Effect of strain on stripe phases in the quantum Hall regime

Preferential orientation of the stripe phases in the quantum Hall (QH) regime has remained a puzzle since its discovery. We show experimentally and theoretically that the direction of high and low resistance of the two-dimensional (2D) hole gas in the QH regime can be controlled by an external strain. Depending on the sign of the in-plane shear strain, the Hartree-Fock energy of holes or electrons is minimized when the charge density wave (CDW) is oriented along the [110] or [110] directions. We suggest that shear strains due to internal electric fields in the growth direction are responsible for the observed orientation of CDW in pristine electron and hole samples.     

Strong coupling effects in binary Yukawa systems

We analyze the acoustic collective excitations in two- and three-dimensional binary Yukawa systems, consisting of two components with different masses. A theoretical analysis reveals a profound difference between the weakly and strongly correlated limits: at weak coupling the two components interact via the mean field only and the oscillation frequency is governed by the light component. In the strongly correlated limit the mode frequency is governed by the combined mass, where the heavy component dominates. Computer simulations in the full coupling range extend and confirm the theoretical results.     

Synthesis of the N,N'-Dibenzoyl Derivative of the Ornithine Decarboxylase Inhibitor (2R,5R)-Hept-6-Yne-2,5-Diamine from D-Glucosamine.

ABSTRACT

The N,N'-dibenzoyl derivative of (2R,5R)-hept-6-yne-2,5-diamine, a very powerful inhibitor of the enzyme ornithine decarboxylase, was prepared from D-glucosamine in fourteen steps.     

SN2 Displacement of Carbohydrate Triflates by 9-Oximes of Erythromycin A and Of a Tylosin Derivative

ABSTRACT

The preparation of 9-O-glycosyloxime derivatives of erythromycin A (1) and tylosin (2) is reported. Access to this new class of macrolides was achieved from (E)-9-oxime of erythromycin A (3) and 9-oxime of tylosin 20-(1,3-dithiane) (4), by successful displacement of triflates of suitably protected carbohydrates.     

Quasiliving Carbocationic Polymerization. IV. Polymerization of p-tert-Butylstyrene

The mechanism of polymerization of p-tert-butylstyrene (ptBuSt) initiated by the cumyl chloride/BCl₃ initiating system in CH₂Cl₂ at -50°C has been investigated. At and below ～0.4 M ptBuSt, quasiliving polymerizations proceed, i.e., initiation is instantaneous, termination is absent or reversible, and chain transfer to monomer can be suppressed or eliminated. In the quasiliving range the M _n versus [ptBuSt]₀ plot is linear and passes through the origin, and a M _w/M _n decreases much below 2.0 with decreasing [ptBuSt]. GPC traces change from broad multimodal to narrow monomodal and the color of polymerization charges change from colorless to golden-yellow with decreasing [ptBuSt]. The effect of temperature jump subsequent to monomer addition has been examined; however, it does not explain the peculiar monomer concentration effect on the mechanism. Changes in the ionicity may be responsible for this phenomenon.     

Minimal simplices inscribed in a convex body

Measuring how far a convex body $\mathcal{K }$ (of dimension $n$ ) with a base point ${O}\in \,\text{ int }\,\mathcal{K }$ is from an inscribed simplex $\Delta \ni {O}$ in “minimal” position, the interior point ${O}$ can display regular or singular behavior. If ${O}$ is a regular point then the $n+1$ chords emanating from the vertices of $\Delta $ and meeting at ${O}$ are affine diameters, chords ending in pairs of parallel hyperplanes supporting $\mathcal{K }$ . At a singular point ${O}$ the minimal simplex $\Delta $ degenerates. In general, singular points tend to cluster near the boundary of $\mathcal{K }$ . As connection to a number of difficult and unsolved problems about affine diameters shows, regular points are elusive, often non-existent. The first result of this paper uses Klee’s fundamental inequality for the critical ratio and the dimension of the critical set to obtain a general existence for regular points in a convex body with large distortion (Theorem A). This, in various specific settings, gives information about the structure of the set of regular and singular points (Theorem B). At the other extreme when regular points are in abundance, a detailed study of examples leads to the conjecture that the simplices are the only convex bodies with no singular points. The second and main result of this paper is to prove this conjecture in two different settings, when (1) $\mathcal{K }$ has a flat point on its boundary, or (2) $\mathcal{K }$ has $n$ isolated extremal points (Theorem C).     

Slipping–rolling transitions of a body with two contact points

Nonlinear Dynamics - In this paper, the general kinematics and dynamics of a rigid body is analysed, which is in contact with two rigid surfaces in the presence of dry friction. Due to the rolling...     

Electrostatic and Non‐covalent Interactions in Dicationic Imidazolium–Sulfonium Salts with Mixed Anions

A series of thioether‐functionalised imidazolium salts have been prepared and characterized. Subsequent reaction of the thioether‐functionalised imidazolium salts with iodomethane affords imidazolium–sulfonium salts composed of doubly charged cations and two different anions. Imidazolium–sulfonium salts containing a single anion type are obtained either by a solvent extraction method or by anion exchange. The imidazolium–sulfonium salts undergo a methyl‐transfer reaction on exposure to water, giving rise to a new, singly charged imidazolium salt with iodide introduced at the 2‐position of the imidazolium ring. Crystal structures of some of the imidazolium–sulfonium salts were determined by X‐ray crystallography providing the topology of the interactions between the dications and the anions. Electrospray ionization mass spectrometry and quantum‐chemical calculations were used to rationalise the relative strength of these interactions.     

Colloidal Metal Nanocatalysts: Synthesis,Characterization, and Catalytic Applications

Metal nanoparticles are key materials in heterogeneous catalysis due to their high catalytic activity and selectivity to the desired product. Accordingly, they are playing a pivotal role in most heterogeneous catalytic reactions that are steeply growing with the development of a colloidal synthetic protocol that enables fine control of size, shape, morphology and composition of metal nanoparticles at an atomic level. These colloidal metal nanoparticles can be dispersed on a rigid support such as mesoporous silica, metal oxide and zeolite, which utilizes metal nanoparticles as model heterogeneous catalysts in industrially important processes involving hydrogenation/dehydrogenation, isomerization and cracking. In this review article, we highlight the recent progress on general colloidal synthetic routes with technological advances in characterization tools that enable the atomic-scale observation of metal nanoparticles. Structure-dependent contributions on the control of product selectivity and turnover rate are also discussed by combining advanced ex situ and in situ surface characterization tools that can monitor the structural change of metal nanocatalysts as well as the evolution of reaction intermediates under the reaction conditions.     

Attaining control by design over the hydrolytic stability of Fe-TAML oxidation catalysts

The iron(III) complexes of tetra amidato macrocyclic ligands (TAMLs) ([Fe{1-X1-2-X2C6H2-4,5-(NCOCMe2NCO)2CR2}(OH2)]- , 1: X1 = X2 = H, R2 = Me2 (a), R2 = (CH2)2 (b); X1 = X2 = Cl, R2 = F2 (c), etc.), which the proton is known to demetalate at pH < 3, are also subject to catalyzed demetalation by Br?nsted acid buffer components at pH 4-9 such as H2PO4-, HSO3-, and CH3CO2H, HO2CCH2CO2-. Buffers based on pyridine (py) and tris(hydroxymethyl)aminomethane (TRIS) are catalytically inactive. Where reactions proceed, the products are demetalated TAMLs and iron species of variable composition. Pseudo-first-order rate constants for the demetalation (kobs) are linear functions of the acid concentrations, and the effective second-order rate constants k1,eff have a hyperbolic dependence on [H+] (k1,eff = a1[H+]/(b1+[H+]). The rate of demetalation of 1a in H2PO4-/HPO42- buffer is appreciable, but the kobs values for 1b and 1c are immeasurably low, showing that the rates are strongly affected by the CR2 or "tail" fragments, which are known to potently affect the TAML basicity. The reactivities of 1 depend insignificantly on the aromatic ring or "head" group of 1. The proposed mechanism involves precoordination of the acidic buffer species followed by hydrolysis. The demetalating abilities of buffer species depend on their structures and acidities. Thus, although pyridine-2-carboxylic (picolinic) acid catalyzes the demetalation, its 3- and 4-isomers (nicotinic and isonicotininc acids) are inactive. The difference is rationalized to result from the ability that only coordinated picolinic acid has to deliver a proton to an amidato nitrogen in an intramolecular manner. The reaction order in picolinic acid equals one for 1a and two for 1b. For 1b, "inactive" pyridine and nicotinic acid speed up the demetalation in the presence of picolinic acid, suggesting that the second order arises from the axial binding of two pyridine molecules, one of which must be picolinic acid. The binding of pyridine- and imidazole-type ligands was confirmed by UV/vis equilibrium measurements and X-ray crystallography. The implications of these mechanistic findings for designing superior Fe-TAML oxidation catalysts and catalyst formulations are discussed using the results of DFT calculations.