全文获取类型
收费全文 | 3157篇 |
免费 | 139篇 |
国内免费 | 20篇 |
专业分类
化学 | 2474篇 |
晶体学 | 7篇 |
力学 | 54篇 |
数学 | 466篇 |
物理学 | 315篇 |
出版年
2023年 | 15篇 |
2022年 | 18篇 |
2021年 | 27篇 |
2020年 | 71篇 |
2019年 | 49篇 |
2018年 | 29篇 |
2017年 | 41篇 |
2016年 | 74篇 |
2015年 | 101篇 |
2014年 | 105篇 |
2013年 | 176篇 |
2012年 | 212篇 |
2011年 | 223篇 |
2010年 | 161篇 |
2009年 | 120篇 |
2008年 | 220篇 |
2007年 | 212篇 |
2006年 | 218篇 |
2005年 | 200篇 |
2004年 | 152篇 |
2003年 | 130篇 |
2002年 | 153篇 |
2001年 | 68篇 |
2000年 | 53篇 |
1999年 | 59篇 |
1998年 | 45篇 |
1997年 | 52篇 |
1996年 | 58篇 |
1995年 | 44篇 |
1994年 | 31篇 |
1993年 | 23篇 |
1992年 | 21篇 |
1991年 | 11篇 |
1990年 | 16篇 |
1989年 | 13篇 |
1988年 | 4篇 |
1987年 | 10篇 |
1986年 | 8篇 |
1985年 | 12篇 |
1983年 | 5篇 |
1982年 | 12篇 |
1981年 | 8篇 |
1979年 | 5篇 |
1978年 | 6篇 |
1977年 | 7篇 |
1975年 | 3篇 |
1974年 | 6篇 |
1973年 | 3篇 |
1972年 | 5篇 |
1971年 | 4篇 |
排序方式: 共有3316条查询结果,搜索用时 15 毫秒
991.
Prof. Dr. Lutz F. Tietze Stefan Jackenkroll Dr. Christian Raith Dr. Dirk A. Spiegl Johannes R. Reiner Maria Claudia Ochoa Campos 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(15):4876-4882
For the synthesis of (?)‐diversonol (ent‐ 1 ), an enantioselective domino‐Wacker/carbonylation/methoxylation reaction and an enantioselective Wacker oxidation were used to give the chroman in high yield and 96 % and 93 % ee, respectively. Dihydroxylation at the vinyl moiety using the Sharpless procedure and a Wittig–Horner reaction followed by hydrogenation, benzylic oxidation, and an intramolecular acylation provided the tetrahydroxanthenone, from which ent‐ 1 is accessible in a few steps. Furthermore, the synthesis of the diastereomeric diversonol rac‐1,9 a‐epi‐diversonol (rac‐ 41 ) is also described. 相似文献
992.
993.
Golo M.J. Meyer Markus R. Meyer Dirk K. Wissenbach Hans H. Maurer 《Journal of mass spectrometry : JMS》2013,48(1):24-41
Glaucine ((S)‐5,6,6a,7‐tetrahydro‐1,2,9,10‐tetramethoxy‐6‐methyl‐4H‐dibenzo [de,g]quinoline) is an isoquinoline alkaloid and main component of Glaucium flavum (Papaveraceae). It was described to be consumed as recreational drug alone or in combination with other drugs. Besides this, glaucine is used as therapeutic drug in Bulgaria and other countries as cough suppressant. Currently, there are no data available concerning metabolism and toxicological analysis of glaucine. To study both, glaucine was orally administered to Wistar rats and urine was collected. For metabolism studies, work‐up of urine samples consisted of protein precipitation or enzymatic cleavage followed by solid‐phase extraction. Samples were afterwards measured by liquid chromatography (LC) coupled to low or high‐resolution mass spectrometry (HR‐MS). The phase I and II metabolites were identified by detailed interpretation of the corresponding fragmentations, which were further confirmed by determination of their elemental composition using HR‐MS. From these data, the following metabolic pathways could be proposed: O‐demethylation at position 2, 9 and 10, N‐demethylation, hydroxylation, N‐oxidation and combinations of them as well as glucuronidation and/or sulfation of the phenolic metabolites. For monitoring a glaucine intake in case of abuse or poisoning, the O‐ and N‐demethylated metabolites were the main targets for the gas chromatography‐MS and LC‐MSn screening approaches described by the authors. Both allowed confirming an intake of glaucine in rat urine after a dose of 2 mg/kg body mass corresponding to a common abuser's dose. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
994.
Alexander Böhm Melanie Gattermayer Christian Trieb Samuel Schabel Dirk Fiedler Frank Miletzky Markus Biesalski 《Cellulose (London, England)》2013,20(1):467-483
We introduce a novel approach for preparing polymer-modified and chemically microstructured paper substrates by a photo-chemical attachment of functional polymers to cellulose microfibers inside model filter papers. Poly(methyl methacrylate), PMMA copolymers, which carry a defined amount of photo-reactive benzophenone functional groups, are adsorbed to paper substrates from solution by a simple dip coating process, followed by covalent attachment of the physisorbed polymers through UV-light irradiation. Non-bound macromolecules can be removed from paper sheets by simple solvent extraction, and the resulting polymer-modified substrates were analysed with respect to chemical identity, attached polymer mass, and homogeneity of the polymer attachment. The amount of paper-attached polymers can be conveniently controlled in a wide range from a few mg/g cellulose fiber up to several tenth of mg/g cellulose fiber, by adjusting the polymer concentration in the coating solution. Polymers are being attached by photo-chemical means, and chemical micro patterns on paper can be designed by lithographical means. In first proof-of-concept studies, millimeter-scale channels were prepared that can be used to control fluid penetration by capillary actions. Because of the modularity in the design of photo-reactive polymers, a number of different chemically microstructured papers can be envisioned which may become potentially interesting in lab-on-paper devices. 相似文献
995.
Dirk Otzen Jürgen Voss Gunadi Adiwidjaja 《Phosphorus, sulfur, and silicon and the related elements》2013,188(6):1249-1270
2,5-anhydro-3-azido-2-thio-D-lyxofuranosides and 3,5-anhydro-2-azido-3-thio-D-lyxofuranosides were prepared from methyl D-xylofuranoside or methyl D-ribofuranoside via corresponding 2,3-epoxysugars or the 5-O-trityl derivative. The sulfur was introduced into molecules by use of the thio-Mitsunobu reaction. Bicyclic azido-thiosugars were transformed into nucleoside analogues, oxidized to sulfoxides and sulfones, and reduced to bicyclic amino-thiosugars. Structures and configurations of products were determined by NMR spectroscopy or X-ray structure analyses. 相似文献
996.
997.
Dr. Alesia A. Tietze Prof. Dr. Frank Bordusa Dr. Ralf Giernoth Prof. Dr. Diana Imhof Prof. Dr. Thomas Lenzer Dr. Astrid Maaß Dr. Carmen Mrestani‐Klaus Prof. Dr. Ines Neundorf Dr. Kawon Oum Prof. Dr. Dirk Reith Dr. Annegret Stark 《Chemphyschem》2013,14(18):4044-4064
During the last decade, ionic liquids (ILs) have revealed promising properties and applications in many research fields, including biotechnology and biological sciences. The focus of this contribution is to give a critical review of the phenomena observed and current knowledge of the interactions occurring on a molecular basis. As opposed to the huge advances made in understanding the properties of proteins in ILs, complementary investigations dealing with interactions between ILs and peptides or oligopeptides are underrepresented and are mostly only of phenomenological nature. However, the field has received more attention in the last few years. This Review features a meta‐analysis of the available data and findings and should, therefore, provide a basis for a scientifically profound understanding of the nature and mechanisms of interactions between ILs and structured or nonstructured peptides. Fundamental aspects of the interactions between different peptides/oligopeptides and ILs are complemented by sections on the experimental (spectroscopy, structural biology) and theoretical (computational chemistry) possibilities to explain the phenomena reported so far in the literature. In effect, this should lead to the development of novel applications and support the understanding of IL–solute interactions in general. 相似文献
998.
999.
王静 邹永存 孙渝 Maximilian Hemgesberg Dirk Schaffner 高洪成 宋晓静 张文祥 贾明君 Werner R. Thiel 《催化学报》2014,35(4):532-539
以含有咪唑阳离子的周期性介孔有机硅(PMO-ILs)材料为载体,制备了一类固载化磷钼酸(PMA)多相催化材料(PMA@PMO-ILs),并采用N2吸附-脱附实验、X射线衍射、原子吸收光谱、差热-热重分析、红外光谱、紫外光谱及固体核磁共振技术研究了材料的结构及物理化学性质. 结果表明,磷钼酸通过静电相互作用被成功固载到PMO-ILs载体表面和孔道中,且在制备过程中磷钼酸及载体基本结构均未发生变化. 反应结果表明,PMA@PMO-ILs材料在以叔丁基过氧化氢为氧化剂的环辛烯环氧化反应中表现出一定的催化活性和很高的选择性. 中断实验结果表明,催化剂的主要活性中心在反应过程中未发生明显流失,且催化剂经多次循环使用后活性及选择性基本保持不变. PMO-ILs中大量的咪唑阳离子能有效稳定磷钼酸阴离子,使该催化材料表现出良好的稳定性. 相似文献
1000.
Jose?L.?Medina-FrancoEmail author Fabian?López-Vallejo Dirk?Kuck Frank?Lyko 《Molecular diversity》2011,15(2):293-304
DNA methyltransferases (DNMTs) represent promising targets for the development of unique anticancer drugs. However, all DNMT
inhibitors currently in clinical use are nonselective cytosine analogs with significant cytotoxic side-effects. Several natural
products, covering diverse chemical classes, have indicated DNMT inhibitory activity, but these effects have yet to be systematically
evaluated. In this study, we provide experimental data suggesting that two of the most prominent natural products associated
with DNA methylation inhibition, (−)-epigallocathechin-3-gallate (EGCG) and curcumin, have little or no pharmacologically
relevant inhibitory activity. We therefore conducted a virtual screen of a large database of natural products with a validated
homology model of the catalytic domain of DNMT1. The virtual screening focused on a lead-like subset of the natural products
docked with DNMT1, using three docking programs, following a multistep docking approach. Prior to docking, the lead-like subset
was characterized in terms of chemical space coverage and scaffold content. Consensus hits with high predicted docking affinity
for DNMT1 by all three docking programs were identified. One hit showed DNMT1 inhibitory activity in a previous study. The
virtual screening hits were located within the biological-relevant chemical space of drugs, and represent potential unique
DNMT inhibitors of natural origin. Validation of these virtual screening hits is warranted. 相似文献