Stability conditions in families

Publications mathématiques de l'IHÉS - We develop a theory of Bridgeland stability conditions and moduli spaces of semistable objects for a family of varieties. Our approach is based...     

Spin Dynamics of Negatively Charged Excitons in InP/(In,Ga)P Quantum Dots in a Magnetic Field

Physics of the Solid State - The dynamics of the photoluminescence negative circular polarization of an ensemble of InP/(In,Ga)P quantum dots is studied. It was found that, the time-resolved...     

Bridging Two Worlds: Colloidal versus Epitaxial Quantum Dots

An overview is given of two distinct classes of semiconductor quantum dots, epitaxial and colloidal structures that have been studied intensely for more than 30 years by now, however, without large interconnection between the two involved research communities. The largely parallel and independent evolution of the two structure classes may be partly related to the origin of colloidal systems from chemistry, while epitaxial quantum dots have been addressed mostly by the physics community. These independent evolutions are somewhat surprising because the interest in optics‐related applications is shared by both communities. Here, a short summary of the development of the two structure classes, the present status of activities, and some perspectives for future developments are presented.     

Synthesis and Pd-catalyzed coupling of1-C-stannylated glycals

Abstract

1-Tributylstannyl glycals were applied as versatile tools in the Pd(0)-mediated synthesis of glycal phthalonitrile conjugates. Likewise, selective homo-coupling of 1-tributylstannyl glycals furnishing C1-C1’ linked glycal dimers was investigated by using Pd(II)-species. For both Stille type couplings the rate-accelerating effect of copper(I) thiophene-2-carboxylate (CuTC) was exploited. The synthesis of stannylated glycal precursors was significantly improved by establishing a one-pot two-step procedure via glycosyl sulfoxides.     

Influence of processing methods on starch properties

Native potato starch was prepared using different processing methods. The samples were characterized by wide-angle X-ray scattering (WAXS), optical microscopy, differential scanning calorimetry (DSC), and microhardness. Compression molding of the starch granules led to sintered relatively brittle materials. Here, the amylopectin crystals of the native powder remained grossly preserved. Preparation of dry films from aqueous gels resulted in disintegration of the structure of the native starch granules and in the formation of a new semicrystalline structure comprised of crystallized amylose molecules. Injection molding of native starch was found to be a processing method that gives rise to amorphous materials with superior mechanical properties.     

Preparation and Characterization of Protonated Fumaric Acid

Fumaric acid was reacted with the binary superacidic systems HF/SbF₅ and HF/AsF₅. The O,O'-diprotonated [C₄H₆O₄]²⁺([MF₆]^–)₂ (M = As, Sb) and the O-monoprotonated [C₄H₅O₄]⁺[MF₆]^– (M = As, Sb) species are formed depending on the stoichiometric ratio of the Lewis acid to fumaric acid. The colorless salts were characterized by low-temperature vibrational spectroscopy. In case of the hexafluoridoantimonates single-crystal X-ray structure analyses were carried out. The [C₄H₆O₄]²⁺([SbF₆]^–)₂ crystallizes in the monoclinic space group C2/c with four formula units per unit cell and [C₄H₅O₄]⁺[SbF₆]^– crystallizes in the triclinic space group P1 with one formula unit per unit cell. The protonation of fumaric acid does not cause a notable change of the C=C bond length. The experimental data are discussed together with quantum chemical calculations of the cations [C₄H₆O₄ · 4 HF]²⁺ and [C₄H₆O₄ · 2 H₂CO · 2 HF]²⁺.     

Experimental and four‐component relativistic DFT studies of tungsten carbonyl complexes

We present a theoretical and experimental study of the structure and nuclear magnetic resonance (NMR) parameters of the pentacarbonyltungsten complexes of η¹‐2‐(trimethylstannyl)‐4,5‐dimethylphosphinine, η²‐norbornene, and imidazolidine‐2‐thione. The three complexes have a pseudo‐octahedral molecular structure with the six ligands bonded to the tungsten atom. The η¹‐2‐(trimethylstannyl)‐4,5‐dimethylphosphinine‐pentacarbonyl tungsten complex was synthesized for the first time. For all compounds, we present four‐component relativistic calculations of the NMR parameters at the Dirac–Kohn–Sham density functional level of theory using hybrid functionals. These large‐scale relativistic calculations of NMR chemical shifts and spin–spin coupling constants were compared with available experimental data, either taken from the literature or measured in this work. The inclusion of solvent effects modeled using a conductor‐like screening model was found to improve agreement between the calculated and experimental NMR parameters, and our best estimates for the NMR parameters are generally in good agreement with available experimental results. The present work demonstrates that four‐component relativistic theory has reached a level of maturity that makes it a convenient and accurate tool for modeling and understanding chemical shifts and indirect spin–spin coupling constants of organometallic compounds containing heavy elements, for which conventional non‐relativistic theory breaks down. Copyright © 2015 John Wiley & Sons, Ltd.