The Missing Link: Au191(SPh-tBu)66 Janus Nanoparticle with Molecular and Bulk-Metal-like Properties.

Understanding the evolution of the structure and properties in metals from molecule-like to bulk-like has been a long sought fundamental question in science, since Faraday's 1857 work. We report the discovery of a Janus nanomolecule, Au191(SPh-tBu)66 having both molecular and metallic characteristics, explored crystallographically and optically and modeled theoretically. Au191 has an anisotropic, singly twinned structure with an Au155 core protected by a ligand shell made of 24 monomeric [-S-Au-S-] and 6 dimeric [-S-Au-S-Au-S-] staples. The Au155 core is composed of an 89-atom inner core and 66 surface atoms, arranged as [Au3@Au23@Au63]@Au66 concentric shells of atoms. The inner core has a monotwinned/stacking-faulted face-centered-cubic (fcc) structure. Structural evolution in metal nanoparticles has been known to progress from multiply twinned, icosahedral, structures in smaller molecular sizes to untwinned bulk-like fcc monocrystalline nanostructures in larger nanoparticles. The monotwinned inner core structure of the ligand capped Au191 nanomolecule provides the critical missing link, and bridges the size-evolution gap between the molecular multiple-twinning regime and the bulk-metal-like particles with untwinned fcc structure. The Janus nature of the nanoparticle is demonstrated by its optical and electronic properties, with metal-like electron-phonon relaxation and molecule-like long-lived excited states. First-principles theoretical explorations of the electronic structure uncovered electronic stabilization through the opening of a shell-closing gap at the top of the occupied manifold of the delocalized electronic superatom spectrum of the inner core. The electronic stabilization together with the inner core geometric stability and the optimally stapled ligand-capping anchor and secure the stability of the entire nanomolecule.

Chiral-Icosahedral ( I) Symmetry in Ubiquitous Metallic Cluster Compounds (145A,60X): Structure and Bonding Principles.

There exists a special kind of perfection-in symmetry, simplicity, and stability-attainable for structures generated from precisely 60 ligands (all of a single type) that protect 145 metal-atom sites. The symmetry in question is icosahedral ( Ih), generally, and chiral icosahedral ( I) in particular. A 60-fold equivalence of the ligands is the smallest number to allow this kind of perfection. Known cluster compounds that approximate this structural ideal include palladium-carbonyls, Ih-Pd145(CO)60; gold-thiolates, I-Au144(SR)60; and gold-alkynyls, I-Au144(C2R)60. Many other variants are suspected. The Pd145 compound established the basic achiral structure-type. However, the Au144-thiolate archetype is prominent, historically in its abundance and ease of preparation and handling, in its proliferation in many laboratories and application areas, and ultimately in the intrinsic chirality of its geometrical structure and organization of its bonding network or connectivity. As discovered by mass spectrometry (the "30-k anomaly") in 1995, it appeared as a broad single peak, as solitary and symmetrical as Mount Fuji, centered near 30 kDa (∼150 Au atoms), provoking these thoughts: Surely this phenomenon requires a unique explanation. It appears to be the Buckminsterfullerene (carbon-60) of gold-cluster chemistry. Herein we provide an elementary account of the unexpected discovery, in which the Pd145-structure played a critical role, that led to the identification and prediction, in 2008, of a fascinating new molecular structure-type, evidently the first one of chiral icosahedral symmetry. Rigorous confirmation of this prediction occurred in early spring 2018, when two single-crystal X-ray crystallography reports were submitted, each one distinguishing both enantiomeric structures and noting profound chirality for the surface (ligand) layer. The emphasis here is on the structure and bonding principles and how these have been elucidated. Our aim has been to present this story in simplest terms, consistent with the radical simplicity of the structure itself. Because it combines intrinsic profound chirality, at several levels, with the highest possible symmetry-type (icosahedral), the structure may attract broader interest also from educators, especially if studied in tandem with the analysis of hollow (shell) metallic systems that exhibit the same chirality and symmetry. Because the shortest (stiffest) bonds follow the chiral 3-way weave pattern of the traditional South-Asian reed football, this cultural artifact may be used to introduce chiral-icosahedral symmetry in a pleasant and memorable way. One may also appreciate easily the bonding and excitations in I-symmetry metallic nanostructures via the golden fullerenes, that is, the proposed hollow Au60,72 spheres. Beyond any aesthetic or pedagogical value, we aim that our Account may provide a firm foundation upon which others may address open questions and the opportunities they present. This Account can scarcely hint at the prospects for further fundamental understanding of these compounds, as well as a widening sphere of applications (chemical, electronic, imaging). The compounds remain crucial to a wider field presently under intense development.

Toward Smaller Aqueous-Phase Plasmonic Gold Nanoparticles: High-Stability Thiolate-Protected ∼4.5 nm Cores.

Most applications of aqueous plasmonic gold nanoparticles benefit from control of the core size and shape, control of the nature of the ligand shell, and a simple and widely applicable preparation method. Surface functionalization of such nanoparticles is readily achievable but is restricted to water-soluble ligands. Here we have obtained highly monodisperse and stable smaller aqueous gold nanoparticles (core diameter ∼4.5 nm), prepared from citrate-tannate precursors via ligand exchange with each of three distinct thiolates: 11-mercaptoundecanoic acid, α-R-lipoic acid, and para-mercaptobenzoic acid. These are characterized by UV-vis spectroscopy for plasmonic properties; Fourier transform infrared (FTIR) spectroscopy for ligand-exchange confirmation; X-ray diffractometry for structural analysis; and high-resolution transmission electron microscopy for structure and size determination. Chemical reduction induces a blueshift, maximally +0.02 eV, in the localized surface plasmon resonance band; this is interpreted as an electronic (-) charging of the monolayer-protected cluster (MPC) gold core, corresponding to a -0.5 V change in electrochemical potential.

Ultrastable silver nanoparticles.

Noble-metal nanoparticles have had a substantial impact across a diverse range of fields, including catalysis, sensing, photochemistry, optoelectronics, energy conversion and medicine. Although silver has very desirable physical properties, good relative abundance and low cost, gold nanoparticles have been widely favoured owing to their proved stability and ease of use. Unlike gold, silver is notorious for its susceptibility to oxidation (tarnishing), which has limited the development of important silver-based nanomaterials. Despite two decades of synthetic efforts, silver nanoparticles that are inert or have long-term stability remain unrealized. Here we report a simple synthetic protocol for producing ultrastable silver nanoparticles, yielding a single-sized molecular product in very large quantities with quantitative yield and without the need for size sorting. The stability, purity and yield are substantially better than those for other metal nanoparticles, including gold, owing to an effective stabilization mechanism. The particular size and stoichiometry of the product were found to be insensitive to variations in synthesis parameters. The chemical stability and structural, electronic and optical properties can be understood using first-principles electronic structure theory based on an experimental single-crystal X-ray structure. Although several structures have been determined for protected gold nanoclusters, none has been reported so far for silver nanoparticles. The total structure of a thiolate-protected silver nanocluster reported here uncovers the unique structure of the silver thiolate protecting layer, consisting of Ag2S5 capping structures. The outstanding stability of the nanoparticle is attributed to a closed-shell 18-electron configuration with a large energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital, an ultrastable 32-silver-atom excavated-dodecahedral core consisting of a hollow 12-silver-atom icosahedron encapsulated by a 20-silver-atom dodecahedron, and the choice of protective coordinating ligands. The straightforward synthesis of large quantities of pure molecular product promises to make this class of materials widely available for further research and technology development.

Liquid Chromatography Separation and Mass Spectrometry Detection of Silver-Lipoate Ag29(LA)12 Nanoclusters: Evidence of Isomerism in the Solution Phase.

Evidence for the existence of condensed-phase isomers of silver-lipoate clusters, Ag29(LA)12, where LA = (R)-α lipoic acid, was obtained by reversed-phase ion-pair liquid chromatography with in-line UV-vis and electrospray ionization (ESI)-MS detection. All components of a raw mixture were separated according to surface chemistry and increasing size via reversed-phase gradient HPLC methods and identified by their corresponding m/z ratio by ESI in the negative ionization mode. Aqueous and methanol mobile-phase mixtures, each containing 400 mM hexafluoroisopropanol (HFIP)-15 mM triethylamine (TEA), were employed to facilitate the interaction between the clusters and stationary phase via formation of ion-pairs. TEA-HFIP (triethylammonium-hexafluoroisopropoxide) had been shown to provide superior chromatographic peak shape and mass spectral signal compared with alternative modifiers such as TEAA (triethylammonium-acetate) for analysis of oligonucleotide samples. Liquid chromatographic separation prior to mass spectrometry detection facilitated sample analysis by production of simplified mass spectra for each eluting cluster species and provided insight into the existence of at least two major solution-phase isomers of Ag29(LA)12. UV-vis detection in-line with ESI analysis provided independent confirmation of the existence of the isomers and their similar electronic structure as judged from their identical optical spectra in the 300-500 nm range. [Diastereomerism provides a possible interpretation for the near-equal abundance of the two forms, based on a structurally defined nonaqueous homologue.].

Capillary Liquid Chromatography Mass Spectrometry Analysis of Intact Monolayer-Protected Gold Clusters in Complex Mixtures.

In some respects, large noble-metal clusters protected by thiolate ligands behave as giant molecules of definite composition and structure; however, their rigorous analysis continues to be quite challenging. Analysis of complex mixtures of intact monolayer-protected clusters (MPCs) by liquid chromatography mass spectrometry (LC-MS) could provide quantitative identification of the various components present. This advance is critical for biomedical and toxicological research, as well as in fundamental studies that rely on the identification of selected compositions. This work expands upon the separate LC and MS results previously achieved, by interfacing the capillary liquid chromatograph directly to the electrospray source of the mass spectrometer, in order to provide an extremely sensitive, quantitative, and rapid means to characterize MPCs and their derivatives far beyond that of earlier reports. Here, we show that nonaqueous reversed-phase chromatography can be coupled to mass-spectrometry detection to resolve complex mixtures in minute (∼100 ng) samples of gold MPCs, of molecular masses up to ∼40 kDa, and with single-species sensitivity easily demonstrated for components on the level of sub-10 ng or picomole (1 pmol).

Ligand-modulated interactions between charged monolayer-protected Au144(SR)60 gold nanoparticles in physiological saline.

In order to determine how functionalized gold nanoparticles (AuNPs) interact in a near-physiological environment, we performed all-atom molecular dynamics simulations on the icosahedral Au144 nanoparticles each coated with a homogeneous set of 60 thiolates selected from one of these five (5) types: 11-mercapto-1-undecanesulfonate -SC11H22(SO3(-)), 5-mercapto-1-pentanesulfonate -SC5H10(SO3(-)), 5-mercapto-1-pentaneamine -SC5H10(NH3(+)), 4-mercapto-benzoate -SPh(COO(-)), or 4-mercapto-benzamide -SPh(CONH3(+)). These thiolates were selected to elucidate how the aggregation behavior of AuNPs depends on ligand parameters, including the charge of the terminal group (anionic vs. cationic), and its length and conformational flexibility. For this purpose, each functionalized AuNP was paired with a copy of itself, placed in an aqueous cell, neutralized by 120 Na(+)/Cl(-) counter-ions and salinated with a 150 mM concentration of NaCl, to form five (5) systems of like-charged AuNPs pairs in a saline. We computed the potential of mean force (the reversible work of separation) as a function of the intra-pair distance and, based on which, the aggregation affinities. We found that the AuNPs coated with negatively charged, short ligands have very high affinities. Structurally, a significant number of Na(+) counter-ions reside on a plane between the AuNPs, mediating the interaction. Each such ion forms a "salt bridge" (or "ionic bonds") to both of the AuNPs when they are separated by its diameter plus 0.2-0.3 nm. The positively charged AuNPs have much weaker affinities, as Cl(-) counter-ions form fewer and weaker salt bridges between the AuNPs. In the case of Au144(SC11H22(SO3(-)))60 pair, the flexible ligands fluctuate much more than the other four cases. The large fluctuations disfavor the forming of salt bridges between two AuNPs, but enable hydrophobic contact between the exposed hydrocarbon chains of the two AuNPs, which are subject to an effective attraction at a separation much greater than the AuNP diameter and involve a higher concentration of counter ions in the inter-pair space.

Gold-copper nano-alloy, "Tumbaga", in the era of nano: phase diagram and segregation.

Gold-copper (Au-Cu) phases were employed already by pre-Columbian civilizations, essentially in decorative arts, whereas nowadays, they emerge in nanotechnology as an important catalyst. The knowledge of the phase diagram is critical to understanding the performance of a material. However, experimental determination of nanophase diagrams is rare because calorimetry remains quite challenging at the nanoscale; theoretical investigations, therefore, are welcomed. Using nanothermodynamics, this paper presents the phase diagrams of various polyhedral nanoparticles (tetrahedron, cube, octahedron, decahedron, dodecahedron, rhombic dodecahedron, truncated octahedron, cuboctahedron, and icosahedron) at sizes 4 and 10 nm. One finds, for all the shapes investigated, that the congruent melting point of these nanoparticles is shifted with respect to both size and composition (copper enrichment). Segregation reveals a gold enrichment at the surface, leading to a kind of core-shell structure, reminiscent of the historical artifacts. Finally, the most stable structures were determined to be the dodecahedron, truncated octahedron, and icosahedron with a Cu-rich core/Au-rich surface. The results of the thermodynamic approach are compared and supported by molecular-dynamics simulations and by electron-microscopy (EDX) observations.

Characterization of size, anisotropy, and density heterogeneity of nanoparticles by sedimentation velocity.

A critical problem in materials science is the accurate characterization of the size dependent properties of colloidal inorganic nanocrystals. Due to the intrinsic polydispersity present during synthesis, dispersions of such materials exhibit simultaneous heterogeneity in density ρ, molar mass M, and particle diameter d. The density increments ∂ρ/∂d and ∂ρ/∂M of these nanoparticles, if known, can then provide important information about crystal growth and particle size distributions. For most classes of nanocrystals, a mixture of surfactants is added during synthesis to control their shape, size, and optical properties. However, it remains a challenge to accurately determine the amount of passivating ligand bound to the particle surface post synthesis. The presence of the ligand shell hampers an accurate determination of the nanocrystal diameter. Using CdSe and PbS semiconductor nanocrystals, and the ultrastable silver nanoparticle (M4Ag44(p-MBA)30), as model systems, we describe a Custom Grid method implemented in UltraScan-III for the characterization of nanoparticles and macromolecules using sedimentation velocity analytical ultracentrifugation. We show that multiple parametrizations are possible, and that the Custom Grid method can be generalized to provide high resolution composition information for mixtures of solutes that are heterogeneous in two out of three parameters. For such cases, our method can simultaneously resolve arbitrary two-dimensional distributions of hydrodynamic parameters when a third property can be held constant. For example, this method extracts partial specific volume and molar mass from sedimentation velocity data for cases where the anisotropy can be held constant, or provides anisotropy and partial specific volume if the molar mass is known.

Optical response of quantum-sized Ag and Au clusters--cage vs. compact structures and the remarkable insensitivity to compression.

Quantum-sized silver and gold clusters show very different spectral characteristics. While silver exhibits a strong localized surface-plasmon resonance (LSPR) band down to very small sizes, the resonance is broadened beyond recognition in Au clusters below about 2 nm. In the present work, we study icosahedral hollow-shell structures, or cages, of about 1.8 nm diameter in comparison with compact clusters and show that the qualitative difference between Ag and Au remains but is reduced, as a significant increase of absorption is found for the Au cage structures. The silver shell Ag92 exhibits a resonance that is red-shifted compared to the compact Ag147, coinciding with the general result found in much larger shells that are amenable to the classical description by Mie theory. However, the electronic structure in particular of the d band is strongly changed. The spectrum of the empty Ag shell is remarkably similar to the spectrum of the respective Au55Ag92 core-shell structure. Inspection of the time-dependent electronic density does not explain this similarity. However, it shows that the overall classical picture of a collective charge oscillation remains valid, although clearly with modifications. We further show a remarkable insensitivity of the absorption spectra of both Ag and Au clusters to even rather extreme values of compression or dilatation.

Interaction between functionalized gold nanoparticles in physiological saline.

The interactions between functionalized noble-metal particles in an aqueous solution are central to applications relying on controlled equilibrium association. Herein, we obtain the potentials of mean force (PMF) for pair-interactions between functionalized gold nanoparticles (AuNPs) in physiological saline. These results are based upon >1000 ns experiments in silico of all-atom model systems under equilibrium and non-equilibrium conditions. Four types of functionalization are built by coating each globular Au144 cluster with 60 thiolate groups: GS-AuNP (glutathionate), PhS-AuNP (thiophenol), CyS-AuNP (cysteinyl), and p-APhS-AuNP (para-amino-thiophenol), which are, respectively, negatively charged, hydrophobic (neutral-nonpolar), hydrophilic (neutral-polar), and positively charged at neutral pH. The results confirm the behavior expected of neutral (hydrophilic or hydrophobic) particles in a dilute aqueous environment, however the PMF curves demonstrate that the charged AuNPs interact with one another in a unique way-mediated by H2O molecules and an electrolyte (Na(+), Cl(-))-in a physiological environment. In the case of two GS-AuNPs, the excess, neutralizing Na(+) ions form a mobile (or 'dynamic') cloud of enhanced concentration between the like-charged GS-AuNPs, inducing a moderate attraction (∼25 kT) between them. Furthermore, to a lesser degree, for a pair of p-APhS-AuNPs, the excess, neutralizing Cl(-) ions (less mobile than Na(+)) also form a cloud of higher concentration between the two like-charged p-APhS-AuNPs, inducing weaker yet significant attractions (∼12 kT). On combining one GS- with one p-APhS-AuNP, the direct, attractive Coulombic force is completely screened out while the solvation effects give rise to moderate repulsion between the two unlike-charged AuNPs.

Structural order in ultrathin films of the monolayer protected clusters based upon 4 nm gold nanocrystals: an experimental and theoretical study.

The structural order in ultrathin films of monolayer protected clusters (MPCs) is important in a number of application areas but can be difficult to demonstrate by conventional methods, particularly when the metallic core dimension, d, is in the intermediate size-range, 1.5 < d < 5.0 nm. Here, improved techniques for the synthesis of monodisperse thiolate-protected gold nanoparticles have made possible the production of dodecane-thiolate saturated ∼4 ± 0.5 nm Au clusters with single-crystal core structure and morphology. An ultrathin ordered film or superlattice of these nanocrystal-core MPCs is prepared and investigated using aberration corrected scanning/transmission electron microscopy (STEM) which allowed imaging of long-range hexagonally ordered superlattices of the nanocrystals, separated by the thiolate groups. The lattice constants determined by direct imaging are in good agreement with those determined by small-angle electron diffraction. The STEM image revealed the characteristic grain boundary (GB) with sigma (Σ) 13 in the interface between two crystals. The formation and structures found are interpreted on the basis of theoretical calculations employing molecular dynamics (MD) simulations and coarse-grained (CG) approach.

Collision-induced dissociation of monolayer protected clusters Au144 and Au130 in an electrospray time-of-flight mass spectrometer.

Gas-phase reactions of larger gold clusters are poorly known because generation of the intact parent species for mass spectrometric analysis remains quite challenging. Herein we report in-source collision-induced dissociation (CID) results for the monolayer protected clusters (MPCs) Au144(SR)60 and Au130(SR)50, where R- = PhCH2CH2-, in a Bruker micrOTOF time-of-flight mass spectrometer. A sample mixture of the two clusters was introduced into the mass spectrometer by positive mode electrospray ionization. Standard source conditions were used to acquire a reference mass spectrum, exhibiting negligible fragmentation, and then the capillary-skimmer potential difference was increased to induce in-source CID within this low-pressure region (∼4 mbar). Remarkably, distinctive fragmentation patterns are observed for each MPC[3+] parent ion. An assignment of all the major dissociation products (ions and neutrals) is deduced and interpreted by using the distinguishing characteristics in the standard structure-models for the respective MPCs. Also, we propose a ring-forming elimination mechanism to explain R-H neutral loss, as separate from the channels leading to RS-SR or (AuSR)4 neutrals.

On the structure of the thiolated Au15 cluster.

The structure of the Au15-thiolate cluster has been elucidated using a DFT approach, and it is demonstrated to comprise a Au4-tetrahedron core protected solely by the combination of two concatenated staple motifs. The longer motif efficiently wraps the core, and threads the shorter one. The structural assignment is supported by comparison to the powder X-ray diffraction pattern and, via time dependent-DFT calculations, to the optical and chiroptical (CD) absorption spectra. The calculated CD spectrum features a characteristic strong peak centered at 3.48 eV in accordance with the experimental profile. These results confirm the existence of long Au(I)-thiolate motifs as protecting units of small thiolated gold clusters with a thiolate-to-gold ratio comparable to the Au15(SR)13 cluster.

Structure & bonding of the gold-subhalide cluster I-Au144Cl60[z].

The structure and bonding of the gold-subhalide compounds Au144Cl60([z]) are related to those of the ubiquitous thiolated gold clusters, or Faradaurates, by iso-electronic substitution of thiolate by chloride. Exact I-symmetry holds for the [z] = [2+,4+] charge-states, in accordance with new electrospray mass spectrometry measurements and the predicted electron shell filling. The high symmetry facilitates analysis of the global structure as well as the bonding network, with some striking results.

Au(67)(SR)(35) nanomolecules: characteristic size-specific optical, electrochemical, structural properties and first-principles theoretical analysis.

The preparation of gold nanomolecules with sizes other than Au(25)(SR)(18), Au(38)(SR)(24), Au(102)(SR)(44), and Au(144)(SR)(60) has been hampered by stability issues and low yields. Here we report a procedure to prepare Au(67)(SR)(35), for either R = -SCH(2)CH(2)Ph or -SC(6)H(13), allowing high-yield isolation (34%, ~10-mg quantities) of the title compound. Product high purity is assessed at each synthesis stage by rapid MALDI-TOF mass-spectrometry (MS), and high-resolution electrospray-ionization MS confirms the Au(67)(SR)(35) composition. Electronic properties were explored using optical absorption spectroscopy (UV-visible-NIR regions) and electrochemistry (0.74 V spacing in differential-pulsed-voltammetry), modes of ligand binding were studied by NMR spectroscopy ((13)C and (1)H), and structural characteristics of the metal atom core were determined by powder X-ray measurements. Models featuring a Au(17) truncated-decahedral inner core encapsulated by the 30 anchoring atoms of 15 staple-motif units have been investigated with first-principles electronic structure calculations. This resulted in identification of a structure consistent with the experiments, particularly, the opening of a large gap (~0.75 eV) in the (2-) charge-state of the nanomolecule. The electronic structure is analyzed within the framework of a superatom shell model. Structurally, the Au(67)(SR)(35) nanomolecule is the smallest to adopt the complete truncated-decahedral motif for its core with a surface structure bearing greater similarity to the larger nanoparticles. Its electronic HOMO-LUMO gap (~0.75 eV) is nearly double that of the larger Au(102) compound and it is much smaller than that of the Au(38) one. The intermediary status of the Au(67)(SR)(35) nanomolecule is also reflected in both its optical and electrochemical characteristics.

Structure of the thiolated Au130 cluster.

The structure of the recently discovered Au130-thiolate and -dithiolate clusters is explored in a combined experiment-theory approach. Rapid electron diffraction in scanning/transmission electron microscopy (STEM) enables atomic-resolution imaging of the gold core and the comparison with density functional theory (DFT)-optimized realistic structure models. The results are consistent with a 105-atom truncated-decahedral core protected by 25 short staple motifs, incorporating disulfide bridges linking the dithiolate ligands. The optimized structure also accounts, via time-dependent DFT (TD-DFT) simulation, for the distinctive optical absorption spectrum, and rationalizes the special stability underlying the selective formation of the Au130 cluster in high yield. The structure is distinct from, yet shares some features with, each of the known Au102 and Au144/Au146 systems.

The superstable 25 kDa monolayer protected silver nanoparticle: measurements and interpretation as an icosahedral Ag152(SCH2CH2Ph)60 cluster.

A cluster obtained in high yield from the reduction of a silver-thiolate precursor, Ag-SCH(2)CH(2)Ph, exhibited a single sharp peak near 25 kDa in the matrix-assisted laser desorption mass spectrum (MALDI MS) and a well-defined metal core of ~2 nm measured with transmission electron microscopy (TEM). The cluster yields a single fraction in high-performance liquid chromatography (HPLC). Increased laser fluence fragments the cluster until a new peak near 19 kDa predominates, suggesting that the parent cluster-Ag(152)(SCH(2)CH(2)Ph)(60)-evolves into a stable inorganic core-Ag(152)S(60). Exploiting combined insights from investigations of clusters and surface science, a core-shell structure model was developed, with a 92-atom silver core having icosahedral-dodecahedral symmetry and an encapsulating protective shell containing 60 Ag atoms and 60 thiolates arranged in a network of six-membered rings resembling the geometry found in self-assembled monolayers on Ag(111). The structure is in agreement with small-angle X-ray scattering (SAXS) data. The protective layer encapsulating this silver cluster may be the smallest known three-dimensional self-assembled monolayer. First-principles electronic structure calculations show, for the geometry-optimized structure, the development of a ~0.4 eV energy gap between the highest-occupied and lowest-unoccupied states, originating from a superatom 90-electron shell-closure and conferring stability to the cluster. The optical absorption spectrum of the cluster resembles that of plasmonic silver nanoparticles with a broad single feature peaking at 460 nm, but the luminescence spectrum shows two maxima with one attributed to the ligated shell and the other to the core.

Adsorption of water molecules on selected charged sodium-chloride clusters.

The adsorption of water molecules (H(2)O) on sodium chloride cluster cations and anions was studied at 298 K over a mass range of 100-1200 amu using a custom-built laser desorption ionization reactor and mass spectrometer. Under the conditions used, the cations Na(3)Cl(2)(+) and Na(4)Cl(3)(+) bind up to three water molecules, whereas the larger cations, Na(5)Cl(4)(+) to Na(19)Cl(18)(+), formed hydrates with one or two only. The overall trend is a decrease in hydration with increasing cluster size, with an abrupt drop occurring at the closed-shell Na(14)Cl(13)(+). As compared to the cluster cations, the cluster anions showed almost no adsorption. Among smaller clusters, a weak adsorption of one water molecule was observed for the cluster anions Na(6)Cl(7)(-) and Na(7)Cl(8)(-). In the higher mass region, a substantial adsorption of one water molecule was observed for Na(14)Cl(15)(-). Density functional theory (DFT) computations were carried out for the adsorption of one molecule of H(2)O on the cations Na(n)Cl(n-1)(+), for n = 2-8, and the anions Na(n)Cl(n+1)(-), for n = 1-7. For each ion, the structure of the hydrate, the hydration energy, and the standard-state enthalpy, entropy, and Gibbs energy of hydration at 298 K were computed. In addition, it was useful to compute the distortion energy, defined as the electronic energy lost due to weakening of the Na-Cl bonds upon adsorption of H(2)O. The results show that strong adsorption of a H(2)O molecule occurs for the linear cations only at an end Na ion and for the nonlinear cations only at a corner Na ion bonded to two Cl ions. An unexpected result of the theoretical investigation for the anions is that certain low-energy isomers of Na(6)Cl(7)(-) and Na(7)Cl(8)(-) bind H(2)O strongly enough to produce the observed weak adsorption. The possible implications of these results for the initial hydration of extended NaCl surfaces are discussed.

Total structure and electronic properties of the gold nanocrystal Au36(SR)24.

A golden opportunity: the total structure of a Au(36)(SR)(24) nanocluster reveals an unexpected face-centered-cubic tetrahedral Au(28) kernel (magenta). The protecting layer exhibits an intriguing combination of binding modes, consisting of four regular arch-like staples and the unprecedented appearance of twelve bridging thiolates (yellow). This unique protecting network and superatom electronic shell structure confer extreme stability and robustness.