The chemistry of alkyl dihalides in zeolite NaX at room temperature

Alkyl dihalides adsorbed in NaX zeolite undergo room temperature, substitutional dehalogenation, single dehydrohalogenation, combined dehalogenation–dehydrohalogenation, or two consecutive dehydrohalogenations. The preference for a particular reaction depends on the nature and relative positions of the halogens, and on alkyl chain length. Nuclear magnetic resonance chemical shifts and mass spectrometric assignments to the reagents and products indicate the particular reactions taking place.All of the alkyl dihalides containing the iodo group (diiodoalkanes) studied (αα, αβ, α,γ) undergo only substitutional dehalogenation to form zeolite bound iodoalkoxy. The same chemistry was observed with both short and long chain diiodoalkanes. The dichloroalkane and dibromoalkane chemistry is dependent on the relative halogen positions and the alkyl chain length. The α, α dichloroalkanes and dibromoalkanes examined by us undergo single dehydrohalogenation, with the exception of 2,2-dichloropropane, which forms a mixture of dehydrohalogenation and substitutional dehalogenation products. The chemistry of α, β alkyl dihalides (Cl, Br) in NaX is dependent on the carbon chain length. The dihaloethanes (Cl, Br) undergo both substitutional dehalogenation and dehydrohalogenation. As the chain length increases to three carbon atoms or more, only dehydrohalogenation reactions take place. No alkyne products were observed with α, β dichloroalkanes and dibromoalkanes, an indication that NaX is too weak a base to initiate sequential dehydrohalogenations. The chemistry of α, γ dichloroalkanes and dibromoalkanes were also dependent on the chain length. The α, γ dihalopropanes (Cl, Br) in NaX form a propene-framework bound product, associated with dehydrohalogenation and subsequent substitutional dehalogenation. The α, γ dihalobutanes (Cl, Br) undergo two consecutive dehydrohalogenations to form dienes.     

Reciprocal and Reciprocal Square Root Units with Operand Modification and Multiplication

Reciprocals and reciprocal square roots are used in several digital signal processing, multimedia, and scientific computing applications. This paper presents high-speed methods for computing reciprocals and reciprocal square roots. These methods use a table lookup, operand modification, and multiplication to obtain an initial approximation. This is followed by a modified Newton-Raphson iteration, which improves the accuracy of the initial approximation. The initial approximation and Newton-Raphson iteration employ specialized hardware to reduce the delay, area, and power dissipation. The application of these methods is illustrated through the design of reciprocal and reciprocal square root units for operands in the IEEE single precision format. These designs are pipelined to produce a new result every clock cycle. Kent Wires received a B.S. degree in Electrical Engineering from Cornell University, Ithaca, NY in 1996, and M.S. and Ph.D. degrees in Electrical Engineering from Lehigh University, Bethlehem, PA, in 1997 and 2001, respectively. From 1998 to 2001, he was a Member of Technical Staff at Lucent Technologies, Allentown, PA, where he was a member of the Advanced DSP Architectures and Compilers Group. He is currently a Senior Member of Technical Staff at Agere Systems, Allentown, PA, where he is a systems architect focusing on media streaming and network protocol techniques. His current research interests include computer arithmetic, media streaming techniques, efficient processor modeling techniques, and network processor architectures and protocols. Michael Schulte received a B.S. degree in Electrical Engineering fromthe University of Wisconsin-Madison in 1991, and M.S. and Ph.D. degrees in Electrical Engineering from the University of Texas at Austin in 1992 and 1996, respectively. From 1996 to 2002, he was an assistant and associate professor at Lehigh University, where he directed the Computer Architecture and Arithmetic Research Laboratory. In 1997, he received a NSF CAREER Award to research hardware support for accurate and reliable numerical computations. Prof. Schulte has consulted for or had joint research projects with Sandbridge Technologies, IBM, Sun Microsystems, ARM, Lucent Technologies, Agere Systems, MIPS Technologies, and Sandia National Laboratories. He is currently an assistant professor at the University of Wisconsin-Madison, where he leads the Madison Embedded Systems and Architectures Group. His research interests include high-performance embedded processors, computer architecture, domain-specific systems, computer arithmetic, and wireless security. He is a senior member of the IEEE and the IEEE Computer Society, and an associate editor for the IEEE Transactions on Computers and the Journal of VLSI Signal Processing.     

Columnar Liquid Crystals with a Central Crown Ether Unit

Novel columnar liquid crystals 5a – f bearing a dibenzo[18]crown[6] central unit have been prepared from bromodialkyloxybenzenes 1 and tetrabromodibenzo[18]crown[6] 2 via Suzuki coupling reactions. Differential scanning calorimetry (DSC) revealed that the length of the alkyl chains in the liquid‐crystalline compounds 5 influenced their mesomorphic properties. Whereas 5a – d gave fan‐shaped textures only upon rapid cooling, 5e and 5f with C9 and C10 side chains formed stable columnar textures upon slow cooling. A hexagonal columnar mesophase of 5a is apparent from X‐ray diffraction studies. Complexation of 5 with potassium significantly shifted the clearing points to higher temperatures and also increased the mesophase stability in the cooling cycle.     

Generation and visualization of four-dimensional MR angiography data using an undersampled 3-D projection trajectory

Time-resolved contrast-enhanced magnetic resonance (MR) angiography (CE-MRA) has gained in popularity relative to X-ray Digital Subtraction Angiography because it provides three-dimensional (3-D) spatial resolution and it is less invasive. We have previously presented methods that improve temporal resolution in CE-MRA while providing high spatial resolution by employing an undersampled 3-D projection (3D PR) trajectory. The increased coverage and isotropic resolution of the 3D PR acquisition simplify visualization of the vasculature from any perspective. We present a new algorithm to develop a set of time-resolved 3-D image volumes by preferentially weighting the 3D PR data according to its acquisition time. An iterative algorithm computes a series of density compensation functions for a regridding reconstruction, one for each time frame, that exploit the variable sampling density in 3D PR. The iterative weighting procedure simplifies the calculation of appropriate density compensation for arbitrary sampling patterns, which improve sampling efficiency and, thus, signal-to-noise ratio and contrast-to-noise ratio, since it is does not require a closed-form calculation based on geometry. Current medical workstations can display these large four-dimensional studies, however, interactive cine animation of the data is only possible at significantly degraded resolution. Therefore, we also present a method for interactive visualization using powerful graphics cards and distributed processing. Results from volunteer and patient studies demonstrate the advantages of dynamic imaging with high spatial resolution.     

Connectivity Probability of Wireless Ad Hoc Networks: Definition, Evaluation, Comparison

The paper presents a new approach investigating mobile ad hoc network connectivity. It is shown how to define and evaluate the connectivity probability of a mobile network where the position of the nodes and the link quality changes over time. The connectivity probability is a measure that can capture the impact of the node movement on the network connectivity. A number of mobility models is considered ranging from the classical Random Direction model to the Virtual World model based on the mobility measurements of a multi–player game. We introduce an Attractor model as a simple way to model non–homogeneous node distribution by incorporating viscosity regions in the simulation area. Methods of ergodic theory are used to show the correctness of the approach and to reduce the computational time. Simulation results show how the node density distribution affects the network connectivity.     

A fuzzy noise reduction method for color images.

A new fuzzy filter is presented for the reduction of additive noise for digital color images. The filter consists of two subfilters. The first subfilter computes fuzzy distances between the color components of the central pixel and its neighborhood. These distances determine in what degree each component should be corrected. All performed corrections preserve the color component distances. The goal of the second subfilter is to correct the pixels where the color components differences are corrupted so much that they appear as outliers in comparison to their environment. Experimental results show the feasibility of the proposed approach. We compare with other noise reduction methods by numerical measures and visual observations. We also illustrate the performance of the proposed method as preprocessing step for edge detection.     

An investigation of the diffraction of waves by complementary infinite double-sectors

Contents In this paper we treat the diffraction by infinite double-sectors. The scalar boundary value problem as well as the electromagnetic boundary value problem is solved using the problem adapted sphero-conal coordinate system. We determine appropriate eigenfunctions and we find a remarkable relation between the eigenvalues of complementary double-sectors due to Babinet's principle. Additionally we consider the diffraction of a plane wave normally incident upon the double-sector for the scalar and for the electromagnetic case.

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Eine Untersuchung der Beugung von Wellen an komplementären unendlich ausgedehnten Doppelsektoren

Übersicht In dieset Arbeit wird die Beugung an unendlich ausgedehnten Doppelsektoren untersucht. Sowohl das skalare wie auch das elektromagneticsche Randwert-problem werden in elliptischen Kegelkoordinaten als problemangepaßtem Koordinatensystem gelöst. Es werden geeignete Eigenfunktionen bestimmt, wobei sich für die Eigenwerte für zueinander komplementäre Doppelsektoren eine Symmetriebeziehung ergibt, welche dem Babinetschen Prinzip entspricht. Desweiteren wird die Beugung einer senkrecht einfallenden ebenen Welle am Doppelsektor für den skalaren wie auch für den elektromagnetischen Fall untersucht.

     

Polychlorinated biphenyl residues in food and human milk: determination of co-planar and mono-ortho substituted congeners

An analytical method is described for determining the residues of coplanar as well as mono-ortho substituted PCB congeners both exhibiting toxicological relevance, in foods of animal origin and in human milk. The unsophisticated procedure, convenient for routine analyses, includes the extraction of lipids, clean-up steps using liquid/liquid partition and column chromatography, fractionation of the congeners using HPLC on a special carbon column with an optimal gradient elution, and capillary column gas chromatography with electron capture or mass spectrometric detection. As preliminary results indicate, the low-chlorinated PCB technical products contribute more to the actual contamination of environment and foods than has been estimated so far. Obviously the co-planar congeners are accumulated during the food chain in a portion different from that found for the other congeners.