全文获取类型
收费全文 | 2953篇 |
免费 | 142篇 |
专业分类
化学 | 1819篇 |
晶体学 | 21篇 |
力学 | 47篇 |
数学 | 603篇 |
物理学 | 605篇 |
出版年
2021年 | 40篇 |
2020年 | 60篇 |
2019年 | 43篇 |
2018年 | 50篇 |
2017年 | 63篇 |
2016年 | 154篇 |
2015年 | 136篇 |
2014年 | 121篇 |
2013年 | 234篇 |
2012年 | 155篇 |
2011年 | 200篇 |
2010年 | 128篇 |
2009年 | 97篇 |
2008年 | 101篇 |
2007年 | 73篇 |
2006年 | 66篇 |
2005年 | 32篇 |
2004年 | 41篇 |
2003年 | 31篇 |
2002年 | 28篇 |
2001年 | 24篇 |
2000年 | 31篇 |
1999年 | 28篇 |
1998年 | 27篇 |
1997年 | 24篇 |
1990年 | 25篇 |
1988年 | 22篇 |
1987年 | 22篇 |
1986年 | 22篇 |
1985年 | 30篇 |
1984年 | 30篇 |
1983年 | 26篇 |
1982年 | 28篇 |
1981年 | 22篇 |
1978年 | 25篇 |
1977年 | 22篇 |
1976年 | 23篇 |
1973年 | 22篇 |
1969年 | 22篇 |
1968年 | 40篇 |
1966年 | 23篇 |
1965年 | 42篇 |
1964年 | 45篇 |
1963年 | 27篇 |
1962年 | 38篇 |
1961年 | 36篇 |
1960年 | 32篇 |
1959年 | 29篇 |
1958年 | 23篇 |
1957年 | 25篇 |
排序方式: 共有3095条查询结果,搜索用时 734 毫秒
131.
Ondřej Rudolf Michal Rouchal Antonín Lyčka Antonín Klásek 《Helvetica chimica acta》2013,96(10):1905-1917
3‐Alkyl/aryl‐3‐hydroxyquinoline‐2,4‐diones were reduced with NaBH4 to give cis‐3‐alkyl/aryl‐3,4‐dihydro‐3,4‐dihydroxyquinolin‐2(1H)‐ones. These compounds were subjected to pinacol rearrangement by treatment with concentrated H2SO4, resulting in 4‐alkyl/aryl‐3‐hydroxyquinolin‐2(1H)‐ones. When a benzyl (Bn) group was present in position 3 of the starting compound, its elimination occurred during the rearrangement, and the corresponding 3‐hydroxyquinolin‐2(1H)‐one was formed. The reaction mechanisms are discussed for all transformations. All compounds were characterized by IR, 1H‐ and 13C‐NMR spectroscopy, as well as mass spectrometry. 相似文献
132.
Ondřej Rudolf Vladimír Mrkvička Antonín Lyčka Michal Rouchal Antonín Klásek 《Journal of heterocyclic chemistry》2013,50(Z1):E100-E110
3‐Chloroquinoline‐2,4‐diones do not react with phosphoryl chloride, however, 2,4‐dichloroquinolines and/or 4‐chloroquinolin‐2‐ones are formed in the presence of N,N‐dimethylaniline. Along with these compounds, small quantities of novel dihydrogen phosphates of 4‐hydroxyquinolin‐2‐ones were isolated. We outline a simple procedure that allows for the preparation of these compounds in moderate to good yields. All compounds were characterized by 1H and 13C NMR, IR, EI‐MS, and ESI‐MS spectroscopy, and in select cases by 31P NMR spectroscopy. 相似文献
133.
Michaela Mačková Michal Himl Jan Budka Michaela Pojarová Ivana Císařová Václav Eigner Petra Cuřínová Hana Dvořáková Pavel Lhoták 《Tetrahedron》2013,69(4):1397-1402
Methylation of 5,11,17,23-tetranitrothiacalix[4]arene with diazomethane leads to the tetramethoxy derivative, which was studied using single-crystal X-ray crystallography. It revealed that this compound, albeit in the 1,3-alternate conformation, can form the inclusion complexes with various solvent molecules possessing acidic methyl groups (ethyl acetate, nitromethane, acetone, acetonitrile) and creates interesting infinite channels filled with solvent molecules. The subsequent transformation of nitro groups into the ureido moieties gave receptors capable of anion recognition even in a highly HB-competitive solvent like DMSO. 相似文献
134.
Galina Karabanovich Jaroslav Roh Zdeňka Padělková Zdeněk Novák Kateřina Vávrová Alexandr Hrabálek 《Tetrahedron》2013
1-Substituted-5-alkylsulfanyl-1H-tetrazoles are well known class of organic substances with various applications in medicinal chemistry or photographic industry. Their selenium analogues, 1-substituted-5-alkylselanyl-1H-tetrazoles are, however, much less explored because of the lack of suitable methods for their preparation. In this work we investigated the synthesis of 1-alkyl/aryl-5-alkylselanyl-1H-tetrazoles from synthetically available alkyl/arylisoselenocyanates. One-pot reactions of arylisoselenocyanates with sodium azide and alkylating agent led to the target 5-alkylselanyl-1-aryl-1H-tetrazoles but also to interesting side products, namely N-alkyl-N-arylcyanamides and (Z)-Se-alkyl-N-cyano-N,N′-diarylisoselenoureas. Nevertheless, when alkylisoselenocyanates were utilized as the substrates, the reactions led exclusively to the formation of 1-alkyl-5-alkylselanyl-1H-tetrazoles in good yields. This simple one-pot procedure brings new possibilities for the preparation of variously substituted selenium compounds. It also opens the way to further investigations of selenium isosteres of the widely utilized 5-thiotetrazole moiety in biomedical applications. 相似文献
135.
Marcel Špulák Zdeněk Novák Karel Palát Jiří Kuneš Jana Pourová Milan Pour 《Tetrahedron》2013,69(6):1705-1711
Contrary to a number of reports, alkylations of the privileged 3,4-dihydroquinazoline scaffold provide N3-alkylated products, and not 4-alkoxyquinazolines. To correctly assign the structure, 13C NMR shifts of the –Z–CHn– (Z=O, N) fragment are necessary; resonances in the 45–55 ppm range are indicative of N3-alkylation. Treatment of 3,4-dihydroquinazoline-4-one with p-TsCl afforded the N3-tosylated compound, whose reaction with an amine yielded the corresponding N3-alkyl derivative. A mechanism corroborated by 15N-labeling involving pyrimidine ring opening and recyclisation is proposed. Finally, the unambiguous preparation of 4-alkoxyquinazolines is described via treatment of 3,4-dihydroquinazoline-4-ones with PCl5 followed by an alkoxide. 相似文献
136.
Silver nanolayers sputtered on polytetrafluoroethylene and their changes induced by post‐deposition annealing at 100–300 °C are studied. Changes in surface morphology and roughness are examined by atomic force microscopy and by measurement of electrical sheet resistance by two‐point technique. Chemical composition was determined by X‐ray photoelectron spectroscopy (XPS) and electrokinetic analysis in dependence on the gold layer thickness. The annealing at 300 °C leads to significant rearrangement of the silver layer, and the transition threshold increases to 35 nm. The presence of oxidized structures on silver‐coated samples is observed in XPS spectra and by electrokinetic analysis, too. Annealing of pristine and silver‐coated poly(tetrafluoroethylene) at 300 °C results in significant change of the sample surface morphology and chemistry. There is observed formation of isolated silver islands on the surface, which could be connected with silver melting. Later, the silver agglomeration takes place. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
137.
Jiřf Skvarla 《Journal of Dispersion Science and Technology》2013,34(3-4):349-380
The Van Oss surface thermodynamic theory of polar and apolar interfacial interactions was extended to the interaction between mineral surfaces and bubbles across liquid media. The acid base (polar) interfacial interactions are supposed to be responsible for the hydration repulsion between a hydrophilic mineral and a bubble as well as for the hydrophobic attraction between a hydrophobic mineral and the bubble. 相似文献
138.
Kateřina Černovská Bedřich Košata Vladimíra Novotná Milada Glogarová 《Liquid crystals》2013,40(9):987-996
Several series of new benzofused thieno[3,2‐b]furan‐ and thieno[3,2‐b]thiophene‐based derivatives have been synthesized and their mesomorphic properties investigated. All the studied compounds exhibit a SmC* phase with very wide temperature interval. Additionally, some types of the compounds show the cholesteric, SmA or blue phase. In the ferroelectric SmC* phase we evaluated physical properties of relevance for possible applications. 相似文献
139.
Hana Macková Daniel Horák Eduard Petrovský Jana Kovářová 《Colloid and polymer science》2013,291(1):205-213
To prepare functionalized magnetic polymer particles that are thermally responsive, inverse emulsion copolymerization of N-isopropylacrylamide, N,N′-methylenebisacrylamide and glycidyl acrylate (GA) was investigated in paraffin oil in the presence of γ-Fe2O3 nanoparticles dispersed in a water/glycerol mixture. The resulting polymer particles were characterized regarding the morphology, size, polydispersity, iron content, and the temperature-dependent phase transition using optical microscopy, transmission electron microscopy, scanning electron microscopy, atomic absorption spectroscopy, and differential scanning calorimetry. Magnetic properties were examined using hysteresis loop measurements and by analyzing the magnetic susceptibility with respect to temperature. We have also investigated the influence of the concentration of γ-Fe2O3 and GA in monomers on properties of the particles (morphology, size, and presence of oxirane groups). The particles possessed a hollow structure as a result of phase separation between water/glycerol hydrophilic solvents in the polymerization feed and the forming polymer. Depending on the concentration of γ-Fe2O3 in the monomer phase, the magnetic hollow particles contained 5–24 wt% iron. In water, the particles gradually collapsed when the temperature was raised to 40 °C because the elevated temperature weakened hydration and the PNIPAAm chains gradually became more hydrophobic. 相似文献
140.
Tomáš Větrovský Petr Baldrian Jiří Gabriel 《Applied biochemistry and biotechnology》2013,169(1):100-109
Production of the lignocellulose-degrading enzymes endo-1,4-β-glucanase, 1,4-β-glucosidase, cellobiohydrolase, endo-1,4-β-xylanase, 1,4-β-xylosidase, Mn peroxidase, and laccase was characterized in a common wood-rotting fungus Fomes fomentarius, a species able to efficiently decompose dead wood, and compared to the production in eight other fungal species. The main aim of this study was to characterize the 1,4-β-glucosidase produced by F. fomentarius that was produced in high quantities in liquid stationary culture (25.9 U?ml?1), at least threefold compared to other saprotrophic basidiomycetes, such as Rhodocollybia butyracea, Hypholoma fasciculare, Irpex lacteus, Fomitopsis pinicola, Pleurotus ostreatus, Piptoporus betulinus, and Gymnopus sp. (between 0.7 and 7.9 U?ml?1). The 1,4-β-glucosidase enzyme was purified to electrophoretic homogeneity by both anion-exchange and size-exclusion chromatography. A single 1,4-β-glucosidase was found to have an apparent molecular mass of 58 kDa and a pI of 6.7. The enzyme exhibited high thermotolerance with an optimum temperature of 60 °C. Maximal activity was found in the pH range of 4.5–5.0, and K M and V max values were 62 μM and 15.8 μmol?min?1?l?1, respectively, when p-nitrophenylglucoside was used as a substrate. The enzyme was competitively inhibited by glucose with a K i of 3.37 mM. The enzyme also acted on p-nitrophenylxyloside, p-nitrophenylcellobioside, p-nitrophenylgalactoside, and p-nitrophenylmannoside with optimal pH values of 6.0, 3.5, 5.0, and 4.0–6.0, respectively. The combination of relatively low molecular mass and low K M value make the 1,4-β-glucosidase a promising enzyme for biotechnological applications. 相似文献