The closed-edge structure of graphite and the effect of electrostatic charging

The properties of graphite, and of few-layer graphene, can be strongly influenced by the edge structure of the graphene planes, but there is still much that we do not understand about the geometry and stability of these edges. We present an experimental and theoretical study of the closed edges of graphite crystals, and of the effect of an electric field on their structure. High-resolution transmission electron microscopy is used to image the edge structure of fresh graphite and of graphite that has been exposed to an electric field, which experiences a separation of the graphene layers. Computer simulations based on density functional theory are used to rationalise and quantify the preference for the formation of multiple concentric loops at the edges. A model is also presented to explain how the application of an electric field leads to the separation of the folded edges

Estudio Teórico Cuanto Mecánico de Cristales Formados por Nanoclusters de Nitruro de Boro [BiNi, i = 12]

Although it has been predicted that boron nitride clusters (i.e., BiNi, i = 12-24) are capable of forming stable periodic structures, little is known about the electronic and vibrational properties of these solids. In this work, a quantum mechanical study of the B12N12 system using periodic models is presented to theoretically characterize this material and determine its potential applications such as the ability to absorb guest molecules such as molecular hydrogen. All the calculations were performed with the program CRYSTAL09 using the B3LYP (HF-DFT hybrid functional) and a localized Gaussian-type basis set of different flexibility. Electrostatic potential maps of different planes of the B12N12 system showed that the crystalline structure has sites capable of hosting molecular species due to their low electron density. Moreover, the computed vibrational frequencies allowed the identification of well-defined zones of the IR-spectrum of this solid.Aunque se ha sugerido que los clusters de nitruro de boro (i.e., BiNi, i = 12-24) son capaces de agregarse y formar estructuras periódicas estables, se conoce poco sobre las propiedades electrónicas y vibracionales de estos sólidos. En este trabajo, un estudio cuanto mecánico del sistema B12N12 es presentado utilizando modelos periódicos para caracterizar teóricamente este material y determinar sus posibles aplicaciones tales como la capacidad para absorber moléculas huéspedes, en particular hidrógeno molecular. Todos los cálculos se realizaron mediante el programa CRYSTAL09 usando el funcional híbrido B3LYP (HF-DFT) y funciones base localizadas de tipo Gaussiano de diferente flexibilidad. Mapas de potencial electrostático de diferentes planos del sistema B12N12 mostraron que la estructura cristalina tiene sitios capaces de alojar especies moleculares debido a su baja densidad electrónica. Las frecuencias vibracionales calculadas permitieron la identificación de zonas bien definidas del espectro IR de este sólido. Además se realizó un estudio de almacenamiento de moléculas huésped en las superficies del cristal en planos seleccionados

Síntesis de 2,8,14,20-tetra-n-butilpirogalol[4]areno y estudio computacional conformacional

This work presents the preparation of mesoporous aluminum silicates from condensation of xerogel obtained from Tetraethyl orthosilicate (TEOS) and other aluminum precursos from various sources. The reagents used were aluminum nitrate (NAN) as inorganic source and the inorganic-alkoxide, Aluminun tris-secbutilate (TBA) as organometallic source. The length of the alkyl chain was evaluated in regards to its influence upon the surface area and in the generation of mesopores in the materials. Additionally, the former parameters were studied to determine their effect on the hydrothermal stability of the aluminosilicates.Este trabajo presenta la obtención de aluminosilicatos mesoporosos generados mediante condensación de xerogel a partir de alcóxido de silicio (TEOS) y precursores de aluminio de diversas fuentes. Se usaron nitrato de aluminio (NAN), como fuente inorgánica y el alcoxido, tris-sec-butilato de aluminio (TBA) como fuente órgano-metálica. Se analizó la importancia del tamaño de cadena de alquilo en el surfactante (Tritón-X) en el proceso de condensación de la red, y su posterior influencia sobre el área superficial y en la obtención de mesoporos en los materiales. Adicionalmente se estudió la influencia de las variables mencionadas sobre la estabilidad hidrotermal de los aluminosilicatos

Caracterización por Espectrometría de Masas MALDI de Cucurbit[n]uriles (n = 6,7,8)

The characterization of a mixture of cucurbit[n]uril (n = 6,7,8) was carried out by MALDI MS (Matrix-assisted Laser Desorption/Ionization Mass Spectrometry Mass Spectrometry). The solid mixture of the titled compounds was synthesized as part of a research study involving supramolecular chemistry at the Departamento de Química e Ingeniería Química at USFQ. For the MALDI analysis, an appropriate procedure was implemented for sample preparation, which includes the effective dissolution of the solid mixture of cucurbit[n]uril in a solution of water and formic acid, and the addition of alpha-cyano-4-hydroxy cinamic acid as MALDI matrix (10g/l in 70:30 (v:v) of methanol: acetonitrile). In the sample preparation process for MALDI, other solvents were employed to try dissolving the cucurbit[n]uril mixture with poor results. The addition of formic acid to deionized water was important to achieve satisfactory dissolution of the cucurbit[n]uril compounds and to assure its compatibility with the matrix. To acquire the corresponding mass spectra, a MALDI-Time-of-Flight Mass Spectrometer (MALDI-TOF MS) was used, which was built in the Departamento de Física at EPN. The mass spectra of the detected cucurbit[n]uril ions were characterized by the presence of the single protonated peaks of three molecular species corresponding to n = 6,7,8 as inferred from their m/z values. Additionally, the mass spectra contained abundant peaks of the MALDI matrix in the mass region up to 500 Da. The mass spectra were satisfactorily internally calibrated with the help of multiple peaks of a PEG600 polymer that was introduced as an internal standard in the sample. The instrumental molecular mass resolution allowed for isotopical resolution over the whole mass range and the overall mass accuracy was about 0.1 %. The most intense signal among the cucurbit[n]uril peaks was assigned to the cucurbit[7]uril compound. A discussion is advanced to rationalize the observed molecular cucurbit[n]uril species under the used methodology.La caracterización de una mezcla de cucurbit[n]uriles (n = 6,7,8) se ha realizado por medio de espectrometría de masas con la técnica de ionización MALDI (Matrix-assisted Laser Desorption/Ionization). Los cucurbit[n]uriles fueron sintetizados como parte de una investigación de química supramolecular dentro del Departamento de Química e Ingeniería Química de la USFQ. Para el análisis con MALDI se ha introducido un procedimiento adecuado para la preparación de muestras, que incluye la disolución efectiva de la mezcla sólida de cucurbit[n]uriles en una solución de agua y ácido fórmico, y el uso del ácido alfa-ciano-4-hidroxi-cinámico como matriz MALDI, en una solución 10g/l en 70:30 (v:v) de metanol y acetronitrilo. En el proceso de preparación de muestras para MALDI se intentó utilizar otros solventes para el cucurbit[n]uril con pobres resultados. La adición del ácido fórmico al agua desionizada fue importante para lograr la correcta disolución del compuesto y asegurar su compatibilidad con la solución de matriz. Para obtener los espectros de masas correspondientes se utilizó un espectrómetro de masas de tiempo de vuelo MALDI (MALDI TOF-MS) construido en el Departamento de Física de la EPN. Los espectros de masa de los cucurbit[n]uriles se caracterizaron por la presencia de los iones protonados con una carga de las especies moleculares correspondientes a n = 6,7,8, según se infiere de los valores m/z medidos. Adicionalmente, los espectros de masa contuvieron picos abundantes de la matriz MALDI en la región de masas hasta 500 Da. Los espectros de masa se calibraron internamente de forma satisfactoria con la ayuda de os múltiples picos del polímero PEG600 que se introdujo como un estándar interno en las muestras. La resolución de masas instrumental permitió separar las componentes isotópicas a lo largo de todo el rango de masas y una precisión general en la detrminación de la masa de alrededor de 0.1 %. La señal más intensa entre los cucurbit[n]uriles se la asignó al compuesto cucurbit[7]uril. Se hace una corta discusión tendiente a racionalizar la observación de las especies observadas con nuestra metodología experimental

2,2,3,3′-Tetra­phenyl-7,7′-biquinoxaline

In the crystal structure of the title compound, C40H26N4, mol­ecules reside on crystallographic centers of inversion and are linked via C—H⋯N inter­actions about inversion centers into one-dimensional chains: longer C—H⋯π(arene) inter­actions complete the inter­molecular inter­actions

Azimuthal anisotropy of neutral pion production in Au+Au collisions at sqrt(s_NN) = 200 GeV: Path-length dependence of jet quenching and the role of initial geometry

We have measured the azimuthal anisotropy of pi0's for 1 < pT < 18 GeV/c for Au+Au collisions at sqrt s_NN = 200 GeV. The observed anisotropy shows a gradual decrease in 3 < pT < 7 - 10 GeV/c, but remains positive beyond 10 GeV/c. The magnitude of this anisotropy is under-predicted, up to at least 10 GeV/c, by current perturbative QCD (pQCD) energy-loss model calculations. An estimate of the increase in anisotropy expected from initial-geometry modification due to gluon saturation effects and initial-geometry fluctuations is insufficient to account for this discrepancy. Calculations which implement a path length dependence steeper than what is implied by current pQCD energy-loss models, show reasonable agreement with the data.Comment: 384 authors, 6 pages text, 3 figures. Submitted to Phys. Rev. Lett. Plain text data tables for the points plotted in figures for this and previous PHENIX publications are (or will be) publicly available at http://www.phenix.bnl.gov/papers.htm

The number of tree species on Earth

One of the most fundamental questions in ecology is how many species inhabit the Earth. However, due to massive logistical and financial challenges and taxonomic difficulties connected to the species concept definition, the global numbers of species, including those of important and well-studied life forms such as trees, still remain largely unknown. Here, based on global ground sourced data, we estimate the total tree species richness at global, continental, and biome levels. Our results indicate that there are ∼73,000 tree species globally, among which ∼9,000 tree species are yet to be discovered. Roughly 40% of undiscovered tree species are in South America. Moreover, almost one-third of all tree species to be discovered may be rare, with very low populations and limited spatial distribution (likely in remote tropical lowlands and mountains). These findings highlight the vulnerability of global forest biodiversity to anthropogenic changes in land use and climate, which disproportionately threaten rare species and thus, global tree richness. Please note an (erratum/corrigendum) for this article is available via https://www.pnas.org/doi/10.1073/pnas.220278411

Evenness mediates the global relationship between forest productivity and richness

1. Biodiversity is an important component of natural ecosystems, with higher species richness often correlating with an increase in ecosystem productivity. Yet, this relationship varies substantially across environments, typically becoming less pronounced at high levels of species richness. However, species richness alone cannot reflect all important properties of a community, including community evenness, which may mediate the relationship between biodiversity and productivity. If the evenness of a community correlates negatively with richness across forests globally, then a greater number of species may not always increase overall diversity and productivity of the system. Theoretical work and local empirical studies have shown that the effect of evenness on ecosystem functioning may be especially strong at high richness levels, yet the consistency of this remains untested at a global scale. 2. Here, we used a dataset of forests from across the globe, which includes composition, biomass accumulation and net primary productivity, to explore whether productivity correlates with community evenness and richness in a way that evenness appears to buffer the effect of richness. Specifically, we evaluated whether low levels of evenness in speciose communities correlate with the attenuation of the richness–productivity relationship. 3. We found that tree species richness and evenness are negatively correlated across forests globally, with highly speciose forests typically comprising a few dominant and many rare species. Furthermore, we found that the correlation between diversity and productivity changes with evenness: at low richness, uneven communities are more productive, while at high richness, even communities are more productive. 4. Synthesis. Collectively, these results demonstrate that evenness is an integral component of the relationship between biodiversity and productivity, and that the attenuating effect of richness on forest productivity might be partly explained by low evenness in speciose communities. Productivity generally increases with species richness, until reduced evenness limits the overall increases in community diversity. Our research suggests that evenness is a fundamental component of biodiversity– ecosystem function relationships, and is of critical importance for guiding conservation and sustainable ecosystem management decisions

The global biogeography of tree leaf form and habit

Understanding what controls global leaf type variation in trees is crucial for comprehending their role in terrestrial ecosystems, including carbon, water and nutrient dynamics. Yet our understanding of the factors influencing forest leaf types remains incomplete, leaving us uncertain about the global proportions of needle-leaved, broadleaved, evergreen and deciduous trees. To address these gaps, we conducted a global, ground-sourced assessment of forest leaf-type variation by integrating forest inventory data with comprehensive leaf form (broadleaf vs needle-leaf) and habit (evergreen vs deciduous) records. We found that global variation in leaf habit is primarily driven by isothermality and soil characteristics, while leaf form is predominantly driven by temperature. Given these relationships, we estimate that 38% of global tree individuals are needle-leaved evergreen, 29% are broadleaved evergreen, 27% are broadleaved deciduous and 5% are needle-leaved deciduous. The aboveground biomass distribution among these tree types is approximately 21% (126.4 Gt), 54% (335.7 Gt), 22% (136.2 Gt) and 3% (18.7 Gt), respectively. We further project that, depending on future emissions pathways, 17–34% of forested areas will experience climate conditions by the end of the century that currently support a different forest type, highlighting the intensification of climatic stress on existing forests. By quantifying the distribution of tree leaf types and their corresponding biomass, and identifying regions where climate change will exert greatest pressure on current leaf types, our results can help improve predictions of future terrestrial ecosystem functioning and carbon cycling

Co-limitation towards lower latitudes shapes global forest diversity gradients

The latitudinal diversity gradient (LDG) is one of the most recognized global patterns of species richness exhibited across a wide range of taxa. Numerous hypotheses have been proposed in the past two centuries to explain LDG, but rigorous tests of the drivers of LDGs have been limited by a lack of high-quality global species richness data. Here we produce a high-resolution (0.025° × 0.025°) map of local tree species richness using a global forest inventory database with individual tree information and local biophysical characteristics from ~1.3 million sample plots. We then quantify drivers of local tree species richness patterns across latitudes. Generally, annual mean temperature was a dominant predictor of tree species richness, which is most consistent with the metabolic theory of biodiversity (MTB). However, MTB underestimated LDG in the tropics, where high species richness was also moderated by topographic, soil and anthropogenic factors operating at local scales. Given that local landscape variables operate synergistically with bioclimatic factors in shaping the global LDG pattern, we suggest that MTB be extended to account for co-limitation by subordinate drivers