887 research outputs found
Nitration of derivatives of 2,5-dichlorobenzenesulfonic acid
The proof that a nitro group enters meta to the sulfonic acid group of 2,5-dichlorobenzenosulfonamide and 2,5-dichlorobenzenosulfonanilide is presented.
The preparation of several new compounds is described.
An unsuccessful attempt to prepare 2,5-dichloro-3-nitrobenzenesulfonyl chloride from 2,5-dichloronitrobenzene is also described
Transmission Oscillator Ultrasonic Spectrometer (TOUS): A new research instrument
TOUS is capable for measuring very small changes in acoustic attenuation and phase velocity. Its high sensitivity to small changes in ultrasonic absorption results in part from operation under marginal conditions. In spite of high sensitivity, TOUS system is relatively simple, inexpensive, and compact
Species–area relationships in continuous vegetation : evidence from Palaearctic grasslands
Aim: Species-area relationships (SARs) are fundamental scaling laws in ecology although their shape is still disputed. At larger areas power laws best represent SARs. Yet, it remains unclear whether SARs follow other shapes at finer spatial grains in continuous vegetation. We asked which function describes SARs best at small grains and explored how sampling methodology or the environment influence SAR shape.
Location: Palaearctic grasslands and other non-forested habitats.
Taxa: Vascular plants, bryophytes and lichens.
Methods: We used the GrassPlot database, containing standardised vegetation-plot data from vascular plants, bryophytes, and lichens spanning a wide range of grassland types throughout the Palaearctic and including 2057 nested-plot series with at least seven grain sizes ranging from 1 cm2 to 1024 m². Using non-linear regression, we assessed the appropriateness of different SAR functions (power, power quadratic, power breakpoint, logarithmic, Michaelis-Menten). Based on AICc, we tested whether the ranking of functions differed among taxa, methodological settings, biomes or vegetation types.
Results: The power function was the most suitable function across the studied taxonomic groups. The superiority of this function increased from lichens to bryophytes to vascular plants to all three taxonomic groups together. The sampling method was highly influential as rooted-presence sampling decreased the performance of the power function. By contrast, biome and vegetation type had practically no influence on the superiority of the power law.
Main conclusions: We conclude that SARs of sessile organisms at smaller spatial grains are best approximated by a power function. This coincides with several other comprehensive studies of SARs at different grain sizes and for different taxa, thus supporting the general appropriateness of the power function for modelling species diversity over a wide range of grain sizes. The poor performance of the Michaelis-Menten function demonstrates that richness within plant communities generally does not approach any saturation, thus calling into question the concept of minimal area
Effective medium theory of elastic waves in random networks of rods
We formulate an effective medium (mean field) theory of a material consisting
of randomly distributed nodes connected by straight slender rods, hinged at the
nodes. Defining novel wavelength-dependent effective elastic moduli, we
calculate both the static moduli and the dispersion relations of ultrasonic
longitudinal and transverse elastic waves. At finite wave vector the waves
are dispersive, with phase and group velocities decreasing with increasing wave
vector. These results are directly applicable to networks with empty pore
space. They also describe the solid matrix in two-component (Biot) theories of
fluid-filled porous media. We suggest the possibility of low density materials
with higher ratios of stiffness and strength to density than those of foams,
aerogels or trabecular bone.Comment: 14 pp., 3 fig
Initial Conditions for a Universe
In physical theories, boundary or initial conditions play the role of
selecting special situations which can be described by a theory with its
general laws. Cosmology has long been suspected to be different in that its
fundamental theory should explain the fact that we can observe only one
particular realization. This is not realized, however, in the classical
formulation and in its conventional quantization; the situation is even worse
due to the singularity problem. In recent years, a new formulation of quantum
cosmology has been developed which is based on quantum geometry, a candidate
for a theory of quantum gravity. Here, the dynamical law and initial conditions
turn out to be linked intimately, in combination with a solution of the
singularity problem.Comment: 7 pages, this essay was awarded First Prize in the Gravity Research
Foundation Essay Contest 200
Quantum state correction of relic gravitons from quantum gravity
The semiclassical approach to quantum gravity would yield the Schroedinger
formalism for the wave function of metric perturbations or gravitons plus
quantum gravity correcting terms in pure gravity; thus, in the inflationary
scenario, we should expect correcting effects to the relic graviton
(Zel'dovich) spectrum of the order (H/mPl)^2
Maximizing Small Group Reading Instruction
In this article, the authors revisit the common practice of small-group reading instruction. They challenge the idea of grouping readers based on text levels and instead review supplemental intervention group research that suggests targeted skill practice as a more optimal use of time in small groups. They then present the ABCs—a focus on assessment, basics & books, and clarity in communication—as the central principles that should guide how we instruct reading in small groups
High-pressure spin shifts in the pseudogap regime of superconducting YBa2Cu4O8 as revealed by 17O NMR
A new NMR anvil cell design is used for measuring the influence of high
pressure on the electronic properties of the high-temperature superconductor
YBaCuO above the superconducting transition temperature . It is found that pressure increases the spin shift at all temperatures in
such a way that the pseudo-gap feature has almost disappeared at 63 kbar. This
change of the temperature dependent spin susceptibility can be explained by a
pressure induced proportional decrease (factor of two) of a temperature
dependent component, and an increase (factor of 9) of a temperature independent
component, contrary to the effects of increasing doping. The results
demonstrate that one can use anvil cell NMR to investigate the tuning of the
electronic properties of correlated electronic materials with pressure.Comment: 4 pages, 4 figures, accepted for publication in Phys. Rev.
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