The "staple" motif: a key to stability of thiolate-protected gold nanoclusters

Recently obtained single-crystal structure of a thiolate-protected gold cluster shows that all thiolate groups form "staple" motifs on the cluster surface. To find out the driving force for such a formation, we use first-principles density functional theory simulations to model formation of "staple" motifs on an Au38 cluster from zero to full coverage. By geometry optimization, molecular dynamics, and simulated annealing, we show that formation of "staples" is strongly preferred on a cluster surface and helps stabilize the cluster by pinning the surface Au atoms and increasing the HOMO-LUMO gap. We devise a method to generate initial structural models for thiolate-protected gold clusters by adding "staples" to the cluster surface. Using this method, we obtain a staple-covered, low-energy structure for Au38(SCH3)24, a much studied cluster whose structure is not yet known. Optical band-edge energy computed from time-dependent DFT for our Au38(SCH3)24 structure shows good agreement with experiment.     

B(14) (2+): A magic number double-ring cluster

B(20) is a "magic number" cluster with double-ring structure. Surprisingly, we also find that B(14) (2+) is a "magic number" cluster with double-ring structure, which has the largest HOMO-LUMO gap (3.31 eV) and the highest aromaticity in double-ring clusters. This double-ring B(14) (2+) cluster is energetically lower than the quasi-planar one by even ～1.2 eV using high level ab initio calculations. B(14) (2+) also has 40 valence electrons as in Al(13) (-) cluster. The reason leading to the unusual properties of B(14) (2+) may be the electronic shell closing as in Al(13) (-) cluster based on the jellium model, besides the double aromaticity in all double-ring clusters.     

Size evolution study of "molecular" and "atom-in-cluster" polarizabilities of medium-size gold clusters

A study on static polarizabilities for a family of gold clusters (Au(n), n = 6, 12, 20, 34, 54) is presented. For each cluster, a density functional theory perturbation theory calculation was performed to compute the cluster polarizability and the polarizability of each atom in the cluster using Bader's "quantum theory of atoms in molecules" formalism. The cluster polarizability tensor, α(cluster), is expressed as a sum of the atom-in-molecule tensors, α(cluster)=∑(Ω)α(Ω). A strong quadratic correlation (R(2) = 0.98) in the isotropic polarizability of atoms in the cluster and their distance to the cluster center of mass was observed. The cluster polarizabilities are in agreement with previous calculations.     

A First-principles Calculation of Structures and Stability of Al13I Cluster

Using first-principles pseudo-potential plane wave method, the energetics, geometrical and electronic structures of three Al13I cluster isomers were calculated. The calculation results of the binding energy indicate Al13I cluster is more stable than Al13 cluster although its electrons are not a magic number as in Al13 cluster, and among Al13I cluster isomers the "Bridge" structure is the most stable, the second is the "Ontop" structure, and the worst is the "Hollow" structure. By analyzing the geometrical structures of Al13I cluster isomers, it is found that after I atom and Al13 cluster combine the geometrical structures of Al13 moieties are changed besides Al13IHollow cluster, in which the Al13 moiety is still a regular icosahedron. For Al13IOntop cluster, the Al13 moiety has a shrinking trend to I, whereas in Al13IBridge cluster it is distorted. Mulliken population analysis shows for the interaction of electrons between Al(I atoms in Al13I cluster not only there exists an ionic bonding but there is a covalent bonding. Part of electrons in the Al13 cluster transfer to I as Al13 cluster and I atom combine. The order of the strength of covalent bonding between Al13 moiety and I in Al13I cluster isomers is Al13IBridge>Al13IHollow>Al13IOntop. Further analysis of electric structures of Al13 and Al13I clusters indicates a higher stability of Al13I cluster than Al13 cluster can be attributed to the s-p hybridization of 3s and 3p electrons of Al in Al13 moiety induced by I doped, which leads to fewer electrons N(EF) at EF in Al13I and a larger energy gap ΔEH-L between HOMO and LUMO levels in Al13I cluster. The distinguish of structural stability of Al13I cluster isomers mainly originates from their different magnitudes in decrease of N(EF) and increase of ΔEH-L relative to Al13 cluster. The fewest N(EF) and the largest ΔEH-L are responsible for the high stability of Al13IBridge cluster .     

Fish-Bite structure by three-dimensional hydrogen-bond acceptor: IR spectroscopy of pyrrole and N-methylpyrrole binary clusters

The N-H[ellipsis (horizontal)]π hydrogen-bonded (H-bonded) structures of pyrrole (Py) and N-methylpyrrole (NMPy) binary clusters have been studied by IR cavity ringdown spectroscopy and density functional theory calculations. The Py(1)-NMPy(1) cluster has an "L-shape" structure, which is formed by an ordinary H-bond between a N-H donor of Py and a π-electron cloud acceptor of NMPy. The Py(2)-NMPy(1) cluster has a "Cyclic" structure, which is also formed by ordinary N-H[ellipsis (horizontal)]π H-bonds as well as the weak C-H[ellipsis (horizontal)]π H-bond between the methyl CH group and the π cloud acceptor of Py. On the other hand, the Py(1)-NMPy(2) cluster shows an extraordinary structure, in which the single donor NH group is surrounded by a three-dimensional H-bond acceptor formed by two aromatic π electron clouds. We call the Py(1)-NMPy(2) cluster as the "Fish-Bite" structure. The Py(1)-NMPy(2) cluster exhibits a redshifted NH stretch by 157 cm(-1) from the Py monomer, which is larger than 94 cm(-1) of the Py(1)-NMPy(1) cluster. However, both Py(1)-NMPy(1) and Py(1)-NMPy(2) clusters have calculated IR intensities of 169 and 163 km∕mol, respectively. This result indicates that not only the N-H[ellipsis (horizontal)]π H-bonds but also the dipole-dipole interaction between Py and NMPy contributes to the Fish-Bite Py(1)-NMPy(2) cluster formation.     

Magnitude and nature of interactions in benzene-X (X=ethylene and acetylene) in the gas phase: significantly different CH/pi interaction of acetylene as compared with those of ethylene and methane

The accurate interaction energies of the CH/pi interaction in the benzene-X clusters (X = ethylene and acetylene) were experimentally and theoretically determined. Two-color multiphoton ionization spectroscopy was applied, and the binding energies in the neutral ground state of the clusters were evaluated from the dissociation threshold measurements of the cluster cations. The experimental binding energies of the clusters (D0) were 1.4+/-0.2 and 2.7+/-0.2 kcal/mol, respectively. Estimated CCSD(T) interaction energies for the clusters at the basis set limit (De) were 2.2 and 2.8 kcal/mol, respectively. Calculated D0 values (1.7 and 2.4 kcal/mol, respectively) are close to the experimental values. Large electron correlation contributions (Ecorr=-3.6 and -2.8 kcal/mol, respectively) show that dispersion is the major source of the attraction in both clusters. The electrostatic interaction in the ethylene cluster is very small (-0.38 kcal/mol), as in the case of the benzene-methane cluster, whereas the electrostatic interaction in the acetylene cluster is large (-1.70 kcal/mol). The shifts of the S1-S0 transition also suggest that the ethylene cluster is a van der Waals-type cluster, but the acetylene cluster is a pi-hydrogen-bonded cluster. The nature of the CH/pi interaction of the "activated" alkyne C-H bond is significantly different from that of the "nonactivated" (or typical) alkane and alkene C-H bonds.     

具有“松散”配体的两个三核钼簇合物的合成和晶体结构｛Mo_3(μ_3-S)(μ-S)_3[μ-S_2P(OC_2H_5)_2][S_2P(OC_2H_5)_2]_3(L)｝(L=C_5H_5N(Ⅰ),C_6H_5CH_2SH(Ⅱ))

报道了两个具有“松散”配体的三核相簇合物,的晶体和分子结构,其中晶体（Ⅰ）属于单斜晶系,空间群晶体（Ⅱ）亦属于单科晶系,Mr空间群结果表明,这两个分别通过松散配体取代法和简单Mo化合物的“自兜”反应法得到的产物均属于具有松散配体的三核相簇合物。簇分子均由Mo3和μ3－S构成[Mo3S],每两个Mo间又被一个μ-S所桥连。其中每个Mo周围进一步被一个μ-dtp中的S或一个松散配体（吮咬（Ⅰ）或卞硫醇分子（Ⅱ））以及端基dtp中的2个S所配位,从而使其形成具有六配位的八面体构型。     

Metalloid group 14 cluster compounds: an introduction and perspectives to this novel group of cluster compounds

Metalloid cluster compounds of group 14 of the general formulae E(n)R(m) with n > m (E = Si, Ge, Sn and Pb, tetrel elements; R = ligand), where "naked" tetrel atoms are present as well as ligand-bound tetrel atoms, represent a novel class of cluster compounds in group 14 chemistry. Since the "naked" tetrel atoms in these clusters exhibit an oxidation state of 0, the average oxidation state of the tetrel atoms in such metalloid group 14 cluster compounds is between 0 and 1. Thus, these cluster compounds may be seen as intermediates on the way to the elemental state. Therefore, interesting properties maybe expected for these compounds which might complement results from nanotechnology. During the last years many different syntheses of such novel cluster compounds have been introduced, leading to several metalloid group 14 cluster compounds which exhibit new and unusual structure and bonding properties. In this tutorial review an account is given of the first steps in this novel field of group 14 chemistry. Special attention is focused on structural features and bonding properties.     

The spectroscopic signature of the "all-surface" to "internally solvated" structural transition in water clusters in the n = 17-21 size regime

The existence of a transitional size regime where preferential stabilization alternates between "all-surface" (all atoms on the surface of a cluster) and "internally solvated" (one water molecule at the center of the cluster, fully solvated) configurations with the addition or the removal of a single water molecule, predicted earlier with the flexible, polarizable (many-body) Thole-type model interaction potential (TTM2-F), has been confirmed from electronic structure calculations for (H2O)n, n = 17-21. The onset of the appearance of the first "interior" configuration in water clusters occurs for n = 17. The observed structural alternation between interior (n = 17, 19, 21) and all-surface (n = 18, 20) global minima in the n = 17-21 cluster regime is accompanied by a corresponding spectroscopic signature, namely, the undulation in the position of the most redshifted OH stretching vibrations according to the trend: interior configurations exhibit more redshifted OH stretching vibrations than all-surface ones. These most redshifted OH stretching vibrations form distinct groups in the intramolecular region of the spectra and correspond to localized vibrations of donor OH stretches that are connected to neighbors via "strong" (water dimer-like) hydrogen bonds and belong to a water molecule with a "free" OH stretch.     

Interrelation between cluster formation time, cluster growth probability, and nucleation rate

Approximate expressions are derived for the mean time tau for formation of a cluster of n molecules in nucleation of single-component phases. The derivation elucidates the interrelation between tau, the cluster growth probability P, and the stationary nucleation rate. The extraction of both tau(n) and P(n) data from individual cluster growth curves obtained in experiments or simulations is discussed. It is shown that the analysis of tau(n) data allows a model-independent determination of the nucleus size, the Zeldovich factor, the stationary nucleation rate, the frequency with which molecules are attached to the nucleus, and the difference between the works to form the nucleus and the smallest "cluster" of one molecule.     

Gold-caged metal clusters with large HOMO-LUMO gap and high electron affinity

We report a series of isoelectronic gold-caged metal clusters, M@Au14 (M = Zr, Hf), and anion clusters, M@Au14- (M = Sc, Y), all having a calculated HOMO-LUMO gap larger than the well-known tetrahedral cluster Au20-the 3D metal cluster with a very large measured HOMO-LUMO gap (1.77 eV). The clusters M@Au14 (M = Sc, Y) also exhibit a calculated electron affinity (EA) and vertical detachment energy (VDE) not only higher than the "superhalogen" icosahedral Al13 cluster but also possibly even higher than a Cl atom which has the highest (measured) elemental EA or VDE (3.61 eV).     

Geometric and electrostatic study of the [4Fe-4S] cluster of adenosine-5'-phosphosulfate reductase from broken symmetry density functional calculations and extended X-ray absorption fine structure spectroscopy

Adenosine-5'-phosphosulfate reductase (APSR) is an iron-sulfur protein that catalyzes the reduction of adenosine-5'-phosphosulfate (APS) to sulfite. APSR coordinates to a [4Fe-4S] cluster via a conserved CC-X(~80)-CXXC motif, and the cluster is essential for catalysis. Despite extensive functional, structural, and spectroscopic studies, the exact role of the iron-sulfur cluster in APS reduction remains unknown. To gain an understanding into the role of the cluster, density functional theory (DFT) analysis and extended X-ray fine structure spectroscopy (EXAFS) have been performed to reveal insights into the coordination, geometry, and electrostatics of the [4Fe-4S] cluster. X-ray absorption near-edge structure (XANES) data confirms that the cluster is in the [4Fe-4S](2+) state in both native and substrate-bound APSR while EXAFS data recorded at ~0.1 ? resolution indicates that there is no significant change in the structure of the [4Fe-4S] cluster between the native and substrate-bound forms of the protein. On the other hand, DFT calculations provide an insight into the subtle differences between the geometry of the cluster in the native and APS-bound forms of APSR. A comparison between models with and without the tandem cysteine pair coordination of the cluster suggests a role for the unique coordination in facilitating a compact geometric structure and "fine-tuning" the electronic structure to prevent reduction of the cluster. Further, calculations using models in which residue Lys144 is mutated to Ala confirm the finding that Lys144 serves as a crucial link in the interactions involving the [4Fe-4S] cluster and APS.     

Light-Force Technique for Measuring Cluster Polarizabilities

We report on a light-force technique to make absolute measurements of cluster polarizabilities in the gas phase. The technique relies on several important experimental components and methods, including a front-ablation cluster source to produce cold clusters, a position-sensitive time-of-flight spectrometer, glass collimating slits, and a reliable technique for timing a particular slice of the cluster beam. The cluster beam is collimated with a pair of slits and the ideal locations of the slits to maximize the observable light force is discussed. To date, the polarizability of C₆₀ at the fundamental wavelength of a Nd:YAG laser (1.064µm) has been measured.     

Aggregation numbers of SDS micelles formed on EHEC. A steady state fluorescence quenching study

The present investigation proves that in the interaction between an uncharged polymer and a negatively charged amphiphilic ion (surfactant) clusters are actually formed and it provides data for the cluster concentration and the cluster size and their variation with composition. The polymer bound cluster size increases after a certain critical surfactant concentration and passes through a maximum. This maximum cluster size decreases with decreasing polymer concentration and attains a limiting value at infinite dilution. For the highest polymer concentration the cluster size is close to the size of normal surfactant micelles. The cluster concentration was determined by a fluorescence quenching technique and the amount surfactant adsorbed to the polymer by dialysis equilibrium measurements. Combining these independent sets of data permits the cluster aggregation number to be unambiguously determined. Solubilization experiments indicate the possibility to regulate the amount solubilized by varying the polymer concentration. The molecular properties of the system are sensitively monitored by the variation in two vibronic peaks in the pyrene fluorescence emission spectrum which defines a hydrophobic index. Very good agreement is found between all three experimental methods. Finally, the model suggested is analyzed in terms of coil size and cluster-cluster distance. Depending upon the degree of adsorption saturation and the density of polymer segments in solution the interaction may switch from being intramolecular to becoming intermolecular.     

Mechanically induced generation of highly reactive excited-state oxygen molecules in cluster scattering

Molecular electronic excitation in (O(2))(n) clusters induced by mechanical collisions via the "chemistry with a hammer" is investigated by a combination of molecular dynamics simulations and quantum chemistry calculations. Complete active space self-consistent field augmented with triple-zeta polarizable basis set quantum chemistry calculations of a compressed (O(2))(2) cluster model in various configurations reveal the emergence of possible pathways for the generation of electronically excited singlet O(2) molecules upon cluster compression and vibrational excitation, due to electronic curve-crossing and spin-orbit coupling. Extrapolation of the model (O(2))(2) results to larger clusters suggests a dramatic increase in the population of electronically excited O(2) products, and may account for the recently observed cluster-catalyzed oxidation of silicon surfaces, via singlet oxygen generation induced by cluster impact, followed by surface reaction of highly reactive singlet O(2) molecules. Extensive molecular dynamics simulations of (O(2))(n) clusters colliding onto a hot surface indeed reveal that cluster compression is sufficient under typical experimental conditions for nonadiabatic transitions to occur. This work highlights the importance of nonadiabatic effects in the "chemistry with a hammer."     

Mixed-Valent Heptairon Complex with a Ground-State Spin Value of S=12/2 Constructed from a Triiron Cluster Ligand

The anionic triiron(III) cluster ligand [Fe(III)(3)(μ(3)-O)(bpca)(2)Cl(4)(EtO)(2)](-) (1; Hbpca=bis(2-pyridylcarbonyl)amine) was prepared as a building block for constructing larger metal assemblies. This "metal cluster complex ligand" was used in the synthesis of the mixed-valent heptairon complex [Fe(II)(1)(2)(EtOH)(2)], which has a ground-state spin value of S=12/2.     

Structural prediction of thiolate-protected Au38: a face-fused bi-icosahedral Au core

The lowest-energy structure of thiolate-group-protected Au38(SR)24 is with ab initio computations. A unique bi-isocahedral Au23 core is predicted for the Au38(SR)24 cluster, consistent with recent experimental and theoretical confirmation of the icosahedral Au13 core for the [Au25(SR)18]- cluster. The computed optical absorption spectrum and X-ray diffraction pattern are in good agreement with experimental measurements. Like the "magic-number" cluster [Au25(SR)18]-, the high stability and selectivity of the magic-number Au38(SR)24 cluster is attributed to high structural compatibility between the bi-isocahedral Au23 core and the 18 exterior staple motifs.     

THF/H~2O二元团簇中“幻数”现象研究

利用分子束、同步辐射光源和飞行时间质谱研究了HF/H~2O二元团簇体系,观察到了一系列组成为(THF)~n·(H~2O)~mH^+(n=2～5,m=0～n-1)离子峰,其中(THF)~n(H~2O)~n~-~2H^+(n=2～5)离子峰强为“幻数”峰。运用从头算(abinitio)分子轨道法,在HF/3-21G基组水平上对(THF)~n(H~2O)~n~-~2H^+(n=2～4)团簇几何构型进行了优化,计算结果表明团簇离子(THF)~n(H~2O)~n~-~2H^+具有较大的稳定性,解释了实验中观察到的“幻数”现象。     

An Unprecedented (1infinity)

A chain of vertex-linked "bare" nido-Ge(9)(2-) clusters (shown in the picture) is featured by the novel polymeric Zintl anion formed from the binary alloy "KGe(4)", ethylenediamine, and [18]crown-6. The polymerization of nido-Ge(9)(4-) clusters is counterintuitive to known two-electron oxidation behavior of nido clusters and the existence of the isolated closo-Ge(9)(2-) ion. The novel semiconducting cluster "wire" establishes direct structural and mechanistic links between molecular Zintl cluster ions with the extended structures of Zintl phases and elemental nanophase materials.     

High-mass cluster ions of ionic liquids in positive-ion and negative-ion DART-MS and their application for wide-range mass calibrations

Eight ionic liquids (ILs) are subjected to both positive-ion and negative-ion direct analyses in real time (DART) Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry (MS). First, their ability to deliver evenly distributed cluster ion series covering a wide m/z range is explored. Then, one of the ILs exhibiting particularly useful cluster ion series in either ion polarity is applied for mass calibration. Using 1-butyl-3-methylimidazolium tricyanomethide delivers positive cluster ions suitable for mass calibration in the m/z 100–4,000 range and covers the m/z 100–2,000 range in negative-ion DART-MS. The corresponding mass reference lists are provided for either polarity. Furthermore, based on 1-butyl-3-methylimidazolium tricyanomethide, a high-mass record of m/z?>?5,000 for positive-ion DART-MS is presented. The mass calibration procedure is finally validated by application to established standard compounds such as polydimethylsiloxanes, perfluorononanoic acid, and Ultramark 1621, a mixture of hexakis (fluoroalkoxy) phosphazenes. Further proof is presented by consistent exact mass differences between adjacent cluster ions.