Tetrahedral Au20 Research Articles

PGA: A new particle swarm optimization algorithm based on genetic operators for the global optimization of clusters.

We have developed a global optimization program named PGA based on particle swarm optimization algorithm coupled with genetic operators for the structures of atomic clusters. The effectiveness and efficiency of the PGA program can be demonstrated by efficiently obtaining the tetrahedral Au20 and double-ring tubular B20, and identifying the ground state clusters through the comparison between the simulated and the experimental photoelectron spectra (PESs). Then, the PGA was applied to search for the global minimum structures of (n = 3-30) clusters, new structures have been found for sizes n = 6, 7, 12, 14, and medium-sized 21-30 were first determined. The high consistency between the simulated spectra and the experimental ones once again demonstrates the efficiency of the PGA program. Based on the ground-state structures of these (n = 3-30) clusters, their structural evolution and electronic properties were subsequently explored. The performance on Au20, B20, , and (n = 3-30) clusters indicates the promising potential of the PGA program for exploring the global minima of other clusters. The code is available for free upon request.

Gas-Phase Structures of [Au21(SR)14]- and [Au17(SR)10]- with Eight Electrons: Can They Support an Icosahedral Au13 Core?

A prototypical thiolate (RS)-protected gold cluster [Au25(SR)18]- has high stability due to specific geometric and electronic structures: an icosahedral (Ih) Au13 core with a closed electronic shell containing eight electrons is completely protected by six units of Au2(SR)3. Nevertheless, collisional excitation of [Au25(SR)18]- in a vacuum induces the sequential release of Au4(SR)4 to form [Au21(SR)14]- and [Au17(SR)10]- both containing eight electrons. To answer a naive question of whether these fragments bear an Ih Au13(8e) core, the geometrical structures of [Au21(SC3H7)14]- and [Au17(SC3H7)10]- in the gas phase were examined by the combination of anion photoelectron spectroscopy and density functional theory (DFT) calculation of simplified models of [Au21(SCH3)14]- and [Au17(SCH3)10]-. We concluded that [Au21(SC3H7)14]- retains a slightly distorted Ih Au13(8e) core, while [Au17(SC3H7)10]- has an amorphous Au13 core composed of triangular Au3, tetrahedral Au4, and prolate Au7 units. DFT calculations on putative species [Au19(SCH3)12]- and [Au18(SCH3)11]- suggested that the Ih Au13(8e) core undergoes dramatic structural deformation due to mechanical stress from μ2 ligation of only one RS.

Proposed Structural Model for Chiral Au40(SC2H4Ph)24 Nanoclusters.

The structural configuration of thiolate-protected gold nanoclusters plays a pivotal role in elucidating the correlation between their structure and properties, comprehending their stability, and guiding experimental synthesis. In this study, utilizing the grand unified model and the ring model, we employed an innovative strategy of fusing triangular Au3 and tetrahedral Au4 elementary blocks by sharing a gold atom to design the gold core, predicting the structure of the Au40(SR)24 nanoclusters. Density functional theory calculations indicate that with the protective ligands simplified to methyl groups the energy of the predicted Au40(SR)24 is 0.45 eV lower than that of the experimentally reported Au40(o-MBT)24 nanoclusters, implying its substantial stability. Furthermore, the calculated UV absorption spectrum and circular dichroism spectrum of predicted Au40(SR)24 are consistent with the experimental results of Au40(SC2H4Ph)24 nanoclusters, suggesting that the predicted structure is a likely candidate for the structure of Au40(SC2H4Ph)24 nanoclusters.

Binding Mechanism and Surface-Enhanced Raman Scattering of the Antimicrobial Sulfathiazole on Gold Nanoparticles.

A combined study using the surface-enhanced Raman scattering (SERS) technique and quantum chemical calculations was carried out to elucidate the adsorption behavior of sulfathiazole, an antibiotic drug, on gold nanoparticles. The tetrahedral Au20 cluster was used as a simple model to mimic a nanostructured gold surface. Computations using density functional theory with the PBE functional were performed in both the gas phase and aqueous medium using a continuum model. The drug is found to bind to the Au metals via the nitrogen of the thiazole ring. The interaction is also partially stabilized by the ring-surface π coupling rather than a sideway adsorption as previously proposed. In an aqueous solution, the drug molecule mainly exists as a deprotonated form, which gives rise to a much greater affinity toward Au nanoparticles as compared to the neutral forms. The drug adsorption further induces a significant alteration on the energy gap of the gold cluster Aun, which could result in an electrical noise. Notable SERS signals below 1600 cm-1, which result from a coupling of several vibrations including the ring breathing, C-C stretching, and N-H bending, could be employed for both qualitative and quantitative detection and assessment of sulfathiazole at trace concentrations.

Plasmon induced hot carrier generation in a pyridine@Au20 composite.

Using time-dependent density functional theory calculations, we have investigated the generation of hot carriers (HCs) in a system comprising a pyridine molecule and a tetrahedral Au20 plasmonic cluster. Our findings indicate that the decay of the localized surface plasmon resonance (LSPR) induced in the pyridine@Au20 system by a laser pulse facilitates the direct transfer of hot electrons from the occupied states of the Au20 cluster to the unoccupied molecular orbitals of pyridine. Notably, we have identified that the interparticle gap distance between the Au20 cluster and the pyridine molecule plays a critical role in controlling the generation of HCs. By precisely controlling the interaction between the plasmonic cluster and the molecule, we can effectively manipulate the energy distribution of the generated HCs. These insights have the potential to drive advancements in the development of more efficient systems for plasmonic catalysis.

Structural predictions of three medium-sized thiolate-protected gold nanoclusters Au44(SR)30, Au56(SR)32, and Au60(SR)34.

The knowledge of structural evolution among thiolate-protected gold nanoclusters is not only helpful for understanding their structure-property relationship but also provides scientific evidence to rule-guided structure predictions of gold nanoclusters. In this paper, three new atomic structures of medium-sized thiolate-protected gold nanoclusters, i.e. Au44(SR)30, Au56(SR)32, and Au60(SR)34, are predicted based on the grand unified model and ring model. Two structural evolution rules, i.e., Au44(SR)28 + [Au12(SR)4] → Au56(SR)32 + [Au12(SR)4] → Au68(SR)36 and Au44(SR)30 + [Au8(SR)2] → Au52(SR)32 + [Au8(SR)2] → Au60(SR)34 + [Au8(SR)2] → Au68(SR)36, are explored. The generic growth patterns underlying both sequences of nanoclusters can be viewed as sequential addition of four and three highly stable tetrahedral Au4 units on the cores, respectively. In addition, density functional theory calculations show that these three newly predicted gold nanoclusters have very close formation energies with their adjacent structures, large highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gaps, and all-positive harmonic vibration frequencies, indicating their high stabilities.

The smallest superatom Au4(PPh3)4I2 with two free electrons: synthesis, structure analysis, and electrocatalytic conversion of CO2 to CO.

Atomically precise metal nanoclusters (NCs) have emerged as a new class of ultrasmall nanoparticles with both free valence electrons and precise structures (from the metal core to the organic ligand shell) and provide great opportunities to understand the relationship between their structures and properties, such as electrocatalytic CO2 reduction reaction (eCO2RR) performance, at the atomic level. Herein, we report the synthesis and the overall structure of the phosphine and iodine co-protected Au4(PPh3)4I2 (Au4) NC, which is the smallest multinuclear Au superatom with two free e- reported so far. Single-crystal X-ray diffraction reveals a tetrahedral Au4 core stabilized by four phosphines and two iodides. Interestingly, the Au4 NC exhibits much higher catalytic selectivity for CO (FECO: > 60%) at more positive potentials (from -0.6 to -0.7 V vs. RHE) than Au11(PPh3)7I3 (FECO: < 60%), a larger 8 e- superatom, and Au(i)PPh3Cl complex; whereas the hydrogen evolution reaction (HER) dominates the electrocatalysis when the potential becomes more negative (FEH2 of Au4 = 85.8% at -1.2 V vs. RHE). Structural and electronic analyses reveal that the Au4 tetrahedron becomes unstable at more negative reduction potentials, resulting in decomposition and aggregation, and consequently the decay in catalytic performance of Au based catalysts towards the eCO2RR.

Porous Supramolecular Assembly of Pentiptycene-Containing Gold(I) Complexes: Persistent Excited-State Aurophilicity and Inclusion-Induced Emission Enhancement.

We report herein a porous supramolecular framework formed by a linear mononuclear Au(I) complex (1) via the tongue-and-groove-like joinery between the pentiptycene U-cavities (grooves) and the rod-shaped π-conjugated backbone and alkyl chains (tongues) with the assistance of C-H···π and aurophilic interactions. The framework contains distorted tetrahedral Au4 units, which undergo stepwise and persistent photoinduced Au(I)-Au(I) bond shortening (excited-state aurophilicity), leading to multicolored luminescence photochromism. The one-dimensional pore channels could accommodate different solvates and guests, and the guest inclusion-induced luminescence enhancement (up to 300%) and/or vapochromism are characterized. A correlation between the aurophilic bonding and the luminescence activity is uncovered by TDDFT calculations. Isostructural derivatives 2 and 3 corroborate both the robustness of the porous supramolecular assembly and the mechanisms of the stimulation-induced luminescence properties of 1. This work demonstrates the cooperation of aurophilicity and structural porosity and adaptability in achieving novel supramolecular photochemical properties.

Geometric and electronic structure analyses on three Au42(SR)26 isomers

In this paper, we present the geometric and electronic structure analyses of three Au42(SR)26 isomers based on a combination of grand unified model (GUM), ring model, superatom network model, and density functional theory calculations. Structural decompositions, Au–Au and Au–S distances, Wiberg bond orders, density of states, composition of the frontier orbitals, and origin of the optical absorption spectra are presented and discussed in detail. The structural decomposition of the Au cores of the three isomers into eight tetrahedral Au4 elementary blocks based on GUM shows that the isomeric mechanism of the three isomers can be ascribed to core and complex isomerism. Inspection of Au–Au and Au–S distances reveals the key role of the aurophilic interactions in the stability of the three Au42(SR)26 isomers. Furthermore, by analyzing the composition of the frontier orbitals, the origin of the optical absorption spectra of the three isomers is attributed to the sp → sp transition. Overall, the geometric and electronic structure analyses of the three Au42(SR)26 isomers illustrate the inherent structure–property correlations among these isomers.

Application of grand unified model and ring model in understanding the isomeric structures of Au28(SR)20 nanoclusters

We present the application of grand unified model (GUM) and ring model in understanding the isomeric structures of six Au28(SR)20 nanoclusters. In light of GUM, three Au14 cores could be attributed to the different packing modes of four tetrahedral Au4 units. The binding of the protection ligands with three Au14 cores can be guided by the ring model, in which the protection ligands and some gold core atoms can form the rings with different radii. Therefore the structural isomerism of six isomers can be understood by different types of ring structures packing, fusing, or interlocking together.

Adsorption/Desorption Behaviors and SERS Chemical Enhancement of 6-Mercaptopurine on a Nanostructured Gold Surface: The Au20 Cluster Model.

Computational approaches are employed to elucidate the binding mechanism and the SERS phenomenon of 6-mercaptopurine (6MP) adsorbed on the tetrahedral Au20 cluster as a simple model for a nanostructured gold surface. Computations are carried out in both vacuum and aqueous environments using a continuum model. In the gaseous phase and neutral conditions, interaction of 6MP with the gold cluster is mostly dominated by a covalent Au−S bond and partially stabilized by the Au⋅⋅⋅H−N coupling. However, in acidic solution, the nonconventional Au⋅⋅⋅H−S hydrogen-bond becomes the most favorable binding mode. The 6MP affinity for gold clusters decreases in the order of vacuum > neutral solution > acidic medium. During the adsorption, the energy gap of Au20 substantially declines, leading to an increase in its electrical conductivity, which can be converted to an electrical noise. Moreover, such interaction is likely a reversible process and triggered by either the low pH in sick tissues or the presence of cysteine residues in protein matrices. While N−H bending and stretching vibrations play major roles in the SERS phenomenon of 6MP on gold surfaces in neutral solution, the strongest enhancement in acidic environment is mostly due to an Au⋅⋅⋅H−S coupling, rather than an aromatic ring-gold surface π overlap as previously proposed.

Au7(SR)7] Ring as a New Type of Protection Ligand in a New Atomic Structure of Au15(SR)13 Nanocluster.

We present a [Au7(SR)7] ring as a new type of protection ligand in a new atomic structure of Au15(SR)13 nanocluster for the first time based on the ring model developed to understand how interfacial interaction dictates the structures of protection motifs and gold cores in thiolate-protected gold nanoclusters. This new Au15(SR)13 model shows a tetrahedral Au4 core protected by one [Au7(SR)7] ring and two [Au2(SR)3] "staple" motifs. Density functional theory (DFT) calculations show that the newly predicted Au15(SR)13 (R = CH3/Ph) has a lower energy of 0.24/0.68 eV than previously proposed isomers. By comparing calculated optical absorption spectra (UV), circular dichroism (CD) spectra, and powder X-ray diffraction (XRD) patterns with related experimental spectra, the calculated CD spectra of the newly predicted Au15(SR)13 (R = CH3/Ph) cannot reproduce the experimental results, indicating that the newly predicted Au15(SR)13 is a new structure that needs to be confirmed by experiment. In addition, DFT calculations also show that the newly predicted Au15(SR)13 (R = CH3/Ph) exhibits a large HOMO-LUMO gap, suggesting its high chemical stability. The proposition of the [Au7(SR)7] ring as a protection ligand in the newly predicted Au15(SR)13 not only enriches the types of protection ligands in thiolate-protected gold nanoclusters but also further confirms the effectiveness and rationality of the ring model for understanding the interfacial interaction between the protection motifs and gold cores in thiolate-protected gold nanoclusters.

Unraveling the Atomic Structures of 10-Electron (10e) Thiolate-Protected Gold Nanoclusters: Three Au32(SR)22 Isomers, One Au28(SR)18, and One Au33(SR)23.

The atomic structures of 10-electron (10e) thiolate-protected gold nanoclusters have not received extensive attention both experimentally and theoretically. In this paper, five new atomic structures of 10e thiolate-protected gold nanoclusters, including three Au32(SR)22 isomers, one Au28(SR)18, and one Au33(SR)23, are theoretically predicted. Based on grand unified model (GUM), four Au17 cores with different morphologies can be obtained via three different packing modes of five tetrahedral Au4 units. Then, five complete structures of three Au32(SR)22 isomers, one Au28(SR)18, and one Au33(SR)23 isomers can be formed by adding the thiolate ligands to three Au17 cores based on the interfacial interaction between thiolate ligands and gold core in known gold nanoclusters. Density functional theory calculations show that the relative energies of three newly predicted Au32(SR)22 isomers are quite close to two previously reported isomers. In addition, five new 10e gold nanoclusters have large highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) gaps and all-positive harmonic vibration frequencies, indicating their high stabilities.

Modulation of the Double-Helical Cores: A New Strategy for Structural Predictions of Thiolate-Protected Gold Nanoclusters.

A fundamental understanding of the structural growth of thiolate-protected gold nanoclusters not only benefits experimental synthesis but also will advance the methodology for structural predictions and for rational design of highly stable nanoclusters. Herein, we report numerous new structures (11 total) of thiolate-protected gold nanoclusters predicted from theoretical modulation of the double-helical cores of experimentally determined nanoclusters. Among these newly predicted structures, Au32(SR)22, Au40(SR)26, and Au48(SR)30 are obtained by adding a defective layer containing 4 gold atoms on a structural sequence of experimentally crystallized nanoclusters, namely, Au28(SR)20, Au36(SR)24, and Au44(SR)28. The generic growth pattern underlying this sequence of nanoclusters can be viewed as adding the highly stable tetrahedral Au4 unit on the double-helical cores. Likewise, the other eight newly predicted structures, including two groups of isomeric structures corresponding to the sequence of experimentally determined Au28(SR)20, Au36(SR)24, Au44(SR)28, and Au52(SR)32 nanoclusters, are successfully predicted. Density functional theory calculations show that these 11 newly predicted nanoclusters exhibit large highest occupied molecular orbital-lowest unoccupied molecular orbital gaps and all-positive harmonic vibrational frequencies, suggesting their high chemical stabilities. Additional analyses on the structures and properties suggest that these newly predicted nanoclusters are very likely to be synthesized in the laboratory. Confirmation by experiments would validate the new strategy for structural prediction of thiolate-protected gold nanoclusters by taking advantage of a large structure database of crystallized ligand-protected gold nanoclusters with a variety of gold cores.

Au54(Et3P)18Cl12: a structurally related cluster to Au32(Et3P)12Cl8 gives insight into the formation process.

The reaction of Et3PAuCl with NaBH4 in EtOH leads to the metalloid gold cluster Au32(Et3P)12Cl8 (Au32) or Au54(Et3P)18Cl12 (Au54) depending on the work-up procedure of the reaction mixture. The molecular structure of Au54 is determined by X-ray diffraction and can be described as a fusion of two Au32 clusters showing a similar solubility. The metalloid cluster Au54 can be either described by a shell model or as a combination of tetrahedral Au4X units (X = Cl, Et3P); edge and face sharing, whereas tetrahedral Au4 units are a central motif in gold cluster chemistry. This novel Au54 gold cluster gives another unique insight into the formation or decomposition process of metalloid clusters, indicating that Au32 and Au54 form from a single yet unknown cluster source.

Toward Solution Syntheses of the Tetrahedral Au20 Pyramid and Atomically Precise Gold Nanoclusters with Uncoordinated Sites.

A long-standing objective of cluster science is to discover highly stable clusters and to use them as models for catalysts and building blocks for cluster-assembled materials. The discovery of catalytic properties of gold nanoparticles (AuNPs) has stimulated wide interests in gaseous size-selected gold clusters. Ligand-protected AuNPs have also been extensively investigated to probe their size-dependent catalytic and optical properties. However, the need to remove ligands can introduce uncertainties in both the structures and sizes of ligand-protected AuNPs for catalytic applications. Ideal model catalysts should be atomically precise AuNPs with well-defined structures and uncoordinated surface sites as in situ active centers. The tetrahedral ( Td) Au20 pyramidal cluster, discovered to be highly stable in the gas phase, provided a unique opportunity for such an ideal model system. The Td-Au20 consists of four Au(111) faces with all its atoms on the surface. Bulk synthesis of Td-Au20 with appropriate ligands would allow its catalytic and optical properties to be investigated and harnessed. The different types of its surface atoms would allow site-specific chemistry to be exploited. It was hypothesized that if the four corner atoms of Td-Au20 were coordinated by ligands the cluster would still contain 16 uncoordinated surface sites as potential in situ catalytically active centers. Phosphine ligands were deemed to be suitable for the synthesis of Td-Au20 to maintain the integrity of its pyramidal structure. Triphenyl-phosphine-protected Td-Au20 was first observed in solution, and its stability was confirmed both experimentally and theoretically. To enhance the synthetic yield, bidentate diphosphine ligands [(Ph)2P(CH2) nP(Ph)2 or L n] with different chain lengths were explored. It was hypothesized that diphosphine ligands with the right chain length might preferentially coordinate to the Td-Au20. Promising evidence was initially obtained by the formation of the undecagold by the L3 ligand. When the L8 diphosphine ligand was used, a remarkable Au22 nanocluster with eight uncoordinated Au sites, Au22(L8)6, was synthesized. With a tetraphosphine-ligand (PP3), a new Au20 nanocluster, [Au20(PP3)4]Cl4, was isolated with high yield. The crystal structure of the new Au20 core did not reveal the expected pyramid but rather an intrinsically chiral gold core. The surface of the new chiral-Au20 was fully coordinated, and it was found to be highly stable chemically. The Au22(L8)6 nanocluster represents the first and only gold core with uncoordinated gold atoms, providing potentially eight in situ catalytically active sites. The Au22 nanoclusters dispersed on oxide supports were found to catalyze CO oxidation and activate H2 without ligand removal. With further understanding about the formation mechanisms of gold nanoclusters in solution, it is conceivable that Td-Au20 can be eventually synthesized, allowing its novel catalytic and optical properties to be explored. More excitingly, it is possible that a whole family of new atomically precise gold nanoclusters can be created with different phosphine ligands.

M16Ni24(CO)40]4–: Coinage Metal Tetrahedral Superatoms as Useful Building Blocks Related to Pyramidal Au20 Clusters (M = Cu, Ag, Au). Electronic and Bonding Properties from Relativistic DFT Calculations

Characterization of the tetrahedral Au20 structure in the gas phase remains a major landmark in gold cluster chemistry, where further efforts to stabilize this bare 20-electron superatom in solution to extend and understand its chemistry have failed so far. Here, we account for the structural, electronic, and bonding properties of [M16Ni24(CO)40]4– (M = Cu, Ag, Au) observed in solution for gold and silver. Our results show a direct electronic relationship with Au20, owing that such species share a common tetrahedral [M16]4– central core with a 1S21P61D102S2 jellium configuration. In the case of Au20, the [Au16]4– core is capped by four Au+ ions, whereas in [M16Ni24(CO)40]4– it is capped by four Ni6(CO)10 units. In both cases, the capping entities are a full part of the superatom entity, where it appears that the free (uncapped) [M16]4– species must be capped for further stabilization. It follows that the Ni6(CO)10 units in [M16Ni24(CO)40]4– should not be considered as external ligands as their bonding wit...

Au19M (M=Cr, Mn, and Fe) as magnetic copies of the golden pyramid

An investigation on structure, stability, and magnetic properties of singly doped Au19M (M=Cr, Mn, and Fe) clusters is carried out by means of density functional theory calculations. The studied clusters prefer forming magnetic versions of the unique tetrahedral Au20. Stable sextet Au19Cr is identified as the least reactive species and can be qualified as a magnetic superatom. Analysis on cluster electronic structures shows that the competition between localized and delocalized electronic states governs the stability and magnetic properties of Au19M clusters.

Structural Evolution of Gold Clusters Aun- (n = 21-25) Revisited.

We performed a combined theoretical and experimental photoelectron spectroscopy study of the structural evolution of gold anion clusters Aun- in the size range n = 21-25, a special size range for gold anion clusters where extensive structural changes from the pyramidal structure at Au20- toward the core-shell structure at Au26- were expected to occur. Density functional theory calculations with inclusion of spin-orbit effects were employed to produce the simulated spectra for the selected low-energy isomers obtained from basin-hopping global minimum search. The comparison of these simulated spectra with reasonably well-resolved experimental photoelectron spectra resulted in the identification of the low-lying structures of the gold clusters. The fused-planar and hollow-tubular structures are found dominant in this special size range. The highly stable tetrahedral Au20 unit (viewed as the fragment of face-centered cubic (FCC) bulk gold) was found intact only in the minor isomer at n = 21, whereas hollow-tubular structures were found prevalent in the n = 22-25 range. At n = 25, the dominant structure is a hollow-tubular one with two of gold pyramids fused together, but not a core-shell one as previously believed.

Au20. Effect of a Strong Tetrahedral Field in a Spherical Concentric Bonding Shell Model

The tetrahedral Au20 cluster represents an outstanding landmark in cluster science. Its electronic structure can be described in terms of superatomic orbitals based on a 1s21p62s21d10 electronic configuration. Here we use the concentric bonding shell approach in order to rationalize Au20 in terms of a multilayered architecture accounting for its magic number of 20 valence electrons, which originates from the 2s antibonding combination between two structural layers. As the number of concentric structures increases from [Au4] → [Au4@Au12] → Au20, the superatomic shells are consequently expanded as, 1s1p → 1s1p1d2s2p1f → 1s1p2s1d2p3s1f3p. The role of spin–orbit coupling in affecting the electronic structure is also described. Our results suggest that Au20 can be conveniently viewed as the combination of concentric structures denoted by [{Au4@Au12}Au4] with considerable sharing of the electron density between the different concentric layers. Thus, the presence of the 2s antibonding combination originates from...