Pt3Co Concave Nanocubes: Synthesis,Formation Understanding,and Enhanced Catalytic Activity toward Hydrogenation of Styrene

We report a facile synthesis route to prepare high‐quality Pt₃Co nanocubes with a concave structure, and further demonstrate that these concave Pt₃Co nanocubes are terminated with high‐index crystal facets. The success of this preparation is highly dependent on an appropriate nucleation process with a successively anisotropic overgrowth and a preservation of the resultant high‐index planes by control binding of oleyl‐amine/oleic acid with a fine‐tuned composition. Using a hydrogenation of styrene as a model reaction, these Pt₃Co concave nanocubes as a new class of nanocatalysts with more open structure and active atomic sites located on their high‐index crystallographic planes exhibit an enhanced catalytic activity in comparison with low‐indexed surface terminated Pt₃Co nanocubes in similar size.     

Biosynthesis and Ether‐Bridge Formation in Nargenicin Macrolides

The nargenicin family of antibiotics are macrolides containing a rare ether‐bridged cis‐decalin motif. Several of these compounds are highly active against multi‐drug resistant organisms. Despite the identification of the first members of this family almost 40 years ago, the genetic basis for the production of these molecules and the enzyme responsible for formation of the oxa bridge, remain unknown. Here, the 85 kb nargenicin biosynthetic gene cluster was identified from a human pathogenic Nocardia arthritidis isolate and this locus is solely responsible for nargenicin production. Further investigation of this locus revealed a putative iron‐α‐ketoglutarate‐dependent dioxygenase, which was found to be responsible for the formation of the ether bridge from the newly identified deoxygenated precursor, 8,13‐deoxynargenicin. Uncovering the nargenicin biosynthetic locus provides a molecular basis for the rational bioengineering of these interesting antibiotic macrolides.     

Essential oils from the Hyptis genus--a review (1909-2009)

Several species belonging to the Hyptis (Lamiaceae) genus represent an important source of bioactive constituents, which are reputed for their wide range of antimicrobial, anticancer and insecticidal activities. The volatile oils obtained from various parts of the Hyptis plants were found to be primarily composed of mono- and sesquiterpenes. Significant differences were observed in the percentage compositions of the major components, which allowed for differentiation among the species. Based on the dominant constituents, phylogenetic relationships were found to be common among some species: 1,8-cineole (H. fruticosa, H. goyazensis, H. martiusii and H. suaveolens); beta-caryophyllene (H. marrubioides, H. pectinata, H. spicigera and H. suaveolens); eugenol (H. recurvata and H. suaveolens); gamma-cadinene (H. glomerata and H. ovalifolia); p-cymene (H. mutabilis and H. pectinata); alpha-pinene (H. crenata and H. emoryi). The monoterpenes, alpha-pinene and p-cymene were detected at various concentrations in all the Hyptis oils investigated. This paper reviews the essential oil compositions of eighteen Hyptis plant species studied in various parts of the world covering the publications of 100 years (1909 to 2009) after the first article appeared in the literature.     

Melt drawing of ultra‐high molecular weight polyethylene: Comparison of Ziegler‐ and metallocene‐catalyzed reactor powders

The drawing behavior of the ultra‐high molecular weight polyethylene (UHMW‐PE) melts has been studied by comparing the stress/strain curves for two types of samples as polymerized using conventional Ziegler and newer metallocene catalyst systems. Two UHMW‐PE samples, having the same viscosity average molecular weight of 3.3 × 10⁶, but different molecular weight distribution, have been drawn from melt at special conditions. The sample films for drawing were prepared by compression molding of reactor powders at 180°C in the melt. Differences in the structural changes during drawing and resultant properties, ascribable to their broad or narrow molecular weight distribution, were estimated from tensile tests, SEM observations, X‐ray measurements and thermal analyses. The metallocene‐catalyzed sample having narrower molecular weight distribution, could be effectively drawn from the melt up to a maximum draw ratio (DR) of 20, significantly lower than that obtained for the Ziegler‐catalyzed sample, ∼ 50. The stress/strain curves on drawing were remarkably influenced by draw conditions, including draw temperature and rate. However, the most effective draw for both was achieved at 150°C and a strain rate of 5 min⁻¹, independent of sample molecular weight distribution. The efficiency of drawing, as evaluated by the resultant tensile properties as a function of DR, was higher for the metallocene‐catalyzed sample having narrower molecular weight distribution. Nevertheless, the maximum achieved tensile modulus and strength for the Ziegler sample, 50–55 and 0.90 GPa, respectively, were significantly higher than those for the metallocene sample, 20 and 0.65 GPa, respectively, reflecting the markedly higher drawability for the former than the latter. The stress/strain behavior indicated that the origin of differences during drawing from the melt could be attributed to the ease of chain relaxation for the lower molecular weight chains in the melt. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1921–1930, 1999     

The structures of [C5H5O]+ ions formed from isomers of nitrophenol in the gaseous phase

Translational energy release measurments on metastable ions are used in the comparison of the structures of isomeric ions. Metastable ions, m₂⁺, formed from m₁⁺ ions as the result of a high energy process in the ion source are compared with isomeric metastable ions formed as daughters from fragmentation of metastable m₁⁺ ions in a field. In the case of o-, m- and p-nitrophenol the structure of the [C₅H₅O]⁺ ions formed from [C₆H₅O]⁺ ions by these two independent methods is different as verified by comparison of the behaviour of [C₅H₅O]⁺ ions formed from several other compounds.     

Functors between shape categories

Permutation groups of prime power degree are investigated here through the study of the corresponding group algebra of the set of all functions from the underlying set on which the permutation group acts to a finite field of characteristic p. For the case when the permutation group is of degree p² acting on a set consisting of the direct product of two elementary abelian p-groups, the structure of a minimal permutation module is obtained under certain conditions. The proofs do not depend on the recent classification results of finite simple groups.     

Angular factorization of matrices

Let Q denote a selfadjoint matrix (or operator of finite rank). The factorization of I + Q into the form $\text{I + Q = (I ? W) D(I ? W}{}^1\text{)}$ is considered. The well-known case where W is constrained to be lower triangular while D is diagonal is extended to quadrangular and other generalized-canonical forms. The case where Q is not selfadjoint is also developed. Applications for these results may be found in the realm of multivariate-stochastic approximation, and signal extraction.     

On a bottleneck bipartition conjecture of Erdős

For a graphG, let (U,V)=max{e(U), e(V)} for a bipartition (U, V) ofV(G) withUV=V(G),UV=Ø. Define (G)=min_(U,V ){(U,V)}. Paul Erds conjectures . This paper verifies the conjecture and shows .This work was part of the author's Ph. D. thesis at the University of New Mexico. Research Partially supported by NSA Grant MDA904-92-H-3050.     

A latent entanglement model for the draw of semicrystalline polymers

A novel molecular model, the latent entanglement, is proposed to explain the relationships between ductilities of polyethylene reactor powders and different conditions of synthesis. According to Hoffman's variable cluster model, irregularities in the crystallite fold surface increase as the synthesis temperature is decreased. The concept of latent entanglement stems from the potentiality of these defects to convert into active entanglements when the involved chain segments are drawn out of the crystals.