全文获取类型
收费全文 | 3561篇 |
免费 | 112篇 |
国内免费 | 22篇 |
专业分类
化学 | 2048篇 |
晶体学 | 50篇 |
力学 | 88篇 |
数学 | 618篇 |
物理学 | 891篇 |
出版年
2023年 | 23篇 |
2022年 | 60篇 |
2021年 | 52篇 |
2020年 | 64篇 |
2019年 | 81篇 |
2018年 | 74篇 |
2017年 | 79篇 |
2016年 | 114篇 |
2015年 | 75篇 |
2014年 | 134篇 |
2013年 | 275篇 |
2012年 | 197篇 |
2011年 | 207篇 |
2010年 | 147篇 |
2009年 | 119篇 |
2008年 | 159篇 |
2007年 | 154篇 |
2006年 | 148篇 |
2005年 | 103篇 |
2004年 | 114篇 |
2003年 | 94篇 |
2002年 | 78篇 |
2001年 | 64篇 |
2000年 | 57篇 |
1999年 | 41篇 |
1998年 | 30篇 |
1997年 | 24篇 |
1996年 | 32篇 |
1995年 | 28篇 |
1994年 | 52篇 |
1993年 | 39篇 |
1992年 | 56篇 |
1991年 | 35篇 |
1990年 | 44篇 |
1989年 | 39篇 |
1988年 | 34篇 |
1987年 | 30篇 |
1986年 | 30篇 |
1985年 | 33篇 |
1984年 | 42篇 |
1983年 | 31篇 |
1982年 | 44篇 |
1981年 | 26篇 |
1980年 | 26篇 |
1979年 | 40篇 |
1978年 | 39篇 |
1977年 | 27篇 |
1976年 | 20篇 |
1975年 | 21篇 |
1973年 | 24篇 |
排序方式: 共有3695条查询结果,搜索用时 31 毫秒
81.
A mild and highly efficient deoxygenation of variety of N-oxides using an inexpensive CuX, or a CuX-Zn or CuX-Al couple is described. 相似文献
82.
The technique of three-phase partitioning (TPP) was used to purify the green fluorescent protein (GFP) in a single step. TPP uses a combination of ammonium sulphate and tert-butanol to precipitate proteins from their crude extracts. In the first round of TPP with 20% ammonium sulphate saturation at the ratio of crude to tert-butanol 1:1 (v/v), most of the GFP remains in the lower aqueous phase. When subjected to a second round of TPP with 60% ammonium sulphate saturation at the ratio of crude to tert-butanol 1:2 (v/v) gives 78% recovery of GFP with a 20-fold purification. The sodium dodecyl sulphate-polyacrylamide gel electrophoretic (SDS-PAGE) analysis of purified preparation shows single band. The fluorescence excitation and emission spectra agreed with values reported in literature. 相似文献
83.
The article reviews in brief, thede novo group additivity approach and, at length, the different topological approaches to obtain predictive and internally consistent
correlations between various properties and structural features of molecules. The stress has mainly been on the molecular
connectivity method. A new rational scheme for nomenclature of connectivity indices of different orders and types is introduced.
The concept of the perturbation connectivity parameter developed by us recently has been applied to obtain correlations for
molar refraction, boiling point, molar volume, heat of atomisation, heat of combustion, heat of vaporisation, magnetic susceptibility
and critical constants of alkanes, alcohols and alkylbenzenes. A comparative study of various approaches reveals that the
present perturbation topological approach has an edge over other similar methods. 相似文献
84.
[structure: see text] Receptors based on a tetraphenylporphyrin scaffold bearing different charged and hydrophobic groups have been synthesized. The interactions of these with horse heart cytochrome c were studied by fluorescence spectroscopy. Receptor 4 was identified to be the strongest synthetic receptor (K(d) = 20 nM) for cytochrome c. The differences in affinity among the receptors reflected a dependence on the number of anionic and hydrophobic groups. 相似文献
85.
l-Proline (10 mol %) was found to be a versatile organocatalyst for the selective synthesis of 2-aryl-1-arylmethyl-1H-benzimidazoles from a wide range of substituted o-phenylenediamines and aldehydes in moderate to excellent isolated yields (32-95%) under mild conditions using chloroform as a solvent at ambient temperature. 相似文献
86.
Maverick AW Laxman RK Hawkins MA Martone DP Fronczek FR 《Dalton transactions (Cambridge, England : 2003)》2005,(1):200-206
The tetradentate Schiff-base ligand SIXH2 (alpha,alpha-bis(salicylimino)-m-xylene), prepared from salicylaldehyde and m-xylylenediamine, forms cofacial binuclear complexes with Pd and Cu. Of the two isomers possible (trans-syn and trans-anti) for M2(SIX)2, these complexes crystallize exclusively as the trans-anti isomer. In ansolvous Pd2(SIX)2, the metal-containing planes are approximately parallel, with PdPd 4.416(1) A. Pd2(SIX)2 also forms a crystalline solvate, in which the molecules adopt a more open conformation with longer metal-metal distances (5.109(1) and 5.112(1) A). The M...M distance is significantly longer in Cu2(SIX)2 (6.653(1) A), because of conformational changes in the m-xylylene moieties and substantial tetrahedral distortion about Cu. 相似文献
87.
A.C. JainS.M. Jain 《Tetrahedron》1972,28(4):981-986
2,4-Dihydroxyphenyl benzyl ketone (I) condenses with 2-methyl but-3-en-2-ol in the presence of boron trifluoride etherate to afford a mixture of 3-C-, 5-C- and 3,5-di C C-prenyl derivatives separable by chromatography. The above mixture without the 5-C-prenyl but with the 4-O-prenyl derivative results when ketone I is treated with prenyl bromide in the presence of methanolic potash. 3-C-(III a) and 5-C-(IV) prenyl derivatives undergo ready oxidative cyclization with DDQ to yield the corresponding pyrano derivatives VIII and XII respectively which form good starting materials for various isoflavone condensations. Thus both linear and angular 6″,6″-dimethyl pyrano-isoflavones and their 2-methyl and 2-phenyl analogues have been synthesized. 相似文献
88.
Summary The stability of emulsion of Toluene in water stabilized by partially flocculated ferric hydroxide sol has been studied. The
size of the emulsion droplets as determined by size frequency analysis is found to be of 1.1 μ radius. The flocculation studies were carried out haemocytometrically at different amount of electrolytes and at different
pH values. The electrokinetic potential ‘ζ’ was determined micro-electrophoretically at different amounts of electrolytes and
also at different pH's. In the case of tri and tetra valent anions the charge reversal was observed. In the light of the D.V.O. theory the interaction
energy profiles were drawn and 2–6 kT deep secondary minima obtained showing thereby the occurrence of reversible flocculation. Degree of aggregation has been
calculated theoretically at different values of ‘A’ and the value of A was found to be 1.0×10−12 erg to fit the value of degree of aggregation obtained haemocytometrically. The charge densities and binding parameters have
also been calculated. 相似文献
89.
E. coli peptide deformylase (PDF) catalyzes the deformylation of nascent polypeptides generated during protein synthesis. While PDF was originally thought to be a zinc enzyme, subsequent studies revealed that the active site metal is iron. In an attempt to understand this unusual metal preference, high-resolution structures of Fe-, Co-, and Zn-PDF were determined in complex with its deformylation product, formate. In all three structures, the formate ion binds the metal and forms hydrogen-bonding interactions with the backbone nitrogen of Leu91, the amide side chain of Gln50, and the carboxylate side chain of Glu133. One key difference, however, is how the formate binds the metal. In Fe-PDF and Co-PDF, formate binds in a bidentate fashion, while in Zn-PDF, it binds in a monodentate fashion. Importantly, these structural results provide the first clues into the origins of PDF's metal-dependent activity differences. On the basis of these structures, we propose that the basis for the higher activity of Fe-PDF stems from the better ability of iron to bind and activate the tetrahedral transition state required for cleavage of the N-terminal formyl group. 相似文献
90.
S. K. Aggarwal G. Chourasiya R. K. Duggal R. Rao P. A. Ramasubramanian H. C. Jain 《Journal of Radioanalytical and Nuclear Chemistry》1987,119(1):1-12
A reverse isotope dilution alpha spectrometric /R-IDAS/ method using239Pu as a spike is described for the determination of plutonium concentration in high burn-up fuel samples wth238Pu/(239Pu+240Pu) alpha activity ratio >0.5, without resorting to any purification from241Am and a bulk of other impurities. It involves the addition of a pre-clibrated spike solution to a known aliquot of the plutonium sample solution followed by source preparation using TEG as a spreading agent. The results obtained on a number of plutonium samples containing 20–80% of241Am /alpha activity wise/ using this method are compared with those achieved by R-IDAS using purification with TTA, with respect to precision and accuracy. Precision and accuracy of 0.5% are demonstrated. This method eliminates the need of any separation and purification of plutonium from241Am and a bulk of other impurities like uranium. 相似文献