全文获取类型
收费全文 | 5222篇 |
免费 | 137篇 |
国内免费 | 42篇 |
专业分类
化学 | 3488篇 |
晶体学 | 64篇 |
力学 | 100篇 |
综合类 | 1篇 |
数学 | 625篇 |
物理学 | 1123篇 |
出版年
2022年 | 26篇 |
2021年 | 34篇 |
2020年 | 60篇 |
2019年 | 61篇 |
2018年 | 37篇 |
2017年 | 45篇 |
2016年 | 94篇 |
2015年 | 100篇 |
2014年 | 111篇 |
2013年 | 243篇 |
2012年 | 245篇 |
2011年 | 392篇 |
2010年 | 152篇 |
2009年 | 145篇 |
2008年 | 310篇 |
2007年 | 327篇 |
2006年 | 336篇 |
2005年 | 324篇 |
2004年 | 263篇 |
2003年 | 195篇 |
2002年 | 194篇 |
2001年 | 71篇 |
2000年 | 76篇 |
1999年 | 72篇 |
1998年 | 69篇 |
1997年 | 59篇 |
1996年 | 103篇 |
1995年 | 67篇 |
1994年 | 81篇 |
1993年 | 93篇 |
1992年 | 80篇 |
1991年 | 59篇 |
1990年 | 44篇 |
1989年 | 32篇 |
1988年 | 39篇 |
1987年 | 29篇 |
1986年 | 36篇 |
1985年 | 66篇 |
1984年 | 50篇 |
1983年 | 40篇 |
1982年 | 66篇 |
1981年 | 55篇 |
1980年 | 52篇 |
1979年 | 42篇 |
1978年 | 54篇 |
1977年 | 44篇 |
1976年 | 30篇 |
1975年 | 43篇 |
1974年 | 22篇 |
1973年 | 22篇 |
排序方式: 共有5401条查询结果,搜索用时 31 毫秒
61.
Rodríguez AD Ramírez C Rodríguez II Barnes CL 《The Journal of organic chemistry》2000,65(5):1390-1398
Four diterpenes and a nor-diterpenoid, all of which possess unusual carbocyclic skeletons, were isolated from the hexane solubles of the West Indian gorgonian Pseudopterogorgia elisabethae. The structures and relative configurations of novel metabolites elisabethin D (2), elisabethin D acetate (3), 3-epi-elisabanolide (5), elisapterosin A (6), and elisapterosin B (7) were elucidated by interpretation of overall spectral data, which included 2D NMR correlation methods, IR, UV, and accurate mass measurements (HREI-MS and HRFAB-MS), chemical reactions, and X-ray diffraction analyses. The tetracyclic carbon skeleton of the elisapterosins is undescribed and constitutes a new class of C(20) rearranged diterpenes. Elisapterosin B displays strong in vitro anti-tuberculosis activity. 相似文献
62.
The use of temperature as a variable in liquid chromatography enables the facile alteration of eluotropic strength without the need to change solvent composition. The ability to change eluotropic strength via temperature alone means that thermal gradients can be used to mimic the effects of solvent gradients but without many of the unwanted effects of changes in solvent composition. Here we illustrate the use of thermal gradients as a means of controlling chromatographic separations using either constant flow or, with the flow rate increased to maintain isobaric conditions, constant pressure, performed using columns packed with 1.7 μm particles. A model is described that can be used to used to predict flow, pressure and temperature under gradient conditions. Practical experimental factors such as the need for post column cooling and the use of frit restrictors in order to obtain optimum results are described. 相似文献
63.
The influence of aryl substituents in dibenzylidene acetone (dba) ligands, for Pd(0) complexes, has been evaluated for Suzuki-Miyaura cross-coupling reactions. Electron-withdrawing substituents such as NO(2) or CF(3) deactivate the catalyst species whereas strongly donating substituents such as OMe increase catalytic activity over that of unsubstituted dba ligands. [reaction: see text] 相似文献
64.
[structure: see text] An improved, third-generation, total synthesis of (+)-discodermolide, a potent microtubule-stabilizing anticancer agent of marine sponge origin, is achieved in 11.1% yield over 21 steps. Key steps include a Still-Gennari HWE olefination, performed using NaH as the base, between C1-C8 beta-ketophosphonate 7 and C9-C24 aldehyde 8, introducing the (8Z)-alkene with 10:1 selectivity, and K-Selectride reduction of the derived enone 16, installing the (7S)-configuration. 相似文献
65.
P. A. Barnes G. M. Parkes P. Sheridan 《Journal of Thermal Analysis and Calorimetry》1994,42(5):841-854
Analysis of very small particles can present problems. This paper describes the application of temperature programmed solid insertion probe mass spectrometry (TP-SIP-MS), scanning electron microscopy and SEM X-ray microanalysis to the identification of foreign particles present in an industrial product. The relative advantages and limitations of the techniques are discussed. It is shown that TP-SIP-MS is a powerful tool for such work and complements the use of more conventional microanalytical methods. 相似文献
66.
Jensen BP Smith CJ Bailey CJ Rodgers C Wilson ID Nicholson JK 《Rapid communications in mass spectrometry : RCM》2005,19(4):519-524
Inductively coupled plasma mass spectrometry (ICPMS) has been used to determine the rate and routes of excretion of bromine following the intraperitoneal administration (50 mg kg(-1)) of 2-, 3- and 4-bromobenzoic acids to male bile-duct-cannulated rats. Analysis of urine and bile for (79/81)Br using ICPMS showed that all three bromobenzoic acids were rapidly excreted (82-98%) within 48 h of dosing, primarily via the urine. High-performance liquid chromatography/inductively coupled plasma mass spectrometry (HPLC/ICPMS) was then used to obtain metabolite profiles for bile and urine. These profiles revealed that extensive metabolism had taken place, with the unchanged bromobenzoic acids forming a minor part of the total of compound-related material detected. Concomitant MS studies, supplemented by alkaline hydrolysis, enabled the identification of the major metabolite of all three of the bromobenzoic acids as a glycine conjugate. Ester glucuronide conjugates were also identified, but formed only a small proportion of total. 相似文献
67.
Gianluca Pozzi Marco Cavazzini Orsolya Holczknecht Silvio Quici Ian Shepperson 《Tetrahedron letters》2004,45(22):4249-4251
A fluorous-tagged TEMPO radical has been prepared and its catalytic activity in the chemoselective oxidation of alcohols to carbonyl compounds has been investigated. The new fluorous radical proved to be an efficient, selective and easily recoverable catalyst, which can be conveniently used in standard organic solvents and then isolated and recycled by fluorous liquid-liquid extraction. 相似文献
68.
Hale KJ Frigerio M Manaviazar S Hummersone MG Fillingham IJ Barsukov IG Damblon CF Gescher A Roberts GC 《Organic letters》2003,5(4):499-502
[structure: see text] The synthesis of two truncated bryostatin analogues 2 and 3 is described. High-field NMR measurements on the C-ring analogue 3 in C(2)H(3)CN containing 25% (2)H(2)O have shown that it binds to the CRD2 of human PKC-alpha at virtually the same position as phorbol-13-acetate (PA) and bryostatin 1 (1). NMR titration studies have also revealed that 3 binds to the CRD2 with a potency similar in magnitude to PA but much less potently than 1. 相似文献
69.
[reaction: see text] An effective strategy has been developed for the rapid and efficient one-pot synthesis of 2-aryl-5-substituted-2,3-dihydrobenzofurans from readily available o-nitrotoluenes and aromatic aldehydes. This strategy allows access to a structurally diverse array of products for further manipulation. 相似文献
70.
Kiyohisa Fujimori Ian E. Craven 《Journal of polymer science. Part A, Polymer chemistry》1986,24(3):559-565
Georgiev and Shirota's simplified terminal complex model was applied to the dilatometrically measured initial rate of copolymerization of sytrene (ST) with maleic anhydride (MA) in methyl ethyl ketone (MEK) at 50°C. The rate was maximum at the feed MA mole fractions of 0.752, 0.769, and 0.806 at the total monomer concentrations of 2M, 1.5M, and 0.5M, respectively. Shirota's method gave the following ratios of propagation rate constants: βA=kAC/kAD = 8.25 and βD = kDC/kDA = 2.70. Georgiev's method gave βA = 14, βD = 2.7, and α = kAD/kDA = 22. The equilibrium constant of the donor-acceptor complexation between ST and MA in MEK was measured to be 0.045 dm3/mol at room temperature. 相似文献