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排序方式: 共有423条查询结果,搜索用时 125 毫秒
91.
Ponnuswamy N Pantoş GD Smulders MM Sanders JK 《Journal of the American Chemical Society》2012,134(1):566-573
Amino-acid functionalized naphthalenediimides self-assemble into hydrogen-bonded supramolecular helical nanotubes via a noncooperative, isodesmic process; the self-assembly of ordered helical systems is usually realized through a cooperative process. This unexpected behavior was rationalized as a manifestation of entropy-enthalpy compensation. Fundamental insights into the thermodynamics governing this self-assembly were obtained through the fitting of the isodesmic model to (1)H NMR spectrometry and circular dichroism spectroscopy measurements. Furthermore, we have extended the application of this mathematical model, for the first time, to quantitatively estimate the effect of guests, solvents, and side chains on the stability of the supramolecular nanotube; most significantly, we demonstrate that C(60) acts as a template to stabilize the nanotube assembly and thereby substantially increase the degree of polymerization. 相似文献
92.
Dr. Martin Wolffs Juliën L. J. van Velthoven Dr. Xianwen Lou Ralf A. A. Bovee Maarten Pouderoijen Joost L. J. van Dongen Dr. Albertus P. H. J. Schenning Prof. Dr. E. W. Meijer 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(47):15057-15064
The self‐assembly of two enantiomerically pure hexa(oligo (p‐phenylene vinylene))‐substituted benzenes having 24 stereocenters was studied in pure methylcyclohexane (MCH) and in a mixture of MCH/toluene (4:1). Irrespective of the solvent a cooperative supramolecular polymerization mechanism was determined for these star‐shaped molecules by using temperature‐dependent CD and UV/Vis spectroscopy. Quite remarkably, a transition from one helical supramolecular state (A) to a second more thermodynamically stable supramolecular helical assembly (B) was observed. The rate of the A→B transition was strongly dependent on the nature of the solvent; being faster in the solvent mixture than in pure MCH. By using size exclusion chromatography we could relate the increased rate to a decreased stability of the supramolecular A state in the solvent mixture. Next, we mixed the two enantiomerically pure hexa‐substituted benzene derivatives in a so‐called majority‐rules experiment, which lead to the anitcipated chiral amplification in the A state. More importantly it appeared that the A→B transition was significantly hampered in these mixed systems. Furthermore, the absence of chiral amplification in the B state revealed the formation of separated enantiomerically pure assemblies. Therefore, by using a wide variety of spectroscopic and chromatographic techniques we determined the influence of solvent and enantiomeric purity on the transition between different supramolecular states. 相似文献
93.
Alayon EM Nachtegaal M Ranocchiari M van Bokhoven JA 《Chemical communications (Cambridge, England)》2012,48(3):404-406
Methane can be converted to methanol over copper-exchanged mordenite at 200 °C. Methanol could be recovered at the end of the reactor. This multi-step reaction opens the possibility for methane to methanol conversion in a closed catalytic cyclic reaction system. 相似文献
94.
Wim Fremout Maarten Dhaenens Steven Saverwyns Jana Sanyova Peter Vandenabeele Dieter Deforce Luc Moens 《Analytica chimica acta》2012
In recent years, the use of liquid chromatography tandem mass spectrometry (LC–MS/MS) on tryptic digests of cultural heritage objects has attracted much attention. It allows for unambiguous identification of peptides and proteins, and even in complex mixtures species-specific identification becomes feasible with minimal sample consumption. Determination of the peptides is commonly based on theoretical cleavage of known protein sequences and on comparison of the expected peptide fragments with those found in the MS/MS spectra. In this approach, complex computer programs, such as Mascot, perform well identifying known proteins, but fail when protein sequences are unknown or incomplete. Often, when trying to distinguish evolutionarily well preserved collagens of different species, Mascot lacks the required specificity. Complementary and often more accurate information on the proteins can be obtained using a reference library of MS/MS spectra of species-specific peptides. Therefore, a library dedicated to various sources of proteins in works of art was set up, with an initial focus on collagen rich materials. This paper discusses the construction and the advantages of this spectral library for conservation science, and its application on a number of samples from historical works of art. 相似文献
95.
Cucu T Devreese B Trashin S Kerkaert B Rogge M De Meulenaer B 《Journal of AOAC International》2012,95(1):149-156
Hazelnuts are widely used nowadays, and can pose a serious threat to allergic consumers due to cross-contamination that may occur during processing. This might lead to the presence of hidden hazelnut in foods. Therefore, reliable tests are needed to detect hazelnut, especially in processed foods. A hazelnut-specific indirect competitive ELISA based on polyclonal chicken antibodies was developed. The polyclonal antibodies were raised against modified hazelnut proteins in order to improve the detectability of hazelnut proteins in processed foods. The assay showed a detection limit of 1.36 microg hazelnut protein/mL of 5 mM urea in phosphate-buffered saline buffer (pH 7.4). Limited cross-reactivity with walnut and pecan nut was observed; no cross-reactivity was observed with other food ingredients. Blank cookies spiked before analysis showed recoveries of 73-107%. However, cookies spiked before baking showed that the detectability was severely decreased. Addition of lactose to the cookies, which led to more severe modification through the Maillard reaction, led to an increase in the detectability. These results indicate that using antibodies developed toward allergens modified through food processing-simulating reactions is a better approach for detection. 相似文献
96.
Grzegorz Łach Maarten DeKieviet Ulrich D. Jentschura 《Central European Journal of Physics》2012,10(4):763-767
The thermal friction force acting on an atom moving relative to a thermal photon bath has recently been calculated on the basis of the fluctuation-dissipation theorem. The thermal fluctuations of the electromagnetic field give rise to a drag force on an atom provided one allows for dissipation of the field energy via spontaneous emission. The drag force exists if the atomic polarizability has a nonvanishing imaginary part. Here, we explore alternative derivations. The damping of the motion of a simple harmonic oscillator is described by radiative reaction theory (result of Einstein and Hopf), taking into account the known stochastic fluctuations of the electromagnetic field. Describing the excitations of the atom as an ensemble of damped harmonic oscillators, we identify the previously found expressions as generalizations of the Einstein-Hopf result. In addition, we present a simple explanation for blackbody friction in terms of a Doppler shift of the thermal radiation in the inertial frame of the moving atom: The atom absorbs blue-shifted photons from the front and radiates off energy in all directions, thereby losing energy. The original plus the two alternative derivations provide for additional confirmation of an intriguing quantum friction effect, and leave no doubt regarding its existence. 相似文献
97.
Hydraulic fracture models typically couple a fracture elasticity model with a geological reservoir model to forecast the rate
of fluid leak-off from the propagating fracture. The most commonly used leak-off model is that originally specified by Carter,
which involves the assumption that the fracture is embedded within an infinite homogenous porous medium where flow only occurs
perpendicular to the fracture plane. The objectives of this paper are: (1) to show that assuming one-dimensional leak-off
can lead to erroneous conclusions, (2) to present a robust numerical methodology for simulating three-dimensional leak-off
from propagating hydraulic fractures, and (3) to present and compare a new analytical method based on assuming three-dimensional
flow of an incompressible fluid through an incompressible porous formation from a circular planar fracture. Provided the fluid
and formation compressibility can be ignored within the reservoir flow model, the three-dimensional leak-off from a circular
planar fracture can be written in closed-form as a function, which depends linearly on fracture pressure and radial extent.
This simple expression for leak-off can be easily coupled to a range of circular fracture elasticity models. As a comparison
example, the Carter model, our new function and a three-dimensional numerical model of the full problem are coupled to the
PK-radial fracture model. Comparison with the numerical model shows that our new function overestimates fracture growth during
intermediate times but accurately predicts both the early and late-time asymptotic behavior. In contrast, the Carter model
fails to replicate both the early and late-time asymptotic behavior. Our new function additionally improves on the Carter
model by not requiring the evaluation of convolution integrals and allowing easy evaluation of both the spatial leakage flux
distribution across the fracture face and the three-dimensional pressure distribution within the porous formation. 相似文献
98.
Unsteady fluctuating velocity fields can contain large-scale periodic motions with frequencies well separated from those of turbulence. Examples are the wake behind a cylinder or the processing vortex core in a swirling jet. These turbulent flow fields contain large-scale, low-frequency oscillations, which are obscured by turbulence, making it impossible to identify them. In this paper, we present an Eulerian time filtering (ETF) technique to extract the large-scale motions from unsteady statistical non-stationary velocity fields or flow fields with multiple phenomena that have sufficiently separated spectral content. The ETF method is based on non-causal time filtering of the velocity records in each point of the flow field. It is shown that the ETF technique gives good results, similar to the ones obtained by the phase-averaging method. In this paper, not only the influence of the temporal filter is checked, but also parameters such as the cut-off frequency and sampling frequency of the data are investigated. The technique is validated on a selected set of time-resolved stereoscopic particle image velocimetry measurements such as the initial region of an annular jet and the transition between flow patterns in an annular jet. The major advantage of the ETF method in the extraction of large scales is that it is computationally less expensive and it requires less measurement time compared to other extraction methods. Therefore, the technique is suitable in the startup phase of an experiment or in a measurement campaign where several experiments are needed such as parametric studies. 相似文献
99.
Davy L S Nieskens Francisco Ample-Navarro Maarten M M Jansen Daniel Curulla-Ferré Josep Ricart Hans Niemantsverdriet 《Chemphyschem》2006,7(5):1068-1074
The decomposition of acetylene on a Rh(100) single crystal was studied by a combination of experimental techniques [static secondary ion mass spectrometry (SSIMS), temperature-programmed desorption (TPD), and low-energy electron diffraction (LEED)] to gain insight into the reaction pathway and the nature of the reaction intermediates. The experimental techniques were combined with a computational approach using density functional theory (DFT). Acetylene adsorbs irreversibly on the Rh(100) surface and eventually decomposes to atomic carbon and gas-phase hydrogen. The combination of experimental and computational results enabled us to determine the most likely reaction pathway for the decomposition process. 相似文献
100.
In reflection seismology one places sources and receivers on the Earth's surface. The source generates elastic waves in the
subsurface, that are reflected where the medium properties, stiffness and density, vary discontinuously. In the field, often,
there are obstructions to collect seismic data for all source-receiver pairs desirable or needed for data processing and application
of inverse scattering methods. Typically, data are measured on the Earth's surface. We employ the term data continuation to describe the act of computing data that have not been collected in the field. Seismic data are commonly modeled by a scattering
operator developed in a high-frequency, single scattering approximation. We initially focus on the determination of the range
of the forward scattering operator that models the singular part of the data in the mentioned approximation. This encompasses
the analysis of the properties of, and the construction of, a minimal elliptic projector that projects a space of distributions
on the data acquisition manifold to the range of the mentioned scattering operator. This projector can be directly used for
the purpose of seismic data continuation, and is derived from the global parametrix of a homogeneous pseudodifferential equation
the solution of which coincides with the range of the scattering operator. We illustrate the data continuation by a numerical
example.
This research was supported in part under NSF CMG grant EAR-0417891.
Partly supported by a John Simon Guggenheim fellowship. 相似文献