The applicability of three-dimensional aromaticity in BiSn(n)- Zintl analogues.

Three-dimensional aromaticity is shown to play a role in the stability of deltahedral Zintl clusters and here we examine the connection between aromaticity and stability. In order to gain further insight, we have studied Zintl analogs comprised of bismuth doped tin clusters with photoelectron spectroscopy and theoretical methods. To assign aromaticity, we examine the ring currents induced around the cage by using the nucleus independent chemical shift. In the current study, BiSn(4)(-) is a stable cluster and fits aromatic criteria, while BiSn(5)(-) is found to fit antiaromatic criteria and has reduced stability. The more stable clusters exhibit an aromatic character which originates from weakly interacting s-states and bonding orbitals parallel to the surface of the cluster, while nonbonding lone pairs perpendicular to the surface of the cluster account for antiaromaticity and reduced stability. The effect of three-dimensional aromaticity on the electronic structure does not result in degeneracies, so the resulting variations in stability are smaller than those seen in conventional aromaticity.

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.

Structural evolution of triniobium carbide clusters: evidence of large Cn chains (n = 3-4) in Nb3Cn- (n = 5-10) clusters.

First-principle density functional calculations and photoelectron spectroscopy experiments show that triniobium carbide clusters exist in multiple motifs. The Nb(3)C(n)(-) (n = 5-10) series have isomers surrounding a triangular Nb(3) base while incorporating Nb-C bonding. We provide evidence of not only C(2) carbon chains but also stable isomers with previously unidentified C(3) and C(4) carbon chains in triniobium carbide clusters.

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.

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.

Effect of charge and composition on the structural fluxionality and stability of nine atom tin-bismuth Zintl analogues.

Synergistic studies of bismuth doped tin clusters combining photoelectron spectra with first principles theoretical investigations establish that highly charged Zintl ions, observed in the condensed phase, can be stabilized as isolated gas phase clusters through atomic substitution that preserves the overall electron count but reduces the net charge and thereby avoids instability because of coulomb repulsion. Mass spectrometry studies reveal that Sn(8)Bi(-), Sn(7)Bi(2)(-), and Sn(6)Bi(3)(-) exhibit higher abundances than neighboring species, and photoelectron spectroscopy show that all of these heteroatomic gas phase Zintl analogues (GPZAs) have high adiabatic electron detachment energies. Sn(6)Bi(3)(-) is found to be a particularly stable cluster, having a large highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap. Theoretical calculations demonstrate that the Sn(6)Bi(3)(-) cluster is isoelectronic with the well know Sn(9)(-4) Zintl ion; however, the fluxionality reported for Sn(9)(-4) is suppressed by substituting Sn atoms with Bi atoms. Thus, while the electronic stability of the clusters is dominated by electron count, the size and position of the atoms affects the dynamics of the cluster as well. Substitution with Bi enlarges the cage compared with Sn(9)(-4) making it favorable for endohedral doping, findings which suggest that these cages may find use for building blocks of cluster assembled materials.

From SiO molecules to silicates in circumstellar space: atomic structures, growth patterns, and optical signatures of SinOm clusters.

SiO is the dominant silicon bearing molecule in the circumstellar medium; however, it agglomerates to form oxygen-rich silicates. Here we present a synergistic effort combining experiments in beams with theoretical investigations to examine mechanisms for this oxygen enrichment. The oxygen enrichment may proceed via two processes, namely, (1) chemically driven compositional separation in (SiO)(n) motifs resulting in oxygen-rich and silicon-rich or pure silicon regions, and (2) reaction between Si(n)O(m) clusters leading to oxygen richer and poorer fragments. While SiO(2) molecules are emitted in selected chemical reactions, they readily oxidize larger Si(n)O(n) clusters in exothermic reactions and are not likely to agglomerate into larger (SiO(2))(n) motifs. Theoretically calculated optical absorption and infrared spectra of Si(n)O(m) clusters exhibit features observed in the extended red emissions and blue luminescence from interstellar medium, indicating that the Si(n)O(m) fragments could be contributing to these spectra.

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.