全文获取类型
收费全文 | 25446篇 |
免费 | 2133篇 |
国内免费 | 6417篇 |
专业分类
化学 | 29384篇 |
晶体学 | 380篇 |
力学 | 254篇 |
综合类 | 205篇 |
数学 | 543篇 |
物理学 | 3230篇 |
出版年
2024年 | 39篇 |
2023年 | 237篇 |
2022年 | 630篇 |
2021年 | 704篇 |
2020年 | 880篇 |
2019年 | 726篇 |
2018年 | 739篇 |
2017年 | 822篇 |
2016年 | 1150篇 |
2015年 | 1115篇 |
2014年 | 1406篇 |
2013年 | 2433篇 |
2012年 | 1670篇 |
2011年 | 1703篇 |
2010年 | 1447篇 |
2009年 | 1694篇 |
2008年 | 1749篇 |
2007年 | 1854篇 |
2006年 | 1860篇 |
2005年 | 1733篇 |
2004年 | 1659篇 |
2003年 | 1432篇 |
2002年 | 983篇 |
2001年 | 691篇 |
2000年 | 622篇 |
1999年 | 601篇 |
1998年 | 540篇 |
1997年 | 476篇 |
1996年 | 367篇 |
1995年 | 371篇 |
1994年 | 313篇 |
1993年 | 296篇 |
1992年 | 274篇 |
1991年 | 162篇 |
1990年 | 104篇 |
1989年 | 105篇 |
1988年 | 91篇 |
1987年 | 54篇 |
1986年 | 39篇 |
1985年 | 53篇 |
1984年 | 31篇 |
1983年 | 13篇 |
1982年 | 14篇 |
1981年 | 21篇 |
1980年 | 10篇 |
1978年 | 10篇 |
1976年 | 15篇 |
1973年 | 9篇 |
1972年 | 7篇 |
1971年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Combinatorial library of isoxazoles was prepared by 1,3-dipolar cycloaddition on solid support. p-Cyano derivatives showed nematic and/or smectic A phases. A bilayer smectic phase for 2h is proposed by the combination of molecular mechanics calculation and X-ray diffraction experiment. 相似文献
52.
Fausi?RassoulEmail author Heinz?L?ster Volker?Richter 《Monatshefte für Chemie / Chemical Monthly》2005,136(8):1501-1507
Summary. L-Carnitine, L-(−)-β-hydroxy-γ-trimethylaminobutyrate, plays an important role as a factor necessary for the transport of long-chain fatty acids into the mitochondria. In order to investigate the influence of L-carnitine on hyperlipidaemias, the experimental model of the sucrose-induced hypertriglyceridaemia of the rat was used. In these experiments L-carnitine in the dose of 11 mg per day and 100 g body weight (over the period of 1 week) was able to antagonize the sucrose-induced hypertriglyceridaemia and the increase of serum free fatty acid level in female rats of the Wistar strain. Carnitine administration did not change the activities of lipogenic enzymes and fatty acid synthesis in the liver. However, L-carnitine increases the rate of hepatic fatty acid oxidation. Our results indicate a hypotriglyceridemic and free fatty acid lowering effect of L-carnitine, and suggest the use of this compound in the therapy of hyperlipidaemias. 相似文献
53.
The stereoselective synthesis of the two segments 3 and 4 of macrolactin A 1 is described. Macrolactin A is a 24-membered polyene macrolide antibiotic, which is of interest due to a strong activity against B16-F10 murine tumor cell and HIV-1 virus. The key step of the synthesis is the 1,2-migration reaction of a (diene)Fe(CO)3 complex with a introduction of the thiol group to construct the unstable (E,Z)-conjugated dienic moiety (C8-C11). 相似文献
54.
N. Lanzetta M. Malinconico E. Martuscelli M. G. Volpe 《Journal of polymer science. Part A, Polymer chemistry》1993,31(5):1315-1322
A series of new aromatic polyetheraroylhydrazides incorporating 4-oxybenzoyl units has been synthesized, whose general formula was [CONHNHCO? O? O–(CH2)x? O? phenyl O-CONHNHCOO]n with x values in the range of 2 to 12. The increasing number of methylene units in the backbone gave rise to polymers which melted before the hydrazidic linkage underwent thermal cyclation to oxadiazole. Moreover, many polymers showed multiple endotherms on melting. X-ray diffraction studies confirmed a crystalline organization of polyhydrazides for methylene units above 4. © 1993 John Wiley & Sons, Inc. 相似文献
55.
56.
Daniel C. Grohs 《Tetrahedron letters》2005,46(25):4373-4376
We have recently described the synthesis of diazabicyclo[4.X.0]alkanes and their use as ligands for the prostate specific membrane antigene (PSMA). The key step of our synthetic route toward these diazabicycloalkanes is an oxidative cleavage of a bicyclic diol moiety followed by the attack of a nitrogen nucleophile to the resulting intermediate bisaldehyde. We herein describe the mechanism of this ring closure and its stereochemical consequences. In addition, we report a convenient method for trapping intermediate bisaldehydes by Wittig reagents. This trapping allows the synthesis of 3,5-disubstituted proline derivatives, which are shown to be versatile precursors for functionalized diazabicycloalkane dipeptide mimetics. 相似文献
57.
IntroductionCatal}ticrcactionsaffcctcdb}'diffosionaIlimitationincatal}'stporcshavcbccnextensive1}dcaltx`ithinlit.rat.rcll~5I.Scvcraltcchniqucsonthcspccialcase0fFischcr-Tropschs}'nthcsis(FTS)t\crcapplicdtocxplorcthebchaviorofthiscatal}ticrcactionundercxpcrimcntalandopcrationaIconditionsI3~5l.It`"ouldbeofvaluctoindicatchcrcthatthcseinvcstigationsx"ercmainl}'bascdonthesimplcstkineticsofthcfirstordcrinH2proposedbyAndersonl6]attheearIystages.However-thesetboents,striCtlyspeaking,areinvalidfor… 相似文献
58.
《Tetrahedron letters》2003,44(46):8445-8448
Fused β-carboline systems, as indolo[2,3-a]indolizidinones, indolo[2,3-a]quinolizidinones, their 2-oxa analogues, and benzo[a]indolo[2,3-a]indolizidinones are prepared efficiently via an RLi addition-N-acyliminium ion cyclisation sequence on readily available imides. In an enantioselective variant of these α-amidoalkylation reactions, the addition of MeLi to a chiral non-racemic imide derived from tryptophan yielded an oxo amide, which was cyclised diastereoselectively upon treatment with BF3·OEt2, to afford 5,11b-trans-indoloindolizidinone in moderate yield and high ee (99%). 相似文献
59.
Synthesis, properties, and structure of dimethylgold(III) complexes [(CH3)2AuI]2 and (CH3)2AuS2CN(C2H5)2 总被引:1,自引:0,他引:1
Volatile diethyldithiocarbamate of dimethylgold(III) was prepared by the interaction of dimethylgold(III) iodide with sodium diethyldithiocarbamate. The complex is examined by the elemental analysis, DTA, IR and electronic spectroscopy. The starting dimeric complex [(CH3)2AuI]2 and a novel monomeric volatile gold(III) complex (CH3)2AuS2CN(C2H5)2 with the AuC2S2 coordination core were investigated by single crystal X-ray diffraction for the first time. 相似文献
60.
Zi-Qiang GuMin Li 《Tetrahedron letters》2003,44(15):3203-3205
A concise, multi-gram scale method for producing the bioactive and enantiomerically pure epimers, (2S,4R)- and (2S,4S)-glutamic acids, in a single synthetic scheme is described. 相似文献