Obesity Is Associated with Oxidative Stress Markers and Antioxidant Enzyme Activity in Mexican Children.

The relationship between metabolic disorders and oxidative stress is still controversial in the child population. The present cross-sectional study aimed to analyze the associations between obesity, cardiometabolic traits, serum level of carbonylated proteins (CPs), malondialdehyde (MDA), and the enzyme activity of catalase (CAT), superoxide dismutase (SOD), and glutathione peroxidase (GPx) in children from Mexico City (normal weight: 120; obesity: 81). Obesity resulted in being positively associated with CAT (ß = 0.05 ± 0.01, p = 5.0 × 10-3) and GPx (ß = 0.13 ± 0.01, p = 3.7 × 10-19) enzyme activity. A significant interaction between obesity and sex was observed in MDA and SOD enzymatic activity (PMDA = 0.03; PSOD = 0.04). The associations between obesity, MDA level, and SOD enzyme activity were only significant in boys (boys: PMDA = 3.0 × 10-3; PSOD = 7.0 × 10-3; girls: p ≥ 0.79). In both children with normal weight and those with obesity, CP levels were positively associated with SOD enzyme activity (PNormal-weight = 2.2 × 10-3; PObesity = 0.03). In conclusion, in Mexican children, obesity is positively associated with CAT and GPx enzyme activity, and its associations with MDA levels and SOD enzyme activity are sex-specific. Therefore, CP level is positively related to SOD enzyme activity independently of body weight.

Incorporating charge transfer effects into a metallic empirical potential for accurate structure determination in (ZnMg)N nanoalloys.

We report the results of a combined empirical potential-density functional theory (EP-DFT) study to assess the global minimum structures of free-standing zinc-magnesium nanoalloys of equiatomic composition and with up to 50 atoms. Within this approach, the approximate potential energy surface generated by an empirical potential is first sampled with unbiased basin hopping simulations, and then a selection of the isomers so identified is re-optimized at a first-principles DFT level. Bader charges calculated in a previous work [A. Lebon, A. Aguado and A. Vega, Corros. Sci., 2017, 124, 35-45] revealed a significant transfer of electrons from Mg to Zn atoms in these nanoalloys; so the main novelty in the present work is the development of an improved EP, termed Coulomb-corrected-Gupta potential, which incorporates an explicit charge-transfer correction term onto a metallic Gupta potential description. The Coulomb correction has a many-body character and is fed with parameterized values of the ab initio Bader charges. The potentials are fitted to a large training set containing DFT values of cluster energies and atomic forces, and the DFT results are used as benchmark data to assess the performance of Gupta and Coulomb-corrected-Gupta EP models. Quite surprisingly, the charge-transfer correction is found to represent only 6% of the nanoalloy binding energies, yet this quantitatively small correction has a sizable beneficial effect on the predicted relative energies of homotops. Zn-Mg bulk alloys are used as the sacrificial material in corrosion-protective coatings, and the long-term goal of our research is to disclose whether those corrosion-protected capabilities are enhanced at the nanoscale.

Computational characterisation of structure and metallicity in small neutral and singly-charged cadmium clusters.

Putative global minimum structures for neutral CdN and singly charged CdN+ and CdN- clusters in the small size regime up to N = 21 atoms are reported. A global optimization approach based on the basin hopping method and a Gupta potential fitted to cluster properties is employed to generate a diverse databank of trial structures, which are then re-optimized at the density-functional level of theory. Novel, previously unreported, structures are found for many sizes. Our results successfully reproduce and interpret the size-dependent stabilities known from mass spectrometry, and strongly suggest that experiments aimed at determining the relative stabilities of neutral cadmium clusters are really measuring cation stabilities. We provide an in-depth analysis of electronic structure and use it to explain the gradual emergence of metallic-like behaviour as the cluster size increases.

Are zinc clusters really amorphous? A detailed protocol for locating global minimum structures of clusters.

We report the results of a conjoint experimental/theoretical effort to assess the structures of free-standing zinc clusters with up to 73 atoms. Experiment provides photoemission spectra for ZnN- cluster anions, to be used as fingerprints in structural assessment, as well as mass spectra for both anion and cation clusters. Theory provides both a detailed description of a novel protocol to locate global minimum structures of clusters in an efficient and reliable way, and its specific application to neutral and charged zinc clusters. Our methodology is based on the well-known hybrid EP-DFT (empirical potential-density functional theory) approach, in which the approximate potential energy surface generated by an empirical Gupta potential is first sampled with unbiased basin hopping simulations, and then a selection of the isomers so identified is re-optimized at a first-principles DFT level. The novelty introduced in our paper is a simple but efficient new recipe to obtain the best possible EP parameters for a given cluster system, with which the first step of the EP-DFT method is to be performed. Our method is able to reproduce experimental measurements at an excellent level for most cluster sizes, implying its ability to locate the true global minimum structures; meanwhile, if exactly the same method is applied based on the existing Gupta potential (fitted to bulk properties), it leads to wrong predicted structures with energies between 1 and 2 eV above the correct ones. Opposite to what was claimed in the past, our work unequivocally demonstrates that Zn clusters are not amorphous, and they rather adopt high symmetry structures for most sizes. We show that Zn clusters have a number of exotic, unprecedented structural and electronic properties which are not expected for clusters of a metallic element, and describe them in detail.

Insulating or metallic: coexistence of different electronic phases in zinc clusters.

How many of the several attributes of the bulk metallic state persist in a nanoparticle containing a finite number of atoms of a metallic element? Do all those attributes emerge suddenly at a well-defined cluster size or do they rather evolve at different rates and in a broad size range? These fundamental questions have been addressed through a conjoint experimental/theoretical investigation of zinc clusters. We report the observation of novel coexistence phenomena involving different electronic phases: for some sizes, metallic and insulating electronic states coexist within a single, Janus-like, nanoparticle; for the rest of sizes, we report the coexistence of two weakly interacting metallic phases with different dimensionalities, localized at the shell and the core of the nanoparticle. These fascinating features are due to an anomalously long core-shell separation that equips the shell and core regions with largely independent structural, vibrational, and thermal properties.

A new magnetic superatom: Cr@Zn17.

We demonstrate, by means of fully unconstrained density functional theory calculations, that cluster Zn17 endohedrally doped with a Cr impurity can be qualified as a magnetic superalkali cluster. We explain the origin of its high stability, its low vertical ionization potential and its high total spin magnetic moment which amounts to 6 µB, exactly the same value as that of the isolated Cr atom. With the aim of exploring the possibility of designing a bistable magnetic nanoparticle, with a corresponding inter-unit exchange coupling, we also consider the assembling of two such units through different contact regions and in different magnetic configurations. Furthermore, we analyze up to which extent is the Zn shell able to preserve the electronic properties of the embedded Cr atom, both against coalescence of the two superatoms forming the magnetically bistable nanoparticle, and upon the adsorption of an O2 molecule or even under an oversaturated O2 atmosphere. Our results are discussed not only emphasizing the fundamental physical and chemical aspects, but also with an eye on the new prospects that those Cr@Zn17 magnetic superalkali clusters (and others of similar kind) may open in spintronics-, molecular electronics- or biomedical-applications.

Síndrome de hipotensión endocraneana espontánea: características imagenológicas y correlación fisiopatológica / Spontaneous intracranial hypotension syndrome. Imaging features and pathophysiologic correlation

Objetivo. Mostrar y describir las principales características imagenológicas del síndrome de hipotensión endocraneana espontánea (SHEE) y su correlación fisiopatológica.Materiales y Métodos. Se analizó retrospectivamente a los pacientes que consultaron en nuestra institución por cefalea ortostática sugestiva de síndrome de hipotensión endocraneana espontánea entre septiembre del 2008 y junio del 2011. Todos los casos fueron evaluados con tomografía computada multicorte (TCMC) (Siemens Sensation 16, Siemens, Erlangen, Alemania) y resonancia magnética (RM) (1.5T Magnetom Symphony, Siemens, Erlangen, Alemania) con inyección de gadolinio (gadopentato de dimeglumina; gadoversetamida, Mallinckrodt Inc.). También se realizó la evaluación del líquido cefalorraquídeo (LCR). Resultados. De la totalidad de casos evaluados, se seleccionaron 5 que presentaron hallazgos clínicos y neurorradiológicos característicos del síndrome de hipotensión endocraneana espontánea. Luego de descartar las etiologías secundarias que justificaban la pérdida de líquido cefalorraquídeo, se incluyó a los pacientes bajo esta entidad, identificando en algunos de ellos el sitio de fuga. Conclusión. El síndrome de hipotensión endocraneana espontánea es una patología poco frecuente, probablemente subdiagnosticada, que debe ser considerada entre los diagnósticos diferenciales en pacientes con cefalea y realce paquimeníngeo, luego de descartar otras causas que puedan justificar la pérdida de líquidoCefalorraquídeo...

Síndrome de hipotensión endocraneana espontánea: características imagenológicas y correlación fisiopatológica / Spontaneous intracranial hypotension syndrome. Imaging features and pathophysiologic correlation

Objetivo. Mostrar y describir las principales características imagenológicas del síndrome de hipotensión endocraneana espontánea (SHEE) y su correlación fisiopatológica.Materiales y Métodos. Se analizó retrospectivamente a los pacientes que consultaron en nuestra institución por cefalea ortostática sugestiva de síndrome de hipotensión endocraneana espontánea entre septiembre del 2008 y junio del 2011. Todos los casos fueron evaluados con tomografía computada multicorte (TCMC) (Siemens Sensation 16, Siemens, Erlangen, Alemania) y resonancia magnética (RM) (1.5T Magnetom Symphony, Siemens, Erlangen, Alemania) con inyección de gadolinio (gadopentato de dimeglumina; gadoversetamida, Mallinckrodt Inc.). También se realizó la evaluación del líquido cefalorraquídeo (LCR). Resultados. De la totalidad de casos evaluados, se seleccionaron 5 que presentaron hallazgos clínicos y neurorradiológicos característicos del síndrome de hipotensión endocraneana espontánea. Luego de descartar las etiologías secundarias que justificaban la pérdida de líquido cefalorraquídeo, se incluyó a los pacientes bajo esta entidad, identificando en algunos de ellos el sitio de fuga. Conclusión. El síndrome de hipotensión endocraneana espontánea es una patología poco frecuente, probablemente subdiagnosticada, que debe ser considerada entre los diagnósticos diferenciales en pacientes con cefalea y realce paquimeníngeo, luego de descartar otras causas que puedan justificar la pérdida de líquidoCefalorraquídeo...(AU)

Neutral and charged gallium clusters: structures, physical properties and implications for the melting features.

We report the putative Global Minimum (GM) structures and electronic properties of Ga(N)(+), Ga(N) and Ga(N)(-) clusters with N = 13-37 atoms, obtained from first-principles density functional theory structural optimizations. The calculations include spin polarization and employ an exchange-correlation functional which accounts for van der Waals dispersion interactions (vdW-DFT). We find a wide diversity of structural motifs within the located GM, including decahedral, polyicosahedral, polytetrahedral and layered structures. The GM structures are also extremely sensitive to the number of electrons in the cluster, so that the structures of neutral and charged clusters differ for most sizes. The main magic numbers (clusters with an enhanced stability) are identified and interpreted in terms of electronic and geometric shell closings. The theoretical results are consistent with experimental abundance mass spectra of Ga(N)(+) and with photoelectron spectra of Ga(N)(-). The size dependence of the latent heats of melting, the shape of the heat capacity peaks, and the temperature dependence of the collision cross-sections, all measured for Ga(N)(+) clusters, are properly interpreted in terms of the calculated cohesive energies, spectra of configurational excitations, and cluster shapes, respectively. The transition from "non-melter" to "magic-melter" behaviour, experimentally observed between Ga(30)(+) and Ga(31)(+), is traced back to a strong geometry change. Finally, the higher-than-bulk melting temperatures of gallium clusters are correlated with a more typically metallic behaviour of the clusters as compared to the bulk, contrary to previous theoretical claims.

Patología orbitocraneana de presentación oftalmológica / Orbitocranial pathology with ophthalmologic symptoms

Objetivos. Comunicar la correlación clínico-etiológica de diferentes entidades orbitocraneanas que se presentaron inicialmente con síntomas oftalmológicos y establecer su aporte para la elección del estudio ideal de cada caso particular.Materiales y Métodos. Se analizaron retrospectivamente 36 pacientes con patología orbitaria y/o intracraneal. Las consultas fueron realizadas entre julio de 2007 y enero de 2011, y todos los casos fueron evaluados con examen oftalmológico, Campimetría Visual Computarizada, Tomografía Computada Multislice (TCMS), Resonancia Magnética (RM) y, en algunos casos, con un estudio histopatológico. Resultados. El síntoma inicial más frecuente fue la disminución o alteración de la agudeza visual, presente en 22 pacientes (61%). Otros síntomas fueron: diplopía en 9 pacientes (25%), exoftalmos en 2 (5,5%), hematoma orbitario en 2 (5,5%) y leucocoria en 1 (3%). En el grupo de pacientes que consultó por alteración visual, los diagnósticos etiológicos fueron variados e incluyeron:meningiomas esfenoidales (n=4), enfermedad de Devic (n=2), glioma mesencefálico (n=1), gliomas ópticos en NF-1 (n=2), metástasis de carcinoma de mama (n=4), linfoma cerebral (n=2), ACV (n=4), hipofisitis linfocitaria (n=1) y pseudotumor cerebri (n=2).Entre los que originalmente manifestaron diplopía, se destacaron: un tumor de lámina cuadrigémina, un quiste pineal con hidrocefalia aguda, dos aneurismas de arteria comunicante posterior, dos aneurismas de arteria carótida interna intracavernosa (uno de ellos gigante y disecante), un aneurisma ventral de la arteria carótida interna supraclinoidea y dos mucoceles fronto-etmoidales. Llama la atención que dos meningiomas el nervio óptico se presentaran inicialmente con exoftalmos y disminución de la agudeza visual en forma secundaria y que la manifestación inicial en un paciente con síndrome de West fuera leucocoria. Conclusión. El abordaje interdisciplinario y la adecuada recomendación de estudios por imágenes en...

Patología orbitocraneana de presentación oftalmológica / Orbitocranial pathology with ophthalmologic symptoms

Objetivos. Comunicar la correlación clínico-etiológica de diferentes entidades orbitocraneanas que se presentaron inicialmente con síntomas oftalmológicos y establecer su aporte para la elección del estudio ideal de cada caso particular. Materiales y Métodos. Se analizaron retrospectivamente 36 pacientes con patología orbitaria y/o intracraneal. Las consultas fueron realizadas entre julio de 2007 y enero de 2011, y todos los casos fueron evaluados con examen oftalmológico, Campimetría Visual Computarizada, Tomografía Computada Multislice (TCMS), Resonancia Magnética (RM) y, en algunos casos, con un estudio histopatológico. Resultados. El síntoma inicial más frecuente fue la disminución o alteración de la agudeza visual, presente en 22 pacientes (61%). Otros síntomas fueron: diplopía en 9 pacientes (25%), exoftalmos en 2 (5,5%), hematoma orbitario en 2 (5,5%) y leucocoria en 1 (3%). En el grupo de pacientes que consultó por alteración visual, los diagnósticos etiológicos fueron variados e incluyeron: meningiomas esfenoidales (n=4), enfermedad de Devic (n=2), glioma mesencefálico (n=1), gliomas ópticos en NF-1 (n=2), metástasis de carcinoma de mama (n=4), linfoma cerebral (n=2), ACV (n=4), hipofisitis linfocitaria (n=1) y pseudotumor cerebri (n=2). Entre los que originalmente manifestaron diplopía, se destacaron: un tumor de lámina cuadrigémina, un quiste pineal con hidrocefalia aguda, dos aneurismas de arteria comunicante posterior, dos aneurismas de arteria carótida interna intracavernosa (uno de ellos gigante y disecante), un aneurisma ventral de la arteria carótida interna supraclinoidea y dos mucoceles fronto-etmoidales. Llama la atención que dos meningiomas del nervio óptico se presentaran inicialmente con exoftalmos y disminución de la agudeza visual en forma secundaria y que la manifestación inicial en un paciente con síndrome de West fuera leucocoria. Conclusión. El abordaje interdisciplinario y la adecuada recomendación de estudios por imágenes en la evaluación de las principales entidades neuro-oftalmológicas que se presentan con signos y síntomas oculares nos permiten arribar a un diagnóstico temprano y, como consecuencia, a la implementación del tratamiento indicado.(AU)

Objectives. To report the clinical and etiological correlation of different orbitocranial lesions in patients initially presenting with ophthalmologic symptoms and to determine its contribution to the selection of the diagnostic test of choice for each individual case. Materials and Methods. We have evaluated retrospectively 36 patients with orbital and/or intracranial conditions, who presented at the ophthalmology department of our institution between july 2007 and january 2011. All patients underwent an ophthalmologic examination, computerassisted campimetry, multislice computed tomography (MSCT), magnetic resonance imaging (MRI) and some had a histopathological test performed. Results. The most common initial symptom was decreased or impaired visual acuity, present in 22 patients (61%). Others symptoms were diplopia in 9 patients (25%), exophthalmos in 2 (5.5%), orbital hematoma in 2 (5.5%) and leukocoria in 1 (3%). In the group of patients with visual impairments, the etiological diagnoses were diverse and included: sphenoid meningioma (n=4), Devics disease (n=2), brainstem glioma (n=1), optic glioma in the context of type 1 neurofibromatosis (n=1), metastasis of breast carcinoma (n= 4), brain lymphoma (n=2), stroke (n=4), lymphocytic hypophysitis (n=1) and pseudotumor cerebri (n=2). In patients with diplopia diagnosis included: a quadrigeminal plate tumor, one pineal cyst with acute hydrocephalus, two posterior communicating artery aneurysms, two intracavernous internal carotid artery aneurysms (one of them giant and dissecting), one ventral supraclinoid internal carotid artery aneurysm and two fronto-ethmoidal mucoceles. We highlight the presence of two optic nerve meningiomas initially presented with exophthalmos and decreased visual acuity, and leukocoria as initial symptom in a West syndrome. Conclusion. The interdisciplinary approach and the proper recommendation of imaging in the evaluation of the major neuro-ophthalmologic entities that present with ophthalmologic symptoms allows us to arrive at an early diagnosis, and hence, to the institution of the appropriate therapy.(AU)

Identifying structural and energetic trends in isovalent core-shell nanoalloys as a function of composition and size mismatch.

We locate the putative global minimum structures of Na(x)Cs(55-x) and Li(x)Cs(55-x) nanoalloys through combined empirical potential and density functional theory calculations, and compare them to the structures of 55-atom Li-Na and Na-K nanoalloys obtained in a recent paper [A. Aguado and J. M. López, J. Chem. Phys. 133, 094302 (2010)]. Alkali nanoalloys are representative of isovalent metallic mixtures with a strong tendency towards core-shell segregation, and span a wide range of size mismatches. By comparing the four systems, we analyse how the size mismatch and composition affect the structures and relative stabilities of these mixtures, and identify useful generic trends. The Na-K system is found to possess a nearly optimal size mismatch for the formation of poly-icosahedral (pIh) structures with little strain. In systems with a larger size mismatch (Na-Cs and Li-Cs), frustration of the pIh packing induces for some compositions a reconstruction of the core, which adopts instead a decahedral packing. When the size mismatch is smaller than optimal (Li-Na), frustration leads to a partial amorphization of the structures. The excess energies are negative for all systems except for a few compositions, demonstrating that the four mixtures are reactive. Moreover, we find that Li-Cs and Li-Na mixtures are more reactive (i.e., they have more negative excess energies) than Na-K and Na-Cs mixtures, so the stability trends when comparing the different materials are exactly opposite to the trends observed in the bulk limit: the strongly non-reactive Li-alkali bulk mixtures become the most reactive ones at the nanoscale. For each material, we identify the magic composition x(m) which minimizes the excess energy. x(m) is found to increase with the size mismatch due to steric crowding effects, and for Li(x)Cs(55-x) the most stable cluster has almost equiatomic composition. We advance a simple geometric packing rule that suffices to systematize all the observed trends in systems with large size mismatch (Na-K, Na-Cs, and Li-Cs). As the size mismatch is reduced, however, electron shell effects become more and more important and contribute significantly to the stability of the Li-Na system.

First-principles determination of the structure of NaN and NaN- clusters with up to 80 atoms.

We have performed an extensive computational search for the global minimum (GM) structures of both neutral and anionic sodium clusters with up to 80 atoms. The theoretical framework combines basin hopping unbiased optimizations based on a Gupta empirical potential (EP) and subsequent reoptimization of many candidate structures at the density functional theory level. An important technical point is that the candidates are selected based on cluster shape descriptors rather than the relative stabilities of the EP model. An explicit comparison of the electronic density of states of cluster anions to experimental photoemission spectra suggests that the correct GM structures have been identified for all but two sizes (N = 47 and 70). This comparison validates the accuracy of the proposed methodology. Furthermore, our GM structures either match or improve over the results of previous works for all sizes. Sodium clusters are seen to accommodate strain very efficiently because: (a) many structures are based on polyicosahedral packing; (b) others are based on Kasper polyhedra and show polytetrahedral order; (c) finally, some (N + 1)-atom structures are obtained by incorporating one adatom into the outermost atomic shell of a compact N-atom cluster, at the cost of increasing the bond strain. GM structures of neutrals and anions differ for most sizes. Cluster stabilities are analyzed and shown to be dominated by electron shell closing effects for the smaller clusters and by geometrical packing effects for the larger clusters. The critical size separating both regimes is around 55 atoms. Some implications for the melting behavior of sodium clusters are discussed.

Melting and freezing of metal clusters.

Recent developments allow heat capacities to be measured for size-selected clusters isolated in the gas phase. For clusters with tens to hundreds of atoms, the heat capacities determined as a function of temperature usually have a single peak attributed to a melting transition. The melting temperatures and latent heats show large size-dependent fluctuations. In some cases, the melting temperatures change by hundreds of degrees with the addition of a single atom. Theory has played a critical role in understanding the origin of the size-dependent fluctuations, and in understanding the properties of the liquid-like and solid-like states. In some cases, the heat capacities have extra features (an additional peak or a dip) that reveal a more complex behavior than simple melting. In this article we provide a description of the methods used to measure the heat capacities and provide an overview of the experimental and theoretical results obtained for sodium and aluminum clusters.

Structure determination in 55-atom Li-Na and Na-K nanoalloys.

The structure of 55-atom Li-Na and Na-K nanoalloys is determined through combined empirical potential (EP) and density functional theory (DFT) calculations. The potential energy surface generated by the EP model is extensively sampled by using the basin hopping technique, and a wide diversity of structural motifs is reoptimized at the DFT level. A composition comparison technique is applied at the DFT level in order to make a final refinement of the global minimum structures. For dilute concentrations of one of the alkali atoms, the structure of the pure metal cluster, namely, a perfect Mackay icosahedron, remains stable, with the minority component atoms entering the host cluster as substitutional impurities. At intermediate concentrations, the nanoalloys adopt instead a core-shell polyicosahedral (p-Ih) packing, where the element with smaller atomic size and larger cohesive energy segregates to the cluster core. The p-Ih structures show a marked prolate deformation, in agreement with the predictions of jelliumlike models. The electronic preference for a prolate cluster shape, which is frustrated in the 55-atom pure clusters due to the icosahedral geometrical shell closing, is therefore realized only in the 55-atom nanoalloys. An analysis of the electronic densities of states suggests that photoelectron spectroscopy would be a sufficiently sensitive technique to assess the structures of nanoalloys with fixed size and varying compositions.

Activation of dinitrogen by solid and liquid aluminum nanoclusters: a combined experimental and theoretical study.

Cross sections for chemisorption of N2 onto Al44(+/-) cluster ions have been measured as a function of relative kinetic energy and the temperature of the metal cluster. There is a kinetic energy threshold for chemisorption, indicating that it is an activated process. The threshold energies are around 3.5 eV when the clusters are in their solid phase and drop to around 2.5 eV when the clusters melt, indicating that the liquid clusters are much more reactive than the solid. Below the melting temperature the threshold for Al44(-) is smaller than for Al44(+), but for the liquid clusters the anion and cation have similar thresholds. At high cluster temperatures and high collision energies the Al44N2(+/-) chemisorption product dissociates through several channels, including loss of Al, N2, and Al3N. Density functional calculations are employed to understand the thermodynamics and the dynamics of the reaction. The theoretical results suggest that the lowest energy pathway for activation of dinitrogen is not dynamically accessible under the experimental conditions, so that an explicit account of dynamical effects, via molecular dynamics simulations, is necessary in order to interpret the experimental measurements. The calculations reproduce all of the main features of the experimental results, including the kinetic energy thresholds of the anion and cation and the dissociation energies of the liquid Al44N2(+/-) product. The strong increase in reactivity on melting appears to be due to the volume change of melting and to atomic disorder.

Photoelectron spectroscopy of cold aluminum cluster anions: comparison with density functional theory results.

Photoelectron spectra of cold aluminum cluster anions Al(n)(-) have been measured in the size range n=13-75 and are compared to the results of density functional theory calculations. Good agreement between the measured spectra and the calculated density of states is obtained for most sizes, which gives strong evidence that the correct structures have been found. In particular the results confirm the occurrence of rather different structural motifs in this size range, from fcc-like stacks over fragments of decahedrons to disordered structures. An analysis of the density of states of representatives of the different structural motifs shows that the electronic structure is strongly influenced by the cluster geometry, and that a clear jelliumlike electron shell structure is present only in some exceptional cases.

Structural and electronic properties of oxidized sodium clusters: A combined photoelectron and density functional study.

Oxygen-doped sodium cluster anions Na(n)O(2) (-) with n=41-148 have been studied by low temperature photoelectron spectroscopy and density functional theory (DFT), with a particular emphasis on those sizes where a spherical electron shell closing is expected. The experimental spectra are in good agreement with the electronic density of states of the DFT lowest energy structures. The cluster structures show segregation between an ionically bonded molecular unit located at the cluster surface and a metallic part. The DFT calculations reveal that each oxygen atom removes two electrons from the metallic electron gas in order to become an O(2-) dianion. A jellium model would therefore predict the electron shell closings to be shifted up by four sodium atoms with respect to pure Na(n) (-) cluster anions. The electron shell closings for Na(n)O(2) (-) are located at n=43, 61, 93, and 139, so the expected four-atom shift is observed only for the small clusters of up to n=61, while a two-atom shift is observed for the larger clusters. The DFT calculations explain this departure from jellium model predictions in terms of a structural transition in the ionically bonded molecular unit.

Electronic effects on melting: comparison of aluminum cluster anions and cations.

Heat capacities have been measured as a function of temperature for aluminum cluster anions with 35-70 atoms. Melting temperatures and latent heats are determined from peaks in the heat capacities; cohesive energies are obtained for solid clusters from the latent heats and dissociation energies determined for liquid clusters. The melting temperatures, latent heats, and cohesive energies for the aluminum cluster anions are compared to previous measurements for the corresponding cations. Density functional theory calculations have been performed to identify the global minimum energy geometries for the cluster anions. The lowest energy geometries fall into four main families: distorted decahedral fragments, fcc fragments, fcc fragments with stacking faults, and "disordered" roughly spherical structures. The comparison of the cohesive energies for the lowest energy geometries with the measured values allows us to interpret the size variation in the latent heats. Both geometric and electronic shell closings contribute to the variations in the cohesive energies (and latent heats), but structural changes appear to be mainly responsible for the large variations in the melting temperatures with cluster size. The significant charge dependence of the latent heats found for some cluster sizes indicates that the electronic structure can change substantially when the cluster melts.