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排序方式: 共有207条查询结果,搜索用时 15 毫秒
61.
Orla A. McNamara N. Rachael Buckley Patrick O'Leary Francis Harrington Norma Kelly Sarah O'Keeffe Angela Stack Shane O'Neill Simon E. Lawrence Catherine N. Slattery Anita R. Maguire 《Tetrahedron》2014
Intramolecular rhodium(II)-catalysed aromatic addition (Buchner) reactions of a range of α- and β-substituted α-diazoketones are reported. Both steric and electronic effects are evident for the aromatic additions investigated. In general, highly efficient aromatic addition is achieved through use of rhodium carboxylates bearing electronegative ligands, such as rhodium trifluoroacetate, while aromatic addition employing rhodium catalysts with more electron-donating ligands, such as rhodium caprolactam, is less efficient. Excellent levels of diastereoselectivity are possible for this process in the presence of rhodium acetate and rhodium caprolactam, however, a reduction in diastereocontrol is generally associated with use of the more reactive, electronegative catalysts. Interestingly, these catalyst effects can be overcome through the steric effects of the substituents on the α-diazoketone substrates, with the presence of sterically bulky substituents at the 2- or 3-position rendering the aromatic addition essentially catalyst independent in terms of efficiency and diastereocontrol. 相似文献
62.
Norma Sbarbati Nudelman Cecilia E. Silvana Alvaro 《Journal of Physical Organic Chemistry》2011,24(11):1067-1071
Inter‐ and intramolecular hydrogen bonding play an important role in determining the arrangement, physical properties, and reactivity of a great diversity of structures in chemical and biological systems. Several aromatic nucleophilic substitutions (ANS) in nonpolar aprotic, (non‐HBD), solvents recently studied in our laboratory have demonstrated the importance of self‐association of amines by hydrogen‐bond interactions. In this paper, we describe 1H‐NMR studies carried out at room temperature on bi‐ and polyfunctionalized amines, namely: N‐(3‐amino‐1‐propyl)morpholine (3‐APMo), histamine, 2‐guanidinobenzimidazole (2‐GB), 1,2‐diaminoethane (EDA), 3‐dimethylamino‐l‐propylamine (DMPA), and 1‐(2‐aminoethyl)piperidine (2‐AEPip). By 1H‐NMR measurements of amine solutions at variable concentrations we have shown that 3‐APMo, histamine and 2‐GB are able to form a six‐membered ring by intramolecular hydrogen bonding, while EDA, DMPA, and 2‐AEPip form dimers by intermolecular hydrogen bonds. Likewise, variable concentration 1H‐NMR studies allowed estimation of the corresponding equilibrium constants for the dimerization. These results are correlated with experimental kinetic results of ANS, confirming hereto the relevance of the “dimer mechanism” in reactions involving these amines. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
63.
Liudas Giraitis Murad S. Taqqu Norma Terrin 《Probability Theory and Related Fields》1998,110(3):333-367
Summary. Let (X
t
,t∈Z) be a linear sequence with non-Gaussian innovations and a spectral density which varies regularly at low frequencies. This
includes situations, known as strong (or long-range) dependence, where the spectral density diverges at the origin. We study
quadratic forms of bivariate Appell polynomials of the sequence (X
t
) and provide general conditions for these quadratic forms, adequately normalized, to converge to a non-Gaussian distribution.
We consider, in particular, circumstances where strong and weak dependence interact. The limit is expressed in terms of multiple
Wiener-It? integrals involving correlated Gaussian measures.
Received: 22 August 1996 / In revised form: 30 August 1997 相似文献
64.
Norma Ortiz 《Journal of Mathematical Analysis and Applications》2005,305(2):513-527
This note uses Clarke's decoupling technique to obtain necessary conditions for the generalized problem of Bolza with Lipschitz continuously varying delay in both the state and velocity variables. 相似文献
65.
The synthesis and electronic and IR spectra of CuL2(NO3)2 and CuL4(NO3)2, where L stands for morpholine, are described. The thermal behaviour of the complexes is also discussed. The solids contain distorted octahedral Cu(II) bonded to morpholine through nitrogen only. In aqueous solutions only the species CuL3(aq)2+ is stable in the concentration range 0.5 < [L] < 2 mol dm?3, its stability constant being log K3 = 14.64 ± 0.15 at 25.00°C and ionic strength 1 mol dm?3; at very high morpholine concentrations, 3.4 < [L] < 6 mol dm?3, evidence is also found for CuL4(aq)2+, the value log K4 = 15.5 being estimated. The aggressiveness of morpholine-H2O2 towards metallic copper is compared with that of ammonia-H2O2, both on thermodynamics and kinetics grounds; experimental results seems to be dominated by kinetics factors. The relevance of these results to water treatment in secondary systems of nuclear power reactors is discussed. 相似文献
66.
Morales W Cason M Aina O de Tacconi NR Rajeshwar K 《Journal of the American Chemical Society》2008,130(20):6318-6319
The energy payback time associated with the semiconductor active material is an important parameter in a photovoltaic solar cell device. Thus lowering the energy requirements for the semiconductor synthesis step or making it more energy-efficient is critical toward making the overall device economics more competitive relative to other nonpolluting energy options. In this communication, combustion synthesis is demonstrated to be a versatile and energy-efficient method for preparing inorganic oxide semiconductors such as tungsten trioxide (WO3) for photovoltaic or photocatalytic solar energy conversion. The energy efficiency of combustion synthesis accrues from the fact that high process temperatures are self-sustained by the exothermicity of the combustion process, and the only external thermal energy input needed is for dehydration of the fuel/oxidizer precursor mixture and bringing it to ignition. Importantly, we show that, in this approach, it is also possible to tune the optical characteristics of the oxide semiconductor (i.e., shift its response toward the visible range of the electromagnetic spectrum) in situ by doping the host semiconductor during the formative stage itself. As a bonus, the resultant material shows enhanced surface properties such as markedly improved organic dye uptake relative to benchmark samples obtained from commercial sources. Finally, this synthesis approach requires only very simple equipment, a feature that it shares with other "mild" inorganic semiconductor synthesis routes such as sol-gel chemistry, chemical bath deposition, and electrodeposition. The present study constitutes the first use of combustion synthesis for preparing WO3 powder comprising nanosized particles. 相似文献
67.
A comprehensive study of the triosephosphate isomerase from the parasite Trypanosoma cruzi (TcTIM) in water, in decane, and in three water/decane mixtures was performed using molecular dynamics (MD) simulations in a time scale of 40 ns. The structure and dynamics of the enzyme, as well as the solvent molecules' distribution and mobility, were analyzed in detail. In the presence of decane, the amplitudes of the most important internal motions of the enzyme backbone were observed to depend on the solvent concentration: the higher the water concentration, the greater the amplitudes. Contrary to this trend, the amplitudes of the TcTIM motions in pure water were similar to those of the simulation with the lowest water concentration. The enzyme was observed to be almost motionless in pure decane due to a sharp increase of the number of intramolecular hydrogen bonds. This caused a contraction of the enzyme structure accompanied by a loss of secondary structure and of a decrease of the hydrophilic solvent accessible surface. A similar behavior, although to a lesser extent, was observed in the simulation at the lowest water concentration. Our results suggest that the presence of decane molecules located at specific sites of the enzyme might accelerate its internal movements, although a minimum number of water molecules is needed for the protein to keep its structure and dynamics. Altogether, this work provides new insight into protein and water behavior in organic solvents as well as into the dynamics of TcTIM itself. 相似文献
68.
Sánchez-Flores NA Pacheco-Malagón G Pérez-Romo P Armendáriz H Valente JS Guzmán-Castillo Mde L Alcaraz J Baños L Blesa JM Fripiat JJ 《Journal of colloid and interface science》2008,323(2):359-364
Well crystallized silicalite-1 has been obtained from three sources of amorphous silica, namely, rice hull ashes, commercial Davisil, and a fume silica from Aldrich. The silicas were first dissolved in glycerol according to a recently described reaction. This reaction transforms rapidly and efficiently large surface area silicates into poly-alkoxide gels. It can be schematized as an etherification of an alcohol function of glycerol by the weakly acid surface silanol groups. The facile hydrolysis of the alkoxide permits the preparation of relatively pure and reactive silica, keeping the mesoporous character of the parent starting material. We insist on the mesoporous character of the solids obtained upon hydrolyzing the organo-silicic gel because we believe the gel plays a role of template in the secondary synthesis of mesoporous structures. The hydrolysis is carried out in presence of a structure directing agent, namely tetra-propylammonium hydroxide, TPAOH. After aging, the residue is dried and calcined. The first advantage of using the organo-silicic gel is probably related to the high degree of depolymerization of silica, witness by the C/Si ratio. The second one, more subtle to define, is to provide an intermediate silica with hydrophilic a hydrophobic regions, interfering differently with the surfactant. After calcination at 500 degrees C, well crystallized silicalite-1 is obtained. The texture of the starting silica influences the textural characteristics of the final silicalite-1. 相似文献
69.
Synthesis of 1,3‐oxazole and benzimidazole pendant groups from polyacrylonitrile hydrolysis products
Isabel N. Vega Leonardo Sánchez Norma D'Accorso 《Journal of heterocyclic chemistry》2008,45(2):429-433
70.
Nadesalingam MP Mukherjee S Somasundaram S Chenthamarakshan CR de Tacconi NR Rajeshwar K Weiss AH 《Langmuir : the ACS journal of surfaces and colloids》2007,23(4):1830-1834
Vacuum anneal induced changes in the surface layers of electrodeposited copper(I) oxide (Cu2O) were probed by time-of-flight positron annihilation induced Auger electron spectroscopy (TOF-PAES) and by electron induced Auger electron spectroscopy (EAES). Large changes in the intensity of the Cu PAES intensity resulting from isochronal in situ vacuum anneals made at increasing temperatures indicated that, before thermal treatment, the surface was completely covered by a carbonaceous overlayer and that this layer was removed, starting at a temperature between 100 and 200 degrees C, to expose an increasing amount of Cu in the top layer as the anneal temperature was increased. The thickness of this overlayer was estimated to be approximately 4 A based on analysis of the EAES data, and its variation with the thermal anneal temperature was mapped. This study demonstrated the order-of-magnitude enhancement in the sensitivity of PAES to the topmost surface layer in Cu2O relative to the EAES counterpart; factors underlying this contrast are discussed. Finally, the implications of ultrathin carbon layers on semiconductor surfaces are discussed. 相似文献