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961.
Relative rate constants for the gas-phase reactions of OH radicals with a series of alkyl nitrates have been determined at 299 ± 2 K, using methyl nitrite photolysis in air as a source of OH radicals. Using a rate constant for the reaction of OH radicals with cyclohexane of 7.57 × 10?12 cm3/molec·s, the rate constants obtained are (× 1012 cm3/molec·s): 2-propyl nitrate, 0.18 ± 0.05; 1-butyl nitrate, 1.42 ± 0.11; 2-butyl nitrate, 0.69 ± 0.10; 2-pentyl nitrate, 1.87 ± 0.12; 3-pentyl nitrate, 1.13 ± 0.20; 2-hexyl nitrate, 3.19 ± 0.16; 3-hexyl nitrate, 2.72 ± 0.22; 3-heptyl nitrate, 3.72 ± 0.43; and 3-octyl nitrate, 3.91 ± 0.80. These rate constants, which are the first reported for the alkyl nitrates, are significantly lower than those for the parent alkanes, and a formula, based on the numbers of the various types of C? H bonds in the alkyl nitrates, is derived for rate constant estimation purposes. 相似文献
962.
Zakaria Benzekri Sara Sibous Houda Serrar Ali Ouasri Said Boukhris Ali Rhandour Abdelaziz Souizi 《Journal of the Iranian Chemical Society》2018,15(12):2781-2787
In this work, the condensation of aromatic aldehydes with different two-substituted aniline catalyzed by NH3(CH2)5NH3BiCl5 as heterogeneous and recyclable catalyst was reported. It was demonstrated that NH3(CH2)5NH3BiCl5 can act as an efficient and active catalyst and is reusable for six runs without a significant loss of their catalytic activity. Simple preparation of the catalyst, high catalytic activity and good reusability are noteworthy advantages of this catalytic system in the synthesis of benzoxazole, benzimidazole and benzothiazole heterocycles at room temperature under solvent-free conditions. 相似文献
963.
Nafise Modanlou Juibari Sara Tarighi 《Journal of Thermal Analysis and Calorimetry》2018,133(3):1317-1326
MnCo2O4 spinel nanoparticles (NPs) have been prepared using Aloe vera gel solution. The characterization of prepared spinel was performed applying Fourier transform infrared spectroscopy, X-ray diffraction, Raman spectroscopy, transmission electron spectroscope, scanning electron microscope and dynamic light scattering. The results manifested that the prepared nanoparticles were mainly spherical plus minor agglomeration with average size distribution between 35 and 60 nm. The catalytic activity of the prepared nanoparticles upon thermal degradation of ammonium perchlorate (AP) was evaluated applying differential scanning calorimetry and thermogravimetry instruments. MnCo2O4 nanoparticles increased the released heat of AP from 450 to 1480 J g?1 and decreased the decomposition temperature from 420 to 293 °C. The kinetic parameters obtained from Kissinger methods showed that the activation energy of AP thermal decomposition in the presence of MnCo2O4 NPs considerably decreased. Also, a mechanism has been proposed in the presence of catalyst for the process of thermal decomposition of AP. 相似文献
964.
Marie Vayer Dr. Sara P. Morcillo Jennifer Dupont Prof. Vincent Gandon Dr. Christophe Bour 《Angewandte Chemie (International ed. in English)》2018,57(12):3228-3232
The borrowing hydrogen strategy has been applied to the ethylation of imines with an air‐stable iron complex as precatalyst. This approach opens new perspectives in this area as it enables the synthesis of unsymmetric tertiary amines from readily available substrates and ethanol as a C2 building block. A variety of imines bearing electron‐rich aryl or alkyl groups at the nitrogen atom could be efficiently reductively alkylated without the need for molecular hydrogen. The mechanism of this reaction, which shows complete selectivity for ethanol over other alcohols, has been studied experimentally and by means of DFT computations. 相似文献
965.
Outstanding Methane Oxidation Performance of Palladium‐Embedded Ceria Catalysts Prepared by a One‐Step Dry Ball‐Milling Method
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Maila Danielis Dr. Sara Colussi Prof. Carla de Leitenburg Dr. Lluís Soler Prof. Jordi Llorca Prof. Alessandro Trovarelli 《Angewandte Chemie (International ed. in English)》2018,57(32):10212-10216
By carefully mixing Pd metal nanoparticles with CeO2 polycrystalline powder under dry conditions, an unpredicted arrangement of the Pd‐O‐Ce interface is obtained in which an amorphous shell containing palladium species dissolved in ceria is covering a core of CeO2 particles. The robust contact that is generated at the nanoscale, along with mechanical forces generated during mixing, promotes the redox exchange between Pd and CeO2 and creates highly reactive and stable sites constituted by PdOx embedded into CeO2 surface layers. This specific arrangement outperforms conventional Pd/CeO2 reference catalysts in methane oxidation by lowering light‐off temperature by more than 50°C and boosting the reaction rate. The origin of the outstanding activity is traced to the structural properties of the interface, modified at the nanoscale by mechanochemical interaction. 相似文献
966.
Numerical Algorithms - This paper introduces a simple variant of the power method. It is shown analytically and numerically to accelerate convergence to the dominant eigenvalue/eigenvector pair,... 相似文献
967.
Ernesto C. Tuazon William P. L. Carter Sara M. Aschmann Roger Atkinson 《国际化学动力学杂志》1991,23(11):1003-1015
The products of the reaction of the hydroxyl (OH) radical with methyl tert-butyl ether (MTBE) in NOx-air systems were identified and measured by Fourier transform infrared absorption spectroscopy and gas chromatography. The products observed, and their yields, were as follows: t-butyl formate, 76 ± 7%; formaldehyde, 37%; methyl acetate, 17 ± 2%, and acetone, 2.1 ± 0.9%, where the stated error limits represent both random (two standard deviations) and estimated systematic uncertainties. These products account for ca. 95% of the MTBE carbon reacted. Infrared absorption bands which may be due to small amounts of organic nitrate formation were observed, but organic nitrate yields could not be quantified. These data allow a chemical mechanism for the reaction of MTBE with the OH radical in the presence of NOx to be formulated. 相似文献
968.
Núñez S Antoniou D Schramm VL Schwartz SD 《Journal of the American Chemical Society》2004,126(48):15720-15729
Crystallographic studies of human purine nucleoside phosphorylase (hPNP) with several transition-state (TS) analogues in the immucillin family showed an unusual geometric arrangement of the atoms O-5', O-4', and O(P), the nucleophilic phosphate oxygen, lying in a close three-oxygen stack. These observations were corroborated by extensive experimental kinetic isotope effect analysis. We propose that protein-facilitated dynamic modes in hPNP cause this stack, centered on the ribosyl O-4' oxygen, to squeeze together and push electrons toward the purine ring, stabilizing the oxacarbenium character of the TS. As the N-ribosidic bond is cleaved during the reaction, the pK(a) values of N-7 and O-6 increase by the electron density expelled by the oxygen-stack compression toward the purine ring. Increased electron density in the purine ring improves electrostatic interactions with nearby residues and facilitates the abstraction of a proton from a solvent proton or an unidentified general acid, making the purine a better leaving group, and accelerating catalysis. Classical and mixed quantum/classical molecular dynamics (MD) simulations of the Michaelis complex of hPNP with the substrates guanosine and phosphate were performed to assess the existence of protein-promoting vibrations (PPVs). Analogous simulations were performed for the substrates in aqueous solution. In the catalytic site, the O-5', O-4', and O(P) oxygens vibrate at frequencies of ca. 125 and 465 cm(-1), as opposed to 285 cm(-1) in the absence of hPNP. The hybrid quantum mechanical/molecular mechanical method was used to assess whether this enzymatic vibration pushing the oxygens together is coupled to the reaction coordinate, and thus has a direct positive impact on catalysis. The potential energy surface for the phosphorolysis reaction for several snapshots taken from the classical MD simulation showed substantial differences in oxygen compression. Our calculations showed the existence of PPVs coupled to the reaction coordinate, which effect electronic alterations in the active site by pushing the three oxygen centers together in proximity, and accelerate substrate turnover in the phosphorolysis reaction catalyzed by hPNP. 相似文献
969.
Sara Porfírio Marco D.R. Gomes da Silva Augusto Peixe Maria J. Cabrita Parastoo Azadi 《Analytica chimica acta》2016
Plant hormones, and especially auxins, are low molecular weight compounds highly involved in the control of plant growth and development. Auxins are also broadly used in horticulture, as part of vegetative plant propagation protocols, allowing the cloning of genotypes of interest. Over the years, large efforts have been put in the development of more sensitive and precise methods of analysis and quantification of plant hormone levels in plant tissues. Although analytical techniques have evolved, and new methods have been implemented, sample preparation is still the limiting step of auxin analysis. In this review, the current methods of auxin analysis are discussed. Sample preparation procedures, including extraction, purification and derivatization, are reviewed and compared. The different analytical techniques, ranging from chromatographic and mass spectrometry methods to immunoassays and electrokinetic methods, as well as other types of detection are also discussed. Considering that auxin analysis mirrors the evolution in analytical chemistry, the number of publications describing new and/or improved methods is always increasing and we considered appropriate to update the available information. For that reason, this article aims to review the current advances in auxin analysis, and thus only reports from the past 15 years will be covered. 相似文献
970.
Laura Bertoletti Julie Schappler Raffaella Colombo Serge Rudaz Rob Haselberg Elena Domínguez-Vega Sara Raimondi Govert W. Somsen Ersilia De Lorenzi 《Analytica chimica acta》2016
In this work we explored the feasibility of different CE-ESI-MS set-ups for the analysis of conformational states of an intact protein. By using the same background electrolyte at quasi physiological conditions (50 mM ammonium bicarbonate, pH 7.4) a sequential optimization was carried out, initially by evaluating a sheath-liquid interface with both a single quadrupole (SQ) and a time-of-flight (TOF) mass spectrometer; then a sheathless interface coupled with high-resolution QTOF MS was considered. Beta2-microglobulin has been taken as a model, as it is an amyloidogenic protein and its conformational changes are strictly connected to the onset of a disease. The separation of two conformers at dynamic equilibrium is achieved all the way down to the MS detection. Notably, the equilibrium ratio of the protein conformers is maintained in the electrospray source after CE separation. Strengths and weaknesses of each optimized set-up are emphasized and their feasibility in unfolding studies is evaluated. In particular, ESI-TOF MS can assign protein forms that differ by 1 Da only and sheathless interfacing is best suited to preserve protein structure integrity. This demonstrates the CE-ESI-MS performance in terms of separation, detection and characterization of conformational species that co-populate a protein solution. 相似文献