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991.
Fernando Loup Paulo Alexandre Santos Dorabella Martins da Silva Santos 《General Relativity and Gravitation》2003,35(11):2035-2044
It is shown that the ansatz of Chung-Freese models a new braneworld, and while superluminal, this braneworld does not suffer from the pathologies of other FTL methods. It is also shown that our universe is a trapped surface inside a universe with large extra dimensions, and in order to preserve compatibility with the Standard Model (SM), we present two trapping mechanisms that keeps SM fields inside our brane. For the Chung-Freese braneworld we develop a similar formalism to Morris domain walls, and we present our universe as a low level state in a potential well with the bulk in the highest levels of potential. Finally, we show that the Israel condition can also act as a trapping mechanism for SM fields inside the brane. Our model uses the idea that fermions are allowed to escape to the bulk inspired by the work of Havoudiasl-Hewett-Rizzo [2]. 相似文献
992.
Alexandre Froda 《Mathematische Zeitschrift》1965,89(2):126-136
Sans résumé 相似文献
993.
Insertion poly(methyl acrylate) and poly(methyl methacrylate) were prepared from monomers adsorbed in monolayers on the surface of montmorillonite clay, both in the presence and in the absence of bifunctional crosslinkers (ethylene glycol dimethacrylate and tetramethylene glycol dimethacrylate). The insertion poly(methyl acrylate) and the crosslinked insertion poly(methyl methacrylate) and dilute-solution properties quite different from conventional polymers of these monomers, the differences including high light-scattering molecular weights combined with low viscosities, low values of the second virial coefficient, unusually large variations of the Huggins' constant k′ with the time-temperature history of the solutions, and low sedimentation velocities. These properties suggest that the insertion polymers have compact structures and are consistent with the postulate of sheetlike macromolecules. The dilute-solution properties of insertion poly(methyl methacrylate) made without crosslinker, unlike those of similarly prepared poly(methyl acrylate), were similar to those of conventional poly(methyl methacrylate). This difference in behavior is attributed to the different tendencies of the two monomers to undergo branching or crosslinking during radical polymerization. 相似文献
994.
Alexandre J. Chorin 《Journal of statistical physics》1994,76(3-4):835-856
A phase diagram is mapped out for a 21/2-dimensional vortex lattice model in which vortex filaments lie in a plane, while both the velocity field and the Green function are three-dimensional. Both positive and negative temperatures are considered. Various qualitative properties of turbulent states and of the super-fluid transition are well verified within the limitations of the model; the percolation properties of vortex transitions are exhibited; the differences between superfluid and classical vortex motion are highlighted, as is the importance of topological constraints in vortex dynamics; an earlier model of intermittency is verified. 相似文献
995.
Alexandre I. Danilenko Valentin Ya. Golodets 《Transactions of the American Mathematical Society》1996,348(12):4857-4882
Let be an ergodic automorphism of a Lebesgue space and a cocycle of with values in an Abelian locally compact group . An automorphism from the normalizer of the full group is said to be compatible with if there is a measurable function such that at a.e. . The topology on the set of all automorphisms compatible with is introduced in such a way that becomes a Polish group. A complete system of invariants for the -outer conjugacy (i.e. the conjugacy in the quotient group is found. Structure of the cocycles compatible with every element of is described.
996.
Arink AM Braam TW Keeris R Jastrzebski JT Benhaim C Rosset S Alexakis A van Koten G 《Organic letters》2004,6(12):1959-1962
[structure: see text] Full conversion and enantioselectivities up to 83% have been obtained in the conjugate addition reactions of diethyl zinc to Michael acceptors catalyzed by well-defined (chiral) copper(I) aminoarenethiolates. Interesting differences between organozinc or Grignard reagents have been found: for cyclic enones R(2)Zn reagents afford better results, whereas earlier work showed that RMgX reagents react more selectively with acyclic enones. 相似文献
997.
Functionalization at C-5 of 4-fluoro- and 4-chloro-1-triisopropylsilyl-7-azaindole, 1 and 2, respectively, leads to a variety of new substituted 7-azaindole derivatives. We also describe two approaches to introduce functionality at C-6. 相似文献
998.
Takaoka LR Buckmelter AJ LaCruz TE Rychnovsky SD 《Journal of the American Chemical Society》2005,127(2):528-529
A synthesis of spiroacetals was developed using a reductive cyclization strategy that leads stereoselectively to spiroacetals with a single anomeric stabilization. The method begins with the synthesis of spiro ortho esters. The ortho ester is converted to a cyano acetal. Reductive lithiation of the cyano acetal generates an axial dialkoxylithium reagent, and intramolecular cyclization produces a new ring with retention of configuration. The strategy is convergent and produces complex spiro acetals in only a few steps. The method will be useful in the synthesis of natural products and will facilitate the synthesis of previously inaccessible contra-thermodynamic acetals. 相似文献
999.
Helio G. Bonacorso Alexandre P. Wentz Cleber A. Cechinel Helena S. Coelho Marcos A.P. Martins Sydney H. Alves 《Journal of fluorine chemistry》2006,127(8):1066-1072
The regiospecific synthesis of a novel series of nine 4-phenyl- and 3-alkyl(aryl)-5-hydroxy-5-trifluoromethyl-4,5-dihydro-1H-1-tosylpyrazoles (pyrazolinyl p-tolyl sulfones), as well as their antimicrobial activity against yeast, such as fungi, bacteria, and alga are reported. The 1-p-tosyl-2-pyrazolines were obtained from the cyclocondensation reaction of 3-phenyl- and 4-alkyl(aryl)-1,1,1-trifluoro-4-alkoxy-3-alken-2-ones, [where alkyl = H, Me and aryl are -C6H5, 4-CH3C6H4, 4-OCH3C6H4, 4-FC6H4, 4-ClC6H4, 4-BrC6H4,] with p-tosylhydrazine in a yield of 58-92% when toluene was employed as solvent. The best activity was obtained when the structure possessed a 4-fluorophenyl substituent linked at the carbon-3 of the pyrazoline ring. Subsequently, the dehydration reaction of 3-(4-fluorophenyl) substituted 2-pyrazoline with thionyl chloride/pyridine in benzene as solvent furnished the corresponding 1H-pyrazole in only a moderate yield (49%). 相似文献
1000.
A dramatic improvement of the catalytic activity was observed when a phosphine was added in allylic alkylation reactions catalyzed by (NHC)Pd(η3-C3H5)Cl complexes. Consequently, several palladium complexes, generated in situ from different NHC-silver complexes, [Pd(η3-C3H5)Cl]2 and PPh3, were tested in this reaction to evaluate their potential. High reaction rates and conversions could be obtained with this catalytic system in the alkylation of allylic acetates with dimethylmalonate, particularly under biphasic conditions using water/dichloromethane and KOH 1 M as the base. These conditions are experimentally more convenient and gave higher reaction rates than the classical anhydrous conditions (NaH/THF). In this system, the phosphine is essential since no conversion was obtained when it is not present. The steric hindrance of the carbene ligand has a great influence on the activity and the stability of the catalytic system. The best NHC ligands for this reaction are either 1-mesityl-3-methyl-imidazol-2-ylidene or 1-(2,6-diisopropylphenyl)-3-methyl-imidazol-2-ylidene which are less bulky among the NHC tested. These two ligands led in 5 min to a complete conversion at 20 °C. The Pd-catalyzed allylic amination reaction using (E)-1,3-diphenylprop-3-en-yl acetate and benzylamine was also tested with (NHC)(PPh3)Pd complexes and under the biphasic conditions. This reaction was found to be slower than the alkylation with dimethylmalonate but a complete conversion could be reached in 6 h at 20 °C using K2CO3 1 M as the base. NMR experiments indicated that mixed (NHC)(PPh3)Pd complexes are formed in situ but their structure could not be established exactly. 相似文献