Crystalline morphologies in segmented copolymers with hard segments of uniform length

The crystal morphological properties of segmented poly(ether ester aramide) elastomers with aromatic hard‐segment amide units of uniform length were studied. Four samples with hard‐segment fractions ranging from 3.4 to 9 wt % were studied by tapping atomic force microscopy (AFM). For one sample, both solution and melt‐processed surfaces were examined, and similar crystal morphologies were found. The semicrystalline morphologies of these polymers had some similarities to other low‐hard‐segment segmented elastomers. The very thin needlelike or ribbonlike crystallites at the surface had a high aspect ratio for all the samples. The main difference observed for the different compositions was a decrease in the surface area density of ribbons with a decrease in the hard‐segment fraction. One sample was chosen for in situ AFM studies during film extension. The details of the crystallite orientation and breakup were studied in increments up to 700% elongation (8× stretch ratio) and after relaxation. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1783–1792, 2004     

Virtual test facilities for building acoustic predictions using FEM

The most important characteristic in building acoustics is the transmission loss which describes the insulation performance of a separating component. This quantity is defined by the ratio of inclined sound power to transmitted power. The transmission loss is usually measured in costly experimental set-ups of either real size facilities or down scaled test models. In order to reduce the effort and to gain further insight into the complex behavior of wave propagation in coupled structure-fluid systems, virtual test facilities using numerical methods (FEM) are established. An appropriate determination of the reverberation time is essential for obtaining good quality simulations. The results of the simulated transmission loss are presented and compared with common analytical prediction methods based on the transfer matrix method. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Resolving the problem of chromatographic overlap by 3D cross correlation (3DCC) processing of LC, MS and NMR data for characterization of complex glycan mixtures

Chromatographic overlap is a common problem in the analysis of complex mixtures. As a result, it is not possible to identify the components because each resulting NMR or MS spectrum contains multiple components. We introduce three-dimensional cross correlation (3DCC) that dissects NMR spectra of a mixture into spectra of the individual components without actually separating them. Correlation of peaks from MS and NMR profiles along a common LC time domain yields 3DCC NMR spectra of pure components correlated with a mass and a retention time. The method requires an LC run followed by fractionation and recording of MS and NMR spectra. The method is applicable to mixtures of any classes of molecules. Here, we demonstrate its application to a mixture of complex glycans obtained from a glycoprotein. Fourteen glycans eluting within only 3 min showed heavy overlap in the chromatographic run. 3DCC allowed their direct characterization without separation. Some of these structures from the glycoprotein bovine fibrinogen had not previously been described. The 3DCC procedure has been implemented in standard software. Actually, 3DCC can be used for any combination of separation techniques, like LC or GC, combined with two characterization methods like UV, IR, Raman, NMR or MS.     

An improved protocol for the RuO4-catalyzed dihydroxylation of olefins

[reaction: see text] Dihydroxylation under ruthenium catalysis provides an easy access to syn-diols, although overoxidation is a common side reaction. Furthermore, the high catalyst loadings offset the lower price of ruthenium compared to osmium. In this paper, we present an improved protocol for the RuO(4)-catalyzed syn-dihydroxylation using only 0.5 mol % catalyst under acidic conditions. A variety of olefins can be hydroxylated in good to excellent yields with only minor formation of side products.     

A comparison of C-F and C-H bond activation by zerovalent ni and pt: a density functional study

Density functional theory indicates that oxidative addition of the C-F and C-H bonds in C6F6 and C6H6 at zerovalent nickel and platinum fragments, M(H2PCH2CH2PH2), proceeds via initial exothermic formation of an eta2-coordinated arene complex. Two distinct transition states have been located on the potential energy surface between the eta2-coordinated arene and the oxidative addition product. The first, at relatively low energy, features an eta3-coordinated arene and connects two identical eta2-arene minima, while the second leads to cleavage of the C-X bond. The absence of intermediate C-F or C-H sigma complexes observed in other systems is traced to the ability of the 14-electron metal fragment to accommodate the eta3-coordination mode in the first transition state. Oxidative addition of the C-F bond is exothermic at both nickel and platinum, but the barrier is significantly higher for the heavier element as a result of strong 5dpi-ppi repulsions in the transition state. Similar repulsive interactions lead to a relatively long Pt-F bond with a lower stretching frequency in the oxidative addition product. Activation of the C-H bond is, in contrast, exothermic only for the platinum complex. We conclude that the nickel system is better suited to selective C-F bond activation than its platinum analogue for two reasons: the strong thermodynamic preference for C-F over C-H bond activation and the relatively low kinetic barrier.     

Immobilization of rhodium complexes at thiolate monolayers on gold surfaces: catalytic and structural studies

Chiral rhodium-diphosphine complexes have been incorporated into self-assembled thiolate monolayers (SAMs) on gold colloids. Catalysts of this type are of interest because they combine properties of homogeneous and heterogeneous systems. In addition, it should be possible to influence the catalytic properties of the metal center by the neighboring thiolate molecules. Colloids with a diameter of ca. 3 nm, coated with a mixed monolayer of n-octanethiolates and thiolates with chiral rhodium-PYRPHOS end groups, were studied as hydrogenation catalysts. With methyl alpha-acetamido-cinnamate as substrate, virtually the same enantioselectivities (up to 93% ee) and full conversion were obtained as with the corresponding homogeneous [Rh(COD)(PYRPHOS)]BAr(F) catalyst. The colloids were easily recovered by filtration and reused as catalysts three times without loss of enantioselectivity. STM studies of analogous SAMs on Au(111) gave a detailed picture of the structure and dynamics of mixed monolayers of this type. The STM images showed that the catalyst-bearing thiolates are distributed statistically on the surface and that the ordered structure of the n-octanethiolate SAM can be retained during incorporation of the catalyst-bearing thiols using the place-exchange methodology.