Structural changes of Pd-13 upon charging and oxidation/reduction

First-principle generalized gradient corrected density functional calculations have been performed to study the stability of cationic and anionic Pd13 +/−, and neutral Pd13O2 clusters. It is found that while cationic Pd13 + favors a C s geometry similar to the neutral Pd13, both anionic Pd13 − and neutral Pd13O2 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 Pd13 in which small energy barriers between 0.3 and 0.4 eV are involved

Re-significación del espacio público mediante acciones participativas y de pedagogía urbana como estrategia para la construcción social de sustentabilidad: caso barrio de Analco

El barrio de Analco es uno de los primeros de Guadalajara, está establecido en el margen oriente del río San Juan de Dios y fue originalmente conformado por pueblos indígenas; con el tiempo su población se transformó a una con mayor mestizaje, lo que generó características socioculturales propias en la que los usos y costumbres le dieron un sentido simbólico e identitario a la vida comunitaria del barrio. El río San Juan de Dios, hoy Calzada Independencia, se estableció como una división territorial y una frontera sociocultural que separó a la ciudad en dos poblaciones con base en criterios étnicos y económicos. Analco se ha visto afectado por intervenciones urbanas, así como por la externalidad de las explosiones del 22 de abril de 1992. Esto ha generado fracturas y un deterioro físico-urbano que resulta evidente en sus espacios públicos, los cuales denotan una transformación en su estructura morfológico-social. El presente estudio de caso abordó el espacio público de este barrio desde el campo del urbanismo y mediante una metodología mixta de triangulación que incluyó la dimensión cuantitativa, la delimitación geográfica tangible y la dimensión cualitativa, la cual se apoyó en la antropología, la psicología social y la pedagogía urbana. La intervención se realizó en conjunto con la comunidad con el fin de incidir en el fortalecimiento del tejido social, de potencializar la calidad de vida y de fomentar la creación de vínculos identitarios como parte del proceso para la construcción social de sustentabilidad que favorecerá la re-significación en el barrio de Analco.Consejo Nacional de Ciencia y Tecnologí

Formation of Silicate Grains in Circumstellar Environments: Experiment, Theory and Observations

Amongst chemical reactions (1) in the molecular universe (2), condensation reaction is probably the most poorly understood. The condensation of a solid from its components in the gas phase occurs in many parts of our galaxy such as stellar mass outflows, the terrestrial region of protoplanetary disks and in primordial solar nebula (3). But how does the transition occur from molecules to intermediate clusters to macroscopic grains? The major focus of the present work is the identification of chemical condensation reaction pathways that lead to the formation of stoichiometry, composition and crystallinity of cosmic silicates from vapor phase species

Photoelectron imaging and theoretical investigation of bimetallic Bi1–2Ga−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 BimGan (m=1–2, n=0–2), and Pbn (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 BimGan and Pbn clusters derived from the photoelectron images. Experiments on the BiGan 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 BiGa2. The measured photoelectron angular distributions of the BimGan and Pbn 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, Bi2Ga−3 and Bi2Ga−5 (anionic), and BiGa3, BiGa5, Bi2Ga4 and Bi2Ga6(neutral) clusters are unusually stable. The stability of the anionic and neutral Bi2Gan 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 BiGa3 cluster is rationalized as being similar to BiAl3, an all-metal aromatic cluster

Closed-shell to split-shell stability of isovalent clusters

Metallic clusters containing 2, 8, 18, and 20 electrons are now known to exhibit enhanced stability that can be reconciled because of filled 1S, 1P, 1D, and 2S electronic shells within a simplified confined nearly free electron (NFE) gas. Here, we present first-principles studies on three isovalent clusters, i.e., ZnMg8, CuMg8−, and AuMg8−, each with 18 valence electrons. All the clusters exhibit local energetic stability but with differing origins. Although the stability of ZnMg8 can be reconciled within the conventional confined NFE picture with filled 1S2, 1P6, and 1D10shells, CuMg8− and AuMg8− are shown to be stable despite the unfilled D-shell. Their stability can be understood as a crystal field–like splitting of the otherwise degenerate D-shell because of internal electric fields of the positive ion cores that lead to a filled 1S2, 1P6, 1D8, 2S2 sequence separated by unfilled D2 states that form a large gap. We also examine the progression toward the metallic character in ZnMgn clusters, because isolated Mg and Zn atoms have filled valence 4s2 and 3s2 atomic states. As Mg atoms are added to a Zn atom, the excited atomic p-states in the Mg atoms hybridize rapidly with Zn and Mg s-states to promote a metallic character that evolves more rapidly than in pure Mgn clusters

Structural, electronic, and chemical properties of multiply iodized aluminum clusters

The electronic structure, stability, and reactivity of iodized aluminum clusters, which have been investigated via reactivity studies, are examined by first-principles gradient corrected density functional calculations. The observed behavior of Al13I−x and Al14I−x clusters is shown to indicate that for x⩽8, they consist of compact Al−13 and Al++14 cores, respectively, demonstrating that they behave as halogen- or alkaline earthlike superatoms. For x\u3e8, the Al cores assume a cagelike structure associated with the charging of the cores. The observed mass spectra of the reacted clusters reveal that Al13I−x species are more stable for even x while Al14I−x exhibit enhanced stability for odd x(x⩾3). It is shown that these observations are linked to the formation and filling of “active sites,” demonstrating a novel chemistry of superatoms

Full-rotation singularity-safe workspace for kinematically redundant parallel robots

This paper introduces and computes a novel type of work-space for kinematically redundant parallel robots that defines the regionin which the end-effector can make full rotations without coming close tosingular configurations; it departs from the traditional full-rotation dex-terous workspace, which considers full rotations without encounteringsingularities but does not take into account the performance problemsresulting from closeness to these locations. Kinematically redundant ar-chitectures have the advantage of being able to be reconfigured withoutchanging the pose of the end-effector, thus being capable of avoidingsingularities and being suitable for applications where high dexterityis required. Knowing the workspace of these robots in which the end-effector is able to complete full, smooth rotations is a key design aspectto improve performance; however, since this singularity-safe workspaceis generally small, or even non-existent, in most parallel manipulators,its characterisation and calculation have not received attention in theliterature. The proposed workspace for kinematically redundant robotsis introduced using a planar parallel architecture as a case study; the for-mulation works by treating the manipulator as two halves, calculatingthe full-rotation workspace of the end-effector for each half whilst ensur-ing singularity conditions are not approached or met, and then findingthe intersection of both regions. The method is demonstrated ontwoexample robot instances, and a numerical analysis is also carried out asa comparison