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.

We recently demonstrated that, in gas phase clusters containing aluminum and iodine atoms, an Al 13 cluster behaves like a halogen atom, whereas an Al 14 cluster exhibits properties analogous to an alkaline earth atom. These observations, together with our findings that Al 13 ؊ is inert like a rare gas atom, have reinforced the idea that chosen clusters can exhibit chemical behaviors reminiscent of atoms in the periodic T he formation of materials with properties different from those of the constituent atoms is a known phenomenon in nature. For example, the formation of NaCl molecules/solid with characteristics different from its constituent elements, Na and Cl, is a classic example. One of the objectives of the research on superatoms (1-3) is to explore if one can carry out, what nature does, in a more facile and controlled manner. Developing an understanding of the factors governing the chemical behavior of clusters (4-10) and demonstrating that this knowledge can be used to design stable building blocks for new materials is critical for translating this concept into practice. For metal clusters, a simple electronic shell model called jellium (11) is routinely used to describe the global features of the electronic structure. In this model, the nuclei together with the innermost electrons form a positive-charged background, whereupon the valence electrons coming from individual atoms are then subjected to this potential. For pure metal clusters, within a spherical jellium background, this approach results in a shell structure where the electrons are arranged in electronic shells 1s 2 , 1p 6 , 1d 10 , 2s 2 , 1f 14 , 2p 6 . . . compared with 1s 2 , 2s 2 , 2p 6 , 3s 2 , 3p 6 , 4s 2 , 3d 10 . . . in individual atoms. Similar shell structure is also obtained for square well and harmonic forms of background potential (12), indicating that the shells derived within a jellium picture represent generic features of electronic states in a confined free electron gas. Clusters containing 2, 8, 18, 20, 34, 40 . . . electrons correspond to filled electronic shells and exhibit enhanced stability as seen via abundances in mass spectra of simple metal clusters, higher ionization potential, lower electron affinity, and chemical inertness seen in reactivity experiments. In this respect, an Al 13 cluster with 39 valence electrons and an electronic structure of 1s 2 , 1p 6 , 1d 10 , 2s 2 , 1f 14 , 2p 5 lacks a single electron as do halogen atoms, which, upon addition of a single electron, acquire a filled shell status (13). Indeed, previous studies (14,15) have shown that Al 13 has an electron affinity comparable to halogen atoms, indicating a chemical behavior reminiscent of halogen atoms. In a similar vein, we had recently shown that in cluster compounds with iodine, an Al 14 cluster exhibits behavior analogous to alkaline earth atoms (3). We had shown that Al 14 I 3 Ϫ is a stable species and that its stability can be reconciled by considering Al 14 in a ϩ 2 valence state (3). The electronic shell structure, outlined above, does ...

The use of Hermite Gaussian auxiliary function densities from the variational fitting of the Coulomb potential for the calculation of exchange-correlation potentials is discussed. The basic working equations for the energy and gradient calculation are derived. The accuracy of this approximation for optimized structure parameters and bond energies are analyzed. It is shown that the quality of the approximation can be systematically improved by enlarging the auxiliary function set. Average errors of 0.5 kcal/mol are obtained with auxiliary function sets including f and g functions. The timings for a series of alkenes demonstrate a substantial performance improvement.

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.

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.