Anionic Gold Clusters Research Articles

Adsorption of Multiple NO Molecules on Anionic Gold Clusters: Electron Transfer and Disproportionation Enhanced by Molecular Aggregation.

The reactions of Aun- clusters with multiple nitric oxide (NO) molecules are explored at 150 K by utilizing a mini-flow-tube reactor and a time-of-flight mass spectrometer. Adsorption of multiple NO molecules is observed on most Aun-, while disproportionation reactions only occur on even-sized Aun- with n = 4, 6, 8, 20 and odd-sized ones with n = 5 and 7. Theoretical calculations reveal the geometric structures and electronic states of the products containing bimolecular and trimolecular NO units, where two NO molecules typically form dimers. Different from NO monomers that weakly interact with odd-sized Aun- and form electron-sharing covalent bonds with Au10-(D3h) and Au16-, NO dimers can extract significant charge from parent Aun-. Regarding the three NO molecules, a predilection toward condensation into trimers on even-sized Aun- is observed, while the tendency is more toward an adsorption pattern of a dimer plus a monomer on odd-sized Aun-. The NO trimers register even higher charge gain from Aun- as compared with the NO dimers, which leads to an elevated degree of activation and induces the progression of disproportionation reactions. Therefore, when considering the reaction between NO and Aun-, it appears that NO has a propensity to form dimers or trimers on Aun-. This behavior of aggregate formation substantially enhances the ability of NO to absorb negative charges from Aun- although the occurrence of disproportionate dissociation reactions is initiated only for specific sizes.

Solvent field regulated superhalogen in pure and doped gold cluster anions

Condensed-phase synthesis of atomically precise clusters has become a vital branch of cluster science, where solvents are indispensable in the synthesis process. Herein, by employing the density functional theory (DFT) calculations and molecular dynamics (MD) simulations, we demonstrated that polar solvents not only provide an important environment to stabilize clusters, but they can also dramatically alter the electronic property of cluster anions forming novel superhalogen anions. Such a regulation effect was first verified in small model gas-phase pure and doped gold cluster anions, which was further evidenced in a real experimentally synthesized Au18 nanocluster. Different solvation models reveal that the solvent field, which is a noninvasive methodology different from conventional electron-counting rules, can be considered as a novel external field to remarkably increase the electron-binding capability of cluster anions while maintaining their geometrical and electronic structures. Considering the indispensability and convenient availability of the solvents, present findings may boost the potential applications of superatoms in constructing super oxidizers in the condensed phase.

Adsorption of multiple NO molecules on Au10- and Au9Zn- planar clusters. A comparative DFT study.

The doping of atomic clusters with transition-metal atoms modifies to a lesser or greater extent the catalytic properties of the pure forms. Here we study by means of density functional theory (DFT) the adsorption of up to six NO molecules on Au10- and Au9Zn- clusters, both with well-tested D3h planar geometry, to learn how precise modifications of the atomic and electronic environment, namely one atom and a valence electron, affect the bonding of multiple NO molecules to anionic gold clusters. First, we confirm that these clusters have D3h symmetry as determined by L. S. Wang and coworkers using photoelectron spectroscopy experiments [Kulichenko et al., J. Phys. Chem. A, 2021, 125, 4606]. Second, we verify that Au10(NO)n- with n ≤ 6 does not form adsorbed (NO)2 dimers, as realized by the experiments of Ma and coworkers [Ma et al., Phys. Chem. Chem. Phys., 2020, 22, 25227] using a mini flow-tube reactor at 150 K. Third, we discover that the ground state of the doped Au9Zn(NO)6- compound forms a (NO)2cis-dimer bridging two non-corner Au atoms of the Au9Zn(NO)4- compound. The discussion of adsorption energies, spin multiplicities, bond lengths, charge trends, vibrational strength frequencies of adsorbed NO's, and projected density of states (PDOS), brings additional testable differences between Au10(NO)n- and Au9Zn(NO)n- compounds (n ≤ 6).

Various bond interactions between NO and anionic gold clusters: a theoretical calculation.

We studied the electronic and geometrical structures of AunNO- (n = 1-20) using the B3LYP method with relatively large basis sets to understand the size-dependent reactivities of Aun- with NO in recent experiments. In most cases, the Aun- in a AunNO- maintains its original geometries, and NO bonds with one gold atom in the N-atop pattern. The theoretical adsorption energy of NO in an even-sized AunNO- is generally larger than those in its odd-sized neighbors, which is consistent with the observed even-odd oscillation in experiments. Various bond interactions are identified between Aun- and NO according to analyses on the NO bond lengths, NO stretching frequencies, charge transfer extents, and densities of states of AunNO-. For the odd-sized AunNO- in their doublet states, the bonds between Aun- and NO can be described as weak dative bonds in the small size range, or even weaker electrostatic interactions for the large ones. For the even-sized AunNO- in their triplet states (with the exception of Au10NO- and Au16NO-), the electron transfer from Aun- to NO forms a triplet NO- and a neutral Aun, which bind together mainly through electrostatic interactions. The lowest-lying structures of Au10NO- and Au16NO- are in their singlets, and the bonds between NO and their gold parts can be described as polar covalent bonds. The stabilities of these two exceptional complexes are enhanced by the closed electron shells formed on their gold parts.

Trinuclear anionic gold clusters bridged by dialkylsilylene and dialkylgermylene.

While the chemistry of gold clusters has been developed rapidly, the knowledge of trinuclear gold clusters with low-valent gold remains scarce. Herein, we report the synthesis, structural characterization, and density functional theory (DFT) calculations of trinuclear anionic gold clusters 3a and 3b ligated by dialkylsilylene (1a) and dialkylgermylene (1b), respectively.

Characterization of atmospheric-pressure spark generated atomic silver and gold clusters by time-of-flight mass spectrometry

The potential of using atmospheric-pressure spark ablation for production of atomic clusters has been shown in the recent past. In this study we used this method to produce atomic clusters in Argon and investigated their chemical composition with an atmospheric pressure interface time-of-flight mass spectrometer (APi-TOF-MS). Both anionic and cationic silver and gold clusters were generated and characterised. The carrier gas was used directly from the gas cylinder without the application of any additional gas purification. As a result, the carrier gas stream contained trace impurities of N2, O2, and H2O, as well as radicals such as O, OH, and H and their ionized forms resulting from the plasma created by the sparks. The experimental results show that different reactivities towards oxygen and water traces can be observed depending on the number of metal atoms in the clusters, their polarity, as well as their compositions; e.g., Aun− shows less reactive behaviour towards oxygen and hydrogen compared to Agn−. The results also show that certain cluster types can be generated at large enough quantities by atmospheric-pressure spark ablation, which considering the low installation and operating cost of the technique can pave the way towards its exploitation for numerous technological applications.

Adsorption and reactions of NO molecules on anionic gold clusters in the size range below 1 nm: effects of clusters' global electronic properties.

We systemically studied adsorption and reactions of NO on Aun- (n≤ 80) using a mini flow-tube reactor running at 150 K. For Aun- (n≤ 11), their reactions with NO mainly formed cluster complexes containing various numbers of NO units; for Aun- (n≥ 12), most active sizes eventually formed specific complexes Aun(NO)3-. The relative rates of the reactions with the first NO were measured. Correlations between these relative rates and the adiabatic detachment energies (ADEs) of Aun- revealed the dominant effect of the clusters' spins and a more complicated electron transfer mechanism than that of reactions with O2. Au20- as well as previously reported Au4,6,8- is an exceptional size, which eventually formed the disproportionate product Au20NO2-, and all these four sizes have very low ADEs. The effects of the clusters' global electronic properties on adsorption and reactions of NO on anionic gold are helpful to understand catalytic mechanisms of gold-based catalysts in NO removal reactions.

Binding of multiple SO2 molecules to small gold cluster anions (AuN−, AuNOH−, N = 1‐8)

AbstractThe binding of SO2 on gas‐phase gold cluster anions, AuN−, and their hydroxide counterparts, AuNOH−, have been studied using density functional theory combined with flow reactor/time‐of‐flight mass spectrometry techniques. SO2 is adsorbed on all of the AuN− and AuNOH− clusters (N = 1‐8) and the hydroxide clusters are more active than the bare anionic clusters. Successive additions of SO2 molecules (up to four) have been analyzed. In all cases, anionic clusters are shown to bind multiple SO2 molecules. Theoretical analyses are in agreement with the experimental results, showing that the addition of more than one molecule is thermodynamically favorable. Larger clusters do not necessarily absorb more molecules, as different SO2 binding motifs on these clusters are present. These results provide important insight for the potential use of these anionic clusters as SO2 hunters.

Adsorption and Desorption of NO and NO2 Molecules on Gold Cluster Anions Observed by Thermal Desorption Spectrometry

The adsorption and thermal desorption of NO and NO2 molecules on gold cluster anions, Au4–, were experimentally observed using gas-phase thermal desorption spectrometry. Multiple collisions of NO molecules with Au4– at 300 K generated Au4(NO)k– (k = 1–3). The NO molecules desorbed from the clusters in a stepwise manner when the clusters were heated, and finally Au4– was regenerated above 600 K. For NO2, Au4N2O4– was found to be thermally unstable. Hence, two NO2 molecules were sequentially released from Au4N3O6–, when Au4N3O6– was heated at 500–800 K, forming Au4NO2–. A gold oxide cluster, Au4O2–, was produced as a result of the decomposition of NO2 to NO and O on Au4–, and the NO molecules were released above 900 K.

Abstraction of the I Atom from CH3I by Gas-Phase Au n - (n = 1-4) via Reductive Activation of the C-I Bond.

Gas-phase reactions of atomic gold anions and small gold cluster anions, Aun– (n = 1–4), with CH3I were investigated to clarify the effect of the cluster size on C–I bond activation and to elucidate the key properties of Au clusters that govern the reactivity. AunI– identified by mass spectrometry was observed as a common reaction product. Photoelectron spectroscopy and density functional theory calculations revealed that Au2I– has a linear structure in which the I atom is bonded to Au2, and Au3I– and Au4I– take a two-dimensional structure in which the I atom is bonded to triangular Au3 moieties. Pseudo-first-order kinetic analyses of the reaction revealed the inverse correlation of the reactivity of Aun– toward CH3I and the electron affinity of Aun, indicating the reductive activation of the C–I bond. Especially, Au2– showed the highest reactivity to form Au2I– as the main product, whereas the adduct compound Au2CH3I– was hardly formed, in sharp contrast to the reaction of Au– reported previously. On the basis of theoretical calculations, we propose that the reaction proceeded dominantly via the I atom abstraction pathway (attack of Au2– from the I atom side), which is highly preferential from the viewpoint of both the energetics and a steric factor. This study demonstrates that not only the reactivity but also the reaction mechanisms and products are governed by the cluster size in C–I bond activation by Au clusters at the smallest size region.

Formation of positive and negative clusters of gold atoms inside helium nanodroplets close to zero K

We report the mass spectrometric detection of positive and negative cluster ions of gold atoms formed by the pickup of gold by helium nanodroplets (HNDs) near zero K and subsequent exposure to electron impact. Both singly and doubly charged positive cluster ions were observed. Ionization is initiated by the helium ions He+ and He*−, metastable helium atoms He* as well as electron attachment. We focus on the size distributions of the ions, electron energy dependence of ion yield, patterns of stability and the presence of “magic” clusters. As reported previously by others using a variety of other methodologies, a clear oscillation in the intensity of (Au)n+/- is seen, especially for n <21, with odd-numbered clusters being more intense than even-numbered clusters. The distributions peak early around n = 8. There is a suggestion of magic numbers at n = 35 and 58. Efficiencies of formation of the gold cluster anions were monitored as a function of the energy of the electrons; the measured intensity distributions exhibit a shift to higher energies with increasing cluster size that is attributed to the need to evaporate helium from the clusters for their observation in the nascent state.

Spontaneous Formation of Gold Cluster Anions on ZnO/Cu(111) Bilayer Films

Recently, bilayer ZnO films grown on Cu, Ag, and Au supports have been reported as new materials with potentially interesting properties. In this work we have investigated the structure and stability of small Aun clusters (n = 2, 3, 4) and of a larger gold aggregate, Au20, deposited on ZnO/Cu(111) ultrathin films using a spin-polarized DFT+U/D2′ approach which includes dispersion corrections. The properties of Au clusters on ZnO/Cu(111) are completely different from those of the corresponding gas-phase units, but also from the same Au clusters deposited on an unsupported ZnO bilayer. On ZnO/Cu(111) small Aun clusters (n = 3, 4) are linear and Au20 is two-dimensional (2D), at variance with the free counterparts. The stability of Au clusters on ZnO/Cu(111) is governed by a spontaneous charge transfer from the underlying Cu(111) substrate via electron tunneling through the thin ZnO film, resulting in negatively charged Au clusters. A Bader charge analysis shows that the excess electrons are localized on the ...

Adsorption of O2 on Anionic Gold Clusters in the 0-1 nm Size Range: An Insight into the Electron Transfer Dynamics from Kinetic Measurements.

We systematically studied the adsorption of O2 on Au n- in the size range of 0-1 nm at low temperatures and determined new active sizes with n = 22, 24, 34, and 36. The kinetic measurements more clearly showed the correlation between the reactivity of Au n- with O2 and their electronic properties: the sizes with a closed electron shell are always inert, and the sizes with an unpaired electron can chemically adsorb one O2 molecule if their adiabatic detachment energies (ADEs) are lower than a threshold around 3.5 eV. This ADE threshold dividing the active and inert Au n- is independent of the clusters' sizes, global geometries, and local adsorption sites. According to the widely accepted electron transfer mechanism, this threshold could stand for the case in which the total energy of the Au n- and an O2 roughly equals that of the spin crossover point of the potential surfaces of Au n-O2- and Au n-···O2.

Photoelectron Spectroscopic and Computational Study of Pyridine-Ligated Gold Cluster Anions.

Pyridine-ligated gold cluster anions were studied through a combination of negative ion photoelectron spectroscopy and density functional theory calculations. Small gold cluster anions ligated by pyridine, Aun(py)- were generated with a ligation cell coupled to a laser vaporization source (LVS). We showed that pyridine is weakly bound (physisorbed) to the Au2- moiety of Au2(py)- by interactions between its gold atoms and either the hydrogen atoms or the π-ring of pyridine. We also found that pyridine's lone electron pair strongly binds (chemisorbs) to both Au3(py)- and Au4(py)- through single gold atoms on each of these clusters. Bonding analysis of two isomers of Au4(py)- supported the presence of two different ligand binding motifs, these differing in terms of the gold atom to which pyridine binds.

Isomers of Anionic Gold Oxide Clusters, AunO2–, Investigated by Thermal Desorption Spectrometry

Anionic gold oxide clusters, AunO2– (n = 2–5), were prepared by sequential O atom transfer reactions from N2O to Aun–. Response of the clusters to heat was investigated by gas-phase thermal desorption spectrometry and was compared with the behavior of AunO2– prepared by the reaction of Aun– with O2. No desorption of O2 was observed from Au4O2– prepared with N2O until 1000 K, but O2 desorbed from Au4O2– prepared with O2 at 500 K, indicating the formation of structural isomers. Theoretical calculations show that the most stable form of Au4O2–, which contains dissociated O atoms, is produced by oxidation with N2O and that the third most stable form of Au4O2–, which contains molecular O2, is produced by oxidation with O2. The desorption energy of O2 from Au5O2–, in which O2 is dissociatively chemisorbed on Au5–, was estimated from the temperature dependence of the cluster signal intensities. The experimentally determined desorption energies were consistent with the values obtained by theoretical calculation.

Excited state nonadiabatic dynamics of bare and hydrated anionic gold clusters Au3(-)[H2O]n (n = 0-2).

We present a joint theoretical and experimental study of excited state dynamics in pure and hydrated anionic gold clusters Au3(-)[H2O]n (n = 0-2). We employ mixed quantum-classical dynamics combined with femtosecond time-resolved photoelectron spectroscopy in order to investigate the influence of hydration on excited state lifetimes and photo-dissociation dynamics. A gradual decrease of the excited state lifetime with the number of adsorbed water molecules as well as gold cluster fragmentation quenching by two or more water molecules are observed both in experiment and in simulations. Non-radiative relaxation and dissociation in excited states are found to be responsible for the excited state population depletion. Time constants of these two processes strongly depend on the number of water molecules leading to the possibility to modulate excited state dynamics and fragmentation of the anionic cluster by adsorption of water molecules.

Density-functional investigation of gold cluster anions doped with gallium: Au n Ga– (1 ⩽ n ⩽ 8)

The equilibrium geometric, relative stabilities, electronic and magnetic properties of small AunGa– clusters (1 ⩽ n ⩽ 8), in comparison with pure gold cluster anions are investigated systematically within the framework of the first-principles density functional calculations at the PW91PW91 level. The calculations reveal that the impure gallium atom changes the structure of pure gold clusters. The most stable structures of AunGa– (4 ⩽ n ⩽ 8) prefer three-dimensional structures. The VDE, fragmentation energies and the secondorder difference energies and the HOMO–LUMO gaps for the lowest-energy structures of AunGa– and Au-n+1 (1 ⩽ n ⩽ 8) clusters show an even-odd oscillation along with the cluster size. Meanwhile the above results indicate the doping gallium atom enhance the stability of the clusters. It is must be pointed out that Au2Ga– cluster is the most stable. Finally, we research the natural population analysis (NPA) of Au-n+1 (1 ⩽ n ⩽ 8) clusters. The results indicate that Ga atom attracts electron from the Au atom except for the Au Ga–cluster.

Probing the structures and electronic properties of dual-phosphorus-doped gold cluster anions ([formula omitted], n = 1–8): A density functional theory investigation

The geometries of gold clusters doped with two phosphorus atoms, AunP2- (n=1–8), were investigated using density functional theory (DFT) methods. Various two-dimensional (2D) and three-dimensional (3D) structures of the doped clusters were studied. The results indicate that the structures of dual-phosphorus-doped gold clusters exhibit large differences from those of pure gold clusters with small cluster sizes. In our study, as for Au6P2-, two cis–trans isomers were found. The global minimum of Au8P2- presents a similar configuration to that of Au20-, a pyramid-shaped unit, and the potential novel optical and catalytic properties of this structure warrant further attention. The higher stability of AunP2- clusters relative to Aun+2- (n=1–8) clusters was verified based on various energy parameters, and the results indicate that the phosphorus atom can improve the stabilities of the gold clusters. We then explored the evolutionary path of AunP2- (n=1–8) clusters. We found that AunP2- clusters exhibit the 2D–3D structural transition at n=6, which is much clearer and faster than that of pure gold clusters and single-phosphorus-doped clusters. The electronic properties of AunP2- (n=1–8) were then investigated. The photoelectron spectra provide additional fundamental information on the structures and molecular orbitals shed light on the evolution of AunP2- (n=1–8). Natural bond orbital (NBO) described the charge distribution in stabilizing structures and revealed the strong relativistic effects of the gold atoms.

At What Size Do Neutral Gold Clusters Turn Three-Dimensional?

Recent experiments and calculations have established the transition from two-dimensional (2D) to three-dimensional (3D) structures at a cluster size of 8 and 12 atoms for gold cluster cations and anions. For neutral gold clusters, however, experimental data are scarce, and existing theoretical studies disagree on the 2D–3D crossover point. We present the results of global structure optimizations of neutral gold clusters Aun for n = 9–13 using a genetic algorithm and meta-generalized density functional theory. The relative energies of the lowest-lying isomers are computed using the revTPSS functional and the random phase approximation (RPA). Thermal, scalar relativistic, and spin–orbit effects are included, and basis set extrapolations are performed for the RPA calculations. For the 2D–3D transition of gold cluster cations and anions, this methodology yields near-quantitative agreement with cross section and electron diffraction measurements. For neutral gold clusters, the 2D and 3D structures are predicte...

Ligand Effects on the Structure and the Electronic Optical Properties of Anionic Au25(SR)18 Clusters

This study addresses how ligands module the structure and the electronic optical properties of a large set of the experimentally known anionic thiolate-protected gold clusters, Au25(SR)18[1−]. Starting from the experimental crystal structure, computational density functional theory calculations reveal that low-polarity R groups do not disturb the Au25S18 framework significantly, such that the inversion symmetry(Ci) of the crystalline state is retained. In the case of p-thiolphenolate ligands, p-SPhX, a major distortion of the Au25S18 framework, destroys the inversion symmetry, the distortion increasing in the order given X = H, Cl, NO2 and CO2H. For branched R groups, linking −CH3 or −NH2 groups at the two-position of the phenylethylthiolate ligand, the inversion symmetry is retained and lost, respectively; similarly, the N-acetyl-cysteine ligand also distorts the framework. These results demonstrate a systematic preference of inversion-symmetric versus nonsymmetric framework depending on the ligand-type. The more distorted structures also exhibit significantly reduced HOMO–LUMO gap values and affect the optical absorption spectra accordingly. This study correlates the distortion of the Au25S18 framework with the structure, electronic, and optical properties among the studied clusters.