全文获取类型
收费全文 | 28252篇 |
免费 | 807篇 |
国内免费 | 228篇 |
专业分类
化学 | 19245篇 |
晶体学 | 128篇 |
力学 | 648篇 |
综合类 | 1篇 |
数学 | 5385篇 |
物理学 | 3880篇 |
出版年
2022年 | 139篇 |
2021年 | 276篇 |
2020年 | 346篇 |
2019年 | 313篇 |
2018年 | 277篇 |
2017年 | 292篇 |
2016年 | 599篇 |
2015年 | 621篇 |
2014年 | 660篇 |
2013年 | 1406篇 |
2012年 | 1408篇 |
2011年 | 1706篇 |
2010年 | 952篇 |
2009年 | 814篇 |
2008年 | 1464篇 |
2007年 | 1466篇 |
2006年 | 1554篇 |
2005年 | 1398篇 |
2004年 | 1328篇 |
2003年 | 1098篇 |
2002年 | 1046篇 |
2001年 | 453篇 |
2000年 | 407篇 |
1999年 | 395篇 |
1998年 | 384篇 |
1997年 | 432篇 |
1996年 | 398篇 |
1995年 | 360篇 |
1994年 | 391篇 |
1993年 | 344篇 |
1992年 | 320篇 |
1991年 | 320篇 |
1990年 | 278篇 |
1989年 | 280篇 |
1988年 | 260篇 |
1987年 | 218篇 |
1986年 | 245篇 |
1985年 | 361篇 |
1984年 | 370篇 |
1983年 | 315篇 |
1982年 | 348篇 |
1981年 | 316篇 |
1980年 | 338篇 |
1979年 | 270篇 |
1978年 | 300篇 |
1977年 | 279篇 |
1976年 | 223篇 |
1975年 | 190篇 |
1974年 | 195篇 |
1973年 | 165篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
Klaus Broschinski Arunachalam Kannan Peter G. Jones Ina Dix Henning Hopf 《Acta Crystallographica. Section C, Structural Chemistry》2007,63(12):o711-o713
In the title compound, C18H16, the [2.2]paracyclophane geometry is restrained to a considerable extent despite the introduction of the extra C=C bridge; typical paracyclophane features, such as the elongated C—C bridges, are still observed. However, the bridgehead atoms of the C=C bridge are forced into unusually close proximity [2.657 (3) Å], which in turn causes the rings to be rotated to an interplanar angle of 13.7 (2)°. The packing involves hexagonally close‐packed layers of molecules parallel to the xy plane, corresponding to the known `7,11' pattern of paracyclophanes, but without significant short intermolecular contacts. 相似文献
972.
Jennifer Albaneze-Walker Jerry A. Murry Arash Soheili Scott Ceglia Shawn A. Springfield Charles Bazaral Peter G. Dormer David L. Hughes 《Tetrahedron》2005,61(26):6330-6336
An efficient synthesis of a potent PDE IV inhibitor 1 is described. The synthesis is highlighted by two practical and efficient catalytic reactions: a highly selective catalytic palladium mediated carbonylation of the pyridine side chain and an efficient palladium-catalyzed Suzuki-Miyaura coupling of a chloropyridine-N-oxide. 相似文献
973.
By14C-labeling it is shown that the ring contraction reaction of 1,4-benzothiazin-2,3-4H-dione (1), leading via benzthiazol-2-carbonic acid (2) finally to benzthiazole (3), proceeds via hydrolytic opening of the thiolactone-bond in1, irrespectively of H+ or OH– catalysis. 相似文献
974.
J rg Enderlein Peter M. Goodwin Alan Van Orden W. Patrick Ambrose Rainer Erdmann Richard A. Keller 《Chemical physics letters》1997,270(5-6):464-470
We have developed a maximum likelihood estimator to distinguish between similar molecules at the single molecule level based upon fluorescence decay measurements. Time resolved fluorescence measurements for single Rhodamine 6G and tetramethylrhodamine isothiocyanate molecules in fluid flow are derived from time-correlated single photon counting. A maximum likelihood estimator is developed and applied to data from a mixture of molecules. Single molecules are identified and distinguished by their fluorescence time decays. Comparison is made between identification error rates and theoretical predictions. To our knowledge, this is the first reported example of single molecule identification by fluorescence decay in a mixture. 相似文献
975.
976.
Peter Rogl Subhash K. Naik Erwin Rudy 《Monatshefte für Chemie / Chemical Monthly》1977,108(5):1189-1211
The system TiC?HfC?WC was investigated by means of melting point, differential thermoanalytical, X-ray diffraction and metallographic techniques on hot pressed and heat treated as well as melted alloy specimens and a complete constitutional diagram from 1500°C through the melting range established. According to the peritectic melting of hexagonal WC both isopleths, TiC?WC as well as HfC?WC show a class II reaction at 2760°C in Ti?W?C and at 2730°C in Hf?W?C. The phase behaviour within the TiC?HfC?WC system is characterized by the presence of a (binary) miscibility gap within TiC?HfC [T c=1780°C, (TiC)0.55(HfC)0.45] which extends into the ternary forming a closed ternary miscibility gap at higher temperatures with an isolated ternary critical point:T c=1800°C, (TiC)0.55(HfC)0.45(WC)0.05. Interaction of the solvus (boundary of the cubic-B 1 monocarbide solid solution) and the ternary miscibility gap was established at 1540°C and (TiC)0.27(HfC)0.41(WC)0.32: Alloys of this composition enter a decomposition reaction on cooling into two isotypic cubic B 1 phases and hexagonal WC. Isothermal sections were calculated assuming regular solutions. 相似文献
977.
Kritzer JA Tirado-Rives J Hart SA Lear JD Jorgensen WL Schepartz A 《Journal of the American Chemical Society》2005,127(1):167-178
Folded polymers are used in Nature for virtually every vital process. Nonnatural folded polymers, or foldamers, have the potential for similar versatility, and the design and refinement of such molecules is of considerable current interest. Here we report a complete and systematic analysis of the relationship between side chain structure and the 14-helicity of a well-studied class of foldamers, beta(3)-peptides, in water. Our experimental results (1) verify the importance of macrodipole stabilization for maintaining 14-helix structure, (2) provide comprehensive evidence that beta(3)-amino acids branched at the first side chain carbon are 14-helix-stabilizing, (3) suggest a novel role for side chain hydrogen bonding as an additional stabilizing force in beta(3)-peptides containing beta(3)-homoserine or beta(3)-homothreonine, and (4) demonstrate that diverse functionality can be incorporated into a stable 14-helix. Gas- and solution-phase calculations and Monte Carlo simulations recapitulate the experimental trends only in the context of oligomers, yielding insight into the mechanisms behind 14-helix folding. The 14-helix propensities of beta(3)-amino acids differ starkly from the alpha-helix propensities of analogous alpha-amino acids. This contrast informs current models for alpha-helix folding, and suggests that 14-helix folding is governed by different biophysical forces than is alpha-helix folding. The ability to modulate 14-helix structure through side chain choice will assist rational design of 14-helical beta-peptide ligands for macromolecular targets. 相似文献
978.
Peter G. Jones Ina Dix Henning Hopf 《Acta Crystallographica. Section C, Structural Chemistry》2007,63(8):o468-o473
The monosubstituted derivative 4‐ethynyl[2.2]paracyclophane, C18H16, (I), and the four disubstituted isomers, 4,12‐, (II), 4,13‐, (III), 4,15‐, (IV), and 4,16‐diethynyl[2.2]paracyclophane, (V), all C20H16, show the usual distortions of the [2.2]paracyclophane framework. The crystal packing is analyzed in terms of C—H...π interactions, some with H...π as short as 2.47 Å, in which the cyclophane rings and/or the triple‐bond systems may act as acceptors. For compounds (I) and (IV), the known `7,11'‐type cyclophane packing is observed, with a herring‐bone pattern of molecules in a layer structure. 相似文献
979.
Jun Liu Kelton A. Schleyer Tyrel L. Bryan Changjian Xie Gustavo Seabra Yongmei Xu Arjun Kafle Chao Cui Ying Wang Kunlun Yin Benjamin Fetrow Paul K. P. Henderson Peter Z. Fatland Jian Liu Chenglong Li Hua Guo Lina Cui 《Chemical science》2021,12(1):239
Heparanase (HPA) is a critical enzyme involved in the remodeling of the extracellular matrix (ECM), and its elevated expression has been linked with diseases such as various types of cancer and inflammation. The detection of heparanase enzymatic activity holds tremendous value in the study of the cellular microenvironment, and search of molecular therapeutics targeting heparanase, however, no structurally defined probes are available for the detection of heparanase activity. Here we present the development of the first ultrasensitive fluorogenic small-molecule probe for heparanase enzymatic activity via tuning the electronic effect of the substrate. The probe exhibits a 756-fold fluorescence turn-on response in the presence of human heparanase, allowing one-step detection of heparanase activity in real-time with a picomolar detection limit. The high sensitivity and robustness of the probe are exemplified in a high-throughput screening assay for heparanase inhibitors.Heparanase, a critical enzyme involved in the remodeling of the extracellular matrix, activates a disaccharide probe HADP to give a strong fluorescence signal. 相似文献
980.
Peter Brown Cecil L. Hornbeck John R. Cronin 《Journal of mass spectrometry : JMS》1972,6(12):1383-1399
The positive ion electron-impact mass spectra of a series of alloxazines, iso-alloxazines and some derivatives have been examined. The compounds employed were lumichrome (7,8-dimethylalloxazine), 1,3-dimethyllumichrome, lumiflavin (7,8,10-trimethyl-iso-alloxazine), 3-methyllumiflavin, riboflavin [7,8-dimethyl-10-(D-1′-ribityl)-iso-alloxazine], riboflavin tetraacetate, 3-methylriboflavin tetraacetate and riboflavin tetrapropionate. By using exact mass measurements, metastable ion defocusing and the mass/composition shifts occurring with derivatives, it has been possible to arrive at detailed interpretations of the mass spectra of all compounds. With lumichrome and lumiflavin, fragmentation commences by elimination of HNCO from the pyrimidine ring. With riboflavin and its derivatives the ribityl chain cleaves off first, followed by decomposition of the iso-alloxazine ring. Application of these methods and findings to the structural analysis of chemically interesting modified flavins is predicted to be rewarding. 相似文献