905 research outputs found
A Demonstration of the ReactivityâSelectivity Principle for the ThiolâDisulfide Interchange Reaction
Equilibrium constants for the reaction of aryl thiol anions with hydroxyethyl disulfide have been measured which, along with literature data, demonstrate a slope of 1.21 for a plot of log K Sâ (R'Sâ + RSSR â R'SSR + âSR) vs pKa. Rate constants were measured also for these endothermic reactions of aryl thiol anions with hydroxyethyl disulfide and also for the exothermic reactions of alkyl thiol anions with the mixed disulfide of mercaptoethanol and 4ânitroâ2,3,5,6âtetrafluorothiophenol. These kinetic data, obtained over a range of K Sâ of 10 21 , show the gradual curvature expected for Hammond postulate type behavior. A quantitative measure of this curvature in terms of the Marcus formalism was carried out for these two data sets along with four others having more moderate values of ÎG. The data were fit with a value for the intrinsic barrier, λ/4 = 11.6 kcal, and a value for the work term W r = 4.0 kcal. A comparison is made of these values with the similar values found for alkyl, proton and acyl transfer reactions. The importance of using a variety of substrates with a series of bases or nucleophiles, rather than a single substrate, is discussed, as are the cause for curvature other than Hammond postulate behavior.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/101845/1/198500126_ftp.pd
Optimierte Bestimmung der unterirdischen Pflanzenbiomasse in Theorie und Praxis
Will man den C oder N Eintrag von Pflanzen ermitteln, muss die unterirdische Pflanzenbiomasse möglichst exakt bestimmt werden. Diese besteht aus der Rhizodeposition und dem Wurzelsystem einer Pflanze. Als Rhizodeposition wird die Abgabe von organischen und anorganischen Verbindungen bezeichnet. Sie setzt sich unter anderem aus Wurzelfragmenten, Wurzelrandzellen, Wurzelexudaten und Lysaten zusammen.
Auf Grund einer fehlenden Wurzelraumbegrenzung, ist die Erfassung des vollstĂ€ndigen Wurzelsystems einer Pflanze im Freiland problematisch. Zur Quantifizierung von Wurzelsystemen sind jedoch Freilandversuche stets GefĂ€Ăversuchen vorzuziehen, da nur so ein ungestörtes Wurzelwachstum erreicht werden kann. Als Konsequenz lassen sich unterschiedliche Wurzel-Spross-VerhĂ€ltnisse in GefĂ€Ă- und Freilandversuchen feststellen. Verlagerungsprozesse innerhalb der Pflanze können zusĂ€tzlich die Berechnung der Rhizodeposition beeinflussen und so zur Ăber- oder UnterschĂ€tzung der unterirdischen Pflanzenbiomasse fĂŒhren. Ziel war daher ein Beprobungsschema zu entwickeln, welches es ermöglicht die Wurzelbiomasse im Freiland zu erfassen und unterschiedliche Berechnungsmethoden der Rhizodeposition zu vergleichen.
HierfĂŒr wurden sowohl im GefÀà als auch im Freiland Erbsen mittels Dochtmethode mit multiplen 13C und 15N-Pulsen markiert, wodurch eine annĂ€hernd kontinuierliche Markierung simuliert wurde. Die Wurzelbiomasse der Erbse wurde im Freiland bestimmt, indem Unterproben mit einem definierten Volumen in 3 festgelegten Positionen im Bestand genommen wurden (direkt auf einer Pflanze; zwischen 2 Pflanzen in der Reihe; in der Mitte von 4 Pflanzen zwischen 2 Reihen). Durch die unterschiedliche Gewichtung der Positionen, die sich aus dem Beprobungsdurchmesser und dem Pflanze-/Reihenabstand ergaben, konnte die vollstĂ€ndige Wurzelbiomasse bestimmt werden. Die Rhizodeposition wurde mit einer Massenbilanz (1) und mit der Janzen und Bruinsma Methode (2) ermittelt.
Zum Zeitpunkt der BlĂŒte waren die Wurzelbiomasse und das Wurzel-Spross-VerhĂ€ltnis im Feld um ein vielfaches GröĂer verglichen mit dem GefĂ€Ă. Bei der Berechnung nach Janzen und Bruinsma können Verlagerungsprozesse wĂ€hrend der BlĂŒte zur ĂberschĂ€tzung der Rhizodeposition fĂŒhren. Erfolgt eine kontinuierliche Markierung ĂŒber den gesamten Vegetationsverlauf, so kann die Rhizodeposition am Kulturende sowohl nach Janzen und Bruinsma als auch mit der Massenbilanz berechnet werden
Antarctic Surface Reflectivity Measurements from the ANITA-3 and HiCal-1 Experiments
The primary science goal of the NASA-sponsored ANITA project is measurement
of ultra-high energy neutrinos and cosmic rays, observed via radio-frequency
signals resulting from a neutrino- or cosmic ray- interaction with terrestrial
matter (atmospheric or ice molecules, e.g.). Accurate inference of the energies
of these cosmic rays requires understanding the transmission/reflection of
radio wave signals across the ice-air boundary. Satellite-based measurements of
Antarctic surface reflectivity, using a co-located transmitter and receiver,
have been performed more-or-less continuously for the last few decades.
Satellite-based reflectivity surveys, at frequencies ranging from 2--45 GHz and
at near-normal incidence, yield generally consistent reflectivity maps across
Antarctica. Using the Sun as an RF source, and the ANITA-3 balloon borne
radio-frequency antenna array as the RF receiver, we have also measured the
surface reflectivity over the interval 200-1000 MHz, at elevation angles of
12-30 degrees, finding agreement with the Fresnel equations within systematic
errors. To probe low incidence angles, inaccessible to the Antarctic Solar
technique and not probed by previous satellite surveys, a novel experimental
approach ("HiCal-1") was devised. Unlike previous measurements, HiCal-ANITA
constitute a bi-static transmitter-receiver pair separated by hundreds of
kilometers. Data taken with HiCal, between 200--600 MHz shows a significant
departure from the Fresnel equations, constant with frequency over that band,
with the deficit increasing with obliquity of incidence, which we attribute to
the combined effects of possible surface roughness, surface grain effects,
radar clutter and/or shadowing of the reflection zone due to Earth curvature
effects.Comment: updated to match publication versio
Onset Rivalry: Brief Presentation Isolates an Early Independent Phase of Perceptual Competition
When the left and right eyes are simultaneously presented with different images, observers typically report exclusive awareness of only one image. This phenomenon is termed binocular rivalry, reflecting the fact that the dominant image alternates every few seconds in a cycle of perceptual competition that continues indefinitely. Despite the apparent continuity in perceptual switching, we now demonstrate that the initial âonsetâ period is fundamentally different to all subsequent rivalry epochs. Using brief intermittent presentations, rivalry dominance shows strong biases such that the same target is perceived with each successive stimulus onset. These biases remain consistent within any given location, but vary across the visual field in a distribution that is stable over multiple weeks but highly idiosyncratic across observers. If the presentation exceeds âŒ1sec at any location, however, the very different and much more balanced alternations of sustained binocular rivalry become apparent. These powerful onset biases are observed with brief intermittent presentations at a single location or with continual smooth motion of the targets. Periods of adaptation to one of the rivaling targets induced local switches in dominance to the non-adapted target. However, these effects were generally limited to the spatial site of adaptation and had less influence over each subsequent cycle of the target. We conclude that onset rivalry is independent of sustained rivalry and cannot be explained by local regions of monocular dominance or memory of past perceptual history, but rather reflects low-level, spatially localized factors that are stable over periods of weeks. These findings suggest that brief presentation paradigms are inappropriate for their current use in studies of the mechanisms underlying sustained rivalry. However, brief presentations are ideal for investigating early stages of perceptual competition
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