149 research outputs found
Insights into the mechanism of energy transfer with poly(heptazine imide)s in deoximation reaction
Following our previous studies on potassium poly(heptazine imide) (K-PHI) – catalyzed photooxidative [3+2] aldoxime-to-nitrile addition to form 1,2,4-oxadiazoles, we discovered that electron-rich oximes yield the parent aldehydes instead of target products. In this work, the mechanism of this singlet oxygen-mediated deoximation process was established using a series of control reactions and spectroscopic measurements such as steady-state and time-resolved fluorescence quenching experiments. Additionally, singlet-triplet energy gap value was obtained for K-PHI in suspension, and the reaction scope was broadened to include ketoximes
Multisite PCET with photocharged carbon nitride in dark
A combination of photochemistry and proton coupled electron transfer (PCET) is a primary strategy employed by biochemical systems and synthetic chemistry to enable uphill reactions under mild conditions. Degenerate nanometer-sized n-type semiconductor nanoparticles (SCNPs) with the Fermi level above the bottom of the conduction band are strongly reducing and act more like metals than semiconductors. Application of the degenerate SCNPs is limited to few examples. Herein, we load microporous potassium poly(heptazine imide) (K-PHI) nanoparticles with electrons (e–) and charge balancing protons (H+) in an illumination phase using sacrificial agents. e–/H+ in the K-PHI nanoparticles are weakly bound and therefore could be used in a range of PCET reactions in dark, such as generation of aryl radicals from aryl halides, ketyl radicals from ketones, and 6e–/6H+ reduction of nitrobenzene to aniline. The integration of several features that until now were intrinsic for plants and natural photosynthesis into a transition metal free nanomaterial composed of abundant elements (C, N, and K) offers a powerful tool for synthetic organic chemistry
Continuum theory of vacancy-mediated diffusion
We present and solve a continuum theory of vacancy-mediated diffusion (as
evidenced, for example, in the vacancy driven motion of tracers in crystals).
Results are obtained for all spatial dimensions, and reveal the strongly
non-gaussian nature of the tracer fluctuations. In integer dimensions, our
results are in complete agreement with those from previous exact lattice
calculations. We also extend our model to describe the vacancy-driven
fluctuations of a slaved flux line.Comment: 25 Latex pages, subm. to Physical Review
The Atomic Slide Puzzle: Self-Diffusion of an Impure Atom
In a series of recent papers van Gastel et al have presented first
experimental evidence that impure, Indium atoms, embedded into the first layer
of a Cu(001) surface, are not localized within the close-packed surface layers
but make concerted, long excursions visualized in a series of STM images. Such
excursions occur due to continuous reshuffling of the surface following the
position exchanges of both impure and host atoms with the naturally occuring
surface vacancies. Van Gastel et al have also formulated an original
lattice-gas type model with asymmetric exchange probabilities, whose numerical
solution is in a good agreement with the experimental data. In this paper we
propose an exact lattice solution of several versions of this model.Comment: Latex, 4 pages, 2 figures, to appear in Phys. Rev. E (RC
Productiemetingen aan mosselzaad-invang-installaties (MZI's) 2009, 2010, 2011: technisch rapport project meerjarige effect- en productiemetingen aan MZI's in de westelijke Waddenzee, Oosterschelde en Voordelta
Ter onderbouwing van de beschrijving van de effecten van de MZI’s op de ecosystemen van de Oosterschelde en Waddenzee zijn gegevens nodig van aanwas, groei, sterfte en totale biomassa van MZI-mosselen gedurende het seizoen. Deze gegevens zijn tevens nodig als input voor modelberekeningen. Met toestemming van de MZI-ondernemers zijn metingen verricht aan bestaande MZI’s, met als doel: het bepalen van individuele groei (in lengte en gewicht) van zaadmosselen; het bepalen van de toename in aantallen en biomassa van zaadmosselen per vierkante meter net of per meter touwlengte; het bepalen van de biomassa aan MZI-systemen gedurende het seizoen. Deze metingen zijn verricht in de Oosterschelde en Waddenzee
Ultra-Slow Vacancy-Mediated Tracer Diffusion in Two Dimensions: The Einstein Relation Verified
We study the dynamics of a charged tracer particle (TP) on a two-dimensional
lattice all sites of which except one (a vacancy) are filled with identical
neutral, hard-core particles. The particles move randomly by exchanging their
positions with the vacancy, subject to the hard-core exclusion. In case when
the charged TP experiences a bias due to external electric field ,
(which favors its jumps in the preferential direction), we determine exactly
the limiting probability distribution of the TP position in terms of
appropriate scaling variables and the leading large-N ( being the discrete
time) behavior of the TP mean displacement ; the latter is
shown to obey an anomalous, logarithmic law . On comparing our results with earlier predictions by Brummelhuis
and Hilhorst (J. Stat. Phys. {\bf 53}, 249 (1988)) for the TP diffusivity
in the unbiased case, we infer that the Einstein relation
between the TP diffusivity and the mobility holds in the leading in order, despite
the fact that both and are not constant but vanish as . We also generalize our approach to the situation with very small but
finite vacancy concentration , in which case we find a ballistic-type law
. We demonstrate that here,
again, both and , calculated in the linear in
approximation, do obey the Einstein relation.Comment: 25 pages, one figure, TeX, submitted to J. Stat. Phy
A One-Dimensional Model for Many-Electron Atoms in Extremely Strong Magnetic Fields: Maximum Negative Ionization
We consider a one-dimensional model for many-electron atoms in strong
magnetic fields in which the Coulomb potential and interactions are replaced by
one-dimensional regularizations associated with the lowest Landau level. For
this model we show that the maximum number of electrons is bounded above by
2Z+1 + c sqrt{B}.
We follow Lieb's strategy in which convexity plays a critical role. For the
case of two electrons and fractional nuclear charge, we also discuss the
critical value at which the nuclear charge becomes too weak to bind two
electrons.Comment: 23 pages, 5 figures. J. Phys. A: Math and General (in press) 199
Nothing moves a surface: vacancy mediated surface diffusion
We report scanning tunneling microscopy observations, which imply that all
atoms in a close-packed copper surface move frequently, even at room
temperature. Using a low density of embedded indium `tracer' atoms, we
visualize the diffusive motion of surface atoms. Surprisingly, the indium atoms
seem to make concerted, long jumps. Responsible for this motion is an ultra-low
density of surface vacancies, diffusing rapidly within the surface. This
interpretation is supported by a detailed analysis of the displacement
distribution of the indium atoms, which reveals a shape characteristic for the
vacancy mediated diffusion mechanism that we propose.Comment: 4 pages; for associated movie, see
http://www-lion.leidenuniv.nl/sections/cm/groups/interface/projects/therm
Inventarisatie van Japanse oesterbanken in de ooesterschelde en Waddenzee in 2011
Het areaal aan oesterbanken is voor de Waddenzee geschat op 909 hectare. Hiervan is 743 hectare in het veld ingemeten en 166 hectare gereconstrueerd in GIS
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