Electronic and Structural Study of ZnxSx [x = 12, 16, 24, 28, 36, 48, 96, and 108] Cage Structures.

We present a density functional study on the structural and electronic properties of ZnS bubble clusters, specifically, hollow cages whose spontaneous formation was previously observed in classical molecular dynamics simulations by Spano et al. [J. Phys. Chem. B 2003, 107, 10337]. The hollow ZnS cages in our study were modeled as ZnxSx [x = 12, 16, 24, 28, 36, 48, 108], and an onionlike structure was modeled as Zn96S96. The study of energetics and stability, performed using large polarized Gaussian basis sets, indicated all structures to be energetically stable with similar binding energy of 5.5-5.6 eV per ZnS pair. Further computation of electronic properties showed that these cages have large vertical ionization energies and relatively low electron affinities in the ranges of 6.8-8.1 and 1.7-3.0 eV, respectively. They have large highest occupied molecular orbital-lowest unoccupied molecular orbital gaps between 2.5 and 3.3 eV, and quasi-particle gaps vary from 6.2 eV for Zn12S12 to 4.19 for Zn108S108. The computed vibrational frequencies for selected cages, that is, Zn12S12, Zn16S16, Zn28S28 (O, S4, and S8 point groups), and Zn36S36 indicate that these cage structures correspond to local minima on the potential energy surface. Finally, the infrared spectra calculated using large basis sets are also reported.

Water inhibits CO oxidation on gold cations in the gas phase. Structures and binding energies of the sequential addition of CO, H2O, O2, and N2 onto Au.

We report a detailed experimental and theoretical study of the gas phase reactivity of Au+ with CO, O2, N2 and their mixtures in the presence of a trace amount of water impurity. The gold cation is found to strongly interact with CO and H2O molecules via successive addition reactions until reaching saturation. The stoichiometry of the formed complex is determined by the strength of the binding energy of the neutral molecule to the gold cation. CO binds the strongest to Au+, followed by H2O, N2 and then O2. We found that the gold cation (Au+) can activate the O2 molecule within the Au+(CO)2(O2) complex which could react with another CO molecule to form Au+(CO)(CO2) + CO2. The product Au+(CO)(CO2) is observed experimentally with a small intensity at room temperature. However, the presence of water leads to the formation of Au+(CO)(H2O)(O2) instead of Au+(CO)2(O2) due to the strong interaction between Au+ and water. The current experiments and calculations might lead to a molecular level understanding of the interactions between the active sites, reactants and impurities which could pave the way for the design of efficient nanocatalysts.

On the existence of designer magnetic superatoms.

The quantum states in small, compact metal clusters are bunched into electronic shells with electronic orbitals resembling those in atoms, enabling classification of stable clusters as superatoms. The filling of superatomic orbitals, however, does not generally follow Hund's rule, and it has been proposed that magnetic superatoms can be stabilized by doping simple metal clusters with magnetic atoms. Here, we present evidence of the existence of a magnetic superatom and the determination of its spin moment. Our approach combines first principles studies with negative ion photoelectron experiments and enables a unique identification of the ground state and spin multiplicity. The studies indicate VNa8 to be a magnetic superatom with a filled d-subshell and a magnetic moment of 5.0 µB. Its low electron affinity is consistent with filled subshell and enhanced stability. The synthesis of this species opens the pathway to investigate the spin-dependent electronics of the new magnetic motifs.

Robust magnetic moments on impurities in metallic clusters: localized magnetic states in superatoms.

Introducing magnetic impurities into clusters of simple metals can create localized states for higher angular momentum quantum numbers (l = 2 or 3) that can breed magnetism analogous to that in virtual bound states in metallic hosts, offering a new recipe for magnetic superatoms. In this work, we demonstrate that MnCa(n) clusters containing 6-15 Ca atoms show a spin magnetic moment of 5.0 µB irrespective of the cluster size. Theoretical analysis reveals that the Mn d states hybridize only partially with superatomic states and introduce extra majority and minority d states, largely localized at the Mn site, with a large gap. Successive addition of Ca atoms introduces superatomic states of varying angular momentum that are embedded in this gap, allowing control over the stability of the motifs without altering the moment. Assemblies of such clusters can offer novel electronic features due to the formation of localized magnetic "quasibound states" in a confined nearly free electron gas.

Spin accommodation and reactivity of silver clusters with oxygen: the enhanced stability of Ag13(-).

Spin accommodation is demonstrated to play a determining role in the reactivity of silver cluster anions with oxygen. Odd-electron silver clusters are found to be especially reactive, while the anionic 13-atom cluster exhibits unexpected stability against reactivity with oxygen. Theoretical studies show that the odd-even selective behavior is correlated with the excitation needed to activate the O-O bond in O(2). Furthermore, by comparison with the reactivity of proximate even-electron clusters, we demonstrate that the inactivity of Ag(13)(-) is associated with its large spin excitation energy, ascribed to a crystal-field-like splitting of the orbitals caused by the bilayer atomic structure, which induces a large gap despite not having a magic number of valence electrons.

Structural changes of Pd13 upon charging and oxidation/reduction.

First-principle generalized gradient corrected density functional calculations have been performed to study the stability of cationic and anionic Pd(13) (+∕-), and neutral Pd(13)O(2) clusters. It is found that while cationic Pd(13) (+) favors a C(s) geometry similar to the neutral Pd(13), both anionic Pd(13)(-) and neutral Pd(13)O(2) favor a compact ~I(h) structure. A detailed analysis of the electronic structure shows that the stabilization of the delocalized 1P and 2P cluster orbitals, and the hybridization of the 1D orbitals with the oxygen atomic p orbitals play an important role in the energetic ordering of C(s) and ~I(h) isomers. A structural oscillation is predicted during an oxidation/reduction cycle of Pd(13) in which small energy barriers between 0.3 and 0.4 eV are involved.

Metallic and molecular orbital concepts in XMg8 clusters, X = Be-F.

The electronic structure and stability of the XMg(8) clusters (X = Be, B, C, N, O, and F) are studied using first principles theoretical calculations to understand the variation in bonding in heteroatomic clusters which mix simple divalent metals with main group dopants. We examine these progressions with two competing models, the first is a distorted nearly free electron gas model and the second is a molecular orbital picture examining the orbital overlap between the dopant and the cluster. OMg(8) is found to be the most energetically stable cluster due to strong bonding of O with the Mg(8) cluster. BeMg(8) has the largest HOMO-LUMO gap due to strong hybridization between the Mg(8) and the Be dopant states that form a delocalized pool of 18 valence electrons with a closed electronic shell due to crystal field effects. Be, B, and C are best described by the nearly free electron gas model, while N, O, and F are best described through molecular orbital concepts.

Ion Mobility Spectrometry Characterization of the Intermediate Hydrogen-Containing Gold Cluster Au7(PPh3)7H52.

We employ ion mobility spectrometry and density functional theory to determine the structure of Au7(PPh3)7H52+ (PPh3 = triphenylphosphine), which was recently identified by high mass resolution mass spectrometry. Experimental ion-neutral collision cross sections represent the momentum transfer between the ionic clusters and gas molecules averaged over the relative thermal velocities of the colliding pair, thereby providing structural insights. Theoretical calculations indicate the geometry of Au7(PPh3)7H52+ is similar to Au7(PPh3)7+, with three hydrogen atoms bridging two gold atoms and two hydrogen atoms forming single Au-H bonds. Collision-induced dissociation products observed during IMS experiments reveal that smaller hydrogen-containing clusters may be produced through fragmentation of Au7(PPh3)7H52+. Our findings indicate that hydrogen-containing species like Au7(PPh3)7H52+ act as intermediates in the formation of larger phosphine ligated gold clusters. These results advance the understanding and ability to control the mechanisms of size-selective cluster formation, which is necessary for scalable synthesis of clusters with tailored properties.

Photoelectron imaging and density-functional investigation of bismuth and lead anions solvated in ammonia clusters.

We present the results of photoelectron velocity-map imaging experiments for the photodetachment of small negatively charged ammonia solvated Bi(n) and Pb(n) (n = 1, 2) clusters at 527 nm. The vertical detachment energies of the observed multiple electronic bands and their respective anisotropy parameters for the solvated Bi and Pb anions and clusters derived from the photoelectron images are reported. The electronic bands of Bi(NH(3))(n=1,2) are distinct from the Bi metal ion in exhibiting a perpendicular distribution whereas the electronic bands in Pb(NH(3))(n=1,2), unlike the Pb anion, show an isotropic distribution with respect to the laser polarization. Density-functional theory calculations with a generalized gradient approximation for the exchange-correlation potential were performed on these clusters to determine their atomic and electronic structures. Calculated geometries show a dramatic change between anionic and neutral ammonia solvated Bi and Pb species. Anionic clusters exhibit van der Waals interactions between the hydrogen atoms of ammonia and the metal core, where it was determined that the negative charge is localized. Neutral clusters, on the other hand, present a covalent bond between the nitrogen atom of ammonia and the metal core. Calculated binding energies show an enhancement in the bonding of the (NH(3))(2) dimer in the presence of the anionic Bi(1,2)(-) and Pb(1,2)(-) metal ions. This is rationalized by the electrostatic interaction between the negative charged metal core and the hydrogen atoms of the ammonia molecule.

Combined experimental and theoretical study of Al(n)X (n = 1-6; X = As, Sb) clusters: evidence of aromaticity and the Jellium model.

The electronic structure of Al(n)X (n = 1-6; X = As, Sb) clusters has been investigated using a synergistic approach combining negative ion photoelectron spectroscopy and first principles electronic structure calculations. It is shown that Al(3)X and Al(5)X exhibit enhanced energetic stability, as evidenced from calculated removal and embedding energies as well as chemical stability manifested through a large gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). However, the stabilities of these species are derived from different mechanisms. Al(3)As and Al(3)Sb, with HOMO-LUMO gaps of 1.86 and 1.73 eV, respectively, are shown to have planar geometries where the p orbitals combine to form one pi and two sigma aromatic orbitals reminiscent of conventional all-metal aromatic species. Al(5)As and Al(5)Sb, with 20 valence electrons, possess a closed electronic shell (1s(2), 1p(6), 1d(10), 2s(2)) within a jellium framework and have HOMO-LUMO gaps of 1.12 and 1.17 eV, respectively.

Enhanced magnetic moments of alkali metal coated Sc clusters: new magnetic superatoms.

It is shown that the magnetic moments of Sc atoms can be significantly enhanced by combining them with alkali atoms. We present results of first principles electronic structure calculations of ScNa(n) (1 < or = n < or = 12) clusters that indicate that a ScNa(12) cluster consisting of a Sc atom surrounded by 12 Na atoms forming a compact icosahedral structure has a spin magnetic moment of 3 micro(B) that is three times that of an isolated Sc atom. This unusual behavior is analyzed in terms of the filling of the supershells 1S, 1P,... controlled by the nature and size of the alkali atoms and the more localized Sc 3d orbitals that hybridize weakly with Na sp orbitals. It is shown that even larger magnetic moments could be attained by controlling the relative position of 1S, 1P, and 3d states. Indeed, our studies indicate large magnetic moment five times that of an isolated Sc atom in the ScK(12) and ScCs(12) clusters, in which the 3d orbitals of Sc adopt a half-filled configuration, while the clusters are stabilized by filled 1S(2), 1P(6), and 2S(2) shells, the features making them as new magnetic superatoms.

Photoelectron imaging and theoretical investigation of bimetallic Bi(1-2)Ga(0-2)(-) and Pb(1-4)(-) cluster anions.

We present the results of photoelectron velocity-map imaging experiments for the photodetachment of small negatively charged Bi(m)Ga(n) (m=1-2, n=0-2), and Pb(n) (n=1-4) clusters at 527 nm. The photoelectron images reveal new features along with their angular distributions in the photoelectron spectra of these clusters. We report the vertical detachment energies of the observed multiple electronic bands and their respective anisotropy parameters for the Bi(m)Ga(n) and Pb(n) clusters derived from the photoelectron images. Experiments on the BiGa(n) clusters reveal that the electron affinity increases with the number of Ga atoms from n=0 to 2. The BiGa(2)(-) cluster is found to be stable, both because of its even electron number and the high electron affinity of BiGa(2). The measured photoelectron angular distributions of the Bi(m)Ga(n) and Pb(n) clusters are dependent on both the orbital symmetry and electron kinetic energies. Density-functional theory calculations employing the generalized gradient approximation for the exchange-correlation potential were performed on these clusters to determine their atomic and electronic structures. From the theoretical calculations, we find that the BiGa(2)(-), Bi(2)Ga(3)(-) and Bi(2)Ga(5)(-) (anionic), and BiGa(3), BiGa(5), Bi(2)Ga(4) and Bi(2)Ga(6) (neutral) clusters are unusually stable. The stability of the anionic and neutral Bi(2)Ga(n) clusters is attributed to an even-odd effect, with clusters having an even number of electrons presenting a larger gain in energy through the addition of a Ga atom to the preceding size compared to odd electron systems. The stability of the neutral BiGa(3) cluster is rationalized as being similar to BiAl(3), an all-metal aromatic cluster.

Influence of stoichiometry and charge state on the structure and reactivity of cobalt oxide clusters with CO.

Cationic and anionic cobalt oxide clusters, generated by laser vaporization, were studied using guided-ion-beam mass spectrometry to obtain insight into their structure and reactivity with carbon monoxide. Anionic clusters having the stoichiometries Co2O3(-), Co2O5(-), Co3O5(-) and Co3O6(-) were found to exhibit dominant products corresponding to the transfer of a single oxygen atom to CO, indicating the formation of CO 2. Cationic clusters, in contrast, displayed products resulting from the adsorption of CO onto the cluster accompanied by the loss of either molecular O 2 or cobalt oxide units. In addition, collision induced dissociation experiments were conducted with N 2 and inert xenon gas for the anionic clusters, and xenon gas for the cationic clusters. It was found that cationic clusters fragment preferentially through the loss of molecular O 2 whereas anionic clusters tend to lose both atomic oxygen and cobalt oxide units. To further analyze how stoichiometry and ionic charge state influence the structure of cobalt oxide clusters and their reactivity with CO, first principles theoretical electronic structure studies within the density functional theory framework were performed. The calculations show that the enhanced reactivity of specific anionic cobalt oxides with CO is due to their relatively low atomic oxygen dissociation energy which makes the oxidation of CO energetically favorable. For cationic cobalt oxide clusters, in contrast, the oxygen dissociation energies are calculated to be even lower than for the anionic species. However, in the cationic clusters, oxygen is calculated to bind preferentially in a less activated molecular O 2 form. Furthermore, the CO adsorption energy is calculated to be larger for cationic clusters than for anionic species. Therefore, the experimentally observed displacement of weakly bound O 2 units through the exothermic adsorption of CO onto positively charged cobalt oxides is energetically favorable. Our joint experimental and theoretical findings indicate that positively charged sites in bulk-phase cobalt oxides may serve to bind CO to the catalyst surface and specific negatively charged sites provide the activated oxygen which leads to the formation of CO 2. These results provide molecular level insight into how size, stoichiometry, and ionic charge state influence the oxidation of CO in the presence of cobalt oxides, an important reaction for environmental pollution abatement.

Al(n)Bi clusters: transitions between aromatic and jellium stability.

An experimental and theoretical study of bismuth-doped aluminum clusters in the gas phase has revealed two particularly stable clusters, namely, Al(3)Bi and Al(5)Bi. We show that their electronic structure can be understood in terms of the aromatic and "Jellium" models, respectively. Negative ion photodetachment spectra provide a fingerprint of the electronic states in Al(n)Bi(-) (n = 1-5) anions, while theoretical investigations reveal the nature of the electronic orbitals involved. Together, the findings reveal that the all-metal Al(3)Bi cluster with 14 valence electrons is a cyclic, planar structure with a calculated large ionization potential of 7.08 eV, a low electron affinity of 1.41 eV, and a large gap of 1.69 eV between the highest occupied and lowest unoccupied molecular orbitals (HOMO-LUMO gap). The Al(3)Bi cluster has molecular orbitals reminiscent of aromatic systems and is a neutral cluster with no need for counterion or ligand support. A slightly larger cluster, Al(5)Bi, has 20 valence electrons and is another highly stable compact structure with a calculated large ionization potential of 6.51 eV and a large HOMO-LUMO gap of 1.15 eV. This cluster's stability is rooted in a Jellium electronic shell closing. The formation of stable species using aromatic bonding allows us to extend the idea of cluster-assembled materials built out of stable clusters with Jellium shell closings (superatoms) to include ones involving aromatic building blocks.

H2O nucleation around Au+.

First principles electronic structure calculations have been carried out to investigate the ground state geometry, electronic structure, and the binding energy of [Au(H2O)n]+ clusters containing up to 10 H2O molecules. It is shown that the first coordination shell of Au+ contains two H2O molecules forming a H2O-Au+-H2O structure with C2 symmetry. Subsequent H2O molecules bind to the previous H2O molecules forming stable and fairly rigid rings, each composed of 4 H2O molecules, and leading to a dumbbell structure at [Au(H2O)8]+. The 9th and the 10th H2O molecules occupy locations above the Au+ cation mainly bonded to one H2O from each ring, leading to structures where the side rings are partially distorted and forming structures that resemble droplet formation around the Au+ cation. The investigations highlight quantum effects in nucleation at small sizes and provide a microscopic understanding of the observed incremental binding energy deduced from collision induced dissociation that indicates that [Au(H2O)n]+ clusters with 7-10 H2O molecules have comparable binding energy. The charge on the Au+ is shown to migrate to the outside H2O molecules, suggesting an interesting screening phenomenon.

Theoretical study of Cu(I)Y zeolite: structure and electronic properties.

The structural and electronic properties of the accessible Cu(I) site of a faujasite-type zeolite have been studied, by use of large cluster models and a density functional theory-based methodology. We demonstrate that the local ideal C(3) symmetry of the Cu(I) site II is broken. The Cu(I) cation is bonded to the zeolite framework by one bond of about 2.26 A and two shorter ones of 2.07 A. We demonstrate that only one cation position exists at this site. This result is also confirmed by a molecular electrostatic potential analysis. We show that local properties at site II, as well as the global properties of the solid (frontier orbitals), do not depend on the Al and cation distribution and only slightly on the cocation nature. Taking into account the present results and well-known experimental data, we propose that specific catalytic behaviors are correlated with local response properties, such as the local acid strength or, in other reactions, specific local architecture or confinement.

Designer magnetic superatoms.

The quantum states in metal clusters are grouped into bunches of close-lying eigenvalues, termed electronic shells, similar to those of atoms. Filling of the electronic shells with paired electrons results in local minima in energy to give stable species called magic clusters. This led to the realization that selected clusters mimic chemical properties of elemental atoms on the periodic table and can be classified as superatoms. So far the work on superatoms has focused on non-magnetic species. Here we propose a framework for magnetic superatoms by invoking systems that have both localized and delocalized electronic states, in which localized electrons stabilize magnetic moments and filled nearly-free electron shells lead to stable species. An isolated VCs(8) and a ligated MnAu(24)(SH)(18) are shown to be such magnetic superatoms. The magnetic superatoms' assemblies could be ideal for molecular electronic devices, as the coupling could be altered by charging or weak fields.

Reactive nature of dopamine as a surface functionalization agent in iron oxide nanoparticles.

Dopamine forms an initial structure coordinated to the surface of the iron oxide nanoparticle as a result of improved orbital overlap of the five-membered ring and a reduced steric environment of the iron complex. However, through transfer of electrons to the iron cations on the surface and rearrangement of the oxidized dopamine, a semiquinone is formed. Because of free protons in the system, oxygens on the surface are protonated, which allows for the Fe2+ to be released into the solution as a hydroxide. This released fragment of the nanoparticle will then eventually oxidize in air to a form of an iron(III) oxyhydroxide. All of the reported results demonstrate that the reactivity between Fe3+ and dopamine quickly facilitates the degradation of the nanoparticles. The energetic modeling studies substantiate our proposed decomposition mechanism and thus conclude that the use of dopamine as a robust anchor for iron oxide or iron oxide shell particles will not fulfill the need for stable ferrofluids in most biomedical applications.

Experimental and theoretical study of the structure and reactivity of Fe1-2O< or =6- clusters with CO.

Synergistic studies employing experiments in the gas phase and theoretical first principles calculations have been carried out to investigate the structure, stability, and reactivity toward CO of iron oxide cluster anions, Fe(x)O(y)- (x = 1-2, y < or = 6). Collision-induced dissociation studies of iron oxide species, employing xenon collision gas, show that FeO3- and FeO2- are the stable building blocks of the larger iron oxide clusters. Theoretical calculations show that the fragmentation patterns leading to the production of O or FeO(n) fragments are governed both by the energetics of the overall process as well as the number of intermediate states and the changes in spin multiplicity. Mass-selected experiments identified oxygen atom transfer to CO as the dominant reaction pathway for most anionic iron oxide clusters. A theoretical analysis of the molecular level pathways has been carried out to highlight the role of energetics as well as the spin states of the intermediates on the oxidation reaction.

Silicon oxide nanoparticles reveal the origin of silicate grains in circumstellar environments.

A synergistic effort combining experiments in beams and first principles theoretical investigations is used to propose mechanisms that could lead to the formation of silicates and nanoparticles with silicon-rich cores through agglomeration of SiO, an abundant oxygen-bearing species in space. The silicon oxygen species involved in the transformation have optical excitations that could contribute to extended red emissions and blue luminescence. Apart from resolving an outstanding astronomical problem, we demonstrate novel silicon architectures.