169 research outputs found

    Dissipation wavenumber and regularity for electron magnetohydrodynamics

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    We consider the electron magnetohydrodynamics (MHD) with static background ion flow. A special situation of B(x,y,t) = ∇× (a~e z) + b~e z with scalar-valued functions a(x,y,t) and b(x,y,t) was studied numerically in the physics paper [7]. The authors concluded from numerical simulations that there is no evidence of dissipation cutoff for the electron MHD. In this paper we show the existence of determining wavenumber for the electron MHD, and establish a regularity condition only on the low modes of the solution. Our results suggest that the conclusion of the physics paper on the dissipation cutoff for the electron MHD is debatable.</p

    3‑Center-5-Electron Boryl Radicals with σ<sup>0</sup>π<sup>1</sup> Ground State Electronic Structure

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    Five- and six-membered boron heterocycle-based three-center-five-electron (<b>3c</b>–<b>5e</b>) type boryl radicals with unusual σ<sup>0</sup>π<sup>1</sup> ground state electronic structures are predicted theoretically. Compared to σ<sup>1</sup>π<sup>0</sup> analogs, their unique electronic structure leads to both lower reactivity toward H-atoms and stronger coordination with Lewis bases. The corresponding Lewis base-stabilized four-center-seven-electron (<b>4c</b>–<b>7e</b>) type boryl radicals are even more unreactive toward H-atoms than the conventional <b>4c</b>–<b>7e</b> ones

    Three-Dimensional Highly Conductive Graphene–Silver Nanowire Hybrid Foams for Flexible and Stretchable Conductors

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    Graphene foams have showed huge application potentials owing to their unique 3D structure and superior properties. Thus, it is highly desired to develop a simple and effective pathway to fabricate high performance graphene-based foams. Here, we present a polymer template-assisted assembly strategy for fabricating a novel class of graphene/AgNW hybrid foams. The hybrid foams show 3D ordered microstructures, high thermal stability, and excellent electrical and mechanical properties, and demonstrate huge application potential in the fields of flexible and stretchable conductors. Importantly, the polymer-template assisted assembly technique is simple, scalable, and low-cost, providing a new synthesis protocol for various multifunctional graphene hybrid foam-based composites

    Role of Mono-N-protected Amino Acid Ligands in Palladium(II)-Catalyzed Dehydrogenative Heck Reactions of Electron-Deficient (Hetero)arenes: Experimental and Computational Studies

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    We report here that mono-N-protected amino acids (MPAAs), an important environmentally compatible structural motif, enable acceleration of Pd­(II)-catalyzed dehydrogenative Heck reactions between pyridines and electron-deficient arenes with simple alkenes, leading to diversely functionalized C3- or <i>meta</i>-selective alkenylated pyridines and benzenes via non-chelate-assisted C–H activation. A comprehensive theoretical study by DFT calculations discloses that the amino scaffold of the MPAA ligand facilely converts to an X-type ligand by an initial N–H activation, resulting in a relatively low activation barrier for the C–H cleavage of pyridine. Then a property reversal of the amino group from X-type to L-type ligand allows the alkene substitution to take place smoothly, while the carboxyl group enables the formation of an intramolecular hydrogen bond, significantly decreasing the activation barrier for the carbopalladation. The results of calculations and the kinetic isotopic effect measurement support a rate-limiting C–H activation by a mechanism involving a concerted metalation/deprotonation pathway, with an endothermicity of 31.0 kcal/mol in the process

    Trapping Behaviors of Photogenerated Electrons on the (110), (101), and (221) Facets of SnO<sub>2</sub>: Experimental and DFT Investigations

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    Spatial separation of photogenerated charges between different crystal facets has been observed in some semiconductor photocatalysts; however, the charge separation mechanism is still ambiguous. As a characteristic parameter of crystal facet, surface energy may be a crucial factor to dictate the flow of photogenerated charges. In this work, the relationship between surface energy and the flow mode of photogenerated charges is investigated by using model photocatalysts, including lance-shaped SnO<sub>2</sub> particles and dodecahedral SnO<sub>2</sub> particles. The former are enclosed by two kinds of crystal facets with a big gap in surface energy, while the latter are composed of two types of crystal facets with nearly equal surface energy. However, the experimental results exhibit that the photogenerated electrons flow to all exposed crystal facets <i>randomly</i> in both two kinds of SnO<sub>2</sub> nanocrystals, which is opposite to what has been observed in extensively investigated semiconductor photocatalysts including TiO<sub>2</sub>, SrTiO<sub>3</sub>, BiVO<sub>4</sub>, BiOCl, and Cu<sub>2</sub>O. Our results disqualify surface energy as an appropriate descriptor of preferential charge flow. Furthermore, the experimental results are confirmed by trapping energies and work functions calculated with the first-principles methods, which are proved to be more relevant parameters for describing the charge flow direction. Additionally, the trapping sites on each crystal facet are determined by charge analysis

    Metabolic Flux Analysis of Lipid Biosynthesis in the Yeast <i>Yarrowia lipolytica</i> Using <sup>13</sup>C-Labled Glucose and Gas Chromatography-Mass Spectrometry - Fig 1

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    <p>Growth curve (A) and residual glucose concentration (B) for <i>Y</i>. <i>lipolytica</i> cultivated in high nitrogen medium (H-N) and low nitrogen medium (L-N) with glucose as a sole carbon source. Each experiment was performed in triplicate, and all data are reported as means ± standard deviations.</p