全文获取类型
收费全文 | 632篇 |
免费 | 23篇 |
专业分类
化学 | 553篇 |
晶体学 | 3篇 |
力学 | 6篇 |
数学 | 43篇 |
物理学 | 50篇 |
出版年
2023年 | 6篇 |
2022年 | 15篇 |
2021年 | 18篇 |
2020年 | 13篇 |
2019年 | 16篇 |
2018年 | 13篇 |
2017年 | 14篇 |
2016年 | 21篇 |
2015年 | 17篇 |
2014年 | 18篇 |
2013年 | 36篇 |
2012年 | 41篇 |
2011年 | 65篇 |
2010年 | 22篇 |
2009年 | 32篇 |
2008年 | 55篇 |
2007年 | 52篇 |
2006年 | 28篇 |
2005年 | 32篇 |
2004年 | 17篇 |
2003年 | 15篇 |
2002年 | 13篇 |
2001年 | 5篇 |
2000年 | 3篇 |
1999年 | 2篇 |
1997年 | 2篇 |
1993年 | 6篇 |
1991年 | 4篇 |
1990年 | 2篇 |
1989年 | 4篇 |
1987年 | 2篇 |
1984年 | 4篇 |
1981年 | 3篇 |
1978年 | 6篇 |
1977年 | 3篇 |
1976年 | 5篇 |
1975年 | 2篇 |
1974年 | 2篇 |
1973年 | 4篇 |
1968年 | 2篇 |
1954年 | 2篇 |
1937年 | 2篇 |
1936年 | 2篇 |
1934年 | 2篇 |
1932年 | 3篇 |
1928年 | 1篇 |
1922年 | 1篇 |
1919年 | 3篇 |
1898年 | 1篇 |
1879年 | 1篇 |
排序方式: 共有655条查询结果,搜索用时 15 毫秒
641.
One of the most powerful approaches for the formation of simple and complex chiral molecules is the metal-catalysed asymmetric allylic alkylation. This reaction has been broadly studied with a great variety of substrates and nucleophiles under different reaction conditions and it has promoted the synthesis of new chiral ligands to be evaluated as asymmetric inductors. Although the mechanism as well as the active species equilibria are known, the performance of the catalytic system depends on the fine tuning of factors such as type of substrate, nucleophile nature, reaction medium, catalytic precursor and type of ligand used. Particularly interesting are chiral phosphines which have proved to be effective asymmetric inductors in several such reactions. The present review covers the application of phosphine-donor ligands in Pd-catalysed asymmetric allylic alkylation in the last decade. 相似文献
642.
Raluy E Favier I López-Vinasco AM Pradel C Martin E Madec D Teuma E Gómez M 《Physical chemistry chemical physics : PCCP》2011,13(30):13579-13584
New catalytic systems based on in situ and preformed palladium nanoparticles in ionic liquids (characterised by TEM) starting from palladium acetate or dipalladiumtris(dibenzylideneacetone) have been applied in the synthesis of 4-phenylbutan-2-one (II), a model compound for the preparation of fragrances. Imidazolium-based ionic liquid containing a methyl hydrogenophosphonate anion leads to an efficient Pd-catalyzed tandem coupling/reduction process, taking advantage of the multi-role of this solvent (nanoparticles stabiliser, base, hydrogen transfer agent). The influence of the mono-phosphine ligands (1-3) on the catalyst has been evaluated, showing that the ligand-free palladium system turns into the most appropriate for the formation of II using Pd(OAc)(2) as precursor. Fine-tuning conditions involved in this multi-parameter process have led us to propose a plausible mechanism based on the hydrogen transfer coming from the methyl hydrogenophosphonate anion. 相似文献
643.
Airoldi C Cardona F Sironi E Colombo L Salmona M Silva A Nicotra F La Ferla B 《Chemical communications (Cambridge, England)》2011,47(37):10266-10268
A small library of glyco-fused benzopyran compounds has been synthesised. Their interaction features with Aβ peptides have been characterised by using STD-NMR and trNOESY experiments. The conformational analysis of the compounds has also been carried out through molecular mechanics (MM) and molecular dynamics (MD) simulations. 相似文献
644.
Erika Cristina Vargas de Oliveira Naira Rezende Maciel Lia Queiroz do Amaral 《Journal of Dispersion Science and Technology》2013,34(12):1656-1662
Oil-in-water emulsions were developed employing the HLB system and emulsion phase inversion (EPI) method. X-ray diffraction revealed that the anisotropic structures around the inner phase globules were lamellar gel network phases. The calculated distances between the lamellae made after preparation and 3 month latter showed that there was no swelling of the lamellar gel network indicating good stability and few changes during storage. The developed emulsions were stable and have potential to be employed for cosmetic and pharmaceutical purposes. The gel phase network and vegetal components seemed to be contributing factors. 相似文献
645.
646.
Gregory B. Kharas Selena M. Russell Robert Cisler Tess L. Capen Erika A. Chlupsa Lisa A. Debellis 《高分子科学杂志,A辑:纯化学与应用化学》2013,50(4):261-264
Electrophilic trisubstituted ethylene monomers, halogen ring‐disubstituted 2‐phenyl‐1,1‐dicyanoethylenes, RC6H3CH?C(CN)2 (where R is 2,3‐diCl, 2,4‐diCl, 2,6‐diCl, 3,4‐diCl, 3,5‐diCl, 2,4‐diF, 2,5‐diF, 2,6‐diF, 3,4‐diF, 3,5‐diF, 2‐Cl, 6‐F) were synthesized by piperidine catalyzed Knoevenagel condensation of ring‐substituted benzaldehydes and malononitrile, and characterized by CHN elemental analysis, IR, 1H‐and 13C‐NMR. Novel copolymers of the ethylenes and vinyl acetate were prepared at equimolar monomer feed composition by solution copolymerization in the presence of a radical initiator (ABCN) at 70°C. The composition of the copolymers was calculated from nitrogen analysis, and the structures were analyzed by IR, 1H and 13C‐NMR, GPC, DSC, and TGA. High Tg of the copolymers, in comparison with that of polyvinyl acetate, indicates a substantial decrease in chain mobility of the copolymer due to the high dipolar character of the trisubstituted ethylene monomer unit. The gravimetric analysis indicated that the copolymers decompose in the 220–800°C range. 相似文献
647.
Cotton and linen fibers were ground in a ball-mill, and the effect of grinding on the microstructure and surface properties of the fibers was determined by combining a couple of simple tests with powerful techniques of surface and structure analysis. Results clearly proved that the effect of grinding on cotton fiber was much less severe than on linen. For both fibers, the degree of polymerization reduced (by 14.5% and 30.5% for cotton and linen, respectively) with a simultaneous increase in copper number. The increased water sorption capacity of the ground substrates was in good agreement with the X-ray results, which proved a less perfect crystalline structure in the ground samples. Data from XPS and SEM-EDS methods revealed that the concentration of oxygen atoms (bonded especially in acetal and/or carbonyl groups) on the ground surfaces increased significantly, resulting in an increase in oxygen/carbon atomic ratio (XPS data: from 0.11 to 0.14 and from 0.16 to 0.29 for cotton and linen, respectively). Although grinding created new surfaces rich in O atoms, the probable higher energy of the surface could not be measured by IGC, most likely due to the limited adsorption of the n-alkane probes on the less perfect crystalline surfaces. 相似文献
648.
Danilo Addari Adriana Mignani Erika Scavetta Domenica Tonelli Antonella Rossi 《Surface and interface analysis : SIA》2011,43(4):816-822
In this work, a Ni/Al hydrotalcite (HT) was used as glucose oxidase (GOx) immobilizer. Small‐area and angle‐resolved X‐ray photoelectron spectra were recorded on HTs electrosynthesized on Pt in the absence and in the presence of GOx, and compared with those obtained for a Pt surface, modified with the electrosynthesized HT, on which a drop of GOx solution was deposited. The simultaneous electrodeposition of HT + GOx resulted in a compact deposit, thicker than the XPS sampling depth (>10 nm), that is not homogeneous in the lateral and in‐depth composition. The presence of GOx can be deduced comparing the N1s spectra of HT and HT + GOx: in the latter, the N1s component at 400 eV binding energy (BE) is predominant whilst, depending on the analyzed point, a small or no contribution from the component at 407.2 eV, due to nitrate, is revealed. Angle‐resolved XPS provides evidence on the in‐depth composition of anions, cations and GOx. The results highlight the crucial role played by nickel in GOx immobilization. On the basis of the results, it can be suggested that enzyme activity is unevenly distributed and is localized in small areas, where Ni concentration is higher. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
649.
van Hove ER Smith DF Fornai L Glunde K Heeren RM 《Journal of the American Society for Mass Spectrometry》2011,22(10):1885-1890
Surface treatment of biological tissue sections improves detection of peptides and proteins for mass spectrometry imaging.
However, liquid surface treatments can result in diffusion of surface analytes and fragile tissue sections can be easily damaged
by typical washing solvents. Here, we present a new surface washing procedure for mass spectrometry imaging. This procedure
uses solvent wetted fiber-free paper to enable local washing of tissue sections for mass spectrometry imaging and tissue profiling
experiments. In addition, the method allows fragile tissues that cannot be treated by conventional washing techniques to be
analyzed by mass spectrometry imaging. 相似文献
650.
Jennifer A. Jamison Erika L. Bryant Shyam B. Kadali Michael S. Wong Vicki L. Colvin Kathleen S. Matthews Michelle K. Calabretta 《Journal of nanoparticle research》2011,13(2):625-636
Gold nanoparticles (AuNP) can interact with a wide range of molecules including proteins. Whereas significant attention has
focused on modifying the nanoparticle surface to regulate protein–AuNP assembly or influence the formation of the protein
“corona,” modification of the protein surface as a mechanism to modulate protein–AuNP interaction has been less explored.
Here, we examine this possibility utilizing three small globular proteins—lysozyme with high isoelectric point (pI) and established
interactions with AuNP; α-lactalbumin with similar tertiary fold to lysozyme but low pI; and myoglobin with a different globular
fold and an intermediate pI. We first chemically modified these proteins to alter their charged surface functionalities, and
thereby shift protein pI, and then applied multiple methods to assess protein–AuNP assembly. At pH values lower than the anticipated
pI of the modified protein, AuNP exposure elicits changes in the optical absorbance of the protein–NP solutions and other
properties due to aggregate formation. Above the expected pI, however, protein–AuNP interaction is minimal, and both components
remain isolated, presumably because both species are negatively charged. These data demonstrate that protein modification
provides a powerful tool for modulating whether nanoparticle–protein interactions result in material aggregation. The results
also underscore that naturally occurring protein modifications found in vivo may be critical in defining nanoparticle–protein
corona compositions. 相似文献