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1.
Understanding the end state of black hole evaporation, the microscopic origin of black hole entropy, the information loss paradox, and the nature of the singularity arising in gravitational collapse - these are outstanding challenges for any candidate quantum theory of gravity. Recently, a midisuperspace model of quantum gravitational collapse has been solved using a lattice regularization scheme. It is shown that the mass of an eternal black hole follows the Bekenstein spectrum, and a related argument provides a fairly accurate estimate of the entropy. The solution also describes a quantized mass-energy distribution around a central black hole, which in the WKB approximation, is precisely Hawking radiation. The leading quantum gravitational correction makes the spectrum non-thermal, thus providing a plausible resolution of the information loss problem. 相似文献
2.
W. A. Rodrigues Jr. Q. A. G. de Souza J. Vaz Jr. P. Lounesto 《International Journal of Theoretical Physics》1996,35(9):1849-1900
In this paper we study Dirac-Hestenes spinor fields (DHSF) on a four-dimensional Riemann-Cartan spacetime (RCST). We prove that these fields must be defined as certain equivalence classes of even sections of the Clifford bundle (over the RCST), thereby being certain particular sections of a new bundle named the spin-Clifford bundle (SCB). The conditions for the existence of the SCB are studied and are shown to be equivalent to Geroch's theorem concerning the existence of spinor structures in a Lorentzian spacetime. We introduce also the covariant and algebraic Dirac spinor fields and compare these with DHSF, showing that all three kinds of spinor fields contain the same mathematical and physical information. We clarify also the notion of (Crumeyrolle's) amorphous spinors (Dirac-Kähler spinor fields are of this type), showing that they cannot be used to describe fermionic fields. We develop a rigorous theory for the covariant derivatives of Clifford fields (sections of the Clifford bundle, CB) and of Dirac-Hestenes spinor fields. We show how to generalize the original Dirac-Hestenes equation in Minkowski spacetime for the case of RCST. Our results are obtained from a variational principle formulated through the multiform derivative approach to Lagrangian field theory in the Clifford bundle. 相似文献
3.
4.
The kinetics of the oxidation of five catecholamines viz., dopamine (A), L-dopa (B), methyldopa (C), epinephrine (D) and norepinephrine
(E) by sodium N-chloro-p-toluenesulfonamide or chloramine-T (CAT) in presence of HClO4 was studied at 30±0.1 °C. The five reactions followed identical kinetics with a first-order dependence on [CAT]
o
, fractional-order in [substrate]
o
, and inverse fractional-order in [H+]. Under comparable experimental conditions, the rate of oxidation of catecholamines increases in the order D>E>A>B>C. The
variation of ionic strength of the medium and the addition of p-toluenesulfonamide or halide ions had no significant effect
on the reaction rate. The rate increased with decreasing dielectric constant of the medium. The solvent isotope effect was
studied using D2O. A Michaelis-Menten type mechanism has been suggested to explain the results. Equilibrium and decomposition constants for
CAT-catecholamine complexes have been evaluated. CH3C6H4SO2NHCl of the oxidant has been postulated as the reactive oxidizing species and oxidation products were identified. An isokinetic
relationship is observed with β=361 K, indicating that enthalpy factors control the reaction rate. The mechanism proposed
and the derived rate law are consistent with the observed kinetics. 相似文献
5.
Juliana Vaz Bevilaqua Lidia M. Lima Aline Gomes Cunha Eliezer J. Barreiro Tito L. M. Alves Lucia Moreira Campos Paiva Denise M.Guimarães Freire 《Applied biochemistry and biotechnology》2005,121(1-3):117-128
The last step of the production of four phthalimide-derived acids, designed to act as antiasthma drugs, was performed by enzymatic hydrolysis of the respective methyl or ethyl esters. The esters 4-ethyl-[2-(1,3-dioxo-1,3-dihydro-2-isoindoylyl)]-phenoxyacetic methyl ester (PHT-MET), 4-ethyl-[2-(1,3-dioxo-1,3-dihydro-2-isoindoylyl)]-phenoxyacetic ethyl ester, 4-(1,3-dioxo-1,3-dihydro-2-isoindoylyl)-phenoxyacetic ethyl ester, and 2-(1,3-dioxo-1, 3-dihydro-2-isoindoylyl)-phenoxyacetic ethyl ester were hydrolyzed by immobilized lipase. The enzymatic reaction could be used only to produce the desired 4-substituted compounds. The best result that was found to hydrolysis of PHT-MET, and, therefore, that ester was selected for optimization experiments in a three-phase system. Reactions were performed with solid biocatalyst (Lipozyme® RM IM), organic solvent phase (ethyl acetate), and aqueous phase (saturated Na2CO3 solution). To optimize the reaction conditions, an experimental design optimization procedure was used. The variables studied were the amount of enzyme, the temperature, and the volume of the aqueous solution. Time course experiments were then performed for different initial enzyme concentrations (0.5, 0.9, and 1.4 UH/mL of solvent). The optimized reaction conditions found were 20 mg of Lipozyme (0.9 UH/mLsolvent) and 5.0 mL of Na2CO3(sat) at 40°C for 6 h. 相似文献
6.
AntnioF.DeC. Alcntara MariaG.F. Vaz HumbertoO. Stumpf Dorila Pil‐Veloso WagnerB. DeAlmeida 《Helvetica chimica acta》2004,87(2):425-438
Conformational studies of 1,3‐dihydroxy‐4,4,5,5‐tetramethyl‐2‐(pyridin‐1‐yl)imidazolidine ( 1a ) and 1,3‐dihydroxy‐4,4,5,5‐tetramethyl‐2‐(pyridin‐3‐yl)imidazolidine ( 1b ), carried out by using 1D 1H‐ and 13C‐NMR and 2D HMQC, HMBC, and NOESY experiments and with the aid of theoretical calculations, indicate that the OH groups are trans to the pyridinyl substituent. Because the two 1H‐NMR signals of the Me groups are distinguishable and do not change between 290 and 380 K, it is proposed that 1a and 1b have each only one conformation in this temperature range. This behavior was not found with 1,3‐dihydroxy‐4,4,5,5‐tetramethyl‐2‐(pyridin‐2‐yl)imidazolidine ( 1c ) because its Me 1H‐NMR signals cross over at 300 K. Hence, more than one conformation must be present, beyond those produced by simple inversions. Theoretical calculations including temperature and solvent effects were performed to provide further information on the conformational analysis and to help to assign the NMR data. The combination of NMR measurements and quantum‐chemical calculations is shown to be a very promising strategy for conformational analysis studies in solution. 相似文献
7.
The kinetics of oxidation of 2-thiouracil (TU) by sodium N-bromobenzenesulphonamide or bromamine-B (BAB) have been studied in an HCl medium, catalyzed by RuCl3, and in a NaOH media with OsO4 as catalyst, at 313 K. The stoichiometry and oxidation products are the same in both cases, but their kinetic patterns were found to be different. In acid medium the rate shows a first order dependence in each of [BAB] and [TU], and is dependent on [RuIII]. The reaction rate is inversely dependent on [H+]. In alkaline medium, the rate is first order in [BAB] and in [OsVIII] and zero order in [TU]. The reaction rate is dependent on [NaOH]. Activation parameters have been evaluated, solvent isotope effects have been studied in D2O medium, and equilibrium constants were calculated. The activation parameters and rate constants indicate that the catalytic efficiency is: OsVIII > RuIII. The proposed mechanisms and the derived rate laws are consistent with the observed kinetics. 相似文献
8.
9.
The compound bis(di-micro-ethoxo-bis(3,5-di-tert-butylsemiquinonato)dicopper(II)) has been synthesized and its structure was determined by single-crystal X-ray diffraction. The compound crystallizes in the monoclinic system, space group C2/c, with a = 37.736(8) A, b = 9.173(2) A, c = 23.270(5) A, beta = 122.24(3) degrees. The structure can be described as a Lewis adduct between two dinuclear [Cu(DBSQ)(C(2)H(5)O)](2) units (DBSQ = 3,5-di-tert-butyl-semiquinonato). The temperature dependence of the magnetic susceptibility was efficiently analyzed by a combined DFT/experimental approach, showing that a rather strong ferromagnetic interaction exists between the DBSQ(-) and the copper(II) ions modulated by an antiferromagnetic interaction between the two copper(II) ions of the dinuclear units. Weak antiferromagnetism between the two units in the unit cell was measured. 相似文献
10.
Charge transport properties of polyimide films implanted with 80 keV Co ions at two different fluences (series I: 1.25 × 10^17 ions/cm^2, series Ⅱ: 1.75 × 10^17 ions/cm^2) are studied in detail. For series I, the temperature dependence of surface resistivity fits Mott's equation very well. It is on the insulating side of the insulator-metal transition (IMT). However, for series Ⅱ, the temperature dependence of surface resistivity is not in agreement with Mott's equation. It is on the metallic side of lMT. The magnetotransport properties of these two series are also studied. No significant magnetoresistive effect is observed for series I at both 5 K and 300 K. For series Ⅱ, an obvious magnetoresistive effect is observed at 5 K, while there is no magnetoresistive effect at 300 K. Rutherford backscattering spectrometry (RBS) is applied to confirm the actual fluence for these two series. 相似文献