10,000 publications from this institution
The authors have concentrated their studies of molecular-scale catalysis on transition metals, which catalyze hydrocarbon conversion reactions, and on the hydrogenation of carbon monoxide. They studied platinum, an excellent catalyst for dehydrocyclization of alkanes to aromatic molecules, and rhodium, which produces oxygenated organic molecules from carbon monoxide and hydrogen selectively. They also investigated iron, that catalyzes the ammonia synthesis from nitrogen and hydrogen as well as the CO-H/sub 2/ reaction. From these studies they identified three necessary ingredients of selective molecular-scale catalysis: atomic surface structure, an active carbonaceous deposit, and the proper oxidation stat of surface atoms.
Greenspan, Alan; Interest rates; Deficit financing
Abstract Mangroves are complex ecosystems subject to periodic and irregular changes in meteorology, tides, water quality and biological factors. Methane dynamics in mangroves and their dependence on biogeochemical controls across scales are poorly characterized using traditional parametric statistics. We measured the ecosystem‐scale methane flux (F CH4 ) using an eddy covariance system in a subtropical estuarine mangrove for two years. A combination of nonparametric statistical approaches, including singular spectrum analysis and information theory, was used to isolate multiscale F CH4 variations and explore their variability and dominant controls at different time scales. F CH4 exhibited the largest variability at the diel scale (57%), followed by seasonal (25%) and multiday (18%) scales. Mutual information metrics suggested that variation in F CH4 was dominantly coupled to plant activities through synchronous processes at the diel scale. Moreover, fluctuations in atmospheric turbulence and soil temperature dominated multiday variation in F CH4 through asynchronous processes. Seasonally, soil temperature and water salinity were dominant variables leading to changes in F CH4 with a time lag of 12 and 17 days, respectively. Weak links between F CH4 and tidal heights were found across all the studied time scales. Transfer entropy metrics indicated that a super typhoon event substantially weakened causal links between biophysical variables and F CH4 by causing severe defoliation and disturbing the soil microbial community. The typhoon also temporarily decoupled the causal link between F CH4 and temperature. These findings call for the inclusion of the properties of scale emergence, nonlinearity, asynchrony and causality for comprehensively understanding and accurately predicting the interactions between mangrove F CH4 and its biophysical controls.
We report a series of palladium diarylamido complexes containing a cyclometalated phosphine ligand and a coordinated THF that form enamine products from migratory insertion of ethylene and octene into the metal-nitrogen bond. The reactions of these complexes were sensitive to the electronic properties of the diarylamido group. The complex with the more electron-donating amido ligand reacted nearly 2 orders of magnitude faster than the complex with the least electron-donating amido group. The mechanism of the reactions with alkenes involves exchange of olefin for THF, as revealed by the positive order of the reaction in alkene and inverse order in the concentration of THF. A syn-aminopalladation that would result from migratory insertion was revealed by the stereochemistry of the enamine product resulting from reaction with cis-ethylene-d(2). Finally, a three-coordinate THF-free species was isolated from a synthesis of the amido complex in aromatic solvents, and spectroscopic evidence for an ethylene amido species was gained, in part by the addition of H(2)(13)C=(13)CH(2) to the three-coordinate amido complex at -100 degrees C.
Human activities affect Earth's life support systems so profoundly as to threaten many of the ecological services that are essential to society. To address this challenge, a new science agenda is needed that integrates people with the rest of nature to help chart a more sustainable trajectory for the relationship between society and the biosphere. This paper describes Earth Stewardship, an initiative of the Ecological Society of America to provide the scientific basis for actively shaping trajectories of social-ecological change to enhance ecosystem resilience and human well-being. Principles for moving toward these goals include simultaneous attention to multiple scales and issues; consideration of both ecological and socioeconomic consequences; alignment of incentives with stewardship behavior; strengthening peoples' connections to valued places; and using demographic transitions as new opportunities for stewardship. Past experience provides guidelines for fostering Earth Stewardship. Early attention to sustainable pathways before problems emerge generally provides more cost-effective solutions than attempting to remediate entrenched problems. Defining sustainable pathways by assessing tradeoffs among alternative options requires careful attention to fine-scale processes, interactions, and feedbacks and to larger-scale controls and constraints. Many opportunities occur locally, through development of practices that match the properties of resources with the needs of their users. Substantial challenges remain at larger scales, including maintaining the diversity, productive capacity, and resilience of nature, which are essential for long-term human welfare. The knowledge needed to inform stewardship requires an interdisciplinary science that draws on the observations, skills, and creativity of a wide range of natural and social scientists, practitioners, and civil society. New questions and solutions will emerge when these groups work together to formulate the issues, design the research, and co-produce the observations, knowledge, and concepts that form the basis for solutions. The goal of Earth Stewardship is not to protect nature from people; rather it is to protect nature for human welfare.
Terrestrial LiDAR scanners have been shown to hold great potential for estimating and mapping three dimensional (3-D) leaf area distribution in forested environments. This is made possible by the capacity of LiDAR scanners to record the 3-D position of every laser\npulse intercepted by plant material. The laser pulses emitted by a LiDAR scanner can be regarded as light probes whose transmission and\ninterception may be used to derive leaf area density at different spatial scales using the Beer-Lambert law or Warren Wilson's contact frequency method among others. Segmenting the canopy into cubic volumes -or voxels provides a convenient means to compute light transmission statistics and describe the spatial distribution of foliage area in tree crowns. In this paper, we investigate the optimal voxel dimensions for estimating the spatial distribution of within crown leaf area density. We analysed LiDAR measurements from two field sites, located in Mali and in California, with trees having different leaf sizes during periods with and without leaves.\n\nWe found that there is a range of voxel sizes, which satisfy three important conditions. The first condition is related to clumping and requires voxels small enough to exclude large gaps between crowns and branches. The second condition requires a voxel size large enough for the conditions postulated by the Poisson law to be valid, i.e., a turbid medium with randomly positioned leaves. And, the third condition relates to the appropriate voxel size to pinpoint the location of those volumes within the canopy which were insufficiently sampled by the LiDAR instrument to derive reliable statistics (occlusion effects). Here, we show that these requirements are a function of leaf size, branching structure, and the predominance of occlusion effects. The results presented provide guiding principles for using voxel volumes in the retrieval of leaf area distributions from terrestrial LiDAR measurements.
The analogy between electromagnetic wave propagation in multidimensionally periodic structures and electron wave propagation in real crystals has proven to be a very fruitful one. Initial efforts were motivated by the prospect of a photonic band gap, a frequency band in three-dimensional dielectric structures in which electromagnetic waves are forbidden, irrespective of propagation direction in space. Today many new ideas and applications are being pursued in two and three dimensions, and in metallic, dielectric and acoustic structures, etc. The author reviews the early motivations for this work, which were derived from the need for a photonic band gap in quantum optics. This led to a series of experimental and theoretical searches for the elusive photonic band-gap structures, those three-dimensionally periodic dielectric structures which are to photon waves what semiconductor crystals are to electron waves. Then he describes how the photonic semiconductor can be 'doped', producing tiny electromagnetic cavities. Finally he summarizes some of the anticipated implications of photonic band structure for quantum electronics and the prospects for the creation of photonic crystals in the optical domain.
Abstract A circuit element is nonenergic if the instantaneous power flow into it is always zero. Well‐known examples include the ideal diode, transformer, gyrator and circulator. Most of the interesting nonenergic elements are nonlinear N ‐ports with N ⩾ 2, and many of their properties are quite counterintuitive. For example, there exists a surprisingly large class of nonenergic multiport capacitors and inductors, all of which, it turns out, are nonlinear and reciprocal. Nonenergic linear N ‐ports, on the other hand, are necessarily resistive and antireciprocal. In this paper, we present a rigorous fundamental theory of nonenergic N ‐ports that results in a general canonical representation. Special canonical forms are developed for nonenergic resistors, capacitors and inductors, and numerous examples are given.
Abstract not Available.
ObjectiveThe Transdiagnostic Sleep and Circadian Intervention for Youth (TranS-C) was developed to improve sleep and circadian functioning in adolescents. This study examined the 12-month effects for TranS-C compared with psychoeducation (PE). We also investigated whether a text messaging intervention can promote maintenance of treatment effects.MethodAt the baseline, adolescents (58% female, average age = 14.8 years) with an eveningness chronotype were randomized to TranS-C (n = 89) or PE (n = 87). At 6-month follow-up, participants were randomized to receive text messages that had repeated treatment information (n = 47), text messages that prompted the recall of treatment information (n = 50), or no text messages (n = 47).ResultsRelative to PE, TranS-C was associated with a reduced eveningness (b = 2.06, p = .005, d = 0.29) from the baseline to a 12-month follow-up. TranS-C treatment effects, relative to PE, were augmented by receiving text messages, compared to no text messages, for eveningness from baseline to 12-month follow-up (b = 1.38, p = .008, d = 0.28) and from 6- to 12-month follow-up (b = 1.07, p = .046, d = 0.21). Neither TranS-C nor text messages were significantly associated with other primary outcomes. TranS-C and text messages were significantly associated with improvements on selected secondary sleep and health outcomes through follow-up.ConclusionsFor adolescents with an eveningness chronotype, improved sleep and circadian functioning on selected outcomes were maintained over 12 months for TranS-C compared with PE. Text messages boosted the effects of TranS-C through 12-month follow-up.
Inflation, after an extended absence, is back. Its resumption reflects a combination of supply and demand factors: supply-side disruptions to trade and production associated with COVID-19; and shifts in the composition of demand to goods from services, along with strong stimulus to spending owing to support payment provided by governments. There is disagreement about what to do in response: whether central banks should continue to respond with increases in interest rates, and whether there is a role for selective wage and price controls. Above all, there is disagreement about how long the inflation problem is likely to persist.
