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991.
992.
993.
Dipl.-Ing. M. Klumpp 《Archive of Applied Mechanics (Ingenieur Archiv)》1961,30(3):153-159
Ohne Zusammenfassung 相似文献
994.
Maire P Deblon S Breher F Geier J Böhler C Rüegger H Schönberg H Grützmacher H 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(17):4198-4205
Iridium(I) complexes containing a (5H-dibenzo[a,d]cyclohepten-5-yl)-phosphane (tropp(R); R = phosphorus-bound substituent = Ph, Cyc) as a rigid, concave-shaped, mixed phosphane olefin ligand were prepared and tested as catalyst precursors in the hydrogenation of imines. With the complex [Ir(tropp(Cyc))(cod)]OTf, turnover frequencies (TOFs) of >6000 h(-1) were reached in the hydrogenation of N-phenyl-benzylidenamine, PhN==CHPh. Lower activities (TOF>80 h(-1)) are observed with N-phenyl-(1-phenylethylidene)amine, PhN==CMePh. Chiral tropp-type ligands were prepared in few simple steps. Monosubstitution of the olefinic unit in the dibenzo[a,d]cycloheptenyl moiety with (R)- or (S)-mentholate gave mixtures of diastereomers that could be separated and isolated in enantiomerically pure form. Iridium(I) complexes with these ligands are rare examples of side-on bonded enolether complexes. In catalytic imine hydrogenations, complete conversion (>98 %) was reached in all cases (conditions: p[H(2)] = 50 bar, T = 50 degrees C, t = 2 h, substrate/catalyst 100:1). The best enantiomeric excess (ee = 86 % S isomer) was reached with PhN==CMePh as substrate and the R,R form of the (10-menthyloxy-5H-dibenzo[a,d]cyclohepten-5-yl)diphenylphosphane ligand. The iridium(I) complex containing the same phosphane gave a 60 % ee (S isomer) with the enamide N-(1-phenylvinyl)acetamide as substrate (conditions: p[H(2)] = 4 bar, T = 50 degrees C, t = 18 h, substrate/catalyst = 50:1). These reactions constitute the first examples in which chiral olefins have been used as steering ligands in catalytic enantioselective hydrogenations. 相似文献
995.
Stulgies B Prinz P Magull J Rauch K Meindl K Rühl S de Meijere A 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,11(1):308-320
Palladium-catalyzed sixfold coupling of hexabromobenzene (20) with a variety of alkenylboronates and alkenylstannanes provided hexaalkenylbenzenes 1 in up to 73 % and 16 to 41 % yields, respectively. In some cases pentaalkenylbenzenes 21 were isolated as the main products (up to 75 %). Some functionally substituted hexaalkenylbenzene derivatives containing oxygen or sulfur atoms in each of their six arms have also been prepared (16 to 24 % yield). The sixfold coupling of the less sterically encumbered 2,3,6,7,10,11-hexabromotriphenylene (24) gave the desired hexakis(3,3-dimethyl-1-butenyl)triphenylene (25) in 93 % yield. The first successful cross-coupling reaction of octabromonaphthalene (26) gave octakis-(3,3-dimethyl-1-butenyl)naphthalene (27) in 21 % yield. Crystal structure analyses disclose that, depending on the nature of the substituents, the six arms are positioned either all on the same side of the central benzene ring as in 1 a and 1 i, making them nicely cup-shaped molecules, or alternatingly above and below the central plane as in 1 h and 23. In 27, the four arms at C-1,4,6,7 are down, while the others are up, or vice versa. Upon catalytic hydrogenation, 1 a yielded 89 % of hexakis(tert-butylethyl)benzene (23). Some efficient accesses to alkynes with sterically demanding substituents are also described. Elimination of phosphoric acid from the enol phosphate derived from the corresponding methyl ketones gave 1-ethynyladamantane (3 b, 62 % yield), 1-ethynyl-1-methylcyclohexane (3 c, 85 %) and 3,3-dimethylpentyne (3 e, 65 %). 1-(Trimethylsilyl)ethynylcyclopropane (7) was used to prepare 1-ethynyl-1-methylcyclopropane (3 d) (two steps, 64 % overall yield). The functionally substituted alkynes 3 f-h were synthesized in multistep sequences starting from the propargyl chloride 11, which was prepared in high yields from the dimethylpropargyl alcohol 10 (94 %). The alkenylstannanes 19 were prepared by hydrostannation of the corresponding alkynes in moderate to high yields (42-97 %), and the alkenylboronates 2 and 4 by hydroboration with catecholborane (27-96 % yield) or pinacolborane (26-69 % yield). 相似文献
996.
How plants keep dry: a physicist's point of view 总被引:5,自引:0,他引:5
This work describes the investigation of the physical basis of the amazing water repellence of some plant leaves, which is in addition to self-cleaning properties known as the "Lotus effect". Two rather different possible mechanisms are proposed, which are suggested to cover the main physics in a majority of superhydrophobic systems. These concepts are illustrated with two different types of plant leaves as well as a model surface carrying carbon nanotube aggregates. 相似文献
997.
We prove that there exists a packing of copies of a tree of size into Kn. Moreover, the proof provides an easy algorithm.Acknowledgments. The research of the second author was partially supported by Deutscher Akademischer Austauschdienst. 相似文献
998.
The thermodynamic behavior of the Angelescu-Verbeure-Zagrebnov
(AVZ) Hamiltonian [1], also called the superstable Bogoliubov model, is solved
for any temperature and any chemical potential. It is found that its thermodynamics
coincides with one for the Mean-Field Gas for small chemical potential
or high temperature. However, for large chemical potential or low temperature,
a non-conventional Bose condensation appears with, even at zero-temperature, a
(non-zero) particle density outside the condensate. Following [2], the analysis in the
present paper corresponds to the main technical step to deduce, in the canonical
ensemble, a new microscopic theory of superfluidity at all temperatures explained
in [3].
Communicated by Vincent Pasquier
Submitted 31/03/03, accepted 01/12/03 相似文献
999.
First-principles total-energy calculations reveal a novel local atomic reconstruction mode around anion vacancies in II-VI and chalcopyrite compounds resulting from the formation of metal dimers. As a consequence, the neutral Se vacancy has an unexpected low symmetry in ZnSe and becomes a deep donor in both ZnSe and CuGaSe2, contrary to the common belief regarding chalcopyrites. The calculated optical transition energies explain the hitherto puzzling absorption bands observed in the classic experiments of the color center in ZnS. 相似文献
1000.
Maas FE Achenbach P Aulenbacher K Baunack S Capozza L Diefenbach J Grimm K Imai Y Hammel T von Harrach D Kabuss EM Kothe R Lee JH Lorente A Lopes Ginja A Nungesser L Schilling E Stephan G Weinrich C Altarev I Arvieux J Collin B Frascaria R Guidal M Kunne R Marchand D Morlet M Ong S van de Wiele J Kowalski S Plaster B Suleiman R Taylor S 《Physical review letters》2004,93(2):022002
We report on a measurement of the parity-violating asymmetry in the scattering of longitudinally polarized electrons on unpolarized protons at a Q2 of 0.230 (GeV/c)(2) and a scattering angle of theta (e) = 30 degrees - 40 degrees. Using a large acceptance fast PbF2 calorimeter with a solid angle of delta omega = 0.62 sr, the A4 experiment is the first parity violation experiment to count individual scattering events. The measured asymmetry is A(phys)=(-5.44+/-0.54(stat)+/-0.26(sys))x10(-6). The standard model expectation assuming no strangeness contributions to the vector form factors is A(0) = (-6.30+/-0.43) x 10(-6). The difference is a direct measurement of the strangeness contribution to the vector form factors of the proton. The extracted value is G(s)(E) + 0.225G(s)(M) = 0.039+/-0.034 or F(s)(1) + 0.130F(s)(2) = 0.032+/-0.028. 相似文献