Highly productive CO2 hydrogenation to methanol – a tandem catalytic approach via amide intermediates

A new system for CO2 reduction to methanol has been demonstrated using homogeneous ruthenium catalysts with a range of amine auxiliaries. Modification of this amine has a profound effect on the yield and selectivity of the reaction. A TON of 8900 and TOF of 4500 h−1 is achieved using a [RuCl2(Ph2PCH2CH2NHMe)2] catalyst with a diisopropylamine auxiliary

Probabilistic fatigue methodology for six nines reliability

Fleet readiness and flight safety strongly depend on the degree of reliability that can be designed into rotorcraft flight critical components. The current U.S. Army fatigue life specification for new rotorcraft is the so-called six nines reliability, or a probability of failure of one in a million. The progress of a round robin which was established by the American Helicopter Society (AHS) Subcommittee for Fatigue and Damage Tolerance is reviewed to investigate reliability-based fatigue methodology. The participants in this cooperative effort are in the U.S. Army Aviation Systems Command (AVSCOM) and the rotorcraft industry. One phase of the joint activity examined fatigue reliability under uniquely defined conditions for which only one answer was correct. The other phases were set up to learn how the different industry methods in defining fatigue strength affected the mean fatigue life and reliability calculations. Hence, constant amplitude and spectrum fatigue test data were provided so that each participant could perform their standard fatigue life analysis. As a result of this round robin, the probabilistic logic which includes both fatigue strength and spectrum loading variability in developing a consistant reliability analysis was established. In this first study, the reliability analysis was limited to the linear cumulative damage approach. However, it is expected that superior fatigue life prediction methods will ultimately be developed through this open AHS forum. To that end, these preliminary results were useful in identifying some topics for additional study

Time travel paradoxes, path integrals, and the many worlds interpretation of quantum mechanics

We consider two approaches to evading paradoxes in quantum mechanics with closed timelike curves (CTCs). In a model similar to Politzer's, assuming pure states and using path integrals, we show that the problems of paradoxes and of unitarity violation are related; preserving unitarity avoids paradoxes by modifying the time evolution so that improbable events bewcome certain. Deutsch has argued, using the density matrix, that paradoxes do not occur in the "many worlds interpretation". We find that in this approach account must be taken of the resolution time of the device that detects objects emerging from a wormhole or other time machine. When this is done one finds that this approach is viable only if macroscopic objects traversing a wormhole interact with it so strongly that they are broken into microscopic fragments.Comment: no figure

Can a circulating light beam produce a time machine?

In a recent paper, Mallett found a solution of the Einstein equations in which closed timelike curves (CTC's) are present in the empty space outside an infinitely long cylinder of light moving in circular paths around an axis. Here we show that, for physically realistic energy densities, the CTC's occur at distances from the axis greater than the radius of the visible universe by an immense factor. We then show that Mallett's solution has a curvature singularity on the axis, even in the case where the intensity of the light vanishes. Thus it is not the solution one would get by starting with Minkowski space and establishing a cylinder of light.Comment: 5 pages, RevTe

Policy Support Within a Target Group: The Case of School Desegregation

This study empirically tests three theoretical approaches to explaining specific support for a policy output among members of its target group. The utilitarian model posits support as a function of objective costs and benefits to the individual stemming directly from the policy. The attitudinal model relates specific support to diffuse predispositions rooted in socialization. The perceptual model holds that specific support derives from beliefs about the character of the political decision process by which the policy was formulated. Tests of these three approaches are based on survey data on specific support for school district desegregation plans among a large sample of black and white parents of public school children in Florida. In both subsamples, the utilitarian approach explained very little of the variance in support, but the attitudinal and perceptual models were corroborated. Implications of these findings are drawn for desegregation policy making and for public policy theory

Policy Support Within a Target Group: The Case of School Desegregation

This study empirically tests three theoretical approaches to explaining specific support for a policy output among members of its target group. The utilitarian model posits support as a function of objective costs and benefits to the individual stemming directly from the policy. The attitudinal model relates specific support to diffuse predispositions rooted in socialization. The perceptual model holds that specific support derives from beliefs about the character of the political decision process by which the policy was formulated. Tests of these three approaches are based on survey data on specific support for school district desegregation plans among a large sample of black and white parents of public school children in Florida. In both subsamples, the utilitarian approach explained very little of the variance in support, but the attitudinal and perceptual models were corroborated. Implications of these findings are drawn for desegregation policy making and for public policy theory

Superluminal travel requires negative energies

I investigate the relationship between faster-than-light travel and weak-energy-condition violation, i.e., negative energy densities. In a general spacetime it is difficult to define faster-than-light travel, and I give an example of a metric which appears to allow superluminal travel, but in fact is just flat space. To avoid such difficulties, I propose a definition of superluminal travel which requires that the path to be traveled reach a destination surface at an earlier time than any neighboring path. With this definition (and assuming the generic condition) I prove that superluminal travel requires weak-energy-condition violation.Comment: 5 pages, RevTeX, 2 figures with epsf. This paper now contains all the material of gr-qc/6805003 and gr-qc/9806091 since these became a single article in Phys. Rev. Let

A dark energy multiverse

We present cosmic solutions corresponding to universes filled with dark and phantom energy, all having a negative cosmological constant. All such solutions contain infinite singularities, successively and equally distributed along time, which can be either big bang/crunchs or big rips singularities. Classicaly these solutions can be regarded as associated with multiverse scenarios, being those corresponding to phantom energy that may describe the current accelerating universe

Promiscuous Partitioning of a Covalent Intermediate Common in the Pentein Superfamily

SummaryMany enzymes in the pentein superfamily use a transient covalent intermediate in their catalytic mechanisms. Here we trap and determine the structure of a stable covalent adduct that mimics this intermediate using a mutant dimethylarginine dimethylaminohydrolase and an alternative substrate. The interactions observed between the enzyme and trapped adduct suggest an altered angle of attack between the nucleophiles of the first and second half-reactions of normal catalysis. The stable covalent adduct is also capable of further reaction. Addition of imidazole rescues the original hydrolytic activity. Notably, addition of other amines instead yields substituted arginine products, which arise from partitioning of the intermediate into the evolutionarily related amidinotransferase reaction pathway. The enzyme provides both selectivity and catalysis for the amidinotransferase reaction, underscoring commonalities among the reaction pathways in this mechanistically diverse enzyme superfamily. The promiscuous partitioning of this intermediate may also help to illuminate the evolutionary history of these enzymes

Carrier capture dynamics of single InGaAs/GaAs quantum-dot layers

Using 800nm, 25-fs pulses from a mode locked Ti:Al2O3 laser, we have measured the ultrafast optical reflectivity of MBE-grown, single-layer In0.4Ga0.6As/GaAs quantum-dot (QD) samples. The QDs are formed via two-stage Stranski-Krastanov growth: following initial InGaAs deposition at a relatively low temperature, self assembly of the QDs occurs during a subsequent higher temperature anneal. The capture times for free carriers excited in the surrounding GaAs (barrier layer) are as short as 140fs, indicating capture efficiencies for the InGaAs quantum layer approaching 1. The capture rates are positively correlated with initial InGaAs thickness and annealing temperature. With increasing excited carrier density, the capture rate decreases; this slowing of the dynamics is attributed to Pauli state blocking within the InGaAs quantum layer