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排序方式: 共有495条查询结果,搜索用时 15 毫秒
31.
John?Ellis Nikolaos?E.?MavromatosEmail author Dimitri?V.?Nanopoulos 《General Relativity and Gravitation》2005,37(10):1665-1670
Generic cosmological models in non-critical string theory have a time-dependent dilaton background at a late epoch. The cosmological
deceleration parameter q
0 is given by the square of the string coupling, g
s
2, up to a negative sign. Hence the expansion of the Universe must accelerate eventually, and the observed value of q
0 corresponds to g
s
2 ∼ 0.6. In this scenario, the string coupling is asymptotically free at large times, but its present rate of change is imperceptibly
small.
First Award in the 2005 Essay Competition of the Gravity Research Foundation. - Ed. 相似文献
32.
This article examines the universal polytope
(of type {5,3,5}) whose facets are dodecahedra, and whose vertex figures are hemi-icosahedra. The polytope is proven to be
finite, and the structure of its group is identified. This information is used to classifiy the quotients of the polytope.
A total of 145 quotients are found, including 69 section regular polytopes with the same facets and vertex figures as
.
Mathematics Subject Classification (2000): 51M20, 20F65, 52B15
An erratum to this article is available at . 相似文献
33.
Khoshtariya DE Wei J Liu H Yue H Waldeck DH 《Journal of the American Chemical Society》2003,125(25):7704-7714
Using nanometer thick tunneling barriers with specifically attached cytochrome c, the electron-transfer rate constant was studied as a function of the SAM composition (alkane versus terthiophene), the omega-terminating group type (pyridine, imidazole, nitrile), and the solution viscosity. At large electrode-reactant separations, the pyridine terminated alkanethiols exhibit an exponential decline of the rate constant with increasing electron-transfer distance. At short separations, a plateau behavior, analogous to systems involving -COOH terminal groups to which cytochrome c can be attached electrostatically, is observed. The dependence of the rate constant in the plateau region on system properties is investigated. The rate constant is insensitive to the mode of attachment to the surface but displays a significant viscosity dependence, change with spacer composition (alkane versus terthiophene), and nature of the solvent (H(2)O versus D(2)O). Based on these findings and others, the conclusion is drawn that the charge-transfer rate constant at short distance is determined by polarization relaxation processes in the structure, rather than the electron tunneling probability or large-amplitude conformational rearrangement (gating). The transition in reaction mechanism with distance reflects a gradual transition between the tunneling and frictional mechanisms. This conclusion is consistent with data from a number of other sources as well. 相似文献
34.
José A. García-Vázquez Jaime Romero Antonio Sousa-Pedrares Antonio Sousa A.D. Garnovskii Dimitri A. Garnovskii 《Journal of chemical crystallography》2000,30(1):23-26
The compound [Fe(C4H6N2)6][C5H4NSO3]2 crystallized in the monoclinic space group, P21/n with unit cell parameters: a = 13.676(3), b = 8.345(2), c = 18.663(4) Å, = 106.40(3)° and Z = 2. The title compound consists of a [Fe(C4H6N2)6]+2 hexacoordinated iron(II) cation and two C5H4NSO–
3 anions. In the cation the iron atom is coordinated to six N-methylimidazole imine nitrogen atoms in a distorted octahedral arrangement. The N1,N5,N1i,N5i atoms are coplanar and the iron lies in this plane. 相似文献
35.
Agadjanov Andria Agadjanov Dimitri Khelashvili Anzor Rusetsky Akaki 《The European Physical Journal A - Hadrons and Nuclei》2013,49(9):1-7
The European Physical Journal A - The divergent part of the one-loop effective action in Chiral Perturbation Theory with virtual photons has been evaluated in an arbitrary covariant gauge. The... 相似文献
36.
Dimitri D.J. Rousseaux Xavier Drooghaag Pierre Godard Jacqueline Marchand-Brynaert 《Polymer Degradation and Stability》2010,95(3):363-3521
Polypropylene ionic thermoplastic elastomers have been prepared by melt radical grafting of maleic anhydride onto polypropylene in the presence of N-bromosuccinimide followed by neutralization of the resulting elastomeric grafted polypropylene using sodium salts. Sodium hydroxide and sodium acetate were compared in aqueous solution, as anhydrous or hydrated powders. The neutralization reaction was followed by Fourier transform infrared spectroscopy, allowing the development of a method to determine the effective neutralization degree. Important physical changes were recorded upon neutralization. Especially thermal stability, shear storage modulus and complex viscosity in the flow region were largely increased as a function of the neutralization degree. 相似文献
37.
Dimitri D.J. Rousseaux 《Polymer Degradation and Stability》2010,95(7):1194-451
Sodium-montmorillonite was intercalated by carboxylate salts to prepare carboxylate clays. The intercalation of sodium acetate doubles the clay basal spacing and no degradation of the carboxylate clay is noticed in the extrusion temperature range. These carboxylate clays were used to synthesize polypropylene-graft-maleic anhydride (PP-g-MA)/clay nanocomposites. Nanocomposites were also produced by a one-pot process using in situ prepared carboxylate clay. The carboxylate salts within the clay layers partially neutralize the maleic anhydride groups of the PP-g-MA matrix, in situ during the melt compounding. The ionic groups of the partially neutralized polymer offer favourable interactions with the clay, hence reinforcing the interfacial bond between the polymer and the clay and improving the composite properties. The use of carboxylate clay clearly improves the clay dispersion into the PP-g-MA matrix and improves the nanocomposite’s thermal and rheological properties. 相似文献
38.
Zoran S. Marković Jasmina M. Dimitrić Marković Ćemal B. Doličanin 《Theoretical chemistry accounts》2010,127(1-2):69-80
The extensive theoretical study of the interaction of one of the most abundant and reactive flavonols, quercetin, with hydroperoxy radical (HOO·), using the M052X/6-31 + Gd, p level of theory, was performed. Results indicating that quercetin is not a planar molecule are in accord with the X-ray analysis. The applied method successfully reproduces the bond dissociation enthalpy, and reveals that the reaction of quercetin with the hydroperoxy radical is governed by a hydrogen atom transfer mechanism. It is confirmed that the 3′OH and 4′OH are the most reactive sites, and that the reaction in the 3′OH position is faster than that in the 4′OH position. 相似文献
39.
40.