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排序方式: 共有72条查询结果,搜索用时 15 毫秒
1.
Structures and cytotoxic activity relationship of casearins, new clerodane diterpenes from Casearia sylvestris Sw 总被引:1,自引:0,他引:1
H Morita M Nakayama H Kojima K Takeya H Itokawa E P Schenkel M Motidome 《Chemical & pharmaceutical bulletin》1991,39(3):693-697
Casearins G-R, new cytotoxic clerodane diterpenes have been isolated from the leaves of Casearia sylvestris Sw. (Flacourtiaceae). Their structures have been elucidated by spectroscopic methods and chemical conversions, and their structure-activity relationships have been discussed. 相似文献
2.
Ulrich Schubert Andreas Schenkel Johannes Müller 《Journal of organometallic chemistry》1985,292(3):c11-c14
In the reaction of Cp2WGl2 with Li[Si(SiMe3)3] the dihydrid tungstenocene derivative [(Me3Si)3SiC5H4]WH2 (3) is formed with a 56% yield. 3 crystallizes in space group P, with a 918.0(4), b 1580.9(4), c 1621.2(7) pm, α 117.63(2), β 89.95(3), γ 94.39(3)° at ?40° C. The dihedral angle between the Cp planes is 140.9°. 相似文献
3.
J.?Magill H.?SchwoererEmail author F.?Ewald J.?Galy R.?Schenkel R.?Sauerbrey 《Applied physics. B, Lasers and optics》2003,77(4):387-390
We report the first successful laser-induced transmutation of 129I, one of the key radionuclides in the nuclear fuel cycle. 129I with a half-life of 15.7 million years is transmuted into 128I with a half-life of 25 min through a (, n) reaction using laser-generated Bremsstrahlung. The integral cross-section value for the (, n) reaction is determined. These experiments offer a new approach to studying transmutation reactions with neutral and charged particles without resource to nuclear reactors or particle accelerators. PACS 52.38.Ph; 25.20.Lj 相似文献
4.
Moldovan L Melinte S Bayot V Papadakis SJ De Poortere EP Shayegan M 《Physical review letters》2000,85(20):4369-4372
We report very low temperature ( T) thermopower and resistivity ( rho) measurements on variable-density, two-dimensional hole systems confined to GaAs quantum wells. As the hole density is lowered from 1.49x10(11) cm(-2) to 0.14x10(11) cm(-2), the system crosses from an insulating ( drho / dT less, similar0) to a metallic regime ( drho / dT>0) and finally displays insulating behavior ( drho / dT<0). Diffusion thermopower shows a striking sign reversal in a narrow range of density in the metallic regime, suggesting a qualitative change in the conduction or the scattering mechanism. 相似文献
5.
Alexander Schenkel 《General Relativity and Gravitation》2011,43(10):2605-2630
We study the quantum field theory (QFT) of a free, real, massless and curvature coupled scalar field on self-similar symmetric
spacetimes, which are deformed by an abelian Drinfel’d twist constructed from a Killing and a homothetic Killing vector field.
In contrast to deformations solely by Killing vector fields, such as the Moyal-Weyl Minkowski spacetime, the equation of motion
and Green’s operators are deformed. We show that there is a *-algebra isomorphism between the QFT on the deformed and the
formal power series extension of the QFT on the undeformed spacetime. We study the convergent implementation of our deformations
for toy-models. For these models it is found that there is a *-isomorphism between the deformed Weyl algebra and a reduced
undeformed Weyl algebra, where certain strongly localized observables are excluded. Thus, our models realize the intuitive
physical picture that noncommutative geometry prevents arbitrary localization in spacetime. 相似文献
6.
We consider an interacting scalar quantum field theory on noncommutative Euclidean space. We implement a family of noncommutative deformations, which – in contrast to the well known Moyal–Weyl deformation – lead to a theory with modified kinetic term, while all local potentials are unaffected by the deformation. We show that our models, in particular, include propagators with anisotropic scaling z=2 in the ultraviolet (UV). For a Φ4-theory on our noncommutative space we obtain an improved UV behaviour at the one-loop level and the absence of UV/IR-mixing and of the Landau pole. 相似文献
7.
We develop a general setting for the quantization of linear bosonic and fermionic field theories subject to local gauge invariance and show how standard examples such as linearised Yang-Mills theory and linearised general relativity fit into this framework. Our construction always leads to a well-defined and gauge-invariant quantum field algebra, the centre and representations of this algebra, however, have to be analysed on a case-by-case basis. We discuss an example of a fermionic gauge field theory where the necessary conditions for the existence of Hilbert space representations are not met on any spacetime. On the other hand, we prove that these conditions are met for the Rarita-Schwinger gauge field in linearised pure $N=1$ supergravity on certain spacetimes, including asymptotically flat spacetimes and classes of spacetimes with compact Cauchy surfaces. We also present an explicit example of a supergravity background on which the Rarita-Schwinger gauge field can not be consistently quantized. 相似文献
8.
Visible Light Activation of Boronic Esters Enables Efficient Photoredox C(sp2)–C(sp3) Cross‐Couplings in Flow 下载免费PDF全文
Fabio Lima Dr. Mikhail A. Kabeshov Dr. Duc N. Tran Dr. Claudio Battilocchio Dr. Joerg Sedelmeier Dr. Gottfried Sedelmeier Dr. Berthold Schenkel Prof. Steven V. Ley 《Angewandte Chemie (International ed. in English)》2016,55(45):14085-14089
We report herein a new method for the photoredox activation of boronic esters. Using these reagents, an efficient and high‐throughput continuous flow process was developed to perform a dual iridium‐ and nickel‐catalyzed C(sp2)–C(sp3) coupling by circumventing solubility issues associated with potassium trifluoroborate salts. Formation of an adduct with a pyridine‐derived Lewis base was found to be essential for the photoredox activation of the boronic esters. Based on these results we were able to develop a further simplified visible light mediated C(sp2)–C(sp3) coupling method using boronic esters and cyano heteroarenes under flow conditions. 相似文献
9.
EP Sheretov VS Gurov MV Dubkov OV Korneeva 《Rapid communications in mass spectrometry : RCM》1999,13(16):1699-1702
In this article we compare the classical monopole mass filter of von Zahn and the monopole mass filter with a hyperbolic V-shaped electrode. The experimental results and those of computer simulation for both mass spectrometers are presented. We show that the replacement of a conventional 90 degrees V-shaped electrode by an electrode with a hyperbolic profile substantially improves the peak shape of any given mass, and increases the mass resolution by a factor of 3-4 and the abundance sensitivity by a factor of 100. The potential of high analytical performance combined with electroforming techniques for electrode manufacture indicate future practical uses of such instruments. Copyright 1999 John Wiley & Sons, Ltd. 相似文献
10.
T. Schlathölter M.W. Newman T.R. Niedermayr G.A. Machicoane J.W. McDonald T. Schenkel R. Hoekstra A.V. Hamza 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2000,12(2):323-327
The emission of small (hydrogenated) carbon cluster ions CnHm
+
(n
=2-22) upon highly charged Xeq+ (q
=20-44) impact on C84 surfaces is studied by means of time-of-flight secondary ion mass spectrometry. The respective stage of hydrogenation/protonation
of a certain carbon cluster ion Cn
+
is a strong indication for its geometrical structure. From the cluster ion yield as a function of cluster size it can be
concluded, that the hydrogenation takes place after the initial carbon cluster formation. The carbon clusters seem to be emitted
as an entity in agreement with “equilibrium” and “shock wave” models.
Received 4 February 2000 相似文献