排序方式: 共有19条查询结果,搜索用时 15 毫秒
1.
van Der Voort M Rella CW van Der Meer LF Akimov AV Dijkhuis JI 《Physical review letters》2000,84(6):1236-1239
We present results of the first vibrational photon-echo, transient-grating, and temperature dependent transient-bleaching experiments on a-Si:H. Using these techniques, and the infrared light of a free electron laser, the vibrational population decay and phase relaxation of the Si-H stretching mode were investigated. Careful analysis of the data indicates that the vibrational energy relaxes directly into Si-H bending modes and Si phonons, with a distribution of rates determined by the amorphous host. Conversely, the pure dephasing appears to be single exponential, and can be modeled by dephasing via two-phonon interactions. 相似文献
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Cruz-Castañeda J. Aguilar-Ovando E. Buhse T. Ramos-Bernal S. Meléndez-López A. Camargo-Raya C. Fuentes-Carreón C. Negrón-Mendoza A. 《Journal of Radioanalytical and Nuclear Chemistry》2017,311(2):1135-1141
Studies in chemical evolution are intended to demonstrate how compounds of biological importance are generated from substances that could have been found in abiotic conditions on the primitive Earth or in extraterrestrial environments. In this context, the aim of the present work was to examine the behavior of dl-glyceraldehyde in both aqueous solution and solid samples under gamma irradiation. We irradiated dl-glyceraldehyde at different doses and temperatures with a gamma source; even at low doses and temperature (77 K), free radicals were detected. Among the products formed were ethylene glycol and glycolaldehyde. Some sugar-like compounds were also detected.
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María E. Noble-Terán Dr. José-Manuel Cruz Dr. Hugo I. Cruz-Rosas Dr. Thomas Buhse Dr. Jean-Claude Micheau 《Chemphyschem》2023,24(18):e202300318
Attrition-enhanced chiral symmetry breaking in crystals, known as Viedma deracemization, is a promising method for converting racemic solid phases into enantiomerically pure ones under non-equilibrium conditions. However, many aspects of this process remain unclear. In this study, we present a new investigation into Viedma deracemization using a comprehensive kinetic rate equation continuous model based on classical primary nucleation theory, crystal growth, and Ostwald ripening. Our approach employs a fully microreversible kinetic scheme with a size-dependent solubility following the Gibbs–Thomson rule. To validate our model, we use data from a real NaClO3 deracemization experiment. After parametrization, the model shows spontaneous mirror symmetry breaking (SMSB) under grinding. Additionally, we identify a bifurcation scenario with a lower and upper limit of the grinding intensity that leads to deracemization, including a minimum deracemization time within this window. Furthermore, this model uncovers that SMSB is caused by multiple instances of concealed high-order autocatalysis. Our findings provide new insights into attrition-enhanced deracemization and its potential applications in chiral molecule synthesis and understanding biological homochirality. 相似文献
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David A. Keire Hongping Ye Michael L. Trehy Wei Ye Richard E. Kolinski Benjamin J. Westenberger Lucinda F. Buhse Moheb Nasr Ali Al-Hakim 《Analytical and bioanalytical chemistry》2011,399(2):581-591
During the 2007–2008 heparin crisis, it was found that the United States Pharmacopeia (USP) testing monograph for unfractionated
heparin sodium (UFH) did not detect the presence of the contaminant, oversulfated chondroitin sulfate (OSCS) in heparin. In
response to this concern, new tests and specifications were developed by the Food and Drug Administration (FDA) and USP and
put in place to not only detect the contaminant OSCS but also to improve assurance of quality and purity of the drug product.
Additional tests were also developed to monitor the heparin supply chain for other possible economically motivated additives
or impurities. In 2009, a new USP monograph was put in place that includes 500 MHz 1H NMR, SAX-HPLC, %galactosamine in total hexosamine, and anticoagulation time assays with purified factor IIa or factor Xa.
These tests represent orthogonal approaches for UFH identification, measurement of bioactivity, and for detection of process
impurities or contaminants in UFH. The FDA has applied these analytical approaches to the study of UFH active pharmaceutical
ingredients in the marketplace. Here, we describe results from a comprehensive survey of UFH collected from seven different
sources after the 2009 monograph revision and compare these data with results obtained on other heparin samples collected
during the 2007–2008 crisis. 相似文献
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Noise added to a chirally autocatalytic model system is usually known to cause mirror-symmetry breaking with statistically equal distributions for the two product enantiomers. We show that if such a system is asymmetrically perturbed by means of a very small undetectable bias in the racemization equilibrium between the two enantiomers, adding Gaussian white noise can lead to an efficient enantioselection. Consequently, within a certain range of the noise amplitude, symmetry breaking gives rise to an entirely biased statistical distribution in favor of one of the enantiomers. In contrast, racemic results will be obtained for the corresponding deterministic case (in the absence of noise). Thus, added noise plays a constructive role by directing the chiral system into a specific enantiomeric direction while being influenced only by a subthreshold asymmetric input. This effect could be of conceptual interest for the impact of weak asymmetric fields on nonlinear chemical reactions. 相似文献
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Irradiation of glyceraldehyde under simulated prebiotic conditions: Study in solid and aqueous state
E. Aguilar-Ovando J. Cruz-Castañeda T. Buhse C. Fuentes-Carreón S. Ramos-Bernal A. Heredia A. Negrón-Mendoza 《Journal of Radioanalytical and Nuclear Chemistry》2018,316(3):971-979
We investigated the radiolysis of dl-glyceraldehyde in a solid state and in an aqueous solution. The catalytic effect of a clay onto glyceraldehyde was also studied. We carried out the irradiation in a 60Co gamma ray source (Gammabeam 650 PT), with doses up to 360 kGy, at three temperatures (77, 198 and 295 K). For the analysis, we used various spectroscopic and chromatographic analytical methods. The results show that this compound is labile under irradiation and that it forms malondialdehyde, glycolaldehyde and other sugar-like compounds that are important in chemical-evolution studies. 相似文献
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The electroacoustical impedance of the quartz crystal microbalance (QCM) in contact with aqueous electrolyte solutions was measured using the transfer function method in a flow injection system . Measurements of both components of the impedance of the QCM, the resistance R and the inductive reactance XL, have been performed for modified and bare gold and silver surfaces and for different concentrations of several aqueous electrolyte solutions. For the experimental concentration range of 0-50 mM, unexpectedly the QCM impedance does not follow the Kanazawa equation, as is usual for bulk newtonian liquids. This behavior indicates the presence of a nanometric sized viscoelastic layer between the piezoelectric crystal and the bulk electrolyte solution. This layer can only be identified as the Gouy-Chapman diffuse double layer (DDL). Its elasticity and viscosity have been estimated by the measurement of R and XL. The viscoelasticity of the DDL appears to be independent of the chemical nature of the surface and of the solution viscosity but strongly dependent on the surface charge, the bulk electrolyte concentration and the dielectric constant of the solvent. 相似文献
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