Bis(perfluorocatecholato)silane Si(cat(F))2 was prepared, and stoichiometric binding to Lewis bases was demonstrated with fluoride, triethylphosphine oxide, and N,N'-diisopropylbenzamide. The potent Lewis acidity of Si(cat(F))2 was suggested from catalytic hydrosilylation and silylcyanation reactions with aldehydes. Mechanistic studies of hydrosilylation using an optically active silane substrate, R-(+)-methyl-(1-naphthyl)phenylsilane, proceeded with predominant stereochemical retention at silicon, consistent with a carbonyl activation pathway. The enantiospecificity was dependent on solvent and salt effects, with increasing solvent polarity or addition of NBu4BAr(F)4 leading to a diminished enantiomeric ratio. The medium effects are consistent with an ionic mechanism, wherein hydride transfer occurs prior to silicon-oxygen bond formation.
This nal report describes the application of computer vision techniques to the lateral and longitudinal control of an autonomous highway vehicle. In the part of the project we focused on an analysis of the vehicle's lateral dynamics and the design of an appropriate controller for lateral control and investigated various static feedback strategies where the measurements obtained from vision, namely o set from the centerline and angle between the road tangent and the orientation of the vehicle at some look-ahead distance, are directly used for control. The role of the look-ahead, its relation to the vision processing delay, longitudinal velocity and road geometry was crucial on the design of the control and their experimental evaluation. We carried out a thorough analysis of the e ects of changing various important system parameters like the vehicle velocity, thelookahead range of the vision sensor and the processing delay associated with the perception and control systems. We also present the results of a series of experiments that were designed to provide a systematic comparison of a number of control strategies. The control strategies that were explored include a lead-lag control law, a full- state linear controller and input-output linearizing control law. Each of these control strategies was implemented and tested at highway speeds on our experimental vehicle platform, a Honda Accord LX sedan. For the longitudinal control problem, we investigated the possibility of using stereo vision to provide the range information, in conjunction with a scanning laser radar sensor. The vision based tracking system utilizes a layered architecture wherein the bottom layer computes motion in both images using a simple correlation algorithm, and the upper level performs stereo xation and reconstruction using an algorithm designed for active vision systems. We present some initial results comparing the quality of range measurements provided by a vision system with the laser radar system. We report the results from the experimental demonstration of the system as part of the National Automated Highway Systems Consortium (NAHSC) Demonstration which took place in August 1997 in San Diego. The overall system was demonstrated as a part of the main highway scenario as well as part of a small public demonstration of the vision based lateral control on a highly curved test track.
Natural killer (NK) cellsexpress receptors specific for class I major histocompatibility complex (MHC) molecules. In the mouse, the class I specific receptors identified to date belong to the polymorphic Ly49 receptor family. Engagement of Ly49 receptors with their respective MHC ligands results in negative regulation of NK cell effector functions, consistent with a critical role of these receptors in "missing self" recognition. The Ly49 receptors analyzed so far are clonally distributed such that multiple distinct Ly49 receptors can be expressed by individual NK cells (for review see refs. 1-3). The finding that most NK cells that express the Ly49A receptor do so from a single Ly49A allele (whereby expression can occur from the maternal or the paternal chromosome) may thus reflect a putative receptor distribution process that restricts the number of Ly49 receptors expressed in a single NK cell (3-5).
Abstract An Erratum has been published for this article in Earthquake Engng. Struct. Dyn. 2004; 33:1429. Based on structural dynamics theory, the modal pushover analysis (MPA) procedure retains the conceptual simplicity of current procedures with invariant force distribution, now common in structural engineering practice. The MPA procedure for estimating seismic demands is extended to unsymmetric‐plan buildings. In the MPA procedure, the seismic demand due to individual terms in the modal expansion of the effective earthquake forces is determined by non‐linear static analysis using the inertia force distribution for each mode, which for unsymmetric buildings includes two lateral forces and torque at each floor level. These ‘modal’ demands due to the first few terms of the modal expansion are then combined by the CQC rule to obtain an estimate of the total seismic demand for inelastic systems. When applied to elastic systems, the MPA procedure is equivalent to standard response spectrum analysis (RSA). The MPA estimates of seismic demand for torsionally‐stiff and torsionally‐flexible unsymmetric systems are shown to be similarly accurate as they are for the symmetric building; however, the results deteriorate for a torsionally‐similarly‐stiff unsymmetric‐plan system and the ground motion considered because (a) elastic modes are strongly coupled, and (b) roof displacement is underestimated by the CQC modal combination rule (which would also limit accuracy of RSA for linearly elastic systems). Copyright © 2004 John Wiley & Sons, Ltd.
Abstract Aus den bei 20°C mit Hilfe der Potentialsprungmethode (d.h.
Abstract The trichloro title compound (II) prepared as shown is converted to the derivatives (III) and (IV) on treatment with one equiv. KOSiMe 3 or LiOtBu, or with 3 equiv. MeMgBr, respectively.
As a unique paradigm for chaos, the various versions of Chua's circuits and equations consists of a three-dimensional autonomous system with a three-segment piecewise-linear function which gives rise to three equilibrium points. This paper considers the possibility of simplifying the system configurations of piecewise-linear chaotic systems based on the structures of Chua's systems. We study a new class of piecewise-linear three-dimensional autonomous system with a three-segment piecewise-linear function. However, unlike Chua's systems, the systems we study in this paper have only single equilibrium points. To find chaotic attractors from this class of systems, we use a systematic random-search process to search the parameter space. The searching process consists of three stages. For the first stage, we simply count the number of points on a Poincaré section and find candidates for chaotic attractors. At the second stage, Lyapunov exponents are calculated for selecting chaotic attractors from the candidates. Finally, bifurcation diagrams constructed around the located chaotic attractors are used to find different types of chaotic attractors. Many qualitatively different chaotic attractors of this class of systems had been found and presented in this paper. Another method to simplify the configurations of a piecewise-linear chaotic system is to reduce the number of segments of the piecewise-linear function. We have developed some chaotic systems with a two-segment piecewise-linear function and which gives rise to two equilibrium points. Many color illustrations of chaotic attractors and bifurcation diagrams are presented.
Electrochemical CO 2 reduction (CO 2 R) is a promising technology that could enable electricity generated from intermittent renewable sources to be stored in the form of carbon-neutral fuels and chemical precursors. Currently, metallic copper (Cu) is the only known electrocatalyst capable of reducing CO 2 to hydrocarbons and alcohols. However, polycrystalline Cu produces up to 16 different reaction products at an applied potential of -1 V vs RHE, with hydrogen, methane, and ethene accounting for the majority of the charge passed. This lack of selectivity has motivated considerable interest in discovering ways to modify the Cu surface that enhance the reaction selectivity to a single desired product. Of particular interest are C 2 products, such as ethanol and ethene, due to their high market value and fuel potential. Prior research efforts have identified several under coordinated Cu single crystal electrodes that exhibit an enhanced selectivity for C 2+ products compared to polycrystalline Cu; however, such electrocatalysts are not scalable and are likely unstable. The selectivity of ethene relative to methane has also been enhanced by nanostructuring the Cu surface by electrochemical cycling and by the electrodeposition of Cu (I) oxide thin-films. However, these electrocatalysts are generally more selective for the formation of hydrogen than hydrocarbons. We note that all reports in the current literature observe a higher selectivity for hydrocarbons than oxygenates over metallic Cu and that there are currently no known methods of enhancing the oxygenate selectivity. Carbon monoxide (CO) reduction has been identified both experimentally and theoretically as the overpotential-determining step in the reduction of CO 2 to hydrocarbons and alcohols over polycrystalline Cu. CO is known to cover a substantial portion of the Cu surface at steady state, resulting in the suppression of the hydrogen evolution reaction (HER) by competitive adsorption for active surface sites. Interestingly, the reaction selectivity observed during CO 2 R over polycrystalline Cu has been reported to change dramatically at potentials negative of -1 V vs RHE, with hydrogen and methane rapidly increasing at the expense of C 2+ products. The onset potential of this selectivity shift agrees well with the calculated onset potential of CO 2 depletion due to concentration polarization within the hydrodynamic boundary layer at the Cu surface. The local depletion of CO 2 results in a lower steady state coverage of adsorbed CO on the Cu surface, reducing the rate of C-C coupling and enhancing the methanation of CO. This analysis suggests that higher CO coverages on Cu would result in an increase in the rate of C-C coupling between CO-derived intermediates, such as formyl, at the expense of C-H bond formation. Based on the aforementioned trends in the current literature, we hypothesized that a phase segregated bimetallic alloy of Cu with a CO-generating metal would enhance the selectivity to oxygenated multi-carbon products at the expense of hydrogen and hydrocarbons. We expect this selectivity shift to occur because the supply of additional CO by spillover will enhance the steady state coverage of CO adsorbed to the Cu surface. While several different metals have been identified as being CO selective, such as Au, Ag, and Zn, only Ag is completely immiscible with Cu and exclusively produces CO at applied potentials where Cu is capable of reducing >95% of the CO that it produces. The reaction selectivity observed during CO 2 R over the phase segregated CuAg bimetallic electrodes has led to the conclusion that they possess a reaction selectivity unlike either metallic constituent. The individual proficiencies of each metal were utilized synergistically to sequentially reduce CO 2 , first to CO over Ag and then to hydrocarbons and alcohols over Cu. The supply of additional CO to Cu by spillover from Ag increases the surface coverage of this key reaction intermediate, resulting in a selectivity shift that favors the formation of multi-carbon oxygenates at the expense of hydrogen and hydrocarbons. This effect is so pronounced that these CuAg bimetallic electrodes are currently the only electrocatalyst yet discovered that is more selective for the formation of multi-carbon oxygenates than hydrocarbons. The selectivity of oxygenates relative to hydrocarbons was found to scale with the relative distribution of Cu and Ag facet terminations present at the electrode surface, in agreement with the CO spillover hypothesis. By tuning the surface composition of the CuAg bimetallic electrode, the selectivity to multi-carbon oxygenates relative to hydrocarbons was increased by a factor of ~6 as compared to polycrystalline Cu.
Objective. Previous research has indicated that cohesive organization of traumatic memories may be necessary for the processing and resolution of post‐trauma symptoms. The present study aimed to evaluate the qualitative features of memory organization, dissociation and perception of threat in traumatic memories recalled by individuals with and without acute stress disorder (ASD). Design. Survivors of motor vehicle accidents (MVA) with either ASD or no ASD participated in a study on traumatic memories within 12 twelve days of the MVA. Method. Participants audiotaped recollections of their memories of the MVA were coded in terms of disorganized structure, dissociative content and perception of threat. Result. The recollections of ASD participants were characterized by disorganization and dissociation more than those of non‐ASD participants. Conclusion. The current findings suggest that disorganized memory structure may be one process that impedes access to, and modification of, trauma‐related cognitive schema.
Als Ausschnitte aus der V 2 O 5 ‐Schichstruktur (Bild rechts; verbrückende O‐Atome große schattierte, terminale O‐Atome große weiße und V‐Atome kleine weiße Kugeln) lassen sich die im Titel erwähnten, in jüngster Zeit hergestellten, neuartigen Polyvanadat‐Strukturen beschreiben und systematisieren. Diese im Gegensatz zu den klassischen Polyoxoamionen vom Keggin – und Schlemper‐Typ „offeneren”︁ Strukturen enthalten leicht zugängliche Vandadiumzentren mit freien Koordinationsstellen, was bereits in manchen Fällen eine Wirt‐Gast‐Chemie ermöglicht hat. magnified image
