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81.
The kinetics of the reaction between permanganate ion and triethylamine in aqueous phosphate buffers (pH 6–8) has been studied. The reaction is autocatalytic, a soluble form of colloidal manganese dioxide temporarily stabilized in solution by adsorption of phosphate ions on its surface having been identified as the autocatalyst. The reaction is also catalyzed by gum arabic. Mechanisms in agreement with the kinetic data are proposed for the reaction pathways taking place in the bulk solution (for both triethylamine and triethylammonium ion as reductants) and on both the gum arabic and manganese dioxide colloidal particles. 相似文献
82.
Od n Arjona Alfonso de Dios Roberto Fernandez de la Pradilla Araceli Mallo Joaquin Plumet 《Tetrahedron》1990,46(24):8179-8186
The regioselectivity of the cycloaddition reactions between acetonitrile oxide and a number of 7-oxabicyclo[ 2.2.1]hept-5-enes is discussed. Acetonitrile oxide adds with complete regioselectivity to 2-endo-acetoxy-5-halo-7-oxabicyclo[2.2.1]hept-5-en-2-exo-carbonitriles. Kinetic measurements indicate a more polar transition state for 2a than for the unsubstituted substrate la 相似文献
83.
Dendrimer-encapsulated Pd nanoparticles as aqueous, room-temperature catalysts for the Stille reaction 总被引:2,自引:0,他引:2
Garcia-Martinez JC Lezutekong R Crooks RM 《Journal of the American Chemical Society》2005,127(14):5097-5103
We report that dendrimer-encapsulated Pd nanoparticles having a diameter of approximately 1.7 nm are effective and general catalysts for coupling aryl halides to organostannanes (the Stille reaction) under mild conditions. The significant results of this study are that the Stille reaction is catalyzed by dendrimer-encapsulated Pd nanoparticles in very good yield, in aqueous solution at 23 degrees C, and using only 0.100 atom % of Pd as catalyst. 相似文献
84.
Direct observation of growth defects in zeolite beta 总被引:2,自引:0,他引:2
Wright PA Zhou W Pérez-Pariente J Arranz M 《Journal of the American Chemical Society》2005,127(2):494-495
High-resolution transmission electron microscopy reveals linear, "double pore" defects in the important zeolite beta. Structural interpretation of these defects gives evidence for the mechanism by which the zeolite crystallizes. 相似文献
85.
Kim YG Garcia-Martinez JC Crooks RM 《Langmuir : the ACS journal of surfaces and colloids》2005,21(12):5485-5491
The electrochemical properties of Au and Pd monolayer-protected clusters (MPCs), prepared by dendrimer-templating and subsequent extraction, are described. Differential pulse voltammetry was used to estimate the size of the MPCs, and the results were compared to microscopic data and calculated values. Purification of the extracted Au and Pd nanoparticles was not required to obtain well-defined differential pulse voltammetry peaks arising from quantized double-layer charging. The calculated sizes of the nanoparticles were essentially identical to those determined from the electrochemical data. The capacitance of the particles was independent of the composition of core metal. Transmission electron microscopy data overestimated the size of the smallest Pd nanoparticles because of inadequate point-to-point resolution. 相似文献
86.
87.
Takagi M Cabrera-Crespo J Baruque-Ramos J Zangirolami TC Raw I Tanizaki MM 《Applied biochemistry and biotechnology》2003,110(2):91-100
Haemophilus influenzae type b (Hib) causes invasive infections in infants and young children. Vaccines consisting of Hib capsular polysaccharide
(polymer of ribosylribitol phosphate [PRP]) conjugated to a protein are effective in the prevention of such infections. The
production of capsular polysaccharide type b was studied in three cultivation conditions: single, glucose pulse, and repeated
batch. Specific polysaccharide production (Y
p/x
) was calculated for all experiments, showing the following values: 67 (single-batch cultivation), 71 (glucose pulse), 75
(repeated-batch cultivation, first batch), and 87 mg of PRP/g of dry cell weight (DCW) (repeated-batch cultivation, second
batch). Biomass concentration reached ∼1.8 g of DCW/L, while polysaccharide concentration was about ∼132 mg/L in the three
fermentation runs. Polysaccharide synthesis is associated with cell growth in all studied conditions as established by Kono's
analysis and Luedeking-Piret's model. 相似文献
88.
Serrano E Pozo OJ Beltrán J Hernández F Font L Miquel M Aragon CM 《Rapid communications in mass spectrometry : RCM》2007,21(7):1221-1229
Acetaldehyde, the main biological metabolite of ethanol, is nowadays considered to mediate some ethanol-induced effects. Previous studies on alcohol effect attenuation have shown that D-(-)-penicillamine (3-mercapto-D-valine), a thiol amino acid, acts as an effective agent for the inactivation of acetaldehyde. In the study reported here, laboratory rats were treated with ethanol and D-(-)-penicillamine at different doses looking for the interaction (in vivo) of D-(-)-penicillamine with metabolically formed acetaldehyde following a condensation reaction to form the stable adduct (4S,2RS)-2,5,5-trimethylthiazolidine-4-carboxylic acid (TMTCA). A novel and rapid procedure based on liquid chromatography coupled to tandem mass spectrometry (LC/MS/MS) was developed for quantification and reliable identification of TMTCA in different rat tissues, including plasma, liver and brain. Firstly, plasma was obtained from whole blood. Then, proteins were precipitated from plasma, brain and liver extracts with acetonitrile and the clarified extracts diluted 10-fold. A 20 microL aliquot of the final extracts was then analyzed using an Atlantis C18 5 microm, 100x2 mm column which was connected to the electrospray source of a LC/triple quadrupole mass spectrometer. The analyte was detected in positive ion mode acquiring four MS/MS transitions in selected reaction monitoring (SRM) mode.The method has been validated and it has proved to be fast, reliable and sensitive. The accuracy and precision were evaluated by means of recovery experiments from plasma, liver and brain samples fortified at two concentration levels obtaining satisfactory recoveries in all cases: 95 and 105% in plasma (at 10 and 100 ng/mL, respectively), 79 and 89% in brain (100 and 1000 ng/g), 85 and 99% in liver (100 and 1000 ng/g). Precision, expressed as repeatability, was in all tissues analyzed lower than 17% at the two concentrations tested. The estimated detection limits were 1 ng/mL in plasma, 4 ng/g in brain and 5 ng/g in liver. The limit of quantitation objective (the lowest concentration that was validated with acceptable results) was set up at 10 ng/mL for plasma and 100 ng/g for brain and liver tissue.The reliable identification of the analyte was ensured by the acquisition of four transitions and by their ion abundance ratio measurement. Due to its excellent selectivity and sensitivity, the method developed in this work provides an excellent tool for the specific determination of this cyclic amino acid in biological samples. 相似文献
89.
The reaction between chromium(VI) and L-ascorbic acid has been studied by spectrophotometry in the presence of aqueous citrate buffers in the pH range 5.69–7.21. The reaction is slowed down by an increase of the ionic strength. At constant ionic strength, manganese(II) ion does not exert any appreciable inhibition effect on the reaction rate. The rate law found is where Kp is the equilibrium constant for protonation of chromate ion and kr is the rate constant for the redox reaction between the active forms of the oxidant (hydrogenchromate ion) and the reductant (L-hydrogenascorbate ion). The activation parameters associated with rate constant kr are Ea = 20.4 ± 0.9 kJ mol?1, ΔH≠ = 17.9 ± 0.9 kJ mol?1, and ΔS≠=?152 ± 3 J K?1 mol?1. The reaction thermodynamic magnitudes associated with equilibrium constant Kp are ΔH0 = 16.5 ± 1.1 kJ mol?1 and ΔS0 = 167 ± 4 J K?1 mol?1. A mechanism in accordance with the experimental data is proposed for the reaction. © 1993 John Wiley & Sons, Inc. 相似文献
90.
Yolanda Orenes Alejandro Rabasa Jesus Javier Rodriguez-Sala Joaquin Sanchez-Soriano 《Entropy (Basel, Switzerland)》2021,23(7)
In the machine learning literature we can find numerous methods to solve classification problems. We propose two new performance measures to analyze such methods. These measures are defined by using the concept of proportional reduction of classification error with respect to three benchmark classifiers, the random and two intuitive classifiers which are based on how a non-expert person could realize classification simply by applying a frequentist approach. We show that these three simple methods are closely related to different aspects of the entropy of the dataset. Therefore, these measures account somewhat for entropy in the dataset when evaluating the performance of classifiers. This allows us to measure the improvement in the classification results compared to simple methods, and at the same time how entropy affects classification capacity. To illustrate how these new performance measures can be used to analyze classifiers taking into account the entropy of the dataset, we carry out an intensive experiment in which we use the well-known J48 algorithm, and a UCI repository dataset on which we have previously selected a subset of the most relevant attributes. Then we carry out an extensive experiment in which we consider four heuristic classifiers, and 11 datasets. 相似文献