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21.
This research originally was aimed at modeling all flows (except free-molecular) by systems of hyperbolic-relaxation equations
(moments of the Boltzmann equation), and developing efficient numerical methods for these. Such systems have many potential
numerical advantages, mainly because there are no second or higher derivatives to be approximated. This avoids accuracy problems
on adaptive unstructured grids, and the source terms, though often stiff, are only local; the compact stencils facilitate
code parallelization. A single code could simulate flows up to intermediate Knudsen numbers, and be hybridized with DSMC where
needed. In this project, one major problem arose that we have not yet solved: the accurate representation of shock structures.
This makes the methodology currently unsuited for, e.g., re-entry flows. We have validated it for subsonic and transonic flows
and are concentrating on applications to MEMS-related flows. We discuss the challenges of our approach, present numerical
algorithms and results based on the 10-moment model, and report progress in our latest research topic: formulating accurate
solid-boundary conditions. 相似文献
22.
Georges Bram Jean Sansoulet Hervé Galons Younes Bensaid Claude Combet-Farnoux Marcel Miocque 《Tetrahedron letters》1985,26(38):4601-4602
Solid-liquid phase transfer catalysis without added organic solvent efficiently promotes MICHAEL reactions. The method is applied here to the addition of acetylacetone, methylacetoacetate and fluorene anions on hindered acceptors. 相似文献
23.
U. Shuali L. Bram M. Steinberg S. Yariv 《Journal of Thermal Analysis and Calorimetry》1991,37(7):1569-1578
The catalytic activity of sepiolite and palygorskite in cracking and disproportionation reactions of cumene was studied. The formation of benzene, diisopropylbenzene and propylene characterizes Brönsted acid sites whereas that of ethylbenzene characterizes Lewis acid sites. At 150 and 250°C the reactions are determined mainly by the presence of acid sites in the channels of these clays. The yields are significantly higher in sepiolite as compared to palygorskite. The concentration of acid sites and their strength increase with the thermal loss of zeolitic and part of the bound water. At 350°C the reactions are determined by the acidity of the external surface of both minerals and yields are only slightly higher in sepiolite. At 150°C most acid sites are of the Brönsted type. With the rise in temperature the relative concentration of Lewis acid sites increases.
Dedicated to Prof. Dr. H. J. Seifert on the occasion of his 60th birthday 相似文献
Zusammenfassung Es wurde die katalytische Aktivität von Sepiolith und Palygorskit in Krack- und Disproportionierungsreaktionen von Kumol untersucht. Die Bildung von Benzol, Diisopropyl-benzol und Propylen charakterisieren Aziditätsstellen nach Bronsted, während die Bildung von Ethylbenzol Aziditätsstellen nach Lewis charakterisiert. Bei 150°C und 250°C werden die Reaktionen hauptsächlich durch die Gegenwart von Aziditätsstellen in den Tunnelgebilden dieser Tonerden bestimmt. Bezogen auf Palygorskit sind die Ausbeuten bei Sepiolith eindeutig höher. Die Konzentration der Aziditätsstellen und ihre Stärke nehmen mit der thermischen Abgabe von zeolithischem und eines Teiles des gebundenen Wassers zu. Bei 350°C werden die Reaktionen durch die Azidität der externen Oberfläche beider Mineralien bestimmt und die Ausbeuten sind in Sepiolith kaum höher. Bei 150°C sind die meisten Aziditätsstellen vom Brönsted-Typ. Mit ansteigender Temperatur nimmt auch die relative Anzahl von Lewis-Aziditätsstellen zu.
Dedicated to Prof. Dr. H. J. Seifert on the occasion of his 60th birthday 相似文献
24.
The solubilizing properties of aqueous solutions of isomeric salts, deoxycholate, chenodeoxycholate and ursodeoxycholate for naphthalene and cholesterol have been studied. For both molecules, ursodeoxycholate micelles have the poorest detergent power. Binding of naphthalene to micelles modifies the chemical shifts of the angular methyl groups of the bile salts as determined by 1H NMR. A greater pertubation of the C19 methyl signal relative to that of the C18 methyl signal is observed with deoxycholate and chenodeoxycholate but not with ursodeoxycholate. These results suggest a distinct micellar structure for ursodeoxycholate. 相似文献
25.
Solid-liquid phase transfer catalysis (PTC) without added solvent efficiently promotes Michael addition of substituted malonates on hindered acceptors. A rearrangement of adducts formed by addition of diethyl N-acetylaminomalonate is described. 相似文献
26.
Dr. Xu Quan Dr. Sutthichat Kerdphon Bram B. C. Peters Dr. Janjira Rujirawanich Dr. Suppachai Krajangsri Dr. Jira Jongcharoenkamol Prof. Pher G. Andersson 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(58):13311-13316
Novel bidentate N-heterocyclic carbene-phosphine iridium complexes have been synthesized and evaluated in the hydrogenation of ketones. Reported catalytic systems require base additives and, if excluded, need elevated temperature or high pressure of hydrogen gas to achieve satisfactory reactivity. The developed catalysts showed extremely high reactivity and good enantioselectivity under base-free and mild conditions. In the presence of 1 mol % catalyst under 1 bar hydrogen pressure at room temperature, hydrogenation was complete in 30 minutes giving up to 96 % ee. Again, this high reactivity was achieved in additive-free conditions. Mechanistic experiments demonstrated that balloon pressure of hydrogen was sufficient to form the activate species by reducing and eliminating the 1,5-cyclooctadiene ligand. The pre-activated catalyst was able to hydrogenate acetophenone with 89 % conversion in 5 min. 相似文献
27.
Dhooge W Arijs K D'Haese I Stuyvaert S Versonnen B Janssen C Verstraete W Comhaire F 《Analytical and bioanalytical chemistry》2006,386(5):1419-1428
In vitro assays are considered as the first step in a tiered approach to compound screening for hormonal activity. Although
many new assays have been developed in recent years, little attention has been paid towards assay validation. Our objective
was to identify critical experimental parameters in a yeast estrogen screen (YES) that affect its sensitivity and specificity.
We investigated the role of incubation time, solvent type, yeast inoculum growth stage and concentration on the outcome of
the YES. Compounds tested included new and established agonists, antagonists and negative controls, and results were evaluated
according to prefixed statistical criteria. In addition, we assessed the assay’s performance in a blind interlaboratory validation
exercise (IVE). An incubation time of five days was necessary to positively identify the estrogenic properties of all agonists
tested, when dissolved in DMSO. Longer incubation times were required when using an ethanol protocol. Similar estrogenic activity
was reported for benzyl butyl phthalate, bisphenol-A, methoxychlor, permethrin and genistein in the IVE. One out of the three
laboratories did not classify α,β-endosulfan, dissolved in DMSO, as an estrogen. The same was true for 4,4′-DDE and lindane, dissolved in ethanol, a result
that might be attributable to an inappropriate yeast start concentration and/or growth stage. These validation experiments
show that under appropriate experimental conditions the YES yields sensitive, specific and reliable results. Therefore it
fulfills the requirements as a first step screening assay to evaluate the capacity of chemicals to interact with the estrogen
receptor. 相似文献
28.
We consider the problem of a travelling merchant who makes money by buying commodities where they are cheap and selling them
in other places where he can make a profit. The merchant ships commodities of his own choice in a van of fixed capacity. Given
the prices of all the commodities in all of the places, and the cost of driving from one place to another, the problem the
merchant faces each day is to select a subset of the cities that he can visit in a day, and to determine the order in which
the cities are visited, such that the total profit is maximised. We call this problem the Merchant Subtour Problem. The MSP models the pricing problem of a rather complex pickup and delivery problem that was given to us by the Dutch logistics
company Van Gend & Loos.
We show that a special case of the MSP has a totally unimodular constraint matrix. This knowledge enables us to develop a
tabu-search algorithm for finding good feasible solutions to the MSP, and a branch-and-price-and-cut algorithm for solving
the MSP to optimality. The relaxations solved in each node of the branch-and-bound tree are strengthened by lifted knapsack
inequalities, lifted cycle inequalities and mod-k cuts. We present computational results on data sets derived from our main instance of the Van Gend & Loos pickup and delivery
problem.
Received: October 25, 2000 / Accepted: January 23, 2002 Published online: September 27, 2002
RID="★"
ID="★" This research was partially supported by EC – Fifth Framework Programme contract IST-1999-14186 (Project ALCOM-FT:
Algorithms and Complexity – Future Technologies). 相似文献
29.
The structure and the nucleophilic reactivity of crowned (18-crown-6) or cryptated {cryptand (2.2.2)} potassium ethyl acetoacetate enolate have been compared in tert-butanol and in DME (or THF). In the protic solvent tert-butanol, the crowned and the cryptated potassium enolate species both exist as loose ion pairs in which the enolate anion, strongly hydrogen-bonded to the solvent, is in a “transoid” (non chelating) conformation. Both species show similar reactivities towards alkylating agents but completely different reactivities are observed in aprotic weakly dissociating media (THF, DME). In contrast to what is observed in tert-butanol, the cryptated species and the crowned species have very different nucleophilic reactivities in THF or DME; in those solvents only the cryptated species retains a loose ion pair structure; the crowned species is a contact ion pair in which the enolate anion chelates the potassium cation. The solvation of this crowned chelate species by tert-butanol has been demonstrated in binary mixtures of solvents (C6D6-t-BuOH, THF-t-BuOH). The oxygen basicity of the enolate anion is very different in the crowned chelated ion pair compared with the cryptand separated ion pair. 相似文献
30.
On the Fourier Spectra of Distributions in Clifford Analysis 总被引:1,自引:1,他引:0
In recent papers by Brackx, Delanghe and Sommen, some fundamental higher dimensional distributions have been reconsidered in the framework of Clifford analysis, eventually leading to the introduction of four broad classes of new distributions in Euclidean space. In the current paper we continue the in-depth study of these distributions, more specifically the study of their behaviour in frequency space, thus extending classical results of harmonic analysis. 相似文献