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Shielding
Kinds of Shielding Terms modified by Shielding Selected AbstractsFurther Conventions for NMR Shielding and Chemical Shifts (IUPAC Recommendations 2008),MAGNETIC RESONANCE IN CHEMISTRY, Issue 6 2008Applied Chemistry Physical, Biophysical Chemistry Division, International Union of Pure Abstract IUPAC has published a number of recommendations regarding the reporting of nuclear magnetic resonance (NMR) data, especially chemical shifts. The most recent publication [Pure Appl. Chem.73, 1795 (2001)] recommended that tetramethylsilane (TMS) serve as a universal reference for reporting the shifts of all nuclides, but it deferred recommendations for several aspects of this subject. This document first examines the extent to which the 1H shielding in TMS itself is subject to change by variation in temperature, concentration, and solvent. On the basis of recently published results, it has been established that the shielding of TMS in solution [along with that of sodium-3-(trimethylsilyl)propanesulfonate, DSS, often used as a reference for aqueous solutions] varies only slightly with temperature but is subject to solvent perturbations of a few tenths of a part per million (ppm). Recommendations are given for reporting chemical shifts under most routine experimental conditions and for quantifying effects of temperature and solvent variation, including the use of magnetic susceptibility corrections and of magic-angle spinning (MAS). This document provides the first IUPAC recommendations for referencing and reporting chemical shifts in solids, based on high-resolution MAS studies. Procedures are given for relating 13C NMR chemical shifts in solids to the scales used for high-resolution studies in the liquid phase. The notation and terminology used for describing chemical shift and shielding tensors in solids are reviewed in some detail, and recommendations are given for best practice. © 2008 IUPAC. Reprinted with permission from Pure Appl. Chem. 2008; 80: 59. This article can be freely downloaded from http://www.iupac.org/publications/pac/80/1/0059/ and can be copied, provided acknowledgement to IUPAC is given. [source] Understanding chemical shielding tensors using group theory, MO analysis, and modern density-functional theoryCONCEPTS IN MAGNETIC RESONANCE, Issue 2 2009Cory M. Widdifield Abstract In this article, the relationships between molecular symmetry, molecular electronic structure, and chemical shielding (CS) tensors are discussed. First, a brief background on the CS interaction and CS tensors is given. Then, the visualization of the three-dimensional nature of CS is described. A simple method for examining the relationship between molecular orbitals (MOs) and CS tensors, using point groups and direct products of irreducible representations of MOs and rotational operators, is outlined. A number of specific examples are discussed, involving CS tensors of different nuclei in molecules of different symmetries, including ethene (D2h), hydrogen fluoride (C,v), trifluorophosphine (C3v), and water (C2v). Finally, we review the application of this method to CS tensors in several interesting cases previously discussed in the literature, including acetylene (D,h), the PtX42, series of compounds (D4h) and the decamethylaluminocenium cation (D5d). © 2009 Wiley Periodicals, Inc. Concepts Magn Reson Part A 34A: 91,123, 2009. [source] Space-Charge Limited Current from Plasma-Facing Material SurfaceCONTRIBUTIONS TO PLASMA PHYSICS, Issue 1-3 2004S. Takamura Abstract We have derived an exact theoretical expression for the space-charge limited current from the solid surfaces adjacent to plasmas that is applicable for an arbitrary sheath voltage. Our expression shows that the spacecharge limited current tends to saturate with the sheath voltage. This new formula is evaluated by 1-D Particle in Cell (PIC) simulation and experiment, and is in a good agreement with the simulation and experimental results. We have also obtained an analytical equation fitted to the new formula based on conventional Child-Langmuir formula by taking into account a more sophisticated dependence of the electrode potential and the plasma density through the effect of Debye shielding and a sheath expansion due to increased voltage across the sheath. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Effects of terrain smoothing on topographic shielding correction factors for cosmogenic nuclide-derived estimates of basin-averaged denudation ratesEARTH SURFACE PROCESSES AND LANDFORMS, Issue 1 2009Kevin P. Norton Abstract Estimation of spatially averaged denudation rates from cosmogenic nuclide concentrations in sediments depends on the surface production rates, the scaling methods of cosmic ray intensities, and the correction algorithms for skyline, snow and vegetation shielding used to calculate terrestrial cosmogenic nuclide production. While the calculation of surface nuclide production and application of latitude, altitude and palaeointensity scaling algorithms are subjects of active research, the importance of additional correction for shielding by topographic obstructions, snow and vegetation is the subject of ongoing debate. The derivation of an additional correction factor for skyline shielding for large areas is still problematic. One important issue that has yet to be addressed is the effect of the accuracy and resolution of terrain representation by a digital elevation model (DEM) on topographic shielding correction factors. Topographic metrics scale with the resolution of the elevation data, and terrain smoothing has a potentially large effect on the correction of terrestrial cosmogenic nuclide production rates for skyline shielding. For rough, high-relief landscapes, the effect of terrain smoothing can easily exceed analytical errors, and should be taken into account. Here we demonstrate the effect of terrain smoothing on topographic shielding correction factors for various topographic settings, and introduce an empirical model for the estimation of topographic shielding factors based on landscape metrics. Copyright © 2008 John Wiley and Sons, Ltd. [source] Long-term landscape evolution: linking tectonics and surface processesEARTH SURFACE PROCESSES AND LANDFORMS, Issue 3 2007Paul Bishop Abstract Research in landscape evolution over millions to tens of millions of years slowed considerably in the mid-20th century, when Davisian and other approaches to geomorphology were replaced by functional, morphometric and ultimately process-based approaches. Hack's scheme of dynamic equilibrium in landscape evolution was perhaps the major theoretical contribution to long-term landscape evolution between the 1950s and about 1990, but it essentially ,looked back' to Davis for its springboard to a viewpoint contrary to that of Davis, as did less widely known schemes, such as Crickmay's hypothesis of unequal activity. Since about 1990, the field of long-term landscape evolution has blossomed again, stimulated by the plate tectonics revolution and its re-forging of the link between tectonics and topography, and by the development of numerical models that explore the links between tectonic processes and surface processes. This numerical modelling of landscape evolution has been built around formulation of bedrock river processes and slope processes, and has mostly focused on high-elevation passive continental margins and convergent zones; these models now routinely include flexural and denudational isostasy. Major breakthroughs in analytical and geochronological techniques have been of profound relevance to all of the above. Low-temperature thermochronology, and in particular apatite fission track analysis and (U,Th)/He analysis in apatite, have enabled rates of rock uplift and denudational exhumation from relatively shallow crustal depths (up to about 4 km) to be determined directly from, in effect, rock hand specimens. In a few situations, (U,Th)/He analysis has been used to determine the antiquity of major, long-wavelength topography. Cosmogenic isotope analysis has enabled the determination of the ,ages' of bedrock and sedimentary surfaces, and/or the rates of denudation of these surfaces. These latter advances represent in some ways a ,holy grail' in geomorphology in that they enable determination of ,dates and rates' of geomorphological processes directly from rock surfaces. The increasing availability of analytical techniques such as cosmogenic isotope analysis should mean that much larger data sets become possible and lead to more sophisticated analyses, such as probability density functions (PDFs) of cosmogenic ages and even of cosmogenic isotope concentrations (CICs). PDFs of isotope concentrations must be a function of catchment area geomorphology (including tectonics) and it is at least theoretically possible to infer aspects of source area geomorphology and geomorphological processes from PDFs of CICs in sediments (,detrital CICs'). Thus it may be possible to use PDFs of detrital CICs in basin sediments as a tool to infer aspects of the sediments' source area geomorphology and tectonics, complementing the standard sedimentological textural and compositional approaches to such issues. One of the most stimulating of recent conceptual advances has followed the considerations of the relationships between tectonics, climate and surface processes and especially the recognition of the importance of denudational isostasy in driving rock uplift (i.e. in driving tectonics and crustal processes). Attention has been focused very directly on surface processes and on the ways in which they may ,drive' rock uplift and thus even influence sub-surface crustal conditions, such as pressure and temperature. Consequently, the broader geoscience communities are looking to geomorphologists to provide more detailed information on rates and processes of bedrock channel incision, as well as on catchment responses to such bedrock channel processes. More sophisticated numerical models of processes in bedrock channels and on their flanking hillslopes are required. In current numerical models of long-term evolution of hillslopes and interfluves, for example, the simple dependency on slope of both the fluvial and hillslope components of these models means that a Davisian-type of landscape evolution characterized by slope lowering is inevitably ,confirmed' by the models. In numerical modelling, the next advances will require better parameterized algorithms for hillslope processes, and more sophisticated formulations of bedrock channel incision processes, incorporating, for example, the effects of sediment shielding of the bed. Such increasing sophistication must be matched by careful assessment and testing of model outputs using pre-established criteria and tests. Confirmation by these more sophisticated Davisian-type numerical models of slope lowering under conditions of tectonic stability (no active rock uplift), and of constant slope angle and steady-state landscape under conditions of ongoing rock uplift, will indicate that the Davis and Hack models are not mutually exclusive. A Hack-type model (or a variant of it, incorporating slope adjustment to rock strength rather than to regolith strength) will apply to active settings where there is sufficient stream power and/or sediment flux for channels to incise at the rate of rock uplift. Post-orogenic settings of decreased (or zero) active rock uplift would be characterized by a Davisian scheme of declining slope angles and non-steady-state (or transient) landscapes. Such post-orogenic landscapes deserve much more attention than they have received of late, not least because the intriguing questions they pose about the preservation of ancient landscapes were hinted at in passing in the 1960s and have recently re-surfaced. As we begin to ask again some of the grand questions that lay at the heart of geomorphology in its earliest days, large-scale geomorphology is on the threshold of another ,golden' era to match that of the first half of the 20th century, when cyclical approaches underpinned virtually all geomorphological work. Copyright © 2007 John Wiley & Sons, Ltd. [source] Disposition of perfluorinated acid isomers in sprague-dawley rats; Part 1: Single doseENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 3 2009Jonathan P. Benskin Abstract Perfluorinated acids (PFAs) and their precursors (PFA-precursors) exist in the environment as linear and multiple branched isomers. These isomers are hypothesized to have different biological properties, but no isomer-specific data are currently available. The present study is the first in a two-part project examining PFA isomer-specific uptake, tissue distribution, and elimination in a rodent model. Seven male Sprague-Dawley rats were administered a single gavage dose of approximately 500 ,g/kg body weight perfluorooctane sulfonate (C8F17SO3,, PFOS), perfluorooctanoic acid (C7F15CO2H, PFOA), and perfluorononanoic acid (C8F17CO2H, PFNA) and 30 ,g/kg body weight perfluorohexane sulfonate (C6F13SO3,, PFHxS). Over the subsequent 38 d, urine, feces, and tail-vein blood samples were collected intermittently, while larger blood volumes and tissues were collected on days 3 and 38 for isomer analysis by high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS). For all PFAs, branched isomers generally had lower blood depuration half-lives than the corresponding linear isomer. The most remarkable exception was for the PFOS isomer containing an alpha-perfluoromethyl branch (1m -PFOS), which was threefold more persistent than linear PFOS, possibly due to steric shielding of the hydrophilic sulfonate moiety. For perfluoromonomethyl-branched isomers of PFOS, a structure,property relationship was observed whereby branching toward the sulfonate end of the perfluoroalkyl chain resulted in increased half-lives. For PFHxS, PFOA, and PFOS, preferential elimination of branched isomers occurred primarily via urine, whereas for PFNA preferential elimination of the isopropyl isomer occurred via both urine and feces. Changes in the blood isomer profiles over time and their inverse correlation to isomer elimination patterns in urine, feces, or both provided unequivocal evidence of significant isomer-specific biological handling. Source assignment based on PFA isomer profiles in biota must therefore be conducted with caution, because isomer profiles are unlikely to be conserved in biological samples. [source] Structural, Spectroscopic, and Proton-Coupled Electron-transfer Behavior of Pyrazolyl-3,5-bis(benzimidazole)-Bridged Homo- and Heterochiral RuIIRuII, OsIIOsII, and OsIIRuII 2,2,-Bipyridine ComplexesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 4 2010Sujoy Baitalik Abstract The homo- and heterobimetallic complexes [(bpy)2MII(H2pzbzim)M,II(bpy)2](ClO4)3·nH2O (1, 3, 5) and their corresponding deprotonated complexes [(bpy)2MII(pzbzim)M,II(bpy)2](ClO4)·nH2O (2, 4, 6) [where MII, M,II = Ru (1, 2) = Os (3, 4); MII = Os and M,II = Ru (3, 5); bpy = 2,2,-bipyridine; H3pzbzim = pyrazole-3,5-bis(benzimidazole)] were synthesized, separated to their heterochiral (a, ,,/,,) and homochiral (b, ,,/,,) diastereoisomers, and characterized by elemental analyses, ESI-MS, and 1H NMR spectroscopy. The X-ray structures of 1a, 3a, and 5a show the involvement of two pyridine rings of two bpy ligands in strong intramolecular nonbonded ,,, interaction. The occurrence of a C,H···, interaction between an aromatic C,H and the ,-cloud of a pyridine ring leads to strong electronic shielding of this proton (1H NMR). In all cases, the two diastereoisomers show practically no differences in their absorption spectra, redox potentials, and pK values. The large shifts in the E1/2 values to less positive potentials and substantial redshifts in the MLCT bands that occur on deprotonation of 1, 3, and 5 are energetically correlated. From the profiles of E1/2(1), (2) vs. pH over the pH range 1,12, the equilibrium constants and standard redox potentials for all the complex species in the metal oxidation states II·II, II·III, and III·III and the bridged ligand in the protonation states H2pzbzim,, Hpzbzim2,, and pzbzim3, have been evaluated. Using these values the bond dissociation free energies for the benzimidazole N,H bonds have been estimated. Spectroelectrochemical studies have been carried out for 1a, 3a, and 5a in the range 400,1100 nm. With stepwise oxidation of the metal centers replacement of MLCT bands by LMCT bands occur gradually with the observation of sharp isosbestic points. In the case of 1a, a band observed at ,max = 910 nm for the RuIIRuIII species has been ascribed to intervalence charge transfer (IVCT) transition. [source] Density-Functional Calculation of the 183W and 17O NMR Chemical Shifts for Large PolyoxotungstatesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 6 2006Jose Gracia Abstract A phosphane sulfate relativistic DFT method (ZORA)has been used to calculate the 183W and 17O NMR chemicalshifts for large polyoxotungstates, including W6O192,,CH3OTiW5O183,, W5O18WIINO3,, W10O324,, ,-,-,-XW12O40n,, ,-PW9O28Br63,, P2W18O626,, PW2O143,, and W7O246,, based on their optimized molecular structures. Despite sizeable deviations between the calculated and experimental values, the calculations correctly reproduce the trends in the change of the chemical shift for both nuclei. As expected, the diamagnetic term is almost constant throughout the whole series. The change in the chemical shifts is shown to be determined by the paramagnetic term, which depends on the electronic structure of the whole anion under study and, in particular, on the local geometry around a given tungsten atom. On the other hand, there is no correlation between the chemical shift and HOMO,LUMO gap, showing that deeper occupied levels and several unoccupied orbitals play an important role in the paramagnetic term. However, analysis of the components of the paramagnetic shielding has shown that the most significant transitions determining the change of both 183W and 17O NMR chemical shifts for anions consisting of tungsten and oxygen atoms are the occupied,occupied and not the occupied,virtual ones.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] Synthesis and Binding Properties of Dendritic Oxybathophenanthroline Ligands towards Copper(II)EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 22 2005Holger Stephan Abstract Dendritic oxybathophenanthroline ligands (generation 0 to 3) have been synthesized by treatment of 4,7-bis(4,-hydroxyphenyl)-1,10-phenanthroline with the corresponding Fréchet-type dendrons carrying a benzylic bromide function at the focal point. The complexation of copper(II) has been studied by liquid,liquid extraction using the radioisotope 64Cu and time-resolved laser-induced fluorescence spectroscopy (TRLFS) in organic media indicating the formation of 1:3 complexes (Cu:dendritic ligand). Electronic and EPR spectroscopy were used to characterize the copper(II) chromophore, which is shown to have the expected distorted square-planar geometry with two phenanthroline donors coordinated to the copper(II) center. The third dendritic ligand therefore is proposed to be bound by secondary interactions. The stability constants of the 1:3 complexes were found to be in the order of log K , 16 in CHCl3. On the other hand, increasing generation of the dendritic Fréchet-type branches leads to enhanced shielding of the copper ion from the environment. Additional information about this behaviour was obtained by the fluorescence lifetimes, which are much less influenced upon addition of copper(II) salt to solutions of the higher generation ligands. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] X-ray Crystal Structure Study of Sterically Congested DiphenyldiazomethanesEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 14 2004Takashi Iikubo Abstract X-ray crystallographic structures were determined for five diphenyldiazomethanes (DDMs) with various ortho substituents, namely, bis(2,4,6-trichlorophenyl)diazomethane, bis(2,4,6-tribromophenyl)diazomethane, bis(4- tert -butyl-2,6-dibromophenyl)diazomethane, (4- tert -butyl-2,6-dimethylphenyl)(2,4,6-tribromophenyl)diazomethane, and [4- tert -butyl-2,6-bis(trifluoromethyl)phenyl](4- tert -butyl-2,6-dibromophenyl)diazomethane, and for bis(4-bromophenyl)diazomethane, a DDM with no ortho substituents. The correlation between the structural parameters, the ESR zero-field splitting (ZFS) parameters and the lifetimes of the triplet diphenylcarbenes (DPCs) generated from these diazo compounds was examined. It is noted that as ortho substituents are introduced onto the phenyl rings of the DDMs, the interplanar angle between the two phenyl rings increases, while the angle of the diazo carbon changes very little. DDM 6 -N2, from which the longest-lived triplet carbene is generated, is shown to have the largest interplanar angle of the DDMs examined. The bond distances between the aromatic carbons and the ortho substituents as well as the van der Waals radii of the substituents were also examined. These studies do not provide a quantitative correlation between the structural parameters of the precursor DDMs and the ZFS parameters and lifetimes of their photoproducts, triplet DPCs, but show how steric shielding of the ortho substituents affects the structures and stabilities of triplet DPCs. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] Sphere contact fatigue of a coarse-grained Al2O3 ceramicFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 11 2006T. FETT ABSTRACT The opposite sphere test is an appropriate tool to determine crack-growth exponents for fatigue under repeated contact loading. Lifetime measurements for a coarse-grained Al2O3 are reported. To explain the fatigue exponents that strongly deviated from those obtained in cyclic bending tests, a fracture mechanics analysis was carried out. It was aimed at determining the correct stress intensity factor solution for the tests, including limited dimensions of test specimens deviating from the case of a cone crack in a half space. Cone crack development was observed microscopically and the related stress intensity factors were computed for the observed crack shape. For modelling the fatigue behaviour, it is assumed that the fatigue effect is influenced by a reduction of the shielding term of crack growth resistance due to periodical friction between the grain-interlock bridges in coarse-grained alumina. This results in a loss of traction at the junctions, crack tip shielding is reduced, and the effective load at the crack tip is increased. [source] Gradual phosphorylation regulates PC4 coactivator functionFEBS JOURNAL, Issue 7 2006Hendrik R. A. Jonker The unstructured N-terminal domain of the transcriptional cofactor PC4 contains multiple phosphorylation sites that regulate activity. The phosphorylation status differentially influences the various biochemical functions performed by the structured core of PC4. Binding to ssDNA is slightly enhanced by phosphorylation of one serine residue, which is not augmented by further phosphorylation. The presence of at least two phosphoserines decreases DNA-unwinding activity and abrogates binding to the transcriptional activator VP16. Phosphorylation gradually decreases the binding affinity for dsDNA. These phosphorylation-dependent changes in PC4 activities correlate with the sequential functions PC4 fulfils throughout the transcription cycle. MS and NMR revealed that up to eight serines are progressively phosphorylated towards the N-terminus, resulting in gradual environmental changes in the C-terminal direction of the following lysine-rich region. Also within the structured core, primarily around the interaction surfaces, environmental changes are observed. We propose a model for co-ordinated changes in PC4 cofactor functions, mediated by phosphorylation status-dependent gradual masking of the lysine-rich region causing shielding or exposure of interaction surfaces. [source] Preparation of High-Performance Conductive Polymer Fibers through Morphological Control of Networks Formed by NanofillersADVANCED FUNCTIONAL MATERIALS, Issue 9 2010Hua Deng Abstract A general method is described to prepare high-performance conductive polymer fibers or tapes. In this method, bicomponent tapes/fibers containing two layers of conductive polymer composites (CPCs) filled with multiwall carbon nanotubes (MWNT) or carbon black (CB) based on a lower-melting-temperature polymer and an unfilled polymer core with higher melting temperature are fabricated by a melt-based process. Morphological control of the conductive network formed by nanofillers is realized by solid-state drawing and annealing. Information on the morphological and electrical change of the highly oriented conductive nanofiller network in CPC bicomponent tapes during relaxation, melting, and crystallization of the polymer matrix is reported for the first time. The conductivity of these polypropylene tapes can be as high as 275,S,m,1 with tensile strengths of around 500,MPa. To the best of the authors' knowledge, it is the most conductive, high-strength polymer fiber produced by melt-processing reported in literature, despite the fact that only ,5,wt.% of MWNTs are used in the outer layers of the tape and the overall MWNT content in the bicomponent tape can be much lower (typically ,0.5,wt.%). Their applications could include sensing, smart textiles, electrodes for flexible solar cells, and electromagnetic interference (EMI) shielding. Furthermore, a modeling approach was used to study the relaxation process of highly oriented conductive networks formed by carbon nanofillers. [source] Brittle-to-Ductile Transition in Uniaxial Compression of Silicon Pillars at Room TemperatureADVANCED FUNCTIONAL MATERIALS, Issue 15 2009Fredrik Östlund Abstract Robust nanostructures for future devices will depend increasingly on their reliability. While great strides have been achieved for precisely evaluating electronic, magnetic, photonic, elasticity and strength properties, the same levels for fracture resistance have been lacking. Additionally, one of the self-limiting features of materials by computational design is the knowledge that the atomistic potential is an appropriate one. A key property in establishing both of these goals is an experimentally-determined effective surface energy or the work per unit fracture area. The difficulty with this property, which depends on extended defects such as dislocations, is measuring it accurately at the sub-micrometer scale. In this Full Paper the discovery of an interesting size effect in compression tests on silicon pillars with sub-micrometer diameters is presented: in uniaxial compression tests, pillars having a diameter exceeding a critical value develop cracks, whereas smaller pillars show ductility comparable to that of metals. The critical diameter is between 310 and 400,nm. To explain this transition a model based on dislocation shielding is proposed. For the first time, a quantitative method for evaluating the fracture toughness of such nanostructures is developed. This leads to the ability to propose plausible mechanisms for dislocation-mediated fracture behavior in such small volumes. [source] Managing Environmental Impacts of Recreation and Tourism in Rainforests of the Wet Tropics of Queensland World Heritage AreaGEOGRAPHICAL RESEARCH, Issue 2 2005Stephen M. Turton Abstract This paper describes environmental impacts of tourism and recreation activities in the world heritage listed rainforests of northeast Australia and presents management strategies for sustainable visitor use of the protected area. Tropical rainforests are characterised by their low resistance and moderate to high resilience to impacts associated with human visitation. Visitor use in the World Heritage Area is mostly associated with walking tracks, camping areas, day use areas and off-road vehicle use of old forestry roads and tracks. Adverse environmental impacts range from vegetation trampling, soil compaction, water contamination and soil erosion at the local scale through to spread of weeds, feral animals and soil pathogens along extensive networks of old forestry roads and tracks at the regional scale. Concentration of visitor use is the most desirable management strategy for controlling adverse impacts at most World Heritage Area visitor nodes and sites, and includes methods such as site hardening and shielding to contain impacts. For dispersed visitor activities, such as off-road vehicle driving and long-distance walking, application of best practice methods by the tourist industry and recreational users such as removal of mud and soils from vehicle tyres and hiking boots before entering pathogen-free catchments, together with seasonal closure of roads and tracks, are the preferred management strategies. Retention of canopy cover at camping areas and day use areas, as well as along walking tracks and forestry roads is a simple, yet effective, management strategy for reduction of a range of adverse impacts, including dispersal of weeds and feral animals, edge effects, soil erosion and nutrient loss, road kill and linear barrier effects on rainforest fauna. [source] Isolation and X-Ray Structures of Reactive Intermediates of Organocatalysis with Diphenylprolinol Ethers and with ImidazolidinonesHELVETICA CHIMICA ACTA, Issue 11 20085-Repulsion, A Survey, Comparison with Computed Structures, the Geminal-Diaryl Effect at Work, with 1-Acyl-imidazolidinones: The Abstract Reaction of 2-phenylacetaldehyde with the Me3Si ether of diphenyl-prolinol, with removal of H2O, gives a crystalline enamine (1). The HBF4 salts of the MePh2Si ether of diphenyl-prolinol and of 2-(tert -butyl)-3-methyl- and 5-benzyl-2,2,3-trimethyl-1,3-imidazolidin-4-one react with cinnamaldehyde to give crystalline iminium salts 2, 3, and 4. Single crystals of the enamine and of two iminium salts, 2 and 3, were subjected to X-ray structure analysis (Figs.,1, 2, and 6), and a 2D-NMR spectrum of the third iminium salt was recorded (Fig.,7). The crystal and NMR structures confirm the commonly accepted, general structures of the two types of reactive intermediates in organocatalysis with the five-membered heterocycles, i.e., D, E (Scheme,2). Fine details of the crystal structures are discussed in view of the observed stereoselectivities of the corresponding reactions with electrophiles and nucleophiles. The structures 1 and 2 are compared with those of other diphenyl-prolinol derivatives (from the Cambridge File CSD; Table,1) and discussed in connection with other reagents and ligands, containing geminal diaryl groups and being used in enantioselective synthesis (Fig.,4). The iminium ions 3 and 4 are compared with N -acylated imidazolidinones F and G (Figs.,9, 12, and 13, and Table,3), and common structural aspects such as minimalization of 1,5-repulsion (the ,A1,3 -effect'), are discussed. The crystal structures of the simple diphenyl-prolinol,HBF4 salt (Fig.,3) and of Boc- and benzoyl-(tert -butyl)methyl-imidazolidinone (Boc-BMI and Bz-BMI, resp.; Figs.,10 and 11) are also reported. Finally, the crystal structures are compared with previously published theoretical structures, which were obtained from high-level-of-theory DFT calculations (Figs.,5 and 8, and Table,2). Delicate details including pyramidalization of trigonal N-atoms, distortions around iminium CN bonds, shielding of diastereotopic faces, and the , -interaction between a benzene ring and a Me group match so well with, and were actually predicting the experimental results that the question may seem appropriate, whether one will soon start considering to carry out such calculations before going to the laboratory for experimental optimizations. [source] Rationalization of the stereochemistry of an addition of dialkyl phosphites to certain chiral aldimines: The experimental and theoretical approachHETEROATOM CHEMISTRY, Issue 2 2002Ryszard B. Nazarski The absolute configuration of an ,-P stereogenic center in two diastereomeric O,O-dialkyl ,-aminophosphonates (3), arising from an induced 1,3-asymmetric phosphite addition to the CN bond of furfural-derived Schiff bases (1), was established from single product 1H NMR data. Such spectra were interpreted with anisotropic shielding in relation to the AM1 and MNDO/d structures of 3; the former ones turned out to be closer to the obtained experimental results (1H NMR spectra of 3, crystallographic database study). Since favored 3-21G geometries of starting imines 1 were modeled as well, it was inferred that a stereochemical outcome of this reaction is governed by Cram selectivity. © 2002 Wiley Periodicals, Inc. Heteroatom Chem 13:120,125, 2002; Published online in Wiley Interscience (www.interscience.wiley.com). DOI 10.1002/hc.10005 [source] Air-Stable Polymer Electronic Devices,ADVANCED MATERIALS, Issue 18 2007K. Lee By introducing a titanium oxide (TiOx) layer between the active layer and the aluminum cathode in polymer-based electronic devices (see figure), devices with excellent air stability and with enhanced performance are demonstrated. The TiOx layer acts as a shielding and scavenging layer that prevents the intrusion of oxygen and humidity into the electronically active polymers, thereby improving the lifetime of unpackaged devices exposed to air by nearly two orders of magnitude. [source] Effect of weak reductant on properties of electroless copper polyacrylonitrile nanocomposites for electromagnetic interference shieldingJOURNAL OF APPLIED POLYMER SCIENCE, Issue 2 2010Keng-Yu Tsao Abstract In this work, the electroless copper method with different reductant compositions (NaHSO3/Na2 S2O3·5H2O and Na2S2O3·5H2O) without sensitizing and activating, was used to deposit copper-sulfide deposition on the polyacrylonitrile (PAN) surface for electromagnetic interference (EMI) shielding materials. The weak reductant, NaHSO3, in the electroless copper method was used to control the phase of copper-sulfide deposition. The Cux(x=1,1.8)S was deposited on the PAN (CuxS-PAN) by reductant composition (NaHSO3/Na2S2O3·5H2O) and the Cux(x=1,1.8)S deposition of CuxS-PAN possesses three kinds of copper-sulfide phases (CuS, Cu1.75S and Cu1.8S). However, the electroless copper with reductant was only Na2S2O3·5H2O (without weak reductant, NaHSO3), the hexagonal CuS deposition was plated on the PAN (CuS-PAN) and increased the EMI shielding effectiveness of CuS-PAN composites about 10,15 dB. In this study, the best EMI SE of CuS-PAN and CuxS-PAN composites were about 27,30 dB and 15,17 dB respectively, as the cupric ion concentration was 0.24 M. The volume resistivity of CuS-PAN composite was about 1000 times lower than that of CuxS-PAN composite and lowest volume resistivity of CuS-PAN composites was 0.012 , cm, as the cupric ion concentration was 0.24 M. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010 [source] A comparative study of some red- and blue-shifted linear H-bonded complexes of N2JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 2 2008Sean A. C. McDowell Abstract Bond length changes, harmonic vibrational frequency shifts, and changes in the proton magnetic shielding of HX and HKrX (X = F, Cl) on complexation with N2 to form the linear red-shifted N2 , HX and linear blue-shifted N2 , HKrX complexes were determined by ab initio computations, with and without counterpoise correction, at the SCF and MP2(full) levels of theory using a 6-311++G(2d,2p) basis set. The MP2 computations agree with predictions from a perturbation theory model involving the first and second derivatives of the interaction energy with respect to displacement of the HX and HKr bond lengths from their equilibrium values in the isolated monomers. The theoretical results agree qualitatively with the experimentally observed frequency shifts, with near quantitative agreement for N2 , HKrCl. The characteristic downfield shift of the isotropic proton magnetic resonance in the red-shifted complexes was obtained, but for the blue-shifted complexes, the proton NMR shifts to higher fields. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2008 [source] A comparison of quantum chemical models for calculating NMR shielding parameters in peptides: Mixed basis set and ONIOM methods combined with a complete basis set extrapolationJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 7 2006Seongho Moon Abstract This article compares several quantum mechanical approaches to the computation of chemical shielding tensors in peptide fragments. First, we describe the effects of basis set quality up to the complete basis set (CBS) limit and level of theory (HF, MP2, and DFT) for four different atoms in trans N -methylacetamide. For both isotropic shielding and shielding anisotropy, the MP2 results in the CBS limit show the best agreement with experiment. The HF values show quite a different tendency to MP2, and even in the CBS limit they are far from experiment for not only the isotropic shielding of carbonyl carbon but also most shielding anisotropies. In most cases, the DFT values differ systematically from MP2, and small basis-set (double- or triple-zeta) results are often fortuitously in better agreement with the experiment than the CBS ones. Second, we compare the mixed basis set and ONIOM methods, combined with CBS extrapolation, for chemical shielding calculations at a DFT level using various model peptides. From the results, it is shown that the mixed basis set method provides better results than ONIOM, compared to CBS calculations using the nonpartitioned full systems. The information studied here will be useful in guiding the selection of proper quantum chemical models, which are in a tradeoff between accuracy and cost, for shielding studies of peptides and proteins. © 2006 Wiley Periodicals, Inc. J Comput Chem 27: 825,836, 2006 [source] Structure, magnetizability, and nuclear magnetic shielding tensors of bis-heteropentalenes.JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 3 2006Abstract The geometry of the heteropentalenes formed by two phosphole units has been determined at the DFT level. The magnetic susceptibility and the nuclear magnetic shielding at the nuclei of these systems have also been calculated using gauge-including atomic orbitals and a large Gaussian basis set to achieve near Hartree,Fock estimates. A comparative study of the various isomers, of their flattened analogs, and of the parent phosphole molecule, shows that the [3,4-c] isomer is the most aromatic system in the set considered, assuming diatropicity and degree of planarity as indicators, even if it is the less stable in terms of total molecular energy. Plots of magnetic field-induced current densities confirm diatropicity of P-containing bis-heteropentalenenes, showing, however, significant differences from the analogous systems with distinct heteroatoms. The maps give evidence of spiral flow nearby CC bonds, compatible with prevalent distortive behavior of , electrons exalted by pyramidalization at P, and competing against the , electron compression, which would favor planar structure. © 2005 Wiley Periodicals, Inc. J Comput Chem 27: 344,351, 2006 [source] The Endogenous Amine 1-Methyl-1,2,3,4- Tetrahydroisoquinoline Prevents the Inhibition of Complex I of the Respiratory Chain Produced by MPP+JOURNAL OF NEUROCHEMISTRY, Issue 1 2000Juan Parrado Abstract : The endogenous monoamine 1-methyl-1,2,3,4-tetrahydroisoquinoline has been shown to prevent the neurotoxic effect of MPP+ and other endogenous neurotoxins, which produce a parkinsonian-like syndrome in humans. We have tested its potential protective effect in vivo by measuring the protection of 1-methyl-1,2,3,4-tetrahydroisoquinoline in the neurotoxicity elicited by MPP+ in rat striatum by tyrosine hydroxylase immunocytochemistry. Because we know that cellular damage caused by MPP+ is primarily the result of mitochondrial respiratory inhibition at the complex I level, we have extended the study further to understand this protective mechanism. We found that the inhibitory effect on the mitochondrial respiration rate induced by MPP+ in isolated rat liver mitochondria and striatal synaptosomes was prevented by addition of 1-methyl-1,2,3,4-tetrahydroisoquinoline. This compound has no antioxidant capacity ; therefore, this property is not involved in its protective effect. Thus, we postulate that the preventive effect that 1-methyl-1,2,3,4-tetrahydroisoquinoline has on mitochondrial inhibition for MPP+ could be due to a "shielding effect," protecting the energetic machinery, thus preventing energetic failure. These results suggest that this endogenous amine may protect against the effect of several parkinsonism-inducing compounds that are associated with progressive impairment of the mitochondrial function. [source] Mechanical properties of femoral cortical bone following cemented hip replacementJOURNAL OF ORTHOPAEDIC RESEARCH, Issue 11 2007G.X. Ni Abstract Femoral bone remodeling following total hip replacement is a big concern and has never been examined mechanically. In this study, six goats underwent unilateral cemented hip hemiarthroplasty with polymethyl methacrylate (PMMA) bone cement. Nine months later animals were sacrificed, and the femoral cortical bone slices at different levels were analysed using microhardness testing and microcomputed tomography (micro-CT) scanning. Implanted femurs were compared to contralateral nonimplanted femurs. Extensive bone remodeling was demonstrated at both the proximal and middle levels, but not at the distal level. Compared with the nonimplanted side, significant decreases were found in the implanted femur in cortical bone area, bone mineral density, and cortical bone hardness at the proximal level, as well as in bone mineral density and bone hardness at the middle level. However, no significant difference was observed in either variable for the distal level. In addition, similar proximal-to-distal gradient changes were revealed both in cortical bone microhardness and bone mineral density. From the mechanical point of view, the results of the present study suggested that stress shielding is an important mechanical factor associated with bone adaptation following total hip replacement. © 2007 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 25:1408,1414, 2007 [source] Activity and regulation of glycoPEGylated factor VIIa analogsJOURNAL OF THROMBOSIS AND HAEMOSTASIS, Issue 9 2008S. GHOSH Summary.,Background:,Recombinant coagulation factor VIIa (rFVIIa) has proven to be a safe and effective drug for treatment of bleeding episodes in hemophilic patients with inhibitors. However, rFVIIa is cleared from the circulation relatively quickly. Protein modification with poly(ethylene glycol) (PEG) can prolong the circulatory lifetime of proteins but it could also impair protein function by molecular shielding of the protein surface. Objectives:, To characterize the interaction of glycoPEGylated rFVIIa , rFVIIa-10K PEG and rFVIIa-40K PEG , with tissue factor (TF), factor X (FX) and plasma inhibitors, tissue factor pathway inhibitor (TFPI) and antithrombin (AT). Methods:, The amidolytic and FX activation assays were employed to investigate the interaction of glycoPEGylated rFVIIa with its macromolecular substrate and inhibitors. Results:, Both the glycoPEGylated rFVIIa analogs exhibited similar amidolytic activity as that of rFVIIa in the absence or the presence of relipidated TF. The analogs were as effective as rFVIIa in activating FX in the absence of TF. In the presence of TF, the glycoPEGylated rFVIIa variants, relative to rFVIIa, were slightly less effective at lower concentrations, but no significant differences were found among them in activating FX at saturating concentrations. Both AT/heparin and TFPI effectively inhibited the glycoPEGylated rFVIIa bound to relipidated TF or TF on stimulated endothelial cells. In contrast to their normal interaction with TF, the glycoPEGylated rFVIIa variants appeared to interact poorly with phospholipids. Conclusions:, The glycoPEGylated rFVIIa variants retained their catalytic activity and interacted efficiently with TF, FX and the plasma inhibitors. Further work with appropriate in vitro and in vivo model systems is needed to determine the feasibility of using glycoPEGylated rFVIIa to improve therapeutic options for bleeding disorders. [source] AIM and NBO analysisMAGNETIC RESONANCE IN CHEMISTRY, Issue 9 2010In the series of diaminoenones, large high-frequency shifts of the 1H NMR of the NH group in the cis -position relative to the carbonyl group suggests strong NH···O intramolecular hydrogen bonding comprising a six-membered chelate ring. The NH···O hydrogen bond causes an increase of the 1J(N,H) coupling constant by 2,4 Hz and high-frequency shift of the 15N signal by 9,10 ppm despite of the lengthening of the relevant NH bond. These experimental trends are substantiated by gauge-independent atomic orbital and density functional theory calculations of the shielding and coupling constants in the 3,3-bis(isopropylamino)-1-(aryl)prop-2-en-1-one (12) for conformations with the Z - and E -orientations of the carbonyl group relative to the NH group. The effects of the NH···O hydrogen-bond on the NMR parameters are analyzed with the atoms-in-molecules (AIM) and natural bond orbital (NBO) methods. The AIM method indicates a weakening of the NH···O hydrogen bond as compared with that of 1,1-di(pyrrol-2-yl)-2-formylethene (13) where NH···O hydrogen bridge establishes a seven-membered chelate ring, and the corresponding 1J(N,H) coupling constant decreases. The NBO method reveals that the LP(O) ,,*NH hyperconjugative interaction is weakened on going from the six-membered chelate ring to the seven-membered one due to a more bent hydrogen bond in the former case. A dominating effect of the NH bond rehybridization, owing to an electrostatic term in the hydrogen bonding, seems to provide an increase of the 1J(N,H) value as a consequence of the NH···O hydrogen bonding in the studied diaminoenones. Copyright © 2010 John Wiley & Sons, Ltd. [source] Structural correlations for 1H, 13C and 15N NMR coordination shifts in Au(III), Pd(II) and Pt(II) chloride complexes with lutidines and collidineMAGNETIC RESONANCE IN CHEMISTRY, Issue 6 2010Leszek Pazderski Abstract 1H, 13C and 15N NMR studies of gold(III), palladium(II) and platinum(II) chloride complexes with dimethylpyridines (lutidines: 2,3-lutidine, 2,3lut; 2,4-lutidine, 2,4lut; 3,5-lutidine, 3,5lut; 2,6-lutidine, 2,6lut) and 2,4,6-trimethylpyridine (2,4,6-collidine, 2,4,6col) having general formulae [AuLCl3], trans -[PdL2Cl2] and trans -/cis- [PtL2Cl2] were performed and the respective chemical shifts (,1H, ,13C, ,15N) reported. The deshielding of protons and carbons, as well as the shielding of nitrogens was observed. The 1H, 13C and 15N NMR coordination shifts (,1Hcoord, ,13Ccoord, ,15Ncoord; ,coord = ,complex , ,ligand) were discussed in relation to some structural features of the title complexes, such as the type of the central atom [Au(III), Pd(II), Pt(II)], geometry (trans - or cis -), metal-nitrogen bond lengths and the position of both methyl groups in the pyridine ring system. Copyright © 2010 John Wiley & Sons, Ltd. [source] Structure determination of slowly exchanging conformers in solution using high-resolution NMR, computational modeling and DFT-GIAO chemical shielding: application to an erythronolide A derivativeMAGNETIC RESONANCE IN CHEMISTRY, Issue 11 2009Dieter Muri Abstract We discuss and demonstrate the potential of HSQC-TOCSY and HSQC-NOESY experiments to offer solutions for overlap problems in COSY and NOESY spectra, leading to improved signals that can be unambiguously assigned to individual carbons. Direct comparison of experimental 1H and 13C chemical shielding with density functional theory (DFT)-calculated values are uninformative; in contrast, the relative differences in experimental shielding between pairs of molecules correlates well with the relative differences in DFT-GIAO shielding for the computed lowest energy conformers. A detailed application of both experimental and theoretical techniques is illustrated for slowly exchanging conformers of an erythronolide A derivative, which demonstrates that structure determination can strongly benefit from the interplay between experiment and theory. Copyright © 2009 John Wiley & Sons, Ltd. [source] Further Conventions for NMR Shielding and Chemical Shifts (IUPAC Recommendations 2008),MAGNETIC RESONANCE IN CHEMISTRY, Issue 6 2008Applied Chemistry Physical, Biophysical Chemistry Division, International Union of Pure Abstract IUPAC has published a number of recommendations regarding the reporting of nuclear magnetic resonance (NMR) data, especially chemical shifts. The most recent publication [Pure Appl. Chem.73, 1795 (2001)] recommended that tetramethylsilane (TMS) serve as a universal reference for reporting the shifts of all nuclides, but it deferred recommendations for several aspects of this subject. This document first examines the extent to which the 1H shielding in TMS itself is subject to change by variation in temperature, concentration, and solvent. On the basis of recently published results, it has been established that the shielding of TMS in solution [along with that of sodium-3-(trimethylsilyl)propanesulfonate, DSS, often used as a reference for aqueous solutions] varies only slightly with temperature but is subject to solvent perturbations of a few tenths of a part per million (ppm). Recommendations are given for reporting chemical shifts under most routine experimental conditions and for quantifying effects of temperature and solvent variation, including the use of magnetic susceptibility corrections and of magic-angle spinning (MAS). This document provides the first IUPAC recommendations for referencing and reporting chemical shifts in solids, based on high-resolution MAS studies. Procedures are given for relating 13C NMR chemical shifts in solids to the scales used for high-resolution studies in the liquid phase. The notation and terminology used for describing chemical shift and shielding tensors in solids are reviewed in some detail, and recommendations are given for best practice. © 2008 IUPAC. Reprinted with permission from Pure Appl. Chem. 2008; 80: 59. This article can be freely downloaded from http://www.iupac.org/publications/pac/80/1/0059/ and can be copied, provided acknowledgement to IUPAC is given. [source] Studying porous materials with krypton-83 NMR spectroscopyMAGNETIC RESONANCE IN CHEMISTRY, Issue S1 2007Zackary I. Cleveland Abstract This report is the first review of 83Kr nuclear magnetic resonance as a new and promising technique for exploring the surfaces of solid materials. In contrast to the spin I = 1/2 nucleus of 129Xe, 83Kr has a nuclear spin of I = 9/2 and therefore possesses a nuclear electric quadrupole moment. Interactions of the quadrupole moment with the electronic environment are modulated by surface adsorption processes and therefore affect the 83Kr relaxation rate and spectral lineshape. These effects are much more sensitive probes for surfaces than the 129Xe chemical shielding and provide unique insights into macroporous materials in which the 129Xe chemical shift is typically of little diagnostic value. The first part of this report reviews the effect of quadrupolar interactions on the 83Kr linewidth in zeolites and also the 83Kr chemical shift behavior that is distinct from that of its 129Xe cousin in some of these materials. The second part reviews hyperpolarized (hp) 83Kr NMR spectroscopy of macroporous materials in which the longitudinal relaxation is typically too slow to allow sufficient averaging of thermally polarized 83Kr NMR signals. The quadrupolar-driven T1 relaxation times of hp 83Kr in these materials are sensitive to surface chemistry, surface-to-volume ratios, coadsorption of other species on surfaces, and surface temperature. Thus, 83Kr T1 relaxation can provide information about surfaces and chemical processes in macroscopic pores and can generate surface-sensitive contrast in hp 83Kr MRI. Copyright © 2007 John Wiley & Sons, Ltd. [source] | ||||||||||||||||||||||||||||||||||||||||