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    Chemistry

    Angewandte Chemie Cover
    Journal of the American Chemical Society

    A Pressure‐Induced Inverse Order–Disorder Transition in Double Perovskites
    Double perovskites Y2CoIrO6 and Y2CoRuO6 show an increase of B‐site disorder with increasing synthesis pressure. This pressure‐induced disorder is accompanied by lattice compression and structure alteration and the long‐range ferrimagnetic orderings gradually collapsed. Calculations suggest that unusual cell compressions under pressures unexpectedly stabilize the disordered phases. Abstract Given the consensus that pressure improves cation ordering in most of known materials, a discovery of pressure‐induced disordering could require recognition of an order–disorder transition in solid‐state physics/chemistry and geophysics. Double perovskites Y2CoIrO6 and Y2CoRuO6 polymorphs synthesized at 0, 6, and 15 GPa show B‐site ordering, partial ordering, and disordering, respectively, accompanied by lattice compression and crystal structure alteration from monoclinic to orthorhombic symmetry. Correspondingly, the long‐range ferrimagnetic ordering in the B‐site ordered samples are gradually overwhelmed by B‐site disorder. Theoretical calculations suggest that unusual unit‐cell compressions under external pressures unexpectedly stabilize the disordered phases of Y2CoIrO6 and Y2CoRuO6. [...]
    Mon, Apr 06, 2020
    Source: Angewandte Chemie Int. edition Category: CHEMISTRY GENERAL
    Boron: Its Role in Energy‐Related Processes and Applications
    The fifth element: Boron has a unique position in the Periodic Table—directly at the apex of the line separating metals and nonmetals. This Review highlights the critical role of boron and boron compounds in the fields of energy conversion and storage, and demonstrates the versatility and potential of boron for energy‐related research. Abstract Boron's unique position in the Periodic Table, that is, at the apex of the line separating metals and nonmetals, makes it highly versatile in chemical reactions and applications. Contemporary demand for renewable and clean energy as well as energy‐efficient products has seen boron playing key roles in energy‐related research, such as 1) activating and synthesizing energy‐rich small molecules, 2) storing chemical and electrical energy, and 3) converting electrical energy into light. These applications are fundamentally associated with boron's unique characteristics, such as its electron‐deficiency and the availability of an unoccupied p orbital, which allow the formation of a myriad of compounds with a wide range of chemical and physical properties. For example, boron's ability to achieve a full octet of electrons with four covalent bonds and a negative charge has led to the synthesis of a wide variety of borate anions of high chemical and electrochemical stability—in particular, weakly coordinating anions. This Review summarizes recent advances in the study of boron compounds for energy‐related processes and applications. [...]
    Mon, Apr 06, 2020
    Source: Angewandte Chemie Int. edition Category: CHEMISTRY GENERAL
    Dynamical Bonding Driving Mixed Valency in a Metal Boride
    Samarium Hexaboride (SmB6) has a plethora of seemingly mutually incompatible properties. Central to all of these is the homogeneous Sm2+/Sm3+ mixed valency. We find that the Sm and B atoms form two thermally accessible bonding minima which control this mixed valency. This is a distinct new mechanism of mixed valency. The new paradigm unifies many properties of SmB6 and predicts a Raman response which was confirmed experimentally. Abstract Samarium hexaboride is an anomaly, having many exotic and seemingly mutually incompatible properties. It was proposed to be a mixed‐valent semiconductor, and later a topological Kondo insulator, and yet has a Fermi surface despite being an insulator. We propose a new and unified understanding of SmB6 centered on the hitherto unrecognized dynamical bonding effect: the coexistence of two Sm−B bonding modes within SmB6, corresponding to different oxidation states of the Sm. The mixed valency arises in SmB6 from thermal population of these distinct minima enabled by motion of B. Our model simultaneously explains the thermal valence fluctuations, appearance of magnetic Fermi surface, excess entropy at low temperatures, pressure‐induced phase transitions, and related features in Raman spectra and their unexpected dependence on temperature and boron isotope. [...]
    Mon, Apr 06, 2020
    Source: Angewandte Chemie Int. edition Category: CHEMISTRY GENERAL
    Selective Electroenzymatic Oxyfunctionalization by Alkane Monooxygenase in a Biofuel Cell
    Biofuel cells: Petroleum‐derived chemicals can undergo regioselective hydroxylation, epoxidation, sulfoxidation, and demethylation in a H2‐fueled biofuel cell. The ability of alkane monooxygenase (alkB) to protect internal functional groups, a very rare and lucrative capability, was demonstrated by the generation of 4‐octen‐1‐ol as the exclusive product from 4‐octene. Abstract Aliphatic synthetic intermediates with high added value are generally produced from alkane sources (e.g., petroleum) by inert carbon–hydrogen (C−H) bond activation using classical chemical methods (i.e. high temperature, rare metals). As an alternative approach for these reactions, alkane monooxygenase from Pseudomonas putida (alkB) is able to catalyze the difficult terminal oxyfunctionalization of alkanes selectively and under mild conditions. Herein, we report an electrosynthetic system using an alkB biocathode which produces alcohols, epoxides, and sulfoxides through bioelectrochemical hydroxylation, epoxidation, sulfoxidation, and demethylation. The capacity of the alkB binding pocket to protect internal functional groups is also demonstrated. By coupling our alkB biocathode with a hydrogenase bioanode and using H2 as a clean fuel source, we have developed and characterized a series of enzymatic fuel cells capable of oxyfunctionalization while simultaneously producing electricity. [...]
    Mon, Apr 06, 2020
    Source: Angewandte Chemie Int. edition Category: CHEMISTRY GENERAL
    Single‐Molecule Study of a Plasmon‐Induced Reaction for a Strongly Chemisorbed Molecule
    The mechanism of the plasmon‐induced dissociation for a strongly chemisorbed molecule was clarified. This was based on both the quantitative analysis of a single‐molecule reaction with a scanning tunneling microscope (STM) combined with light irradiation and the qualitative analysis of the electronic structures, responsible for the reaction, which were revealed by STM action spectroscopy and density functional theory calculations. Abstract Chemical reactions induced by plasmons achieve effective solar‐to‐chemical energy conversion. However, the mechanism of these reactions, which generate a strong electric field, hot carriers, and heat through the excitation and decay processes, is still controversial. In addition, it is not fully understood which factor governs the mechanism. To obtain mechanistic knowledge, we investigated the plasmon‐induced dissociation of a single‐molecule strongly chemisorbed on a metal surface, two O2 species chemisorbed on Ag(110) with different orientations and electronic structures, using a scanning tunneling microscope (STM) combined with light irradiation at 5 K. A combination of quantitative analysis by the STM and density functional theory calculations revealed that the hot carriers are transferred to the antibonding (π*) orbitals of O2 strongly hybridized with the metal states and that the dominant pathway and reaction yield are determined by the electronic structures formed by the molecule–metal chemical interaction. [...]
    Mon, Apr 06, 2020
    Source: Angewandte Chemie Int. edition Category: CHEMISTRY GENERAL
    The world generates over six million tons of coffee grounds, according to the International Coffee Organization. The journal Agriculture and Food Chemistry reported in 2012 that over half of spent coffee grounds end up in landfills. Cellulose nanofibers are the building blocks for plastic resins that can be made into biodegradable plastic products. [...]
    Mon, Apr 06, 2020
    Source: PhysOrg Chemistry Category: CHEMISTRY GENERAL
    Lead-based perovskites are very promising materials for the production of solar panels. They efficiently turn light into electricity but they also present some major drawbacks: the most efficient materials are not very stable, while lead is a toxic element. University of Groningen scientists are studying alternatives to lead-based perovskites. Two factors that significantly affect the efficiency of these solar cells are the ability to form thin films and the structure of the materials in the solar cells. Therefore, it is very important to investigate in situ how lead-free perovskite crystals form and how the crystal structure affects the functioning of the solar cells. The results of the study were published in the journal Advanced Functional Materials on 31 March. [...]
    Mon, Apr 06, 2020
    Source: PhysOrg Chemistry Category: CHEMISTRY GENERAL
    Two-dimensional (2-D) materials, which show excellent physical and chemical properties, have received unprecedented attention and become a research hotspot in scientific fields such as physics, chemistry and materials. Organic modification onto 2-D materials by covalently bonding or physically adsorbing organic molecules can greatly regulate and optimize the properties of 2-D materials. However, the organic modification methods reported so far are exfoliation first and then an organic modification (E-M) strategy, which usually possess some disadvantages such as less modification ratio, uncertain type, number and position of the functional groups, a tendency for defects and so on. Therefore, the development of functionalization of 2-D materials is greatly limited. [...]
    Mon, Apr 06, 2020
    Source: PhysOrg Chemistry Category: CHEMISTRY GENERAL
    Scientists from the Institute of Physical Chemistry of the Polish Academy of Sciences (IPC PAS) in cooperation with the Faculty of Chemistry of the Warsaw University of Technology (WUT) have developed a new, solvent-free method for the encapsulation of drug molecules in MOF (metal-organic framework) porous materials. [...]
    Mon, Apr 06, 2020
    Source: PhysOrg Chemistry Category: CHEMISTRY GENERAL
    University of Groningen scientists have observed the characteristics of a single enzyme inside a nanopore. They learned that the enzyme can exist in four different folded states, or conformers, that play an active role in the reaction mechanism. These results will have consequences for enzyme engineering and the development of inhibitors. The study was published in Nature Chemistry on 6 April. [...]
    Mon, Apr 06, 2020
    Source: PhysOrg Chemistry Category: CHEMISTRY GENERAL

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