Conspectus There exists a special kind of perfectionin symmetry, simplicity, and stabilityattainable for structures generated from precisely 60 ligands (all of a single type) that protect 145 ...metal-atom sites. The symmetry in question is icosahedral (Ih ), generally, and chiral icosahedral (I) in particular. A 60-fold equivalence of the ligands is the smallest number to allow this kind of perfection. Known cluster compounds that approximate this structural ideal include palladium-carbonyls, Ih -Pd145(CO)60; gold-thiolates, I-Au144(SR)60; and gold-alkynyls, I-Au144(C2R)60. Many other variants are suspected. The Pd145 compound established the basic achiral structure-type. However, the Au144-thiolate archetype is prominent, historically in its abundance and ease of preparation and handling, in its proliferation in many laboratories and application areas, and ultimately in the intrinsic chirality of its geometrical structure and organization of its bonding network or connectivity. As discovered by mass spectrometry (the “30-k anomaly”) in 1995, it appeared as a broad single peak, as solitary and symmetrical as Mount Fuji, centered near 30 kDa (∼150 Au atoms), provoking these thoughts: Surely this phenomenon requires a unique explanation. It appears to be the Buckminsterfullerene (carbon-60) of gold-cluster chemistry. Herein we provide an elementary account of the unexpected discovery, in which the Pd145-structure played a critical role, that led to the identification and prediction, in 2008, of a fascinating new molecular structure-type, evidently the first one of chiral icosahedral symmetry. Rigorous confirmation of this prediction occurred in early spring 2018, when two single-crystal X-ray crystallography reports were submitted, each one distinguishing both enantiomeric structures and noting profound chirality for the surface (ligand) layer. The emphasis here is on the structure and bonding principles and how these have been elucidated. Our aim has been to present this story in simplest terms, consistent with the radical simplicity of the structure itself. Because it combines intrinsic profound chirality, at several levels, with the highest possible symmetry-type (icosahedral), the structure may attract broader interest also from educators, especially if studied in tandem with the analysis of hollow (shell) metallic systems that exhibit the same chirality and symmetry. Because the shortest (stiffest) bonds follow the chiral 3-way weave pattern of the traditional South-Asian reed football, this cultural artifact may be used to introduce chiral-icosahedral symmetry in a pleasant and memorable way. One may also appreciate easily the bonding and excitations in I-symmetry metallic nanostructures via the golden fullerenes, that is, the proposed hollow Au60,72 spheres. Beyond any aesthetic or pedagogical value, we aim that our Account may provide a firm foundation upon which others may address open questions and the opportunities they present. This Account can scarcely hint at the prospects for further fundamental understanding of these compounds, as well as a widening sphere of applications (chemical, electronic, imaging). The compounds remain crucial to a wider field presently under intense development.
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Chirality has been found as a relevant property of nanomaterials, including ligand-protected metal clusters and nanorods. This property not only is crucial in nanotechnology developments related to ...asymmetric catalysis and chiroptical phenomena but also generates fundamental questions on the existence of chirality at the nanoscale. In fact, X-ray total structure determination, electron diffraction studies, NMR, and circular dichroism spectroscopies as well as theoretical calculations performed on gold clusters protected with thiolate or phosphine ligands have confirmed the existence of chiral structures in the size range of 18–144 Au atoms. In this work, we realize a comparative analysis of the degree or amount of chirality existing in chiral ligand-protected gold clusters (LPGC), through a geometric quantification, using the Hausdorff chirality measure (HCM). The calculated HCM values provide a quantitative framework to compare, classify, and gain insight into the origin of chirality. Interestingly, these values are consistent with the current knowledge on the different sources of chirality: achiral cores and chiral arrangement of ligands in, for example, Au102(SR)44 and Au38(SR)24 or intrinsically chiral cores, like in Au52(SR)32 and Au20 protected with phosphine ligands. Our calculations are also helpful to assign an index of chirality and classify as chiral several recently synthesized and structurally solved LPGC that in first instance were not identified as such. The calculated HCM values are used to extract trends on how chirality is spatially distributed in LPGC and correlate them with optical activity measurements. The main trend indicates that the Au–S interface has the dominant role in the chirality of LPGC.
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Chirality is a fundamental property of matter with profound impact in physics, chemistry, biology, and medicine. It is present at several scales going from elementary particles, to molecules, to ...macroscopic materials, and even to astronomical objects. During the last 30 years, chirality has also been investigated at the nanoscale, being a hot research topic in nanoscience. The importance of chirality at the nanoscale is due, in part, to the potential applications that chiral nanomaterials could have in nanotechnology. Great interest exists nowadays in the study of chirality in bare and ligand-protected metal nanoclusters. These are aggregates of n metal atoms (n ~ 10-300) that can be in gas phase or stabilized by organic ligands, covering the cluster surface. Chirality in bare and thiolate-protected gold clusters (TPGC) has received special attention because of the important progress achieved in their synthesis, size separation, and precise structural characterization. Here, we review the recent experimental and theoretical developments on the origin and physicochemical manifestations of chirality in bare and TPGC . Since chirality is a geometrical property, we also discuss the proposal for its quantification, and the correlation of this geometric measure with the chiroptical response, like the circular dichroism spectrum, calculated from quantum mechanical methods.
The vibrational properties of atomic clusters are a fingerprint of their structures and can be used to investigate their thermodynamic behavior at low temperatures. In this work, we report a ...theoretical study, using density functional theory, on the vibrational spectrum and density of states (VDOS) of the cationic sodium cluster Na139 +. Our study focuses on the most stable isomer, which corresponds to a truncated icosahedron. This isomer displays an electronic density of states that is in good agreement with photoelectron spectroscopy data previously published. After validation of the sodium cluster structure, its vibrational frequency spectrum was obtained in the harmonic approximation through a diagonalization of the dynamical matrix. The calculated vibrational frequencies were used to evaluate the cluster caloric curve and heat capacity at low temperatures. An excellent agreement was obtained between the calculated caloric curve and experimental data recently reported down to 6 K. A fit to the bulk Debye model of the calculated and measured cluster thermal energy yields a large variation at low temperatures of the equivalent Debye temperature as compared with a weaker temperature dependence found in bulk materials. Moreover, a further analysis shows that the calculated heat capacity of the 139-atom cationic sodium cluster does not follow the bulk Debye T 3 law at very low temperatures, due to the discreteness of the cluster frequency spectrum, and to the finite value of its acoustic gap (lowest frequency value). These results, indicating a finite size effect on the cluster vibrational spectrum, reflect the difference in the VDOS between clusters and bulk, and confirm the limitation of the bulk Debye model (and Debye temperature) to describe the low-temperature thermal behavior of metal clusters in the size range of around 139 atoms. The calculated vibrational frequency spectrum also provides the temperature dependence of the total vibrational excitation for the 139-atom sodium clusters, indicating that at 6 K, ∼ 92% of them are in their vibrational ground state.
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An effectiveway of enhancing hydrogen storage on adsorbent materials can be induced by the hydrogen spill-over mechanism, although to date there is no general consensus which satisfactorily explains ...the mechanism. In this work, a possible reaction path to explain hydrogen adsorption is shown. Density-functional calculations were used to study the dissociation of molecular hydrogen near to a stressed region, as a consequence of chemisorbed hydrogen at the graphene-nitrogen surface. We found that as a result of the buckling induced by the chemisorbed hydrogen, the dissociation barrier of molecular hydrogen diminished by 0.84
e
V
. The chemisorbed hydrogen is the final state in the spill-over mechanism on a graphene-nitrogen decorated with palladium clusters. This effect helps to create hydrogen nanoislands that may change the diffusion and detrapping of H. An electronic structure analysis suggests that these systems occasionally present metallic or semiconductor behavior.
Graphical Abstract
Hydrogen dissociation and adsorption process via buckling defect
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
An effectiveway of enhancing hydrogen storage on adsorbent materials can be induced by the hydrogen spill-over mechanism, although to date there is no general consensus which satisfactorily explains ...the mechanism. In this work, a possible reaction path to explain hydrogen adsorption is shown. Density-functional calculations were used to study the dissociation of molecular hydrogen near to a stressed region, as a consequence of chemisorbed hydrogen at the graphene-nitrogen surface. We found that as a result of the buckling induced by the chemisorbed hydrogen, the dissociation barrier of molecular hydrogen diminished by 0.84 eV. The chemisorbed hydrogen is the final state in the spill-over mechanism on a graphene-nitrogen decorated with palladium clusters. This effect helps to create hydrogen nanoislands that may change the diffusion and detrapping of H. An electronic structure analysis suggests that these systems occasionally present metallic or semiconductor behavior. Graphical Abstract Hydrogen dissociation and adsorption process via buckling defect.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
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A systematic theoretical study on graphene oxide model systems was performed with Density Functional Theory (DFT), and supported by experimental evidence. The results revealed that ...graphene is highly susceptible to be decorated with organic functional groups, which induced the formation of a band gap, and the rising of a novel semiconducting character. This novel property was used to explore the possible photocatalytic potential in the model systems under study. That is, we evaluated work functions to theoretically obtain the energies of the valence band maximum and the conduction band minimum with respect to the normal hydrogen electrode potential. The assessment of UV-vis profile via Time-dependent DFT also showed the potential of the model systems to efficiently absorb sun light irradiation under photocatalytic conditions. Moreover, the results showed that it is possible to tune the photocatalytic potential of the graphene oxide models under study by interchanging the functional groups anchored on the graphene surface, and their corresponding contents ratio. Experimental evidence obtained via the measurement of optoelectronic properties, revealed that it is possible to classify a graphene oxide powder into one of the model systems under study, while a photocatalytic procedure performed in our laboratory, showed the facile photoreduction of formic acid into methanol with such a graphene oxide. Consequently, the prediction of the electronic structure properties is expected. This may represent a tool to design materials based on graphene oxide to be implemented in reactors for photocatalytic applications.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Time and temperature, besides pressure in a lesser extent, represent the most significant variables influencing the rheological behavior of viscoelastic materials. These magnitudes are each other ...related through the well-known Time-Temperature Superposition (TTS) principle, which allows the master curve referred to relaxation (or creep) behavior to be derived as a material characteristic. In this work, a novel conversion law to interrelate relaxation curves at different temperatures is proposed by assuming they to be represented by statistical cumulative distribution functions of the normal or Gumbel family. The first alternative responds to physical considerations while the latter implies the fulfillment of extreme value conditions. Both distributions are used to illustrate the suitability of the model when applied to reliable derivation of the master curve of Polyvinil-Butyral (PVB) from data of experimental programs. The new approach allows not only the TTS shift factors to be estimated by a unique step, but the whole family of viscoelastic master curves to be determined for the material at any temperature. This represents a significant advance in the characterization of viscoelastic materials and, consequently, in the application of the TTS principle to practical design of viscoelastic components.
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Partiendo de la dificultad que entraña la descripción documental del fondo cartográfico antiguo, se recogen y analizan aquellos aspectos más problemáticos de este proceso, tomando como marco ...referencial la normativa española al respecto, contenida en las Reglas de Catalogación (RC), así como los códigos de ámbito internacional ISBD (CM) y AACR2. Se distinguen cuatro grandes bloques temáticos de problemas: normativos, de presentación de los datos en el documento, de codificación matemática del dibujo cartográfico y de indización. Se plantea la necesidad de una revisión del capítulo 4 de RC, que incorpore todos los avances surgidos tanto de la propia experiencia profesional en este campo como del desarrollo de los sistemas electrónicos de codificación y recuperación de la información. En este sentido se aportan algunas sugerencias y soluciones. Finalmente se incluye una bibliografía seleccionada sobre todas las facetas que componen el proceso de descripción.
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