2,268 research outputs found

    Enantioselective Organocatalytic Intramolecular Diels−Alder Reactions. The Asymmetric Synthesis of Solanapyrone D

    Get PDF
    The first direct enantioselective organocatalytic intramolecular Diels−Alder reaction has been accomplished. The use of iminium catalysis has provided a new catalytic strategy for the enantioselective [4 + 2] cycloisomerization of a wide variety of tethered diene-enal systems. The use of imidazolidinones 1 and 2 as the asymmetric catalysts has been found to mediate the enantioselective construction of [4.4.0] and [4.3.0] ring systems. Application of this methodology to the highly efficient asymmetric synthesis of the marine metabolite solanpyrone D has also been accomplished. A diverse spectrum of aldehyde substrates can also be accommodated in this new organocatalytic transformation. Importantly, this technology has been utilized to execute the first enantioselective, catalytic Type II IMDA reaction

    High order magnon bound states in the quasi-one-dimensional antiferromagnet α\alpha-NaMnO2_2

    Full text link
    Here we report on the formation of two and three magnon bound states in the quasi-one-dimensional antiferromagnet α\alpha-NaMnO2_2, where the single-ion, uniaxial anisotropy inherent to the Mn3+^{3+} ions in this material provides a binding mechanism capable of stabilizing higher order magnon bound states. While such states have long remained elusive in studies of antiferromagnetic chains, neutron scattering data presented here demonstrate that higher order n>2n>2 composite magnons exist, and, specifically, that a weak three-magnon bound state is detected below the antiferromagnetic ordering transition of NaMnO2_2. We corroborate our findings with exact numerical simulations of a one-dimensional Heisenberg chain with easy-axis anisotropy using matrix-product state techniques, finding a good quantitative agreement with the experiment. These results establish α\alpha-NaMnO2_2 as a unique platform for exploring the dynamics of composite magnon states inherent to a classical antiferromagnetic spin chain with Ising-like single ion anisotropy.Comment: 5 pages, 4 figure

    Passive Wireless SAW Sensors for IVHM

    Get PDF
    NASA aeronautical programs require integrated vehicle health monitoring (IVHM) to ensure the safety of the crew and the vehicles. Future IVHM sensors need to be small, light weight, inexpensive, and wireless. Surface acoustic wave (SAW) technology meets all of these constraints. In addition it operates in harsh environments and over wide temperature ranges, and it is inherently radiation hardened. This paper presents a survey of research opportunities for universities and industry to develop new sensors that address anticipated IVHM needs for aerospace vehicles. Potential applications of passive wireless SAW sensors from ground testing to high altitude aircraft operations are presented, along with some of the challenges and issues of the technology

    The Role of Ligand Steric Bulk in New Monovalent Aluminum Compounds

    Get PDF
    The article of record as published may be located at https://doi.org/10.1021/acs.jpca.7b02075The tetrameric Al(I) cyclopentadienyl compound Al4Cp*4 (Cp* = C5Me5) is a prototypical low-valence Al compound, with delocalized bonding between four Al(I) atoms and η5 ligands bound to the cluster exterior. The synthesis of new [AlR]4 (R = C5Me4Pr, C5Me4iPr) tetramers is presented. Though these systems failed to crystallize, comparison of variable-temperature 27Al NMR data with density functional theory (DFT) calculations indicate that these are Al4R4 tetramers analogous to Al4Cp*4 but with increased ligand steric bulk. NMR, DFT, and Atoms in Molecules analyses show that these clusters are enthalpically more stable as tetramers than the Cp* variant, due in part to noncovalent interactions across the bulkier ligand groups. Thermochemistry calculations for the low-valence metal interactions were found to be extremely sensitive to the DFT methodology used; the M06-2X functional with a cc-pVTZ basis set is shown to provide very accurate values for the enthalpy of tetramerization and 27Al NMR shifts. This computational method is then used to predict geometrical structures, noncovalent ligand interactions, and monomer/tetramer equilibrium in solution for a series of Al(I) cyclopentadienyl compounds of varying steric bulk

    First-order melting of a weak spin-orbit Mott insulator into a correlated metal

    Full text link
    The electronic phase diagram of the weak spin-orbit Mott insulator (Sr(1-x)Lax)3Ir2O7 is determined via an exhaustive experimental study. Upon doping electrons via La substitution, an immediate collapse in resistivity occurs along with a narrow regime of nanoscale phase separation comprised of antiferromagnetic, insulating regions and paramagnetic, metallic puddles persisting until x~0.04. Continued electron doping results in an abrupt, first-order phase boundary where the Neel state is suppressed and a homogenous, correlated, metallic state appears with an enhanced spin susceptibility and local moments. As the metallic state is stabilized, a weak structural distortion develops and suggests a competing instability with the parent spin-orbit Mott state.Comment: 5 pages, 4 figure

    Child educational progress in Born in Bradford pregnancies affected by gestational diabetes and also exposed to maternal common mental disorders

    Get PDF
    Abstract Gestational diabetes and the maternal mental disorders of anxiety and depression have been implicated in adverse offspring neuro-behavioural outcomes but these exposures have only been studied in isolation. 1051 children whose mothers were diagnosed with gestational diabetes in UK’s Born in Bradford cohort had linkage to maternal primary care records, providing diagnostic and treatment codes for depression and anxiety. Education record linkage provided results of the Early Years Foundation Stage Profile from the first year of school, aged five. Risk of not attaining a ‘Good level of development’ was analysed using multivariable Poisson regression within a generalised estimating equation framework. Multiple imputation was implemented for missing data. There was limited evidence of increased risk of failure to attain a ‘good level of development’ in those additionally exposed to maternal mental disorders (adjusted RR 1.21; 95% CI 0.94, 1.55). However, there was more evidence in children of Pakistani maternal ethnicity (adjusted RR 1.36; 95% CI 1.04, 1.77) than White British; this may have been driven by English not being the primary language spoken in the home. Therefore there may be groups with GDM in whom it is particularly important to optimise both maternal physical and mental health to improve child outcomes
    • …
    corecore