The crystal growth process in colloidal nanocrystal systems is usually associated with the Ostwald-ripening mechanism. Here, we report on experimental evidence indicating that another crystal growth process took place in a colloidal nanocrystal system at room temperature. This crystal growth process is based on grain rotation among neighboring grains, resulting in a coherent grain–grain interface, which, by eliminating common boundaries, causes neighboring grains to coalesce, thereby forming a single larger nanocrystal. This phenomenon was observed in SnO2 nanocrystals (particle size ranging from 10 to 30 Å).

In this letter, we report, for the first time, the real-time in situ nucleation and growth of Ag filaments on α-Ag2WO4 crystals driven by an accelerated electron beam from an electronic microscope under high vacuum. We employed several techniques to characterise the material in depth. By using these techniques combined with first-principles modelling based on density functional theory, a mechanism for the Ag filament formation followed by a subsequent growth process from the nano- to micro-scale was proposed. In general, we have shown that an accelerated electron beam from an electronic microscope under high vacuum enables in situ visualisation of Ag filaments with subnanometer resolution and offers great potential for addressing many fundamental issues in materials science, chemistry, physics and other fields of science.

In this work, we utilise a combination of theory, computation and experiments to understand the early events related to the nucleation of Ag filaments on α-Ag2WO4 crystals, which is driven by an accelerated electron beam from an electron microscope under high vacuum. The growth process and the chemical composition and elemental distribution in these filaments were analysed in depth at the nanoscale level using TEM, HAADF, EDS and XPS; the structural and electronic aspects were systematically studied in using first-principles electronic structure theory within QTAIM framework. The Ag nucleation and formation on α-Ag2WO4 is a result of the order/disorder effects generated in the crystal by the electron-beam irradiation. Both experimental and theoretical results show that this behavior is associated with structural and electronic changes of the [AgO2] and [AgO4] clusters and, to a minor extent, to the [WO6] cluster; these clusters collectively represent the constituent building blocks of α-Ag2WO4.

Registro de acceso restringido Este recurso no está disponible en acceso abierto por política de la editorial. No obstante, se puede acceder al texto completo desde la Universitat Jaume I o si el usuario cuenta con suscripción. Registre d'accés restringit Aquest recurs no està disponible en accés obert per política de l'editorial. No obstant això, es pot accedir al text complet des de la Universitat Jaume I o si l'usuari compta amb subscripció. Restricted access item This item isn't open access because of publisher's policy. The full--text version is only available from Jaume I University or if the user has a running suscription to the publisher's contents.

Theoretical and experimental studies were performed on the structure, optical properties, and growth of silver nanostructures in silver phosphate (Ag 3 PO 4 ). This material was synthesized by the coprecipitation method and processed in a microwave-assisted hydrothermal system at 150 °C for different times. The structural behavior was analyzed by means of X-ray diffraction, Rietveld refinement, and Raman spectroscopy. Field emission gun scanning electron microscopy as well as transmission electron microscopy revealed the presence of irregular spherical-like Ag 3 PO 4 microparticles; metallic silver nanostructures were found on their surfaces. The growth processes of Ag nanostructures when irradiated with an electron beam were explained by theoretical calculations. First-principles calculations, within a quantum theory of atoms in molecules framework, have been carried out to provide deeper insight and understanding of the observed nucleation and early evolution of Ag nanoparticles on Ag 3 PO 4 crystals, driven by an accelerated electron beam from an electronic microscope under high vacuum. The Ag nucleation and formation is a result of structural and electronic changes of the AgO 4 tetrahedral cluster as a constituent building block of Ag 3 PO 4 , consistent with Ag metallic formation. The optical properties were investigated by ultraviolet−visible spectroscopy and photoluminescence (PL) measurements at room temperature. PL properties of this phosphate were explained by the recombination phenomenon of electron−hole pairs via cluster-to-cluster charge transfer.

The paper collects the answers of the authors to the following questions: Is the lack of precision in the definition of many chemical concepts one of the reasons for the coexistence of many partition schemes? Does the adoption of a given partition scheme imply a set of more precise definitions of the underlying chemical concepts? How can one use the results of a partition scheme to improve the clarity of definitions of concepts? Are partition schemes subject to scientific Darwinism? If so, what is the influence of a community's sociological pressure in the “natural selection” process? To what extent does/can/should investigated systems influence the choice of a particular partition scheme? Do we need more focused chemical validation of Energy Decomposition Analysis (EDA) methodology and descriptors/terms in general? Is there any interest in developing common benchmarks and test sets for cross‐validation of methods? Is it possible to contemplate a unified partition scheme (let us call it the “standard model” of partitioning), that is proper for all applications in chemistry, in the foreseeable future or even in principle? In the end, science is about experiments and the real world. Can one, therefore, use any experiment or experimental data be used to favor one partition scheme over another? © 2019 Wiley Periodicals, Inc.

In this tutorial review we show how the methods and techniques of computational chemistry have been applied to the understanding of the physical basis of the rate enhancement of chemical reactions by enzymes. This is to answer the question: Why is the activation free energy in enzyme catalysed reactions smaller than the activation free energy observed in solution? Two important points of view are presented: Transition State (TS) theories and Michaelis Complex (MC) theories. After reviewing some of the most popular computational methods employed, we analyse two particular enzymatic reactions: the conversion of chorismate to prephenate catalysed by Bacillus subtilis chorismate mutase, and a methyl transfer from S-adenosylmethionine to catecholate catalysed by catechol O-methyltransferase. The results and conclusions obtained by different authors on these two systems, supporting either TS stabilisation or substrate preorganization, are presented and compared. Finally we try to give a unified view, where a preorganized enzyme active site, prepared to stabilise the TS, also favours those reactive conformations geometrically closer to the TS.

The molecular mechanisms for the inverse-electron-demand Diels−Alder reactions between nitroethene and three substituted ethenes (propene, methyl vinyl ether, and dimethylvinylamine) to give the corresponding nitroso cycloadducts have been characterized with density functional theory methods using the B3LYP/6-31G* calculational level. On the basis of stability arguments and molecular orbital analysis relative rates, regioselectivity, and stereoselectivity, the presence of Lewis acid catalyst modeled by the BH3 system and the inclusion of solvent effects as a function of the nature of substituent in the dienophile fragment are analyzed and discussed. The ortho attack mode presents transition structures more stable than the meta one. For the former, reactivity, endo selectivity, and asynchronicity are enhanced with the increase of the electron-releasing character of the substituent on dienophile fragment. The reaction between nitroethene and propene has dissymmetric concerted transition structures associated with a pericyclic process, while the reaction between nitroethene and dimethylvinylamine takes place along an asynchronous transition structure corresponding to a nucleophilic attack to nitroethene, with concomitant ring closure and without participation of zwitterionic intermediates. For the most unfavorable meta attack modes, the reactions have synchronous mechanisms that are not sensible to the substitution on the dienophile system. For the ortho channels, the inclusion of Lewis acid catalyst and solvent effects contributes to the charge-transfer process from the substituted ethenes to nitroethene and rate acceleration, as well as a significant increase of the endo stereoselectivity.