全文获取类型
收费全文 | 10454篇 |
免费 | 419篇 |
国内免费 | 62篇 |
专业分类
化学 | 7912篇 |
晶体学 | 81篇 |
力学 | 227篇 |
数学 | 1296篇 |
物理学 | 1419篇 |
出版年
2023年 | 72篇 |
2022年 | 89篇 |
2021年 | 126篇 |
2020年 | 206篇 |
2019年 | 167篇 |
2018年 | 129篇 |
2017年 | 115篇 |
2016年 | 269篇 |
2015年 | 251篇 |
2014年 | 284篇 |
2013年 | 513篇 |
2012年 | 750篇 |
2011年 | 922篇 |
2010年 | 436篇 |
2009年 | 285篇 |
2008年 | 673篇 |
2007年 | 727篇 |
2006年 | 770篇 |
2005年 | 754篇 |
2004年 | 595篇 |
2003年 | 505篇 |
2002年 | 445篇 |
2001年 | 114篇 |
2000年 | 108篇 |
1999年 | 100篇 |
1998年 | 91篇 |
1997年 | 99篇 |
1996年 | 121篇 |
1995年 | 84篇 |
1994年 | 89篇 |
1993年 | 73篇 |
1992年 | 67篇 |
1991年 | 63篇 |
1990年 | 49篇 |
1989年 | 34篇 |
1988年 | 44篇 |
1987年 | 34篇 |
1986年 | 21篇 |
1985年 | 82篇 |
1984年 | 49篇 |
1983年 | 37篇 |
1982年 | 61篇 |
1981年 | 59篇 |
1980年 | 48篇 |
1979年 | 49篇 |
1978年 | 43篇 |
1977年 | 38篇 |
1976年 | 30篇 |
1975年 | 38篇 |
1974年 | 23篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
211.
Burke AJ Davies SG Garner AC McCarthy TD Roberts PM Smith AD Rodriguez-Solla H Vickers RJ 《Organic & biomolecular chemistry》2004,2(9):1387-1394
Conjugate addition of lithium (S)-N-benzyl-N-alpha-methylbenzylamide to a range of alpha, beta-unsaturated Weinreb amides proceeds with high levels of diastereoselectivity (>95% de). The beta-amino Weinreb amide products may be transformed into beta-amino ketones via reactions with Grignard reagents, while treatment with DIBAL-H furnishes beta-amino aldehydes. Trapping of the aldehyde via Wadsworth-Emmons reaction and subsequent manipulation offers an efficient route to homochiral delta-amino acid derivatives and 2-substituted piperidines. The application of this methodology for the synthesis of (S)-coniine is demonstrated. 相似文献
212.
Bulloch EM Jones MA Parker EJ Osborne AP Stephens E Davies GM Coggins JR Abell C 《Journal of the American Chemical Society》2004,126(32):9912-9913
(6S)-6-Fluoroshikimate has antimicrobial activity. The molecular basis of this effect had not been identified, but there was speculation that (6S)-6-fluoroshikimate is first converted in vivo into 2-fluorochorismate, which then could inhibit 4-amino-4-deoxychorismate synthase (ADCS). 2-Fluorochorismate was prepared from E-fluorophosphoenolpyruvate and erythose-4-phosphate by the sequential reactions of DAHP synthase, dehydroquinate synthase, dehydroquinase, shikimate dehydrogenase, EPSP synthase, and chorismate synthase. Inhibition studies on ADCS showed that it was inhibited rapidly and irreversibly by 2-fluorochorismate. Electrospray mass spectrometry of the inactivated enzyme showed an additional mass of 198 +/- 10 Da. A novel peptide of 1087.6 Da was identified in the HPLC trace for the tryptic digest of 2-fluorochorismate-inactivated ADCS. Sequencing of this peptide by MS/MS showed that the peptide corresponded to residues 272-279 with a modification of 206.1 Da on Lys-274. This observation is particularly exciting in the context of a recent proposal for the catalytic mechanism of ADCS. 相似文献
213.
Ghazali NF Patterson DA Livingston AG 《Chemical communications (Cambridge, England)》2004,(8):962-963
Organic solvent nanofiltration (OSN) was used to investigate the mechanism of chiral selectivity in diastereomeric salt formation of alpha-phenylethylamine with D-tartaric acid and di-p-toluoyl-D-tartaric acid as resolving agents; results indicate that for these systems chiral selectivity occurs only upon crystallisation and chiral interactions in solution were negligible. 相似文献
214.
The intermolecular Pauson-Khand reaction between norbornene and dicobalt carbonyl complexes of phenylacetylene substituted with chiral phosphorus ligands has been investigated. High yields (?98%) and enantiomeric excesses of up to 56% have been observed. 相似文献
215.
Contribution to the study of the behaviour of catechin in alkaline medium When catechin ( 1 ) is warmed with aqueous alkali it first undergoes rapid epimerisation. After some time a novel bridged carbocyclic enol, catechinic acid ( 3 ) separates out and may be isolated in excellent yield. Eventually catechinic acid isomerises to 2-(3′,4′-dihydroxyphenyl)-3,9-epoxy-6-oxo-bicyclo[3,3,1]-non-7-en-8,9-diol ( 4 ) via an open chain anion ( 5 ) which is also involved in the epimerisation of catechin. Assignments of structure are given and the equilibria observed are discussed. 相似文献
216.
Alexander Chernega Dirk. L. Elend Christian A.P. Smethurst Paul M. Roberts Andrew D. Smith G. Darren Smyth 《Tetrahedron》2007,63(30):7036-7046
A three-step protocol for the asymmetric synthesis of a range of β-substituted Baylis-Hillman products has been developed. This procedure involves the diastereoselective conjugate addition of lithium (R)-N-methyl-N-(α-methylbenzyl)amide to an α,β-unsaturated ester to generate an N-protected β-amino ester in high de. Subsequent asymmetric aldol reaction via deprotonation with LDA, transmetallation with B(OMe)3 and addition of an aldehyde gives a range of syn-aldol products in moderate to high de. Purification of the syn-aldol products to homogeneity followed by tandem N-oxidation and Cope elimination gives the desired β-substituted Baylis-Hillman products in good yield and high de and ee. 相似文献
217.
Bradley D. Fahlman Andrew D. Daniels Gustavo E. Scuseria Andrew R. Barron 《Journal of Cluster Science》2002,13(4):587-599
Thermodynamic and mechanistic features of the chalcogen exchange reaction between [RGa(
3-Te)]4 and elemental sulfur or selenium have been studied employing density functional theory (DFT) calculations using the BL3YP basis set and Stuttgart pseudopotentials. For [MeGa(
3-E)]4 (E=S, Se, Te) the correlation between the calculated parameters and diffraction data for their isolable analogs is greater than 98%. Each step of the conversion of [MeGa(
3-Te)]4 to [MeGa(
3-E)]4 via [Me4Ga4(
3-Te)4–x
(
3-E)
x
] (E=S, Se) is predicted to occur as a series of isolated reactions. The entropy change for each chalcogen exchange is small in magnitude and corresponds to the degree of cage distortion within the cubane molecules. Calculations performed on [MeGa(
3-Te)]4...S8 and [MeGa(
3-Te)]4-S suggest that an increase in electrophilicity of the gallium next to a surface bound tellurium may result in nucleophilic cage opening for which intermediate structures are calculated. 相似文献
218.
Dimitris K. Agrafiotis Brian Tansy Andrew Streitwieser 《Journal of computational chemistry》1990,11(9):1101-1110
Two new computational methods for the evaluation and partitioning of projected, planar, and averaged slab electron densities, implemented in the program PRODEN, are presented. The new algorithms for the projection, demarcation, and integration of electron densities are described and evaluated in terms of speed and numerical accuracy. Integrated Projected Populations are analyzed and some of the advantages and limitations of the methods are discussed. 相似文献
219.
The light and metal catalysed rearrangement of 3,4-disubstituted-Δ2-1,2,4-oxadiazolin-5-thiones (3) to the corresponding Δ2-1,2,4-thiodiazolin-5-ones (4) is described. The thermal stability of (3) is in great great contrast to the ease of rearrangement of the acyclic analogues. The rearrangement of (3) and (4) under electron impact are described. 相似文献
220.
Summary A popular first step in the problem of structure-based, de novo molecule design is to identify regions where specific functional groups or chemical entities would be expected to interact strongly. When the three-dimensional structure of the receptor is not available, it may be possible to derive a pharmacophore giving the three-dimensional relationships between such chemical groups. The task then is to design synthetically feasible molecules which not only contain the required groups, but which can also position them in the desired relative orientation. One way to do this is to first link the groups using an acyclic chain. We have investigated the application of the tweak algorithm [Shenkin, P.S. et al., Biopolymers, 26 (1987) 2053] for generating families of acyclic linkers. These linking structures can subsequently be braced using a ring-joining algorithm [Leach, A.R. and Lewis, R.A., J. Comput. Chem., 15 (1994) 233], giving rise to an even wider variety of molecular skeletons for further studies. 相似文献