Midrapidity neutral-pion production in proton-proton collisions at square root s = 200 GeV

The invariant differential cross section for inclusive neutral-pion production in p+p collisions at sqrt[s]=200 GeV has been measured at midrapidity (|eta|<0.35) over the range 1相似文献   

Elliptic flow of identified hadrons in Au+Au collisions at sqrt sNN =200 GeV

The anisotropy parameter (v(2)), the second harmonic of the azimuthal particle distribution, has been measured with the PHENIX detector in Au+Au collisions at sqrt[s(NN)]=200 GeV for identified and inclusive charged particle production at central rapidities (|eta|<0.35) with respect to the reaction plane defined at high rapidities (|eta|=3-4 ). We observe that the v(2) of mesons falls below that of (anti)baryons for p(T)>2 GeV/c, in marked contrast to the predictions of a hydrodynamical model. A quark-coalescence model is also investigated.     

Scaling properties of proton and antiproton production in sqrt[s(NN)]=200 GeV Au+Au collisions

We report on the yield of protons and antiprotons, as a function of centrality and transverse momentum, in Au+Au collisions at sqrt[s(NN)]=200 GeV measured at midrapidity by the PHENIX experiment at the BNL Relativistic Heavy Ion Collider. In central collisions at intermediate transverse momenta (1.5相似文献   

Large magnetic entropy change in Nd<Subscript>2/3</Subscript>Sr<Subscript>1/3</Subscript>MnO<Subscript>3</Subscript>

In order to search for new materials for the application of magnetic refrigeration, the polycrystalline perovskite compound Nd_2/3Sr_1/3MnO₃ was prepared by a solid-state method. The dependence of the magnetization on the applied field and temperature was measured near the Curie temperature. In terms of Maxwells equation, the temperature dependence of the absolute value of the isothermal magnetic entropy change |S_M| at various applied fields from 1 T to 5 T was determined. The results showed that a large magnetic entropy change was observed in this compound. The maximum magnetic entropy change |S_M^max|can reach 3.25 J/kgK with an applied field of 1 T at the Curie temperature of 257.5 K, which equals that of Gd. At 5 T applied field, it is 7.57 J/kgK. Such good magnetocaloric properties make this compound a promising candidate for the application of magnetic refrigeration in the room-temperature range. PACS 74.25.Ha; 75.30.-m; 75.30.Sg; 75.50.-y; 75.60.-d     

Temperature-controlled catalytic growth of one-dimensional gallium nitride nanostructures using a gallium organometallic precursor

Catalytic growth of 1-D GaN nanostructures is achieved at temperatures from 550 to 850 °C using NH₃ and gallium acetylacetonate. Structural characterization of the 1-D GaN nanostructures by HRTEM shows that straight GaN nanowires, needle-like nanowires (nanoneedles), and bamboo-shoot-like nanoneedles are synthesized at 750, 650, and 550 °C, respectively. In addition to selecting a proper catalyst, providing sufficient precursors has been demonstrated to be a crucial factor for the low-temperature growth of 1-D GaN nanostructures via the VLS mechanism. Possible mechanisms for forming nanoneedles at low temperatures are proposed. PACS 61.46.+w; 68.65.–k; 81.07.–b     

Quantitative comparison of kinetic stabilities of metallomacrocycle-based rotaxanes

Four mononuclear metallomacrocycles with identical cavities but different transition metals (Os(VI), Pd(II), Pt(II), and Re(I)) were prepared. With these metallomacrocycles, the corresponding rotaxanes 2-Os, 2-Pd, 2-Pt, and 2-Re were self-assembled by hydrogen-bonding interactions. The kinetic stabilities of the rotaxanes were determined quantitatively and compared with each other by (1)H NMR spectroscopic techniques, including two-dimensional exchange spectroscopy (2D-EXSY) experiments. The activation free energies (DeltaG( not equal )) for the exchange between the rotaxanes 2-Os, 2-Pd and 2-Pt and their free components were determined to be 15.5, 16.0, and 16.4 kcal mol(-1), respectively. These magnitudes imply that the rotaxanes 2-Os, 2-Pd and 2-Pt are kinetically labile at room temperature and exist only as equilibrium mixtures with free components in solution. In contrast, the rotaxane 2-Re is kinetically stable enough to be isolated in pure form by silica gel chromatography under ordinary laboratory conditions. However, at higher temperatures (>60 degrees C) 2-Re was slowly disassembled into its components until the equilibrium was established. The rate constants were measured at three different temperatures, and the Eyring plot yielded the activation enthalpy DeltaH(not equal)=35 kcal mol(-1) and the activation entropy DeltaS(not equal)=27 eu for the disassembly of the rotaxane 2-Re in Cl(2)CDCDCl(2). These thermodynamic parameters gave the activation free energy DeltaG(not equal)(off)=27.1 kcal mol(-1) at 25 degrees C. Consequently, 2-Re is one example of a novel metallomacrocycle-based rotaxane that contains a coordination bond with enough strength to allow both for isolation in pure form around room temperature and for self-assembly at higher temperatures.     

Lasonolide A: structural revision and total synthesis

The proposed structure of lasonolide A was synthesized employing radical cyclization reactions of beta-alkoxyacrylates for preparation of the tetrahydropyranyl units A and B, but the spectroscopic data did not match those of the natural product. Both enantiomers of a revised structure featuring 17E,25Z double bonds were synthesized, and the (-)-isomer was found to be the biologically active enantiomer.     

UCST behavior in blend systems of isotactic polystyrene/poly(4-methyl styrene) in comparison with atactic polystyrene/poly(4-methyl styrene)

Blends of poly(4-methylstyrene) (P4MS) with polystyrene (iPS) exhibit an upper critical solution temperature (UCST) at ca. 270 °C. The overall phase behavior and trend of variation in the phase diagrams for the iPS/P4MS blend system with respect to molecular weights of iPS is similar to an earlier studied blend system of atactic PS with P4MS. This suggests that the crystal phase-related tacticity and crystallinity in iPS does not influence the amorphous phase behavior and UCST behavior of the polymer mixtures. A model based on a modified Flory-Huggins expression for binary interactions was constructed to describe the UCST-type behavior of the iPS/P4MS blend and to compare the qualitative effects of molecular weights on iPS/P4MS blend vs. atactic PS/P4MS systems.     

Synthesis of novel 2,6-disubstituted-3,4-dimethylidene tetrahydropyrans via Prins-type cyclization

Synthesis of novel substituted tetrahydropyrans with adjacent exo-methylene groups at the C3 and C4 positions via Prins-type cyclization has been described.     

Polymeric enzyme mimics: catalytic activity of ribose-containing polymers for a phosphate substrate

The polymers containing ribose rings: poly(5'-acrylamido-5'-deoxy-1',2'-O-isopropylidene-alpha-D-ribose) (11), poly(5'-acrylamido-5'-deoxy-alpha-D-ribose) (12) and poly(5'-acrylamido-5'-deoxy-1'-O-methyl-D-ribose) (13) were prepared as enzyme mimics. Polymers 12 and 13 with free vic-cis-diol groups catalyzed the hydrolysis of phosphodiester (ethyl p-nitrophenyl phosphate and N-methylpyridinium 4-tert-butylcatechol cyclic phosphate) and phosphomonoester substrates with a rate acceleration of 10 approximately equal to 10(3) compared with the uncatalyzed reaction. They also catalyzed the reverse reactions, i.e., the esterification of phosphomonoester to phosphodiester and the phosphorylation of alcohols with phosphate ions. The catalytic activity was attributable to the vic-cis-diols of riboses on polymer chains, which formed hydrogen bonds with two phosphoryl oxygen atoms of phosphates so as to activate the phosphorus atoms to be attacked by nucleophiles. The catalytic activity was negligible for polymer 11 where vic-cis-diol groups were blocked with isopropylidene groups. The catalytic activity was attributable to the vic-cis-diols of riboses on polymer chains, which formed hydrogen bonds with two phosphoryl oxygen atoms of phosphates so as to activate the phosphorus atoms to be attacked by nucleophiles.