Enantioselective Catalysts for the Synthesis of α‐Substituted Allylboronates—An Accelerated Approach towards Isomerically Pure Homoallylic Alcohols

The use of a convenient protecting group for boronates allows a selective, catalyzed S_N2′ reaction to generate allylboronates which are applied for the synthesis of enantiomerically pure homoallylic alcohols. Depending on the configuration of both catalyst and the protecting group any of the four possible stereoisomers can be formed. The rationale behind the selective addition is supported by density functional theory calculations.     

Nondisruptive Dissolution of Hyperpolarized 129Xe into Viscous Aqueous and Organic Liquid Crystalline Environments

Studies of hyperpolarized xenon‐129 (hp‐¹²⁹Xe) in media such as liquid crystals and cell suspensions are in demand for applications ranging from biomedical imaging to materials engineering but have been hindered by the inability to bubble Xe through the desired media as a result of viscosity or perturbations caused by bubbles. Herein a device is reported that can be reliably used to dissolve hp‐¹²⁹Xe into viscous aqueous and organic samples without bubbling. This method is robust, requires small sample volumes (<60 μL), is compatible with existing NMR hardware, and is made from readily available materials. Experiments show that Xe can be introduced into viscous and aligned media without disrupting molecular order. We detected dissolved xenon in an aqueous liquid crystal that is disrupted by the shear forces of bubbling, and we observed liquid‐crystal phase transitions in (MBBA). This tool allows an entirely new class of samples to be investigated by hyperpolarized‐gas NMR spectroscopy.     

Versatile variable temperature insert at the DEIMOS beamline for in situ electrical transport measurements

The design and the first experiments are described of a versatile cryogenic insert used for its electrical transport capabilities. The insert is designed for the cryomagnet installed on the DEIMOS beamline at the SOLEIL synchrotron dedicated to magnetic characterizations through X‐ray absorption spectroscopy (XAS) measurements. This development was spurred by the multifunctional properties of novel materials such as multiferroics, in which, for example, the magnetic and electrical orders are intertwined and may be probed using XAS. The insert thus enables XAS to in situ probe this interplay. The implementation of redundant wiring and careful shielding also enables studies on operating electronic devices. Measurements on magnetic tunnel junctions illustrate the potential of the equipment toward XAS studies of in operando electronic devices.     

Haar type and Carleson constants

For a collection of dyadic intervals, a Banach space X, andp(1, 2], we assume the upper ^p estimates where x_IX, and h_I denotes the L normalized Haar function supportedon I. We determine the minimal requirement on the size of suchthat these estimates imply that X is of Haar type p. The characterizationis given in terms of the Carleson constant of .     

Dispersions of internally liquid crystalline systems stabilized by charged disklike particles as pickering emulsions: basic properties and time-resolved behavior

The present paper reports on dispersions of internally liquid crystalline particles, formed from monoglyceride and oil mixtures, stabilized with discrete disklike particles of Laponite clay. Small-angle X-ray scattering (SAXS) was used to probe the presence of dispersed particles as well as their internal liquid crystalline structure. The data were compared to scattering results of reference systems, namely, from the bulk as well as from well-defined particles formed with a polymer as the emulsifier. The submicrometer sizes of the various particles could be derived using dynamic light scattering (DLS). The possible mechanisms involved in the stabilization of each of the different phases by the Laponite platelets, including the role of the residual salt, are discussed. Time-resolved experiments were performed over 60 days in order to follow the evolution of both the internal structure and size of the particles. In particular, we discuss the peculiar behavior of the sample without added oil, where the cubosomes transform into hexosomes over time. The effect of the high pH induced by the Laponite platelets in water, which could result in a hydrolysis of the monoglycerides, was shown to be responsible for the observed cubosome-to-hexasome transition, as well as for the decrease in the lattice parameters.     

Controlling the melting of kinetically frozen poly(butyl acrylate-b-acrylic acid) micelles via addition of surfactant

We have studied the melting of polymeric amphiphilic micelles induced by small-molecule surfactant and explained the results by experimental determination of the interfacial tension between the core of the micelles and the surfactant solutions. Poly(n-butyl acrylate-b-acrylic acid) (PBA-b-PAA) amphiphilic diblock copolymers form kinetically frozen micelles in aqueous solutions. Strong interactions with surfactants, either neutral or anionic [C12E6, C6E4, sodium dodecyl sulfate (SDS)], were revealed by critical micelle concentration (cmc) shifts in specific electrode and surface tension measurements. Since both polymer and surfactant are either neutral or bear negative charges, the attractive interactions are not due to electrostatic interactions. Light scattering, neutron scattering, and capillary electrophoresis experiments showed important structural changes in mixed PBA-b-PAA/surfactant systems. Kinetically frozen micelles of PBA-b-PAA, that are hardly perturbed by concentration, ionization, ionic strength, and temperature stresses, can be disintegrated by addition of small-molecule surfactants. The interfacial energy of the PBA in surfactant solutions was measured by drop shape analysis with h-PBA homopolymer drops immersed in small-molecule surfactant solutions. The PBA/water interfacial energy gammaPBA/H2O of 20 mN/m induces a high energy cost for the extraction of unimers from micelles so that PBA-b-PAA micelles are kinetically frozen. Small-molecule surfactants can reduce the interfacial energy gammaPBA/solution to 5 mN/m. This induces a shift of the micelle-unimer equilibrium toward unimers and leads, in some cases, to the apparent disintegration of PBA-b-PAA micelles. Before total disintegration, polymer/surfactant mixtures are dispersions of polydisperse mixed micelles. Based on core interfacial energy arguments, the disintegration of kinetically frozen polymeric micelles was interpreted by gradual fractionation of objects (polydisperse dispersion mechanism), whereas the disintegration of polymeric micelles in a thermodynamically stable state was interpreted by an exchange between a population of large polymer-rich micelles and a population of small surfactant-rich micelles (bidisperse dispersion mechanism). Finally, in our system and other systems from the literature, interfacial energy arguments could explain why the disintegration of polymer micelles is either partial or total as a function of the surfactant type and concentration and the hydrophobic block molar mass of the polymer.     

Manager-worker-based model for the parallelization of quantum Monte Carlo on heterogeneous and homogeneous networks

A manager-worker-based parallelization algorithm for Quantum Monte Carlo (QMC-MW) is presented and compared with the pure iterative parallelization algorithm, which is in common use. The new manager-worker algorithm performs automatic load balancing, allowing it to perform near the theoretical maximal speed even on heterogeneous parallel computers. Furthermore, the new algorithm performs as well as the pure iterative algorithm on homogeneous parallel computers. When combined with the dynamic distributable decorrelation algorithm (DDDA) [Feldmann et al., J Comput Chem 28, 2309 (2007)], the new manager-worker algorithm allows QMC calculations to be terminated at a prespecified level of convergence rather than upon a prespecified number of steps (the common practice). This allows a guaranteed level of precision at the least cost. Additionally, we show (by both analytic derivation and experimental verification) that standard QMC implementations are not "perfectly parallel" as is often claimed.     

Resonance fluorescence from a coherently driven semiconductor quantum dot in a cavity

We show that resonance fluorescence, i.e., the resonant emission of a coherently driven two-level system, can be realized with a semiconductor quantum dot. The dot is embedded in a planar optical microcavity and excited in a waveguide mode so as to discriminate its emission from residual laser scattering. The transition from the weak to the strong excitation regime is characterized by the emergence of oscillations in the first-order correlation function of the fluorescence, g(tau), as measured by interferometry. The measurements correspond to a Mollow triplet with a Rabi splitting of up to 13.3 microeV. Second-order correlation measurements further confirm nonclassical light emission.     

Valence changes and structural distortions in "charge ordered" manganites quantified by atomic-scale scanning transmission electron microscopy

We investigate the microscopic nature of the "charge ordering" modulation in mixed-valent manganites in real space using scanning transmission electron microscopy. The modulation in Bi0.5Sr0.4Ca0.1MnO3 has a uniform periodicity appearing as stripes in high angle annular dark field images. Geometric phase analysis shows the modulation to be a displacement wave with transverse amplitude (0.008+/-0.001)a and longitudinal amplitude (0.003+/-0.001)a. Series of energy loss spectra taken across the stripes show no periodic changes and place an upper bound of +/-0.04 on any valence changes of the Mn ions.