5 ring configuration

New mixed metal clusters with M19 metal frameworks have been synthesized by NaBH4 reduction of Au(NO3)(PMe2Ph) together with AgNO3 in ethanol. Single crystal X-ray diffraction has revealed Au12Ag7 and Au17Ag2 metal skeletons for these clusters, which are best described in terms of bicapped pentagonal antiprismatic cages with a staggered-staggered M(5) ring configuration. These clusters connect the missing link between M13 icosahedral and M25 biicosahedral clusters providing a view of the cluster growth process. A TEM image of this cluster has been observed, which has clearly demonstrated single-sized nano-particles of less than 1.0 nm.     

CO-induced formation of an interpenetrating bicuboctahedral Au2Pd18 kernel in nanosized Au2Pd28(CO)26(PEt3)10: formal replacement of an interior (μ12-Pd)2 fragment in the corresponding known isostructural homopalladium Pd30(CO)26(PEt3)10 with nonisovalent (μ12-Au)2 and resulting experimental/theoretical implications

Initially isolated from Pd(10)(CO)(12)(PEt(3))(6) (5) and Au(SMe(2))Cl precursors in a two-step carbon monoxide (CO)-involved procedure, the nanosized interpenetrating bicuboctahedral gold (Au)-palladium (Pd) Au(2)Pd(28)(CO)(26)(PEt(3))(10) (1) was then directly obtained in 25-30% yield from the CO-induced reaction of the CO-stable Au-centered cuboctahedral Au(2)Pd(21)(CO)(20)(PEt(3))(10) (3) with the structurally analogous CO-unstable Pd(23)(CO)(20)(PEt(3))(10) (4). Our hypothesis that this latter synthesis is initiated by the reaction of 3 with coordinatively unsaturated homopalladium species resulting from CO-induced fragmentation of 4 was subsequently substantiated by the alternatively designed synthesis of 1 (～25% yield) from the CO-induced reaction of 3 with the structurally dissimilar CO-unstable Pd(38)(CO)(28)(PEt(3))(12) (6). The composition of 1, unambiguously established from a 100 K CCD X-ray diffractometry study, is in accordance with single-crystal X-ray Au-Pd field-emission microanalysis. The pseudo-C(2h) 30-atom Au(2)Pd(28) geometry of 1 may be formally derived via substitution of the interior (μ(12)-Pd)(2) moiety in the interpenetrating bicuboctahedral Pd(20) kernel of the known isostructural Pd(30)(CO)(26)(PEt(3))(10) (2) with the corresponding interior (μ(12)-Au)(2) moiety, in which the otherwise entire metal-core geometry and CO/PR(3)-ligated environment are essentially not altered. Of major significance is that this interior nonisovalent Pd-by-Au replacement in 2 produces CO-stable 1, whereas nanosized CO/PR(3)-ligated homopalladium Pd(n) clusters with n > 10 are generally unstable under CO. Because the two adjacent encapsulated Au atoms of 2.811(1) ? separation are not present on the metal surface, isolation of 1 under CO is ascribed to an electronic property. The virtually ideal geometrical site-occupancy fit between 1 and 2 provides definite crystallographic evidence for extensive delocalization in 1 of the two valence Au 6s electrons over the entire cluster (instead of a "localized" covalent Au-Au electron-pair interaction). Gradient-corrected (pseudo-scalar-relativistic) density functional theory (DFT) calculations were performed on the isostructural Au(2)Pd(28)(CO)(26)(PH(3))(10) (1-H) and Pd(30)(CO)(26)(PH(3))(10) (2-H) model clusters along with hypothetical [Au(2)Pd(28)(1-H)](2+) and [Pd(30)(2-H)](2-) analogues (with phosphine ethyl substituents replaced by hydrogen ones). Natural population analysis of these four model clusters revealed similar highly positively charged metal surfaces of 28 Pd atoms relative to the two negatively charged interior metal atoms, which reflect a partially oxidized metal surface due to dominant CO back-bonding. The surprising observation that each less electronegative interior Pd atom in 2-H is more negatively charged by 0.30e than each interior Au atom in 1-H points to a more cationic Au in 1 than interior Pd in 2; this unexpected (opposite) charge difference is consistent with delocalization of each Au 6s valence electron toward a Au(+) configuration. This premise is in agreement with the calculated Wiberg bond index (WBI) value of 0.055 for the Au-Au bond order in 1-H versus the WBI single-bond value of 1.01 obtained from analogous DFT calculations for the bare, neutral Au(2) dimer, which has a much shorter spectroscopically determined gas-phase distance of 2.472 ? (that corresponds to a "localized" electron-pair interaction). Isolation of 1 under CO is of prime importance in nanoscience/nanotechnology in establishing relative stabilizations toward CO in well-defined CO/PEt(3)-ligated nonisovalent Pd(2)-by-Au(2)-substituted Au(2)Pd(n-2) clusters [namely, n = 30 (1) and 23 (3)]. These important stereochemical implications have a direct relevance to the recent report of the higher tolerance to CO poisoning of highly active Au-Pd nanoparticle catalysts used for the complete conversion of formic acid into high-purity hydrogen (and CO(2)) for chemical hydrogen storage.     

Chemisorption sites of CO on small gold clusters and transitions from chemisorption to physisorption

Gold clusters adsorbed with CO, Au(m)(CO)(n) (-) (m=2-5; n=0-7), were studied by photoelectron spectroscopy (PES). The first few CO adsorptions were observed to induce significant redshifts to the PES spectra relative to pure gold clusters. For each Au cluster, a critical CO number (n(c)) was observed, beyond which the PES spectra of Au(m)(CO)(n) (-) change very little with increasing n. n(c) was shown to correspond exactly to the available low coordination apex sites in each Au cluster. CO first chemisorbs to these sites and additional CO then only physisorbs to the chemisorption-sautrated Au(m)(CO)(n) (-) complexes.     

Synthesis and electrochemical and spectroscopic characterization of biicosahedral Au25 clusters

The synthesis and electrochemical and spectroscopic characterization of biicosahedral Au(25) clusters with a composition of [Au(25)(PPh(3))(10)(thiolate)(5)Cl(2)](2+) are described. The biicosahedral Au(25) clusters protected with various types of thiol ligands including alkanethiols, 2-phenylethanethiol, 11-mercaptoundecanoic acid, and 11-mercapto-1-undecanol were synthesized in high yields using a one-step, one-phase procedure in which Au(PPh(3))Cl is reduced with tert-butylamine-borane in the presence of the thiol ligand in a 3:1 v/v chloroform/ethanol solution. All biicosahedral Au(25) clusters prepared exhibit characteristic optical absorption and photoluminescence properties. The emission energy is found to be substantially smaller than the optical absorption energy gap of 1.82 eV, indicating a subgap energy luminescence. The electrochemical HOMO-LUMO gap (~1.54 eV) of the clusters is also substantially smaller than the optical absorption energy gap but rather similar to the emission energy. These electrochemical and optical properties of the biicosahedral Au(25) clusters are distinctly different from those of the Au(25)(thiolate)(18) clusters.     

Growth format, electronic architecture, magnetic, and optical properties of aromatic cyclo-Cu3Au3 homotops

Bimetallic Cu(3)Au(3) clusters have been investigated using electronic structure calculation techniques (DFT) to understand their electronic, magnetic, and optical properties as well as the geometrical structures. The most stable homotop is the planar cyclo-[Cu(3)(micro-Au)(3)] form consisting of a triangular positively charged Cu(3) structural core with negatively charged Au atoms occupying exposed positions. This structure is characterized by the maximum number of heterobonds and peripheral positions of Au atoms. Possible growth formats of the cyclo-[Cu(3)(micro-Au)(3)] homotops have been explored following both the edge-capping and the stepwise metal atom substitution mechanism. The bonding pattern along with the density of states (DOS) plots of the cyclo-[Cu(3)(micro-Au)(3)] homotop are thoroughly analyzed and compared with those of the pure cyclo-[Cu(3)(micro-Cu)(3)] and cyclo-[Au(3)(micro-Au)(3)] clusters. Particular attention was paid on the stability of these bimetallic clusters in relation with the ring-shaped electron density distribution (aromaticity). It was found that all 3-membered metal rings exhibit significant aromatic character, which was verified by a number of established criteria of aromaticity, such as structural, energetic, magnetic (NICS profiles), and out-of-plane ring deformability criteria. The NICS (1) values correlate well with the out-of-plane ring deformation energy. Finally, a comprehensive analysis of the optical spectra of the CuAu, Cu(2), and Au(2) diatomics and the cyclo-[Cu(3)(micro-Au)(3)], cyclo-[Cu(3)(micro-Cu)(3)], and cyclo-[Au(3)(micro-Au)(3)] clusters placed the electronic assignments of the optical transitions on a firm footing.     

Interlocked catenane-like structure predicted in Au24(SR)20: implication to structural evolution of thiolated gold clusters from homoleptic gold(I) thiolates to core-stacked nanoparticles

Atomic structure of a recently synthesized ligand-covered cluster Au(24)(SR)(20) [J. Phys. Chem. Lett., 2010, 1, 1003] is resolved based on the developed classical force-field based divide-and-protect approach. The computed UV-vis absorption spectrum and powder X-ray diffraction (XRD) curve for the lowest-energy isomer are in good agreement with experimental measurements. Unique catenane-like staple motifs are predicted for the first time in core-stacked thiolate-group (RS-) covered gold nanoparticles (RS-AuNPs), suggesting the onset of structural transformation in RS-AuNPs at relatively low Au/SR ratio. Since the lowest-energy structure of Au(24)(SR)(20) entails interlocked Au(5)(SR)(4) and Au(7)(SR)(6) oligomers, it supports a recently proposed growth model of RS-AuNPs [J. Phys. Chem. Lett., 2011, 2, 990], that is, Au(n)(SR)(n-1) oligomers are formed during the initial growth of RS-AuNPs. By comparing the Au-core structure of Au(24)(SR)(20) with other structurally resolved RS-AuNPs, we conclude that the tetrahedral Au(4) motif is a prevalent structural unit for small-sized RS-AuNPs with relatively low Au/SR ratio. The structural prediction of Au(24)(SR)(20) offers additional insights into the structural evolution of thiolated gold clusters from homoleptic gold(I) thiolate to core-stacked RS-AuNPs. Specifically, with the increase of interfacial bond length of Au(core)-S in RS-AuNPs, increasingly larger "metallic" Au-core is formed, which results in smaller HOMO-LUMO (or optical) gap. Calculations of electronic structures and UV-vis absorption spectra of Au(24)(SR)(20) and larger RS-AuNPs (up to ~2 nm in size) show that the ligand layer can strongly affect optical absorption behavior of RS-AuNPs.     

Selective synthesis of organogold magic clusters Au54(C≡CPh)26

Organogold clusters Au(54)(C(2)Ph)(26) were selectively synthesized by reacting polymer-stabilized Au clusters (1.2 ± 0.2 nm) with excess phenylacetylene in chloroform.     

Au(20) nanocluster protected by hemilabile phosphines

A novel phosphine-protected Au(20) nanocluster was isolated through the reduction of Au(PPhpy(2))Cl by NaBH(4) (PPhpy(2) = bis(2-pyridyl)-phenylphosphine). Its composition was determined to be [Au(20)(PPhpy(2))(10)Cl(4)]Cl(2), and single crystal X-ray structural analysis revealed that the Au(20) core can be viewed as being generated from the fusion of two Au(11) clusters via sharing two vertices. Optical absorption spectroscopy indicated this Au(20) has a large HOMO-LUMO gap (E(g) ≈ 2.24 eV). This is the first example of a ligand-protected gold nanocluster with a core generated from incomplete icosahedral Au(11) building units.     

Incremental binding energies of gold(I) and silver(I) thiolate clusters

Density functional theory is used to find incremental fragmentation energy, overall dissociation energy, and average monomer fragmentation energy of cyclic gold(I) thiolate clusters and anionic chain structures of gold(I) and silver(I) thiolate clusters as a measure of the relative stability of these systems. Two different functionals, BP86 and PBE, and two different basis sets, TZP and QZ4P, are employed. Anionic chains are examined with various residue groups including hydrogen, methyl, and phenyl. Hydrogen and methyl are shown to have approximately the same binding energy, which is higher than phenyl. Gold-thiolate clusters are bound more strongly than corresponding silver clusters. Lastly, binding energies are also calculated for pure Au(25)(SR)(18)(-), Ag(25)(SR)(18)(-), and mixed Au(13)(Ag(2)(SH)(3))(6)(-) and Ag(13)(Au(2)(SH)(3))(6)(-) nanoparticles.     

Structure evolution of gold cluster anions between the planar and cage structures by isoelectronic substitution: Au(n)- (n = 13-15) and MAu(n)- (n = 12-14; M = Ag, Cu)

The structural and electronic effects of isoelectronic substitution by Ag and Cu atoms on gold cluster anions in the size range between 13 and 15 atoms are studied using a combination of photoelectron spectroscopy and first-principles density functional calculations. The most stable structures of the doped clusters are compared with those of the undoped Au clusters in the same size range. The joint experimental and theoretical study reveals a new C(3v) symmetric isomer for Au(13)(-), which is present in the experiment, but has hitherto not been recognized. The global minima of Au(14)(-) and Au(15)(-) are resolved on the basis of comparison between experiment and newly computed photoelectron spectra that include spin-orbit effects. The coexistence of two isomers for Au(15)(-) is firmly established with convincing experimental evidence and theoretical calculations. The overall effect of the isoelectronic substitution is minor on the structures relative to those of the undoped clusters, except that the dopant atoms tend to lower the symmetries of the doped clusters.     

Size-dependent dynamics in excited states of gold clusters: from oscillatory motion to photoinduced melting

Femtosecond time resolved photoelectron spectroscopy in combination with direct ab initio molecular dynamics "on the fly" based on density functional theory has been used to study the relaxation dynamics of optically excited states in small mass selected anionic gold clusters (Au(n) (-); n = 5-8). The nature of the dynamics strongly depends on the cluster size and structure. Oscillatory wavepacket motion (Au(5)(-)), a long lived excited state (Au(6)(-)), as well as photoinduced melting (Au(7)(-),Au(8)(-)) is observed in real time. This illustrates nonscalable properties of excited states in clusters in the size regime, in which each atom counts.     

Synthesis and electrospray mass spectrometry determination of thiolate-protected Au55(SR)31 nanoclusters

Since the pioneering work of Schmid et al. on phosphine-capped Au(55) clusters, the search for thiolated Au(55) has long been of major interest. This work reports the synthesis and electrospray ionization mass spectrometry (ESI-MS) evidence of Au(55)(SCH(2)CH(2)Ph)(31) clusters.     

Unraveling the mechanisms of O2 activation by size-selected gold clusters: transition from superoxo to peroxo chemisorption

The activation of dioxygen is a key step in CO oxidation catalyzed by gold nanoparticles. It is known that small gold cluster anions with even-numbered atoms can molecularly chemisorb O(2) via one-electron transfer from Au(n)(-) to O(2), whereas clusters with odd-numbered atoms are inert toward O(2). Here we report spectroscopic evidence of two modes of O(2) activation by the small even-sized Au(n)(-) clusters: superoxo and peroxo chemisorption. Photoelectron spectroscopy of O(2)Au(8)(-) revealed two distinct isomers, which can be converted from one to the other depending on the reaction time. Ab initio calculations show that there are two close-lying molecular O(2)-chemisorbed isomers for O(2)Au(8)(-): the lower energy isomer involves a peroxo-type binding of O(2) onto Au(8)(-), while the superoxo chemisorption is a slightly higher energy isomer. The computed detachment transitions of the superoxo and peroxo species are in good agreement with the experimental observation. The current work shows that there is a superoxo to peroxo chemisorption transition of O(2) on gold clusters at Au(8)(-): O(2)Au(n)(-) (n = 2, 4, 6) involves superoxo binding and n = 10, 12, 14, 18 involves peroxo binding, whereas the superoxo binding re-emerges at n = 20 due to the high symmetry tetrahedral structure of Au(20), which has a very low electron affinity. Hence, the two-dimensional (2D) Au(8)(-) is the smallest anionic gold nanoparticle that prefers peroxo binding with O(2). At Au(12)(-), although both 2D and 3D isomers coexist in the cluster beam, the 3D isomer prefers the peroxo binding with O(2).     

Aurophilicity in gold(I) thiosemicarbazone clusters

The reaction of the phosphine thiosemicarbazone ligands HLPH and HLPMe with Au(I) ions yields the gold complexes [Au(3)(HLPH)(2)Cl(2)]Cl·2MeOH (1·2MeOH) and [Au(2)(HLPMe)Cl(2)] (2). The structures determined by X Ray diffraction, [Au(3)(HLPH)(2)Cl(2)]Cl·4MeOH (1·4MeOH) and [Au(2)(HLPMe)Cl(2)](2) (2), are the first examples of gold(I) thiosemicarbazone clusters showing aurophilicity. The structure of the trinuclear cation 1 contains the Au(1) atom located in an inversion centre, being connected to another gold(I) atom, Au(2), through a phosphino thiosemicarbazone molecule which acts as a S,P-bridging ligand. Additionally, every gold(I) atom in the trinuclear cation 1 assembles into trinuclear linear cluster units by means of close gold-gold interactions, being connected through the crystal cell in a 2D zigzag mode. The crystal structure of [Au(2)(HLPMe)Cl(2)](2) (2) contains one discrete molecule [(AuCl)(2)(HLPMe)] in the asymmetric unit, which is further assembled into tetranuclear [(AuCl)(2)(HLPMe)](2) units by means of close gold-gold interactions. Both clusters are highly luminescent in solution.     

Facile syntheses of monodisperse ultrasmall Au clusters

During our effort to synthesize the tetrahedral Au20 cluster, we found a facile synthetic route to prepare monodisperse suspensions of ultrasmall Au clusters AuN (N < 12) using diphosphine ligands. In our monophasic and single-pot synthesis, a Au precursor ClAu(I)PPh3 (Ph = phenyl) and a bidentate phosphine ligand P(Ph)2(CH2)(M)P(Ph)2 are dissolved in an organic solvent. Au(I) is reduced slowly by a borane-tert-butylamine complex to form Au clusters coordinated by the diphosphine ligand. The Au clusters are characterized by both high-resolution mass spectrometry and UV-vis absorption spectroscopy. We found that the mean cluster size obtained depends on the chain length M of the ligand. In particular, a single monodispersed Au11 cluster is obtained with the P(Ph)2(CH2)3P(Ph)2 ligand, whereas P(Ph)2(CH2)(M)P(Ph)2 ligands with M = 5 and 6 yield Au10 and Au8 clusters. The simplicity of our synthetic method makes it suitable for large-scale production of nearly monodisperse ultrasmall Au clusters. It is suggested that diphosphines provide a set of flexible ligands to allow size-controlled synthesis of Au nanoparticles.     

How cationic gold clusters respond to a single sulfur atom

Results describing the interaction of a single sulfur atom with cationic gold clusters (Au(n) (+), n=1-8) using density functional theory are described. Stability of these clusters is studied through their binding energies, second order differences in the total energies, fragmentation behavior, and atom attachment energies. The lowest energy structures for these clusters appear to be three dimensional right from n=3. In most cases the sulfur atom in the structure of Au(n)S(+) is observed to displace the gold atom siting at the peripheral site of the Au(n) (+) cluster. The dissociation channels of Au(n)S(+) clusters follow the same trend as Au(n) (+) cluster, based on the even/odd number of gold atoms in the cluster, with the exception of Au(3)S(+). This cluster dissociates into Au and Au(2)S(+), signifying the relative stability of Au(2)S(+) cluster regardless of having an odd number of valence electrons. Clusters with an even number of gold atoms dissociate into Au and Au(n-1)(S)(+) and clusters with an odd number of gold atoms dissociate into Au(2) and Au(n-2)(S)(+) clusters. An empirical relation is found between the conduction molecular orbital and the number of atoms in the Au(n)S(+) cluster.     

Tuning cluster reactivity by charge state and composition: experimental and theoretical investigation of CO binding energies to Ag(n)Au(m)(+/-) (n + m = 3)

Temperature-dependent gas-phase reaction kinetics measurements and equilibrium thermodynamics under multicollision conditions in conjunction with ab initio DFT calculations were employed to determine the binding energies of carbon monoxide to triatomic silver-gold binary cluster cations and anions. The binding energies of the first CO molecule to the trimer clusters increase with increasing gold content and with changing charge from negative to positive. Thus, the reactivity of the binary clusters can be sensitively tuned by varying charge state and composition. Also, multiple CO adsorption on the clusters was investigated. The maximum number of adsorbed CO molecules was found to strongly depend on cluster charge and composition as well. Most interestingly, the cationic carbonyl complex Au(3)(CO)(4)(+) is formed at cryogenic temperature, whereas for the anion, only two CO molecules are adsorbed, leading to Au(3)(CO)(2)(-). All other trimer clusters adsorb three CO molecules in the case of the cations and are completely inert to CO in our experiment in the case of the anions.