bluuebunny/crossref_metadata_2025_split huggingface dataset:
Furyl‐Substituted Group‐4 Metallocene Catalysts for Elastomeric Polypropylene Formation—The Role of the Remote Substituents
Authors: Dreier Thorsten , Erker Gerhard 
Published: April 2004

AbstractTreatment of a series of 2‐alkylfurans (1) and related systems with n‐butyllithium, followed by addition of the resulting lithiofurans to 2‐indanone, H3O+‐catalyzed elimination, deprotonation, and transmetallation to zirconium tetra‐chloride, gave a series of differently substituted bis [(5‐alkyl‐2‐furyl) indenyl] ZrCl2 complexes [alkyl = n‐butyl (5d), n‐hexyl (5e), n‐octyl (5f), tert‐butyl (5g)], as well as the corresponding benzofuryl (5h) and 4,5‐dimethylfuryl (5i) complexes. Their treatment with methylalumoxane (>1000‐fold excess) generated active homogeneous Ziegler‐Natta polymerization catalysts. Propene polymerization at temperatures between −20 °C and +20 °C gave elastomeric polypropylenes. The elastomeric features were characterized by stress/strain curves that revealed a pronounced dependence on the actual alkyl substituent attached at the furyl rings at the periphery of the bent metallocene complexes. Increasing steric bulk of these substituents at the catalyst framework seems to result in increased isotacticities and a corresponding improvement in elastomeric properties of the polymers obtained by these systems.
The Diversity of B0 and B1 Gradient NMR Diffusometry Techniques
Authors: Ardelean Ioan , Kimmich Rainer 
Published: April 2004

AbstractThe conventional pulsed gradient Hahn or stimulated echo methods for diffusion measurements are briefly reviewed. Compared to these techniques, unconventional NMR diffusometry variants based on gradients of the amplitude of the radio frequency field, on nonlinear (or multiple) echoes, and on imaging techniques open promising aspects depending on the application envisaged. A series of typical pulse sequences representing the diversity of NMR diffusometry methods is described and discussed with respect to advantages and disadvantages.
High Molecular Weight Atactic Polypropylene prepared by Zirconium Complexes of an Amine Bis(phenolate) Ligand
Authors: Groysman Stanislav , Tshuva Edit Y. , Reshef Dvora , Gendler Shimrit , Goldberg Israel , Kol Moshe , Goldschmidt Zeev , Shuster Michael , Lidor Gershon 
Published: April 2004

AbstractThe polymerization of propylene employing the recently developed amine bis(phenolate) zirconium catalysts is reported for the first time. The polymerizations were conducted in liquid propylene employing two different experimental setups, and two different precatalysts, based on the same amine bis(phenolate) ligand having a dimethylamino donor on a sidearm. The first precatalyst studied was a dibenzyl zirconium complex, which was found to lead to high molecular weight atactic polypropylene. The effects of the cocatalyst, the ratio between the cocatalyst and the precatalyst, and the polymerization temperature on the nature of the resulting polymer and the activity of the catalyst were studied. Reducing the relative amount of the cocatalyst and lowering the polymerization temperature increased the molecular weight of the polymer, and a temperature of 50 °C led to the most active catalyst. A dichloro zirconium complex of the same ligand, expected to show a higher hydrolytic and thermal stability was developed, its X‐ray structure was solved, and its activity in polymerization of propylene was studied and found to be similar to that of the dibenzyl precatalyst.
“Chain‐End‐Controlled Isotactic” and “Stereoblock‐Isotactic” Polypropylene: Where Is the Difference?
Authors: Busico Vincenzo , Cipullo Roberta , Talarico Giovanni , Castelli Valeria Van Axel 
Published: April 2004

AbstractThis communication describes the microstructure of “stereoblock‐isotactic” polypropylene obtained with sterically hindered “oscillating” metallocene catalysts, points out in which respects it differs from that of “chain‐end‐controlled isotactic” polypropylene, and explains why the two definitions cannot be used as synonymous (as is commonly found in the literature).
Application of the triisobutylaluminum‐promoted reductive rearrangement to sucrose‐5‐enes
Authors: Du Roizel Bérengère , Henriques Ana M. P. , Pearce Alan J. , Sinaÿ Pierre 
Published: April 2004

AbstractCarbohydrate‐based vinyl acetals (5‐hex‐enopyranosides) undergo reductive rearrangement with triisobutylaluminum (TIBAL) to afford highly functionalized cyclohexanes in which both the aglycon and anomeric stereochemistry are retained. Here, we report the first application of this process to the rearrangement of hex‐5‐enopyranosides of sucrose in which the interglycosidic oxygen atom of the vinyl acetal system links the anomeric centers of both monosaccharide units. The sucrose‐derived 5‐hex‐enopyranoside 1 undergoes smooth reductive rearrangement with TIBAL to afford the (1 → 2′) ether‐linked pseudo‐disaccharide 2 in 34% yield. The rearrangement is accompanied by some loss of stereochemical integrity at C‐2′ due to a competitive exo‐cleavage of the interglycosidic (O‐C2′) bond, hence diastereomers at C‐2′ are also obtained in 12% yield. The 4‐O‐allyl‐protected sucrose‐5‐ene 3 is similarly transformed into the corresponding (1 → 2′) ether‐linked pseudo‐disaccharide 4, illustrating the compatibility of the allyl group with the TIBAL reaction conditions.magnified image
Diffusion MR in Biological Systems: Tissue Compartments and Exchange
Authors: Stanisz Greg J. 
Published: April 2004

AbstractThe processes of diffusion in multi‐compartmental tissue models are discussed using an analytical model of diffusion with exchange. Diffusion in the intracellular compartment has been found to be highly restricted, with the apparent diffusion coefficient (ADC1) dependent on the experimental parameters. Extracellular diffusion mediated by tortuosity, depends on tissue geometry, but not on the experimental conditions. Both the intra‐ and extracellular diffusion can be anisotropic and exhibit non‐monoexponential behavior. The cell membrane permeability is shown to significantly influence the diffusion measurement in tissues. In consequence, the phenomenological, bi‐exponential models of diffusion cannot directly estimate the intrinsic characteristics of intra‐ and extracellular water. The mechanisms of the ADC decrease after stroke are also discussed.
Mismatch discrimination in oligonucleotide hybridization reactions using single strand binding protein. A surface plasmon fluorescence study
Authors: Neumann Thomas , Knoll Wolfgang 
Published: April 2004

AbstractWe describe the use of surface plasmon‐ and surface plasmon fieldenhanced fluorescence spectroscopy for the detection of hybridization reactions between surface‐attached probe oligonucleotides and complement strands binding from solution. These targets, exhibiting different base mismatches relative to the probe 15‐mer sequences, carry a fluorophore at their 5'‐end thus allowing for sensitive detection and quantification of association, kon, and dissociation, koff, rate constants, as well as affinity constants, Ka. We demonstrate that by the competitive binding / replacement of single strand binding proteins the mismatch discrimination can be further enhanced.
Optically detected magnetic resonance of thiol‐capped CdTe nanocrystals
Authors: Glozman Alexander , Lifshitz Efrat , Hoppe Kathrin , Rogach Ahdrey L. , Weller Horst , Echymüller Alexander 
Published: April 2004

AbstractThe optical properties of thiol‐stabilized CdTe nanocrystals have been examined. The thiol groups ‐SR generate a CdS shell at the interface, leading to a CdTe/CdS core—shell structure. The present paper describes our efforts to identify the influence of the CdTe–SR interface on the optical properties of the nanocrystals, utilizing photoluminescence and optically detected magnetic resonance (ODMR) spectroscopy. The photoluminescence spectrum consists of an excitonic peak, overlapped by a broad band at lower energies. The ODMR spectrum, in the spectral regime of the broad band, showed two resonance signals. They are associated with a trapped hole at an anisotropic site of a cadmium vacancy at the Cd–SR interface and an electron in the conduction band.
Chemical applications of near‐field scanning optical microscopy: Surface and near‐surface chemical imaging with conventional near‐field optical probes and externally illuminated chemically active ion sensors
Authors: Strinkovski Alina , Shalom Shmuel , Radko Anna , Ignatov Andre , Lewis Aaron , Ottolenghi Michael , Sukenik Chaim 
Published: April 2004

AbstractNear‐field optics using force sensing cantilevered optical fibers and micropipettes is used to investigate the chemical distribution of protein complexing dyes in chromosomes and the sensing and chemical imaging of surfaces and nearsurface regions. These results are put in the context of what presently can be considered as a broad picture of near‐field optical chemical applications.
Spin Echo Analysis of Restricted Diffusion under Generalized Gradient Waveforms for Spherical Pores with Relaxivity and Interconnections
Authors: Ryland Brett N. , Callaghan Paul T. 
Published: April 2004

AbstractThe problem of restricted diffusion in spherical pores is examined under conditions of finite gradient pulse width in pulsed gradient spin echo (PGSE)‐NMR experiments. Closed form analytic expressions are derived, and the case of interconnected pores is briefly examined. An expression, based on the pore hopping approximation, is presented that predicts the echo attenuation for diffusion between pores under any gradient waveform.
Synthesis, Characterization, and Reactivity of Lanthanide Complexes with Bulky Silylallyl Ligands
Authors: Woodman Timothy J. , Schormann Mark , Bochmann Manfred 
Published: April 2004

AbstractThe synthesis of new lanthanide allyl complexes of enhanced stability and solubility in saturated hydrocarbons based on silyl‐substituted allyl ligands is reported. Thus the potassium salt K(CH2CHCHSiMe3) (1) reacts with YCl3 in tetrahydrofuran to give the tris‐allyl complex Y(CH2CHCHSiMe3)3 (2), while K(CH2CHCHSiMe2tBu) (3) affords Y(CH2CHCHSiMe2tBu)3(THF)1.5 (4). Slow re‐crystallization of 4 from light petroleum in the presence of tert‐butylcyanide led to multiple insertion to give the sec‐amido complex Y{NHC(tBu)(CH)3SiMe2tBu}2{η2‐NHC(tBu)CH=CHCH2SiMe2tBu)CH(CHCHSiMe2tBu)CtBuNH}(THF)·(CH3CH(Me)(CH2)2CH3) (5), which was crystallographically characterized. The reaction of ScCl3(THF)3 with two equivalents of Li{1,3‐C3H3(SiMe3)2} in tetrahydrofuran gives the bis‐allyl complex {1,3‐C3H3(SiMe3)2}2Sc(μ‐Cl)2Li(THF)2 (6), while the analogous reaction of K{1,3‐C3H3(SiMe3)2} (7) with either LaCl3 or YCl3 in tetrahydrofuran affords the bis‐allyl complexes MCl{1,3‐C3H3(SiMe3)2}2(THF)x (8, M = La, x = 1; 9, M = Y, x = 0). An attempt to prepare the similar neodymium complex gave the mono‐allyl complex NdI2{1,3‐C3H3(SiMe3)2}(THF)1.25 (10). The reactions of 8 and 9 with triisobutyl aluminum in benzene‐d6 show allyl exchange between lanthanide and aluminum. Complexes 8, 9, and 10 have been tested with a variety of activator systems as catalysts for the polymerization of 1,3‐butadiene.
Partitioning and pore‐filling: Solubility‐normalized sorption isotherms of nonionic organic contaminants in soils and sediments
Authors: Grathwohl Peter , Rahman Md. Mokhlesur 
Published: April 2004

AbstractNonlinear sorption of hydrophobic organic pollutants in soils and sediments can be described by a combination of partitioning and adsorption (pore‐filling) mechanisms. Both the partitioning and the pore‐filling models plot sorptive uptake vs. solubility‐normalized aqueous concentrations, which yields “collapsing” sorption isotherms for similar compounds. With this new concept, sorption of a variety of similar compounds could be predicted based on measured data of one probe chemical. The solubility‐normalized Freundlich model predicts an inverse linear relationship between the sorption coefficient measured at a given relative concentration vs. solubility, S, which is demonstrated for 3 partitioning dominated samples from Bangladesh (soil, aquifer sediment, peat) and 4 compounds (carbofuran, trichloroethene, 1,2‐dichlorobenzene, phenanthrene) as well as for 2 activated carbons (F300, F400), and a wide variety of compounds (literature data). Future research will focus on the improvement of prediction of sorption in soils and sediments by including other normalization parameters, such as molar volume, polarizability, etc., in addition to the water solubility.
Photoconductivity of poly(N‐vinylcarbazole) (PVK) doped with the metallofullerene Dy@C82 and the fullerenes C84 and C60
Authors: Li Xiaogang , Yang Shihe , Xu Yu , Yang Shangfeng , Liu Yunqi , Zhu Daoben , Yang Chunlei , Wang Jiannong , Ge Weikun 
Published: April 2004

AbstractPhotoconductivity has been measured on films of pure poly(Nvinylcarbazole) (PVK) and PVK doped with the metallofullerene Dy@C82 and the fullerenes C84 and C60. The photo‐induced discharge rate of the PVK film increased dramatically when doped with the metallofullerene or the fullerenes. Comparatively, the film doped with Dy@C82 displayed better photoconductivity than that doped with C84, which is attributed to the Dy@C82 being a better electron acceptor than C84. However, the film doped with C60 showed the best photoconductivity. This is attributed to the better electron accepting ability of excited C60* or the better miscibility of the hollow fullerenes with the PVK polymer and organic solvents.
Quality indicators of the state of chemical pollution in israel
Authors: Haran Miriam , Samuels Rana , Gabbay Shoshana , Mingelgrin Uri 
Published: April 2004

AbstractThe state of chemical pollution in Israel is evaluated with the aid of environmental indicators adapted from a list of indicators compiled by the European Union. The precise definition of the individual indicators was modified to suit the specific environmental conditions in Israel. The environmental topics covered are those that have a strong chemical aspect to them, including groundwater quality, pollution of rivers, air pollution by acidifying substances and particulate matter, CO2 emissions, disposal of hazardous waste and handling of hazardous substances, air pollution originating from transportation, wastewater treatment and reuse, and solid waste disposal, as well as the status of environmental research.The environmental indicators chosen reveal that in recent years there has been a steady improvement in the state of chemical pollution, excepting a deterioration in the quality of groundwater and an increase in vehicular emission of pollutants. The overall situation is, however, still unsatisfactory. A comparison of the state of chemical pollution in Israel to that in the EU and other countries demonstrated that the rate of improvement in environmental quality as determined by the examined indicators is slower in Israel than in most developed countries.
Pressure and gas composition effects on the operation of the pulsed flame photometric detector
Authors: Frishman Gad , Amirav Aviv , Barak Haim 
Published: April 2004

AbstractThe effect of pressure and hydrogen/oxygen ratio of a burning gas mixture on pulsed flame emission time‐dependence was investigated in the range of 0.1–5 atm using a specially designed pulsed flame photometric detector (PFPD). We studied the pressure and gas composition effect on the pulsed flame delayed light emission of sulfur, phosphorus, and nitrogen‐containing organic compounds. The optimal pressure conditions for nitrogen detection, intensity, and emission time delay was found to be 0.4 bar, at which the detection sensitivity could be improved by a factor of 2. For phosphorus, the optimal pressure obtained was 1.3 bar with 40% sensitivity improvement (compared with 1 bar). In the case of sulfur detection, two emission maxima were obtained, at 1.1 and 0.6 bar, at H/O ratio of 5. Increasing the H/O ratio resulted in the appearance of only one peak at 1 bar, and enhancement of the sensitivity by a factor of 2.4 at H/O ratio of 10.3. From the analytical point of view, we found that emission intensity is practically unchanged by the pressure and the H/O ratio for all three elements investigated in the range of 0.8–1.1 bar and H/O of 5–6. Thus, in addition to excellent sensitivity and improved selectivity, the PFPD can be applied under a variety of atmospheric pressure conditions in field environmental applications.
Diffusion MRI and MRS of Skeletal Muscle
Authors: Strijkers Gustav J. , Drost Maarten R. , Heemskerk Anneriet M. , Kruiskamp Marijn J. , Nicolay Klaas 
Published: April 2004

AbstractMR offers powerful tools to measure molecular diffusion in intact tissues under in vivo conditions. This review focuses on the use of diffusion‐weighted MR imaging (DW‐MRI) and spectroscopy (DW‐MRS) to noninvasively study structural and functional aspects of skeletal muscle. It will be shown that DW‐MRI allows for a high‐resolution reconstruction of the orientation of the muscle fibers, which represents vital input for mathematical models of muscle biomechanics and an improved understanding of contractile dysfunction in muscle disease. DW‐MRS can be used to quantitatively measure the translational displacement of endogenous metabolites in intact muscle. DW‐MRS is of fundamental interest because it enables one to probe the in situ status of the intracellular space from the diffusion characteristics of the metabolites, while at the same time providing information on the intrinsic diffusion properties of the metabolites themselves. This yields information on cellular architecture and on the intrinsic capacity for diffusional mass transport.
The Kinetic Stability of Cationic Benzyl Titanium Complexes that Contain a Linked Amido‐Cyclopentadienyl Ligand: The Influence of the Amido‐Substituent on the Ethylene Polymerization Activity of “Constrained Geometry Catalysts”
Authors: Okuda Jun , Musikabhumma Kittichote , Sinnema Piet‐Jan 
Published: April 2004

AbstractCationic benzyl titanium complexes [Ti(η5: η1‐C5Me4SiMe2NR')‐(CH2Ph)]+ were cleanly formed by the reaction of the dibenzyl titanium complexes [Ti(η5: η1‐C5Me4SiMe2NR')(CH2Ph)2] with B(C6F5)3 and [Ph3C][B(C6F5)4] in bromobenzene. NMR spectroscopic studies suggest that the benzyl titanium cations contain a fluxional η2‐coordinated benzyl ligand. Kinetic analysis showed that the benzyl titanium cations decompose according to first‐order kinetics and that the amido substituents R' (R' = Me, iPr, tBu) in the linked amido‐cyclopentadienyl ligand influence the lability of these benzyl titanium cations. The order of the kinetic stability of the benzyl titanium cations was found for both anions to follow the order R' = Me > iPr > tBu. The benzyl titanium cations generated with [Ph3C][B(C6F5)4] were found to undergo faster decomposition than those generated with B(C6F5)3. The ethylene polymerization activity order for both systems was found to be the reverse: R' = tBu > iPr > Me. The decomposition of the benzyl titanium cations was suggested to occur via C—H activation with concomitant toluene elimination.
Centroid dynamics with quantum statistics
Authors: Roy Pierre‐Nicholas , Blinov Nicholas 
Published: April 2004

AbstractThis paper discusses the recent developments of a new molecular dynamics approach for the study of quantum dynamics for systems obeying Bose—Einstein statistics. The formalism is based on the mapping of a quantum mechanical system onto a set of phase space variables. These phase space variables are associated with the centroid (center‐of‐mass) of a Feynman path. We present the essential features of the formalism for the representation of operators and correlation functions and discuss some aspects of its practical implementation. We also present a recently developed simplified model for superfluid environments, based on an effective centroid potential approximation to the partition function. In the context of this model, we introduce a new expression for the calculation of the condensate fraction based on the off‐diagonal one‐particle density matrix. We perform calculations for a many‐particle system where the masses and pair interactions correspond to those of helium. The results show that this simplified model exhibits Bose—Einstein condensation below a certain characteristic temperature.
Synthesis of three tethered trisaccharides to probe entropy contributions in carbohydrate—protein interactions
Authors: Zhang Ping , Bundle David R. 
Published: April 2004

AbstractCrystal structure data show that the branched trisaccharide 1 constitutes the complete antigenic determinant of the Salmonella serogroup B antigen that is recognized by monoclonal antibody SE155.4. In an effort to characterize the entropic costs associated with immobilization of glycosidic torsional angles in the bound state, three distinct intramolecularly tethered analogues of this trisaccharide 2‐4 have been synthesized. Two trisaccharides are tethered by a methylene acetal via the O‐2 position of the 3,6‐dideoxy‐hexose, abequose to either O‐2 of galactose (2) or O‐4 of mannose (3). The third tether, α,α′‐di‐thio‐p‐xylene, spans the C‐6 atoms of the mannose and galactose residues to create trisaccharide 4. The acetal tethers of 2 and 3 span hydroxyl centers that are known to be involved in intramolecular sugar‐sugar hydrogens bonds, but both trisaccharides are biologically inactive due to distorted conformations that cannot be accommodated in the antibody binding site. Trisaccharide 4 is active since both hydroxymethyl groups of galactose and mannose are solvent exposed in the bound state and the constrained conformation of 4 is virtually superimposable on the bound conformation of 1. Despite the retained complimentary and significant reduction of torsional flexibility, trisaccharide 4 exhibits a ΔG° = ‐7.6 kcal mol−1 compared to ΔG° = ‐7.1 kcal mol−1 for 1. The modest free energy gain for the tethered trisaccharide 4 arises from a small entropy gain (TΔΔS = 0.3 kcal mol−1) and an even smaller enthalpic change (ΔΔH = ‐0.2 kcal mol−1).