全文获取类型
收费全文 | 648篇 |
免费 | 29篇 |
国内免费 | 3篇 |
专业分类
化学 | 511篇 |
晶体学 | 8篇 |
力学 | 6篇 |
数学 | 68篇 |
物理学 | 87篇 |
出版年
2021年 | 10篇 |
2020年 | 12篇 |
2019年 | 5篇 |
2018年 | 6篇 |
2017年 | 8篇 |
2016年 | 20篇 |
2015年 | 15篇 |
2014年 | 15篇 |
2013年 | 28篇 |
2012年 | 37篇 |
2011年 | 42篇 |
2010年 | 23篇 |
2009年 | 21篇 |
2008年 | 32篇 |
2007年 | 41篇 |
2006年 | 27篇 |
2005年 | 29篇 |
2004年 | 18篇 |
2003年 | 22篇 |
2002年 | 16篇 |
2001年 | 6篇 |
2000年 | 14篇 |
1999年 | 11篇 |
1997年 | 15篇 |
1996年 | 8篇 |
1995年 | 5篇 |
1994年 | 10篇 |
1993年 | 11篇 |
1992年 | 5篇 |
1991年 | 5篇 |
1990年 | 6篇 |
1987年 | 5篇 |
1986年 | 4篇 |
1984年 | 4篇 |
1983年 | 9篇 |
1981年 | 8篇 |
1980年 | 4篇 |
1979年 | 3篇 |
1978年 | 11篇 |
1977年 | 9篇 |
1976年 | 5篇 |
1975年 | 5篇 |
1974年 | 4篇 |
1973年 | 3篇 |
1972年 | 3篇 |
1971年 | 4篇 |
1968年 | 3篇 |
1961年 | 6篇 |
1956年 | 3篇 |
1904年 | 4篇 |
排序方式: 共有680条查询结果,搜索用时 156 毫秒
661.
Cu‐Based Catalyst Resulting from a Cu,Zn,Al Hydrotalcite‐Like Compound: A Microstructural,Thermoanalytical, and In Situ XAS Study 下载免费PDF全文
Dr. Stefanie Kühl Dr. Andrey Tarasov Dr. Stefan Zander Dr. Igor Kasatkin Dr. Malte Behrens 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(13):3782-3792
A Cu‐based methanol synthesis catalyst was obtained from a phase pure Cu,Zn,Al hydrotalcite‐like precursor, which was prepared by co‐precipitation. This sample was intrinsically more active than a conventionally prepared Cu/ZnO/Al2O3 catalyst. Upon thermal decomposition in air, the [(Cu0.5Zn0.17Al0.33)(OH)2(CO3)0.17] ? mH2O precursor is transferred into a carbonate‐modified, amorphous mixed oxide. The calcined catalyst can be described as well‐dispersed “CuO” within ZnAl2O4 still containing stabilizing carbonate with a strong interaction of Cu2+ ions with the Zn–Al matrix. The reduction of this material was carefully analyzed by complementary temperature‐programmed reduction (TPR) and near‐edge X‐ray absorption fine structure (NEXAFS) measurements. The results fully describe the reduction mechanism with a kinetic model that can be used to predict the oxidation state of Cu at given reduction conditions. The reaction proceeds in two steps through a kinetically stabilized CuI intermediate. With reduction, a nanostructured catalyst evolves with metallic Cu particles dispersed in a ZnAl2O4 spinel‐like matrix. Due to the strong interaction of Cu and the oxide matrix, the small Cu particles (7 nm) of this catalyst are partially embedded leading to lower absolute activity in comparison with a catalyst comprised of less‐embedded particles. Interestingly, the exposed Cu surface area exhibits a superior intrinsic activity, which is related to a positive effect of the interface contact of Cu and its surroundings. 相似文献
662.
Menche D Arikan F Perlova O Horstmann N Ahlbrecht W Wenzel SC Jansen R Irschik H Müller R 《Journal of the American Chemical Society》2008,130(43):14234-14243
A potent novel analogue of the natural macrolide antibiotic etnangien, a structurally unique RNA polymerase inhibitor from myxobacteria, is reported. It may be readily obtained from fermentation broths of Sorangium cellulosum and shows high antibiotic activity, comparable to that of etnangien. However, it is much more readily available than the notoriously labile authentic natural product itself. Importantly, it is stable under neutral conditions, allowing for elaborate NMR measurements for assignment of the 12 hydroxyl- and methyl-bearing stereogenic centers. The full absolute and relative stereochemistries of these complex polyketides were determined by a combination of extensive high-field NMR studies, including J-based configuration analysis, molecular modeling, and synthetic derivatization in combination with an innovative method based on biosynthetic studies of this polyketide which is also presented here. A first look into the solution conformation and 3D structure of these promising macrolide antibiotics is reported. Finally, the complete biosynthetic gene cluster was analyzed in detail, revealing a highly unusual and complex trans-AT type polyketide biosynthesis, which does not follow colinearity rules, most likely performs programmed iteration as well as module skipping, and exhibits HMG-CoA box-directed methylation. 相似文献
663.
Unsymmetrical 1-(arylimino)-3-(2-hetarylimino)isoindolines have been prepared from 1,3-diiminoisoindoline, an arylamine (aniline, 2-methylaniline, 2-iodoaniline), and a heteroaromatic amine (2-amino-6-methylpyridine, 2-amino-4-methylthiazole) in a stepwise manner by two consecutive condensations. The metalation reactions of these compounds with palladium(II) acetate proceed upon cyclopalladation of the carbocyclic aryl moieties and yield unsymmetrical C, N, N pincer complexes in all cases. X-ray crystallographic analysis were performed on single crystals of hydrogen{acetato[1-phenylimino-3-(6-methylpyridylimino)isoindolinato]palladate(II)} H[(phpi)Pd(OAc)] and pyridine[1-(2-tolylimino)-3-(4-methylthiazolylimino)isoindolinato]palladium(II) [(2-tolti)Pd(py)] by which the coordination mode, the conformation, the protonation site, and the trans influence of the carbon donor were established. For one more C, N, N pincer complex, hydrogen{acetato[1-(2-iodophenylimino)-3-(6-methylpyridylimino)isoindolinato]palladate(II)} H[(2-Iphpi)Pd(OAc)], a similar mononuclear coordination mode was confirmed by (1)H NMR spectroscopy, whereas for the product of an oxidative addition reaction of a palladium(0) precursor to the iodoaryl derivative a product with exo coordination was found. First experiments showed the effectivity of one of these complexes as a precatalyst in CC coupling reactions (Heck and Stille coupling). 相似文献
664.
Silke Taubmann 《Journal of organometallic chemistry》2008,693(10):1808-1814
A series of new 1,5-cyclooctadiene iridium(I) complexes with chelating ligands has been synthesized. The ligands are naphthoxyimines, carboxylates and alcoholates. The complexes catalyze the homogeneous dehydrogenation of cyclooctane to give cyclooctene and hydrogen without an external hydrogen acceptor up to rates of 75 turnovers. The catalysts are active for at least 48 h at a temperature of 300 °C. The ligand structure has an influence on the activity and selectivity of the corresponding catalysts. 相似文献
665.
Arnar Sigurdsson Dr. Berta M. Martins Dr. Simon A. Düttmann Martin Jasyk Benjamin Dimos-Röhl Felix Schöpf Manuel Gemander Dr. Caroline H. Knittel Dr. Romina Schnegotzki Bianca Schmid Dr. Simone Kosol Lea Pommerening María Gonzáles-Viegaz Maria Seidel Manuela Hügelland Prof. Dr. Silke Leimkühler Prof. Dr. Holger Dobbek Dr. Andi Mainz Prof. Dr. Roderich D. Süssmuth 《Angewandte Chemie (International ed. in English)》2023,62(23):e202302490
Lanthipeptides are ribosomally-synthesized natural products from bacteria featuring stable thioether-crosslinks and various bioactivities. Herein, we report on a new clade of tricyclic class-IV lanthipeptides with curvocidin from Thermomonospora curvata as its first representative. We obtained crystal structures of the corresponding lanthipeptide synthetase CuvL that showed a circular arrangement of its kinase, lyase and cyclase domains, forming a central reaction chamber for the iterative substrate processing involving nine catalytic steps. The combination of experimental data and artificial intelligence-based structural models identified the N-terminal subdomain of the kinase domain as the primary site of substrate recruitment. The ribosomal precursor peptide of curvocidin employs an amphipathic α-helix in its leader region as an anchor to CuvL, while its substrate core shuttles within the central reaction chamber. Our study thus reveals general principles of domain organization and substrate recruitment of class-IV and class-III lanthipeptide synthetases. 相似文献
666.
Waraich SM Hering B Burghard Z Bill J Behrens P Menzel H 《Journal of colloid and interface science》2012,367(1):74-82
Composite materials with an ordered layered structure resembling that of nacre were fabricated by layer-by-layer assembly making use of presynthesized α-zirconium hydrogenphosphate hydrate (ZrP) platelets and chitosan. These two biocompatible materials were chosen in view of possible applications in the biomedical field, e.g., as bone or joint replacement implants. The effect of different concentrations of the inorganic ZrP platelets and the organic components (chitosan) on the composite assembly and structure was investigated. A high concentration of chitosan (0.1 wt.%) resulted in a misalignment of the inorganic platelets, while at very low concentrations (0.001 wt.%), the substrate was not fully covered by the polymer, again leading to misalignment. Also, the concentration of the α-ZrP platelets affected the composite assembly and structure. The number of dipping cycles was varied between 70 and 220, yielding a maximum thickness of approximately 6 μm. The pH value of the chitosan solution was also varied to investigate its influence on the composite assembly. The mechanical properties of the composites were tested with a nanoindenter. For samples prepared with the same number of dipping cycles, higher values of Young's modulus and hardness were obtained with improved alignment of the platelets in the samples. For samples prepared with 220 dipping cycles, a Young's modulus of 2.6 GPa and a hardness of 70 MPa were observed. Important general relationships are recognized between the preparation parameters, the degree of order within the nacre-like films and the resulting mechanical properties. 相似文献
667.
668.
Schumacher S Nestler J Otto T Wegener M Ehrentreich-Förster E Michel D Wunderlich K Palzer S Sohn K Weber A Burgard M Grzesiak A Teichert A Brandenburg A Koger B Albers J Nebling E Bier FF 《Lab on a chip》2012,12(3):464-473
A novel innovative approach towards a marketable lab-on-chip system for point-of-care in vitro diagnostics is reported. In a consortium of seven Fraunhofer Institutes a lab-on-chip system called "Fraunhofer ivD-platform" has been established which opens up the possibility for an on-site analysis at low costs. The system features a high degree of modularity and integration. Modularity allows the adaption of common and established assay types of various formats. Integration lets the system move from the laboratory to the point-of-need. By making use of the microarray format the lab-on-chip system also addresses new trends in biomedicine. Research topics such as personalized medicine or companion diagnostics show that multiparameter analyses are an added value for diagnostics, therapy as well as therapy control. These goals are addressed with a low-cost and self-contained cartridge, since reagents, microfluidic actuators and various sensors are integrated within the cartridge. In combination with a fully automated instrumentation (read-out and processing unit) a diagnostic assay can be performed in about 15 min. Via a user-friendly interface the read-out unit itself performs the assay protocol, data acquisition and data analysis. So far, example assays for nucleic acids (detection of different pathogens) and protein markers (such as CRP and PSA) have been established using an electrochemical read-out based on redoxcycling or an optical read-out based on total internal reflectance fluorescence (TIRF). It could be shown that the assay performance within the cartridge is similar to that found for the same assay in a microtiter plate. Furthermore, recent developments are the integration of sample preparation and polymerase chain reaction (PCR) on-chip. Hence, the instrument is capable of providing heating-and-cooling cycles necessary for DNA-amplification. In addition to scientific aspects also the production of such a lab-on-chip system was part of the development since this heavily affects the success of a later market launch. In summary, the Fraunhofer ivD-platform covers the whole value chain ranging from microfluidics, material and polymer sciences, assay and sensor development to the production and assembly design. In this consortium the gap between diagnostic needs and available technologies can be closed. 相似文献
669.
Franziska Debatin Karsten Behrens Dr. Jens Weber Dr. Igor A. Baburin Prof. Dr. Arne Thomas Dr. Johannes Schmidt Dr. Irena Senkovska Prof. Dr. Stefan Kaskel Alexandra Kelling Dr. Niklas Hedin Dr. Zoltan Bacsik Dr. Stefano Leoni Prof. Dr. Gotthard Seifert Prof. Dr. Christian Jäger Dr. Christina Günter Prof. Dr. Uwe Schilde Dr. Alwin Friedrich Prof. Dr. Hans‐Jürgen Holdt 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(37):11630-11640
We report on a new series of isoreticular frameworks based on zinc and 2‐substituted imidazolate‐4‐amide‐5‐imidate (IFP‐1–4, IFP=imidazolate framework Potsdam) that form one‐dimensional, microporous hexagonal channels. Varying R in the 2‐substitued linker (R=Me (IFP‐1), Cl (IFP‐2), Br (IFP‐3), Et (IFP‐4)) allowed the channel diameter (4.0–1.7 Å), the polarisability and functionality of the channel walls to be tuned. Frameworks IFP‐2, IFP‐3 and IFP‐4 are isostructural to previously reported IFP‐1. The structures of IFP‐2 and IFP‐3 were solved by X‐ray crystallographic analyses. The structure of IFP‐4 was determined by a combination of PXRD and structure modelling and was confirmed by IR spectroscopy and 1H MAS and 13C CP‐MAS NMR spectroscopy. All IFPs showed high thermal stability (345–400 °C); IFP‐1 and IFP‐4 were stable in boiling water for 7 d. A detailed porosity analysis was performed on the basis of adsorption measurements by using various gases. The potential of the materials to undergo specific interactions with CO2 was investigated by measuring the isosteric heats of adsorption. The capacity to adsorb CH4 (at 298 K), CO2 (at 298 K) and H2 (at 77 K) at high pressure were also investigated. In situ IR spectroscopy showed that CO2 is physisorbed on IFP‐1–4 under dry conditions and that both CO2 and H2O are physisorbed on IFP‐1 under moist conditions. 相似文献
670.
Milroy LG Rizzo S Calderon A Ellinger B Erdmann S Mondry J Verveer P Bastiaens P Waldmann H Dehmelt L Arndt HD 《Journal of the American Chemical Society》2012,134(20):8480-8486
We have characterized rationally designed and optimized analogues of the actin-stabilizing natural products jasplakinolide and chondramide C. Efficient actin staining was achieved in fixed permeabilized and non-permeabilized cells using different combinations of dye and linker length, thus highlighting the degree of molecular flexibility of the natural product scaffold. Investigations into synthetically accessible, non-toxic analogues have led to the characterization of a powerful cell-permeable probe to selectively image static, long-lived actin filaments against dynamic F-actin and monomeric G-actin populations in live cells, with negligible disruption of rapid actin dynamics. 相似文献