Structural Evolution of Doped Gold Clusters: MAux− (M = Si, Ge, Sn; x = 5−8)

Mechanisms for Catalytic CO Oxidation on SiAun (n = 1-5) Cluster

Significant progress has been made in understanding the reactivity and catalytic activity of gas-phase and loaded gold clusters for CO oxidation. However, little research has focused on mixed silicon/gold clusters (SiAun) for CO oxidation. In the present work, we performed density function theory (DFT) calculations for a SiAun (n = 1-5) cluster at the CAM-B3LYP/aug-cc-pVDZ-PP level and investigated the effects on the reactivity and catalytic activity of the SiAun cluster for CO oxidation. The calculated results show that the effect is very low for the activation barriers for the formation of OOCO intermediates on SiAu clusters, SiAu3 clusters, and SiAu5 clusters in the catalytic oxidation of CO and the activation energy barriers for the formation of OCO intermediates on OSiAu3, OSiAu4, and OSiAu5. Our calculations show that, compared with the conventional small Au cluster, the incorporation of Si enhances the catalytic performance towards CO oxidation.

Large-Sized Aun- Core-Shell Clusters (n = 61-66): Enduring Structure of the Icosahedral Au13 Core

Large-sized gold Au_n^- anion clusters exhibit structural characteristics drastically different from other coinage metals. Typically, coinage metal nanoclusters exhibit a 13-atom icosahedral core at the cluster size of 55. Gold clusters, contrarily, do not entail this core until the size reaches 60. Here, we investigated the robustness of the icosahedral core within the large-sized Au_n^- anion clusters. We found that the icosahedral core persists over the size of range of n = 61-66. To adapt the exceptional robustness of the icosahedral core, the shells of the clusters tend to undergo notable structural deformations with polygonal defects. As the cluster size increases from 61 to 66, the core starts to become distorted at n = 64 and the space between the core and shell becomes enlarged. To our knowledge, this is the first theoretical study that provides the simulated photoelectron spectra of the two largest sized gold clusters: Au₆₅^- and Au₆₆^-.

Physico-Chemical Insights into Gas-Phase and Oxide-Supported Sub-Nanometre AuCu Clusters

Catalysis by AuCu nanoclusters is a promising scientific field. However, our fundamental understanding of the underlying mechanisms of mixing in AuCu clusters at the sub-nanometre scale and their physico-chemical properties in both the gas-phase and on oxide supports is limited. We have identified the global minima of gas-phase and MgO(100)-supported AuCu clusters with 3–10 atoms using the Mexican Enhanced Genetic Algorithm coupled with density functional theory. Au and Cu adatoms and supported dimers have been also simulated at the same level of theory. The most stable composition, as calculated from mixing and binding energies, is obtained when the Cu proportion is close to 50%. The structures of the most stable free AuCu clusters exhibit Cu-core/Au-shell segregation. On the MgO surface however, there is a preference for Cu atoms to lie at the cluster-substrate interface. Due to the interplay between the number of interfacial Cu atoms and surface-induced cluster rearrangement, on the MgO surface 3D structures become more stable than 2D structures. The O-site of MgO surface is found to be the most favourable adsorption site for both metals. All dimers favour vertical (V) configurations on the surface and their adsorption energies are in the order: AuCu < CuCu < AuAu < AuCu (where the underlined atom is bound to the O-site). For both adatoms and AuCu dimers, adsorption via Cu is more favourable than Au-adsorbed configurations, but, this disagrees with the ordering for the pure dimers due to a combination of electron transfer and the metal-on-top effect. Binding energy (and second difference) and HOMO-LUMO gap calculations show that even-atom (even-electron) clusters are more stable than the neighbouring odd-atom (odd- electron) clusters, which is expected for closed- and open-shell systems. Supporting AuCu clusters on the MgO(100) surface decreases the charge transfer between Au and Cu atoms calculated in free clusters. The results of this study may serve as a foundation for designing better AuCu catalysts.

Structural exploration of AuxM− (M = Si, Ge, Sn; x = 9–12) clusters with a revised genetic algorithm

We used a revised genetic algorithm (GA) to explore the potential energy surface (PES) of Au_xM⁻ (x = 9–12; M = Si, Ge, Sn) clusters. The most interesting finding in the structural study of Au_xSi⁻ (x = 9–12) is the 3D (Au₉Si⁻ and Au₁₀Si⁻) → quasi-planar 2D (Au₁₁Si⁻ and Au₁₂Si⁻) structural evolution of the Si-doped clusters, which reflects the competition of Au–Au interactions (forming a 2D structure) and Au–Si interactions (forming a 3D structure). The Au_xM⁻ (x = 9–12; M = Ge, Sn) clusters have quasi-planar structures, which suggests a lower tendency of sp³ hybridization and a similarity of electronic structure for the Ge or Sn atom. Au₉Si⁻ and Au₁₀Si⁻ have a 3D structure, which can be viewed as being built from Au₈Si⁻ and Au₉Si⁻ with an extra Au atom bonded to a terminal gold atom, respectively. In contrast, the quasi-planar structures of Au_xM⁻ (x = 9–12; M = Ge, Sn) reflect the domination of the Au–Au interactions. Including the spin–orbit (SO) effects is very important to calculate the simulated spectrum (structural fingerprint information) in order to obtain quantitative agreement between theoretical and future experimental PES spectra.

We used a revised genetic algorithm (GA) to explore the potential energy surface (PES) of Au_xM⁻ (x = 9–12; M = Si, Ge, Sn) clusters.

Stabilization of golden cages by encapsulation of a single transition metal atom

Golden cage-doped nanoclusters have attracted great attention in the past decade due to their remarkable electronic, optical and catalytic properties. However, the structures of large golden cage doped with Mo and Tc are still not well known because of the challenges in global structural searches. Here, we report anionic and neutral golden cage doped with a transition metal atom MAu16 (M = Mo and Tc) using Saunders 'Kick' stochastic automation search method associated with density-functional theory (DFT) calculation (SK-DFT). The geometric structures and electronic properties of the doped clusters, MAu16q (M = Mo and Tc; q = 0 and -1), are investigated by means of DFT theoretical calculations. Our calculations confirm that the 4d transition metals Mo and Tc can be stably encapsulated in the Au16- cage, forming three different configurations, i.e. endohedral cages, planar structures and exohedral derivatives. The ground-state structures of endohedral cages C2v Mo@Au16--(a) and C1 Tc@Au16--(b) exhibit a marked stability, as judged by their high binding energy per atom (greater than 2.46 eV), doping energy (0.29 eV) as well as a large HOMO-LUMO gap (greater than 0.40 eV). The predicted photoelectron spectra should aid in future experimental characterization of MAu16- (M = Mo and Tc).

Probing the structural, electronic and magnetic properties of AgnSc (n = 1–16) clusters

The structural, electronic and magnetic properties of Ag_nSc (n = 1–16) clusters have been studied on the basis of density functional theory and the CALYPSO structure prediction method.

Sugar-peptidic bond interactions: spectroscopic characterization of a model system

Sugars are small carbohydrates which play numerous roles in living organisms such as storage of energy or as structural components. Modifications of specific sites within the glycan chain can modulate a carbohydrate's overall biological function as it happens with nucleic acids and proteins. Hence, identifying discrete carbohydrate modifications and understanding their biological effects is essential. A study of such processes requires of a deep knowledge of the interaction mechanism at the molecular level. Here, we use a combination of laser spectroscopy in jets and quantum mechanical calculations to characterize the interaction between phenyl-β-d-glucopyranoside and N-methylacetamide as a model to understand the interaction between a sugar and a peptide bond. The most stable structure of the molecular aggregate shows that the main interaction between the peptide fragment and the sugar proceeds via a C[double bond, length as m-dash]OH-O2 hydrogen bond. A second conformer was also found, in which the peptide establishes a C[double bond, length as m-dash]OH-O6 hydrogen bond with the hydroxymethyl substituent of the sugar unit. All the conformers present an additional interaction point with the aromatic ring. This particular preference of the peptide for the hydroxyl close to the aromatic ring could explain why glycogenin uses tyrosine in order to convert glucose into glycogen by exposing the O4H hydroxyl group for the other glucoses for the polymerization to take place.

Probing the structures of gold-aluminum alloy clusters AuxAly(-): a joint experimental and theoretical study

Besides the size and structure, compositions can also dramatically affect the properties of alloy nanoclusters. Due to the added degrees of freedom, determination of the global minimum structures for multi-component nanoclusters poses even greater challenges, both experimentally and theoretically. Here we report a systematic and joint experimental/theoretical study of a series of gold-aluminum alloy clusters, AuxAly(-) (x + y = 7,8), with various compositions (x = 1-3; y = 4-7). Well-resolved photoelectron spectra have been obtained for these clusters at different photon energies. Basin-hopping global searches, coupled with density functional theory calculations, are used to identify low-lying structures of the bimetallic clusters. By comparing computed electronic densities of states of the low-lying isomers with the experimental photoelectron spectra, the global minima are determined. It is found that for y ≥ 6 there is a strong tendency to form the magic-number square bi-pyramid motif of Al6(-) in the AuxAly(-) clusters, suggesting that the Al-Al interaction dominates the Au-Au interaction in the mixed clusters. A closely related trend is that for x > 1, the gold atoms tend to be separated by Al atoms unless only the magic-number Al6(-) square bi-pyramid motif is present, suggesting that in the small-sized mixed clusters, Al and Au components do not completely mix with one another. Overall, the Al component appears to play a more dominant role due to the high robustness of the magic-number Al6(-) square bi-pyramid motif, whereas the Au component tends to be either "adsorbed" onto the Al6(-) square bi-pyramid motif if y ≥ 6, or stays away from one another if x < y < 6.

Geometric and electronic properties of gold clusters doped with a single oxygen atom

A systematic theoretical study on single oxygen atom doped gold clusters showed that a single oxygen atom can be adsorbed on various sites of gold surfaces, and obtain nearly one electron from gold atoms.

Structural assignment, and electronic and magnetic properties of lanthanide metal doped silicon heptamers Si7M0/− with M = Pr, Gd and Ho

The ground state geometries of neutral and anionic lanthanide-metal-doped silicon clusters Si₇M^0/− with M = Pr, Gd and Ho were determined by quantum chemical (DFT) computations and the previous experimental photoelectron spectra were assigned.

Investigation on the neutral and anionic BxAlyH2 (x + y = 7, 8, 9) clusters using density functional theory combined with photoelectron spectroscopy

The photoelectron experimental spectra measured at 266 nm and simulated spectra of B₂Al₅H₂⁻ and B₂Al₆H₂⁻ clusters.

Dopant induced modulation in the structure and electronic properties of Au10 cluster

DFT calculations at PBE0/SDD ∪ 6-31++G(d,p) level suggest that doped Au₁₀ clusters (with alkali and alkaline earth metals as dopants) are better potential candidates for use in heterogeneous catalysis.

Three-Dimensional Assignment of the Structures of Atomic Clusters: an Example of Au8M (M=Si, Ge, Sn) Anion Clusters

Identification of different isomer structures of atomic and molecular clusters has long been a challenging task in the field of cluster science. Here we present a three-dimensional (3D) assignment method, combining the energy (1D) and simulated (2D) spectra to assure the assignment of the global minimum structure. This method is more accurate and convenient than traditional methods, which only consider the total energy and first vertical detachment energies (VDEs) of anion clusters. There are two prerequisites when the 3D assignment method is ultilized. First, a reliable global minimum search algorithm is necessary to explore enough valleys on the potential energy surface. Second, trustworthy simulated spectra are necessary, that is to say, spectra that are in quantitative agreement. In this paper, we demonstrate the validity of the 3D assignment method using Au₈M⁻ (M = Si, Ge, Sn) systems. Results from this study indicate that the global minimum structures of Au₈Ge⁻ and Au₈Sn⁻ clusters are different from those described in previous studies.

Structure identification of endohedral golden cage nanoclusters

The endohedral structures of MAu₁₆⁻ (M = Y, Zr and Nb) nanoclusters.

Density functional study on the structural, electronic, and magnetic properties of 3d transition-metal-doped Au5 clusters

Density functional calculations have been performed for the structural, electronic, and magnetic properties of Au5M (M = Sc-Zn) clusters. Geometry optimizations indicate that the M atoms in low-energy Au5M isomers prefer to occupy the most highly coordinated position. The ground-state clusters except Au5Sc possess a planar structure. The vibrational spectra of the doped clusters are completely different from that of a pure gold cluster. The relative stability and chemical activity are investigated through the averaged binding energy and energy gap for the most stable Au5M clusters. It is found that the impurity atoms (not including the Zn atom) can enhance the thermal stability of the host cluster. The chemical activity of Au5M clusters is higher than that of the Au6 cluster. The calculated energy gaps are in accord with available approximate experimental data. The vertical ionization potential, the electron affinity, and photoelectron spectrum are computed and simulated theoretically for all of the ground-state clusters. The magnetism analyses show that the magnetic moment of these Au5M clusters varies from 0 to 5 μB by substituting a Au atom in a Au6 cluster with various M atoms and is mainly localized on the M atom for M = Ti-Ni.

A combined stochastic search and density functional theory study on the neutral and charged silicon-based clusters MSi6 (M = La, Ce, Yb and Lu)

Structures and simulated photoelectron spectra of MSi₆⁻ (M = La, Ce, Yb and Lu).

Selected AB4(2-/-) (A = C, Si, Ge; B = Al, Ga, In) ions: a battle between covalency and aromaticity, and prediction of square planar Si in SiIn4(2-/-)

CAl(4)(2-/-) (D(4h), (1)A(1g)) is a cluster ion that has been established to be planar, aromatic, and contain a tetracoordinate planar C atom. Valence isoelectronic substitution of C with Si and Ge in this cluster leads to a radical change of structure toward distorted pentagonal species. We find that this structural change goes together with the cluster acquiring partial covalency of bonding between Si/Ge and Al(4), facilitated by hybridization of the atomic orbitals (AOs). Counter intuitively, for the AAl(4)(2-/-) (A = C, Si, Ge) clusters, hybridization in the dopant atom is strengthened from C, to Si, and to Ge, even though typically AOs are more likely to hybridize if they are closer in energy (i.e. in earlier elements in the Periodic Table). The trend is explained by the better overlap of the hybrids of the heavier dopants with the orbitals of Al(4). From the thus understood trend, it is inferred that covalency in such clusters can be switched off, by varying the relative sizes of the AOs of the main element and the dopant. Using this mechanism, we then successfully killed covalency in Si, and predicted a new aromatic cluster ion containing a tetracoordinate square planar Si, SiIn(4)(2-/-).

Photoelectron spectroscopy of silicon doped gold and silver cluster anions

Photoelectron spectra of low temperature silicon doped gold cluster anions Au(n)Si(-) with n = 2-56 and silver cluster anions Ag(n)Si(-) with n = 5-82 have been measured. Comparing the spectra as well as the general size dependence of the electron detachment energies to the results on undoped clusters shows that the silicon atom changes the apparent free electron count in the clusters. In the case of larger gold clusters (with more than about 30 gold atoms) the silicon atom seems to consistently delocalize all of its four valence electrons, while in the case of the silver clusters a less uniform behavior is observed. Here the silicon atoms act partly as electron donors, partly as electron acceptors, without following an obvious simple principle. Additionally some structural information can be obtained from the measured spectra: while Ag(54)Si(-) seems to adopt an icosahedral structural motif, Au(54)Si(-) seems to take on a low symmetry structure, much like the corresponding pure 55 atom clusters. This indicates that for such larger clusters the incorporation of a single silicon atom does not change the ground state geometry significantly.

A systematic search for the structures, stabilities, and electronic properties of bimetallic Ca₂-doped gold clusters: comparison with pure gold clusters

The local meta-GGA exchange correlation density functional (TPSS) with a relativistic effective core potential was employed to systematically investigate the geometric structures, stabilities, and electronic properties of bimetallic Ca(2)Au( n ) (n = 1-9) and pure gold Au( n ) (n ≤ 11) clusters. The optimized geometries show that the most stable isomers for Ca(2)Au( n ) clusters have 3D structure when n > 2, and that one Au atom capping the Ca(2)Au( n-1) structure for different-sized Ca(2)Au( n ) (n = 1-9) clusters is the dominant growth pattern. The average atomic binding energies and second-order difference in energies show that the Ca(2)Au(4) isomer is the most stable among the Ca(2)Au( n ) clusters. The same pronounced even-odd alternations are found in the HOMO-LUMO gaps, VIPs, and hardnesses. The polarizabilities of the Ca(2)Au( n ) clusters show an obvious local minimum at n = 4. Moreover, the inverse corrections to the polarizabilities versus the ionization potential and hardness were found for the gold clusters.

Gold behaves as hydrogen in the intermolecular self-interaction of metal aurides MAu4 (M = Ti, Zr, and Hf)

We performed density functional calculations to examine the intermolecular self-interaction of metal tetraauride MAu(4) (M = Ti, Zr, and Hf) clusters. We found that the metal auride clusters have strong dimeric interactions (2.8-3.1 eV) and are similar to the metal hydride analogues with respect to structure and bonding nature. Similarly to (MH(4))(2), the (μ-Au)(3) C(s) structures with three three-center two-electron (3c-2e) bonds were found to be the most stable. Natural orbital analysis showed that greater than 96 % of the Au 6s orbital contributes to the 3c-2e bonds, and this predominant s orbital is responsible for the similarity between metal aurides and metal hydrides (>99 % H 1s). The favorable orbital interaction between occupied Au 6s and unoccupied metal d orbitals leads to a stronger dimeric interaction for MAu(4)-MAu(4) than the interaction for MH(4)-MH(4). There is a strong relationship between the dimeric interaction energy and the chemical hardness of its monomer for (MAu(4))(2) and (MH(4))(2).