Publications

Implications of Pre-eruptive Zircon Saturation in Silicic Magmas for High-Precision U/Pb Geochronology

Entanglement Entropy Scaling in the Disordered Golden Chain

Mechanistic Studies on Rhodium-Catalyzed Enantioselective Silylation of Aryl C–H Bonds

Several classes of enantioselective silylations of C-H bonds have been reported recently, but little mechanistic data on these processes are available. We report mechanistic studies on the rhodium-catalyzed, enantioselective silylation of aryl C-H bonds. A rhodium silyl dihydride and a rhodium norbornyl complex were prepared and determined to be interconverting catalyst resting states. Kinetic isotope effects indicated that the C-H bond cleavage step is not rate-determining, but the C-H bond cleavage and C-Si bond-forming steps together influence the enantioselectivity. DFT calculations indicate that the enantioselectivity originates from unfavorable steric interactions between the substrate and the ligand in the transition state leading to the formation of the minor enantiomer.

Active sites and states in the heterogeneous catalysis of carbon–hydrogen bonds

C–H bond activation for several alkenes (ethylene, propylene, isobutene, cyclohexene and 1-hexene) and alkanes (methane, ethane, n -hexane, 2-methylpentane and 3-methylpentane) has been studied on the (111) crystal face of platinum as a function of temperature at low (<10 −6 Torr) and high (≥1 Torr) pressures in the absence and presence of hydrogen pressures (≥10 Torr). Sum frequency generation (SFG) vibrational spectroscopy has been used to characterize the adsorbate structures and high pressure scanning tunnelling microscopy (HP-STM) has been used to monitor their surface mobility under reaction conditions during hydrogenation, dehydrogenation and CO poisoning. C–H bond dissociation occurs at low temperatures, approximately 250 K, for all of these molecules, although only at high pressures for the weakly bound alkanes because of their low desorption temperatures. Bond dissociation is known to be surface structure sensitive and we find that it is also accompanied by the restructuring of the metal surface. The presence of hydrogen slows down dehydrogenation and for some of the molecules it influences the molecular rearrangement, thus altering reaction selectivity. Surface mobility of adsorbates is essential to produce catalytic activity. When surface diffusion is inhibited by CO adsorption, ordered surface structures form and the reaction is poisoned. Ethylene hydrogenation is surface structure insensitive, while cyclohexene hydrogenation and dehydrogenation are structure sensitive. n -Hexane and other C 6 alkanes form either upright or flat-lying molecules on the platinum surface which react to produce branched isomers or benzene, respectively.

Enantioselective Total Syntheses of (−)-Taiwaniaquinone H and (−)-Taiwaniaquinol B by Iridium-Catalyzed Borylation and Palladium-Catalyzed Asymmetric α-Arylation

We report a concise, enantioselective total synthesis of (−)-taiwaniaquinone H and the first enantioselective total synthesis of (−)-taiwaniaquinol B by a route that includes asymmetric palladium-catalyzed α-arylation of a ketone with an aryl bromide that was generated by sterically controlled halogenation via iridium-catalyzed C−H borylation. This asymmetric α-arylation creates the benzylic quaternary stereogenic center present in the taiwaniaquinoids. The synthesis was completed efficiently by developing a Lewis acid-promoted cascade to construct the [6,5,6] tricyclic core of an intermediate common to the synthesis of a number of taiwaniaquinoids. Through the preparation of these compounds, we demonstrate the utility of constructing benzylic quaternary stereogenic centers, even those lacking a carbonyl group in the α-position, by asymmetric α-arylation.

Fluoride-Induced Hierarchical Ordering of Mesoporous Silica in Aqueous Acid-Syntheses

Hierarchical ordering of mesoporous silica induced by fluoride addition is described for aqueous acid-syntheses. The authors report that small amounts of fluoride make it possible to control the organization of the mesoporous materials on different length scales. At low pH, fluoride addition brings about the formation of large hydrothermally stable silica rods (see Figure) consisting of aligned bundles of mesoporous silica fibers.

Natural killer cell receptors: The offs and ons of NK cell recognition

CCDC 690693: Experimental Crystal Structure Determination

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Ultrafast Optical Nanoscopy of Carrier Dynamics in Silicon Nanowires

Carrier distribution and dynamics in semiconductor materials often govern their physical properties that are critical to functionalities and performance in industrial applications. The continued miniaturization of electronic and photonic devices calls for tools to probe carrier behavior in semiconductors simultaneously at the picosecond time and nanometer length scales. Here, we report pump-probe optical nanoscopy in the visible-near-infrared spectral region to characterize the carrier dynamics in silicon nanostructures. By coupling experiments with the point-dipole model, we resolve the size-dependent photoexcited carrier lifetime in individual silicon nanowires. We further demonstrate local carrier decay time mapping in silicon nanostructures with a sub-50 nm spatial resolution. Our study enables the nanoimaging of ultrafast carrier kinetics, which will find promising applications in the future design of a broad range of electronic, photonic, and optoelectronic devices.

Mars or Mercury redux: The geopolitics of bilateral trade agreements

Abstract We analyse the role of economic and security considerations in bilateral trade agreements. We use the pre‐World War I period to test whether trade agreements are governed by standard gravity variables, or by instead—or in addition—geopolitical factors. While we like others find support for standard gravity variables, we also find that defence pacts boost the probability of trade agreements by as much as 20 percentage points. Our estimates imply that were the U.S. to alienate its geopolitical allies, the likelihood and benefits of successful bilateral agreements would fall significantly. Trade creation from an agreement between the U.S. and E.U. countries would decline by about 0.6 per cent of total U.S. exports.

STAR CELLULAR NEURAL NETWORKS FOR ASSOCIATIVE AND DYNAMIC MEMORIES

In this paper, we propose a Star cellular neural network (Star CNN) for associative and dynamic memories. A Star CNN consists of local oscillators and a central system. All oscillators are connected to a central system in the shape of a Star, and communicate with each other through a central system. A Star CNN can store and retrieve given patterns in the form of synchronized chaotic states with appropriate phase relations between the oscillators (associative memories). Furthermore, the output pattern can occasionally travel around the stored patterns, their reverse patterns, and new relevant patterns which are called spurious patterns (dynamic memories).

A keystone ecologist: Robert Treat Paine, 1933–2016

Abstract Robert T. Paine, who passed away on 13 June 2016, is among the most influential people in the history of ecology. Paine was an experimentalist, a theoretician, a practitioner, and proponent of the “ecology of place,” and a deep believer in the importance of natural history to ecological understanding. His scientific legacy grew from the discovery of a link between top‐down forcing and species diversity, a breakthrough that led to the ideas of both keystone species and trophic cascades, and to our early understanding of the mosaic nature of biological communities, causes of zonation across physical gradients, and the intermediate‐disturbance hypothesis of species diversity. Paine's influence as a mentor was equally important to the growth of ecological thinking, natural resource conservation, and policy. He served ecology as an Ecological Society of America president, an editor of the Society's journals, a member of and contributor to the National Academy of Sciences and the National Research Council, and an in‐demand advisor to various state and federal agencies. Paine's broad interests, enthusiasm, charisma, and humor deeply affected our lives and the lives of so many others.

Measuring and Modeling Ecosystem Photosynthetic Productivity and Respiration in Responses to Climatic Fluctuations in the Past 60 Years at an Oak-Grass Savanna in California

Microstructure and Solvent Distribution in Cross-Linked Diblock Copolymer Gels

Gels were created by irradiating poly(α-methylstyrene-block-isoprene) (PaMS−I) and poly(vinylferrocenium triflate-block-isoprene) (PVFT−I) copolymers with an electron beam and swelling the resulting cross-linked solids in good solvents. The radiation cross-links the polyisoprene (PI) block, induces chain scission of the poly(α-methylstyrene) block (PaMS), and has no effect on the poly(vinylferrocenium triflate) (PVTF) block. After extraction of un-cross-linked chains, the PaMS−I gels comprise solvent-filled, open channels inside a swollen PI network, while the swollen PVFT−I gels comprise channels of solvated PVFT chains inside a swollen PI network. The dependence of the structure, composition, and cross-linking density of the swollen microphases on radiation dose is determined by combining small-angle neutron scattering experiments with conventional gel characterization methods.

Coherent phonon motions and ordered vacancy compound mediated quantum path interference in Cu-poor CuIn$_{x}$Ga$_{(1-x)}$Se$_2$ (CIGS) with attosecond transient absorption

In this study, coherent phonon motion is observed in bandgap excited CuIn$_{x}$Ga$_{(1-x)}$Se$_2$ (CIGS) utilizing extreme ultraviolet (XUV) attosecond transient absorption spectroscopy across the Se M$_{4,5}$ absorption edge. Two frequencies of coherent phonon motion are resolved, a low frequency mode attributed through Raman measurements to the $A_{1g}$ phonon motion of a Cu-deficient ordered vacancy compound (OVC), while the high frequency mode originates from the $A_{1g}$ phonon motion in the chalcopyrite phase. The two oscillations lead to modulations in the XUV differential absorption $ΔA(ε,τ)$ due to energy shifts of the Se M$_{4,5}$ edge, with a minima occuring approximately 1 ps after the band gap excitation. The hot carrier cooling time of holes and electrons are disentangled and the observed slower cooling of holes is attributed to the higher density of hole states in the valence band. We also observe fast oscillations (18.6(3) fs period) across the Se absorption edge, which are interpreted to originate from quantum path interference between the electronic conduction bands of the chalcopyrite CIGS and OVC phases, opening the possibility towards quantum coherent metrology in photovoltaics on the femtosecond timescale. The complex interplay between the chalcopyrite and OVC phases are revealed in this investigation through both coherent vibrational and electronic motions.

Systematic softening in universal machine learning interatomic potentials

Assessing the Implementation of the Chronic Care Model in Quality Improvement Collaboratives: Does Baseline System Support for Chronic Care Matter?

While collaboratives are an increasingly popular approach to facilitating quality improvement (QI) in healthcare organizations, little is known about the effective implementation of collaborative processes or organizational change activities motivated by collaborative participation.

Low-temperature pyrolytic transformations of tri-tert-butoxysiloxy derivatives of aluminum to aluminosilicate materials

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTLow-temperature pyrolytic transformations of tri-tert-butoxysiloxy derivatives of aluminum to aluminosilicate materialsKarl W. Terry, Peter K. Ganzel, and T. Don TilleyCite this: Chem. Mater. 1992, 4, 6, 1290–1295Publication Date (Print):November 1, 1992Publication History Published online1 May 2002Published inissue 1 November 1992https://pubs.acs.org/doi/10.1021/cm00024a032https://doi.org/10.1021/cm00024a032research-articleACS PublicationsRequest reuse permissionsArticle Views270Altmetric-Citations61LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-AlertscloseSupporting Info (1)»Supporting Information Supporting Information Get e-Alerts