全文获取类型
收费全文 | 22688篇 |
免费 | 812篇 |
国内免费 | 23篇 |
专业分类
化学 | 14958篇 |
晶体学 | 164篇 |
力学 | 367篇 |
数学 | 3867篇 |
物理学 | 4167篇 |
出版年
2023年 | 145篇 |
2022年 | 156篇 |
2021年 | 256篇 |
2020年 | 331篇 |
2019年 | 346篇 |
2018年 | 457篇 |
2017年 | 409篇 |
2016年 | 841篇 |
2015年 | 660篇 |
2014年 | 689篇 |
2013年 | 1456篇 |
2012年 | 1434篇 |
2011年 | 1454篇 |
2010年 | 879篇 |
2009年 | 822篇 |
2008年 | 1271篇 |
2007年 | 1318篇 |
2006年 | 1184篇 |
2005年 | 1085篇 |
2004年 | 981篇 |
2003年 | 763篇 |
2002年 | 658篇 |
2001年 | 519篇 |
2000年 | 468篇 |
1999年 | 365篇 |
1998年 | 242篇 |
1997年 | 232篇 |
1996年 | 305篇 |
1995年 | 219篇 |
1994年 | 237篇 |
1993年 | 200篇 |
1992年 | 192篇 |
1991年 | 179篇 |
1990年 | 182篇 |
1989年 | 155篇 |
1988年 | 143篇 |
1987年 | 136篇 |
1986年 | 135篇 |
1985年 | 194篇 |
1984年 | 196篇 |
1983年 | 126篇 |
1982年 | 129篇 |
1981年 | 143篇 |
1980年 | 138篇 |
1979年 | 95篇 |
1978年 | 96篇 |
1977年 | 103篇 |
1976年 | 84篇 |
1975年 | 88篇 |
1973年 | 71篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
901.
Nieto-Oberhuber C López S Echavarren AM 《Journal of the American Chemical Society》2005,127(17):6178-6179
New Au(I) complexes with bulky, biphenyl phosphines are the most reactive catalysts for the cyclizations of enynes. 1,6-Enynes with an aryl ring at the alkyne give 2,3,9,9a-tetrahydro-1H-cyclopenta[b]naphthalenes by a 5-exo-dig cyclization followed by a Nazarov-type ring expansion. 1,8-Dien-3-ynes also cyclize by a 5-exo-dig pathway to form hydrindanes. 相似文献
902.
Canosa P Morales S Rodríguez I Rubí E Cela R Gómez M 《Analytical and bioanalytical chemistry》2005,383(7-8):1119-1126
The degradation of 2-(2,4-dichlorophenoxy)-5-chlorophenol (triclosan) in chlorinated water samples was investigated. Sensitive
determination of the parent compound and its transformation products was achieved by gas chromatography with mass spectrometry
detection after sample concentration, using a solid-phase extraction sorbent and silylation of the target compounds. Experiments
were accomplished using ultrapure water spiked with chlorine and triclosan concentrations in the low mg/l and ng/ml ranges
respectively. Chlorination of the phenolic ring and cleavage of the ether bond were identified as the main triclosan degradation
pathways. Both processes led to the production of two tetra- and a penta-chlorinated hydroxylated diphenyl ether, as well
as 2,4-dichlorophenol. The formation of 2,3,4-trichlorophenol was not detected in any experiment; however, significant amounts
of 2,4,6-trichlorophenol were noticed. All of these five compounds were also identified when triclosan was added to tap-water
samples with free chlorine concentrations below 1 mg/l. Minor amounts of three di-hydroxylated phenols, containing from one
to three atoms of chlorine in their structures, were also identified as unstable triclosan chlorination by-products. The analysis
of several raw wastewater samples showed the co-existence of important concentrations of triclosan and its most stable by-products
(2,4-dichlorophenol and 2,4,6-trichlorophenol), reinforcing the potential occurrence of the described transformations when
products containing triclosan are mixed with chlorinated tap water. 相似文献
903.
Bérces A Fekete A Gáspár S Gróf P Rettberg P Horneck G Rontó G 《Journal of photochemistry and photobiology. B, Biology》1999,53(1-3):36-43
To determine the impact of environmental UV radiation, biological dosimeters that weight directly the incident UV components of sunlight have been developed, improved and evaluated in the frame of the BIODOS project. Four DNA-based biological dosimeters ((i) phage T7, (ii) uracil thin layer, (iii) spore dosimeter and (iv) DLR-biofilm) have been assessed from the viewpoint of their biological relevance, spectral response and quantification of their biological effectiveness. The biological dosimeters have been validated by comparing their readings with weighted spectroradiometer data, by comparison with other biological doses, as well as with the determined amounts of DNA UV photoproducts. The data presented here demonstrate that the biological dosimeters are potentially reliable field dosimeters for measuring the integrated biologically effective irradiance for DNA damage. 相似文献
904.
1,5-Bis[(2-pyridyl)-3-sulphophenyl methylene] thiocarbonohydrazide (PSTH) immobilized on an anion-exchange resin (Dowex) has been used for the on-line preconcentration of mercury from biological samples and waters prior to its determination by inductively coupled plasma atomic emission spectroscopy. The metal was eluted from the column using a solution of 2 M HNO(3) and mixed on-line with SnCl(2). The optimum experimental conditions were evaluated for the continuous preconcentration of Hg, the direct generation of mercury vapour and the final determination of this element by ICP-AES. The enrichment, together with low blank levels of the optimized procedure, allow the simple determination of this toxic element at concentrations down to a few nanograms per milliliter. The proposed method has a linear calibration range 5-1000 ng ml(-1) of mercury, with a detection limit of 4 ng ml(-1) (S/N=3) and a sampling rate of 40 h(-1), investigated with a 9 ml sample volume. The precision of the method (evaluated as the relative standard deviation obtained after analyzing ten series of ten replicates) was +/-3.6% at the 10 ng ml(-1) level of Hg(II) and +/-1.3% at the 100 ng ml(-1) level. The accuracy of the method was examined by the analysis of certified reference materials. 相似文献
905.
Highly enantioselective copper(I)-fesulphos-catalyzed 1,3-dipolar cycloaddition of azomethine ylides
Cabrera S Gómez Arrayás R Carretero JC 《Journal of the American Chemical Society》2005,127(47):16394-16395
The catalyst system formed by Cu(CH3CN)4ClO4 and the planar chiral P,S-ligand Fesulphos behaves as a very efficient chiral Lewis acid in the catalytic asymmetric 1,3-dipolar cycloaddition of azomethine ylides. This catalyst shows a remarkable reactivity at low catalyst loading (0.5-3 mol %), affording in good yields the endo adducts with exceptional levels of enantioselectivity (up to >99% ee). This catalytic asymmetric procedure has a broad structural scope with regard to both azomethine and dipolarophile substitution. The first examples of catalytic asymmetric 1,3-dipolar cycloaddition with ketimine-derived azomethines are reported. 相似文献
906.
Alkyl substituted chromium Fischer carbene complexes react with 1,1-diphenylallene in the presence of rhodium(I) catalysts (10 mol%) to yield highly substituted dienyl indenone derivatives. In this process a catalytic chromium(0)-rhodium(I) exchange occurs, four new C-C bonds are created, and four-components (two allenes, the carbene ligand and one CO ligand) are joined in a chemo- and regioselective manner. 相似文献
907.
Rodríguez Flores J Berzas Nevado JJ Durán Merás I Rodríguez Gómez MJ 《Journal of separation science》2005,28(7):658-664
A capillary zone electrophoresis (CZE) method using a fused-silica capillary (60.2 cm x 75 microm ID) was investigated for the determination of triamterene (TRI), methotrexate (MTX), and creatinine (CREA) in human urine. The separation was performed using a hydrodynamic injection time of 7 s (0.5 psi), a voltage of 25 kV, a capillary temperature of 30 degrees C, and 40 mM phosphoric acid adjusted to pH 2.25 by addition of triethanolamine as separation electrolyte. Under these conditions, analysis takes about 15 min. A linear response over the 0.5-15.0 mg L(-1) concentration range was found for TRI and MTX, and 0.5-80.0 mg L(-1) for CREA. Dilution of the sample (water:urine, 1:1 for TRI and MTX, and 1:25 for CREA determination) was the only step necessary prior to analysis by electrophoresis. The developed method is easy, rapid, and sensitive and has been applied to determine triamterene,methotrexate, and creatinine in urine samples with satisfactory results. 相似文献
908.
Muñoz MA Ferrero R Carmona C Balón M 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2004,60(1-2):193-200
The NH-pi interactions of indole with benzene, naphthalene, phenanthrene, toluene, m-xylene, and mesitilene, in carbon tetrachloride solutions, have been studied by Fourier transform infrared spectroscopy. The experiments, carried out on the NH stretching band of indole, prove the formation of 1:1 complexes in which the NH bond of indole is engaged. The NH frequency shifts are independent of the number of rings in the base, but they progressively increase as the electron density is enhanced by methylation. The association constants increase with the increase of both, the number of rings and the methyl groups on the base. At higher base concentrations, further shifts on the free NH and associated bands indicate the formation of 1:2 complexes, which suggest hybride NH-pi and van der Waals interactions between one indole ring and two benzene acceptor molecules. 相似文献
909.
Gyula Simig Gábor Doleschall Gyula Hornyák József Fetter Károly Lempert József Nyitrai Péter Huszthy Tibor Gizur Mária Kajtár-Peredy 《Tetrahedron》1985,41(2):479-484
Acylation of the N-substituted diethyl aminomalonates 3a–3d with diketene furnished the ring tautomers 6a–6d of the expected acetoacetyl derivatives 5. By treatment with iodine and sodium ethoxide compounds 6a–6d are smoothly converted into the β-lactam derivatives 2a–2d. Deethoxycarbonylation of the ethylene ketals 7a–7d of the latter furnishes mixtures of the corresponding diastereomeric monoesters 8 and10. The ethoxycarbonyl groups of the trans esters 8 are more reactive than those of the cis isomers 10. This permits, under appropriate conditions, selective alkaline hydrolysis and NaBH4 reduction of the trans esters 8 in the presence of the cis esters 10. Reduction of the cis ester 10c under more forceful conditions furnishes the trans hydroxymethyl derivative 11c. 相似文献
910.
A. Negrón-Mendoza G. Albarrán S. Castillo-Rojas 《Journal of Radioanalytical and Nuclear Chemistry》1992,160(2):461-466
The purpose of this work is to study the radiolytic behavior of aqueous solutions of14C-pyruvic acid (oxygen-free). Several parameters were varied such as radiation dose, pH, etc. Gas chromatography, mass spectrometry and high performance liquid chromatography were used extensively for the analysis of radiolytic products. The more abundant product was dimethyltartaric acid. Other products identified were acetic, lactic, succinic, malonic and tricarballylic acids, acetaldehyde, carbon dioxide, and hydrogen. The yields of these products were influenced by the irradiation dose, concentration of the target compound and pH of the solution. 相似文献