全文获取类型
收费全文 | 3847篇 |
免费 | 265篇 |
国内免费 | 27篇 |
专业分类
化学 | 2383篇 |
晶体学 | 50篇 |
力学 | 120篇 |
数学 | 480篇 |
物理学 | 698篇 |
无线电 | 408篇 |
出版年
2023年 | 27篇 |
2022年 | 103篇 |
2021年 | 121篇 |
2020年 | 121篇 |
2019年 | 160篇 |
2018年 | 128篇 |
2017年 | 121篇 |
2016年 | 197篇 |
2015年 | 145篇 |
2014年 | 162篇 |
2013年 | 330篇 |
2012年 | 233篇 |
2011年 | 258篇 |
2010年 | 173篇 |
2009年 | 185篇 |
2008年 | 229篇 |
2007年 | 240篇 |
2006年 | 170篇 |
2005年 | 133篇 |
2004年 | 119篇 |
2003年 | 104篇 |
2002年 | 99篇 |
2001年 | 63篇 |
2000年 | 50篇 |
1999年 | 39篇 |
1998年 | 23篇 |
1997年 | 20篇 |
1996年 | 24篇 |
1995年 | 15篇 |
1994年 | 13篇 |
1993年 | 18篇 |
1992年 | 17篇 |
1991年 | 11篇 |
1990年 | 13篇 |
1989年 | 7篇 |
1988年 | 10篇 |
1987年 | 18篇 |
1986年 | 12篇 |
1985年 | 16篇 |
1984年 | 29篇 |
1983年 | 11篇 |
1981年 | 16篇 |
1980年 | 22篇 |
1979年 | 11篇 |
1978年 | 12篇 |
1977年 | 8篇 |
1976年 | 7篇 |
1975年 | 9篇 |
1973年 | 7篇 |
1862年 | 8篇 |
排序方式: 共有4139条查询结果,搜索用时 765 毫秒
41.
Mustafa Odabaolu idem Albayrak Reit
zkanca Fatma Zehra Aykan Peter Lonecke 《Journal of Molecular Structure》2007,840(1-3):71-89
Some new substituted polyhydroxy azo–azomethine compounds were prepared by reaction of tris(hydroxymethyl)aminomethane with (E)-2-hydroxy-5-(phenyldiazenyl) benzaldehyde and its substituted derivatives. The structures of azo and azo–azomethine compounds were determined by IR, UV–vis, 1H NMR and 13C NMR spectroscopic techniques, and/or X-ray diffraction studies. According to IR spectra, all azo–azomethine compounds adopt keto form in solid state. UV–vis analysis has shown the presence of keto–enol tautomerism in solution for all azo–azomethine compounds, except that for nitro substituted derivative, enol form is dominantly favored in solution. At the same time, above mentioned derivative compounds were studied in vitro for their antimicrobial properties. Among the phenylazosalicylaldehyde series compound tested, 4-phenylazosalicylaldehyde, 4-(3-chlorophenylazo)salicylaldehyde, 4-(2-chlorophenylazo)salicylaldehyde, 4-(4-fluorophenylazo)salicylaldehyde, 4-(3-chlorophenylazo)salicylaldehyde and 4-(4-ethylphenylazo)salicylaldehyde showed a weak antimicrobial activity only against gram positive bacteria. On the contrary, phenylazosalicylaldehyde series compounds were reacted tris(hydroxmethyl)aminomethane, that exhibited a strong antimicrobial activity against gram positive bacteria, yeast and mould. Moreover, while the 2-{[1,3-dihydroxy-2-(hydroxymethyl)propan-2-ylimino]methyl}phenol did not show an inhibition on tested microorganism, the addition of phenyldiazine groups to 2-{[1,3-dihydroxy-2-(hydroxymethyl)propan-2-ylimino]methyl}phenol resulted in a strong increases in antimicrobial activity. 相似文献
42.
E. Abdel-Latif H. M. Mustafa H. A. Etman A. A. Fadda 《Russian Journal of Organic Chemistry》2007,43(3):443-448
The reaction of malononitrile dimer with phenyl isothiocyanate gave (6-amino-1-phenyl-2-thioxo-1,2,3,4-tetrahydropyrimidin-4-ylidene)malononitrile
which was then used as starting material in the synthesis of pharmacologically important fused pyrimidine derivatives, such
as 4-dicyanomethylidene-1,5-diphenyl-2-thioxo-1,2,3,4-tetrahydropyrrolo[2,3-d]pyrimidine, 6-cyano-4-dicyanomethylidene-7-methylsulfanyl-5-oxo-1-phenyl-2-thioxo-1,2,3,4,5,8-hexahydropyrido[2,3-d]pyrimidine, 6-dicyanomethylidene-3-phenyl-2-thioxo-1,2,3,6-tetrahydro-9H-purine, and 6-substituted 4-dicyanomethylidene-7-oxo-1-phenyl-2-thioxo-1,2,3,4,7,8-hexahydropteridines.
Published in Russian in Zhurnal Organicheskoi Khimii, 2007, Vol. 43, No. 3, pp. 443–449.
The text was submitted by the authors in English. 相似文献
43.
Polymer colloids based on 3‐O‐methacryloyl‐1,2:5,6‐di‐O‐isopropylidene‐α‐D ‐glucofuranose (3‐MDG) and butyl acrylate (BA) were prepared via free radical mini‐emulsion polymerization. The kinetic and colloidal features of the copolymerization were investigated. The final particle size (D) of the sugar latexes is inversely proportional to the concentration of the anionic emulsifier (sodium dodecyl sulphate, SDS) and the non‐ionic one (alkyl polyglucoside, APG). It was also found that D is independent of the concentration of either the water‐soluble initiator (potassium persulfate, KPS), or the oil‐soluble initiator (2,2′‐azobisisobutyronitrile, AIBN). The rate of mini‐emulsion polymerization is lower in comparison with the conventional emulsion polymerization under the same conditions. The polymerization rate (Rp) and the total number of particles (Np) are proportional to the 0.72th and 0.93th power of the SDS, and to the 1.40th and 2.22th of the APG concentration. Following reaction orders, 0.79/0.06 were obtained for Rp/Np versus the concentration of KPS, and 0.22/?0.01 for AIBN, respectively. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
44.
Preparation of ultradispersed transition metals (20–100 nm) by reduction of metal complexes in molecular microreactors (cavities of immobilized calixarenes, polyelectrolytes) was studied.Translated from Zhurnal Prikladnoi Khimii, Vol. 77, No. 11, 2004, pp. 1914–1916.Original Russian Text Copyright © 2004 by Altshuler, Sapozhnikova. 相似文献
45.
In the work described here, a biosensor was developed for the determination of sulfite in food. Malva vulgaris tissue homogenate containing sulfite oxidase enzyme was used as the biological material. M. vulgaris tissue homogenate was crosslinked with gelatin using glutaraldehyde and fixed on a pretreated Teflon membrane. Sulfite was enzymatically converted to sulfate in the presence of the dissolved oxygen, which was monitored amperometrically. Sulfite determination was carried out by standard curves, which were obtained by the measurement of consumed oxygen level related to sulfite concentration. Several operational parameters had been investigated: the amounts of plant tissue homogenate and gelatin, percentage of glutaraldehyde, optimum pH and temperature. Also, some characterization studies were done. There was linearity in the range between 0.2 and 1.8 mM at 35 °C and pH 7.5. The results of real sample analysis obtained with the biosensor agreed well with the enzymatic reference method using spectrophotometric detection. 相似文献
46.
Kayed A. Abu Safieh Ahmad M. Abu Mahthieh Mustafa M. El-Abadelah Mikdad T. Ayoub Wolfgang Voelter 《Monatshefte für Chemie / Chemical Monthly》2007,138(2):157-160
Summary. A novel method for the synthesis of a new series of 5-substituted 1,3-dimethyl pyrazolo[4,3-e][1,2,4]triazines is described. The new synthetic strategy is based on the classical Bischler 1,2,4-benzotriazine synthesis. This approach involves the preparation of 5-hydrazinopyrazole from 5-chloro-1,3-dimethyl-4-nitropyrazole
followed by acylation and nitro group reduction to form the corresponding 4-amino-3-(acylhydrazino)pyrazoles. Intramolecular
oxidative cyclization of the latter derivatives, using polyphosphoric acid, produced the respective target pyrazolotriazines. 相似文献
47.
Kayed A. Abu Safieh Ahmad M. Abu Mahthieh Mustafa M. El-Abadelah Mikdad T. Ayoub Wolfgang Voelter 《Monatshefte für Chemie / Chemical Monthly》2007,21(9):157-160
A novel method for the synthesis of a new series of 5-substituted 1,3-dimethyl pyrazolo[4,3-e][1,2,4]triazines is described. The new synthetic strategy is based on the classical Bischler 1,2,4-benzotriazine synthesis. This approach involves the preparation of 5-hydrazinopyrazole from 5-chloro-1,3-dimethyl-4-nitropyrazole
followed by acylation and nitro group reduction to form the corresponding 4-amino-3-(acylhydrazino)pyrazoles. Intramolecular
oxidative cyclization of the latter derivatives, using polyphosphoric acid, produced the respective target pyrazolotriazines. 相似文献
48.
Hans R. Kricheldorf Majdi Al Masri 《Journal of polymer science. Part A, Polymer chemistry》1995,33(15):2667-2671
Attempts were made to synthesize poly(ether-sulfone)s from aliphatic diols or bissilylated diols on the one hand, and 4,4′-dichlorodiphenylsulfone or 4,4′-difluorodiphenylsulfone on the other hand. The reaction conditions and the catalyst were varied. Polycondensations of silylated diols with 4,4′-difluorodiphenylsulfone and powdered K2 CO3 in N-methylpyr-rolidone proved to give the best results. Using silylated isosorbide and isomannide as mono-mers chiral poly(ether-sulfone)s were prepared. GPC measurements indicate weight-average molecular weights in the range of 27×103–200×103. © 1995 John Wiley & Sons, Inc. 相似文献
49.
Renata Ga<parová Daniel Zbojek Margita Lácová Katarína Král'ová Anton Gatial Branislav Horváth Alžbeta Krutošíková 《Central European Journal of Chemistry》2005,3(4):622-646
The reactions of substituted furo[3,2-b]pyrrole-5-carboxhydrazides 1 with 5-arylfuran-2-carboxaldehydes 2, 4,5-disubstituted furan-2-carboxaldehydes 3 and thiophene-2-carboxaldehyde 4 has been studied. The advantage of microwave irradiation on some of these reactions was reflected in the reduced reaction
time and increased yields. Reactions of 1 with 4-substituted 1,3-oxazol-5(4H)-ones 11 led to diacylhydrazines 13 or to imidazole derivatives 14 depending on the temperature. 1,2,4-Triazole-3-thione 17 was synthesized by two-step reaction of 1 with phenylisothiocyanate and subsequent base-catalyzed cyclization of thiosemicarbazide 16. The effects of hydrazones 5–10 on inhibition of photosynthetic electron transport in spinach chloroplasts and chlorophyll content in the antialgal suspensions
of Chlorella vulgaris were investigated. 相似文献
50.
In the presence of triethylamine, 2-mercaptobenzoic acid readily adds onto acylhydrazonoyl chlorides (1a-c) (precursors of the reactive nitrile imine 1,3-dipolar species) to afford good yields of the corresponding 2-[(2-oxo-1-arylhydrazonopropan-1-yl)mercapto]benzoic acids (2a-c). The latter acyclic adducts, in THF in the presence of 1,1'-carbonyldiimidazole, undergo intramolecular cyclization involving the activated carboxy and the enol functionality to deliver the respective 2-(N-arylhydrazono)-3-oxobenzothiophenes (3a-c). In the solid state, the latter compounds adopt the (Z)-geometry around the C=N double bond as evidenced by single crystal X-ray structure determination for 3b. 相似文献