全文获取类型
收费全文 | 1270篇 |
免费 | 44篇 |
国内免费 | 5篇 |
专业分类
化学 | 975篇 |
晶体学 | 5篇 |
力学 | 16篇 |
数学 | 163篇 |
物理学 | 160篇 |
出版年
2024年 | 3篇 |
2023年 | 9篇 |
2022年 | 29篇 |
2021年 | 56篇 |
2020年 | 34篇 |
2019年 | 29篇 |
2018年 | 22篇 |
2017年 | 16篇 |
2016年 | 42篇 |
2015年 | 35篇 |
2014年 | 46篇 |
2013年 | 60篇 |
2012年 | 67篇 |
2011年 | 97篇 |
2010年 | 69篇 |
2009年 | 54篇 |
2008年 | 70篇 |
2007年 | 86篇 |
2006年 | 65篇 |
2005年 | 59篇 |
2004年 | 48篇 |
2003年 | 53篇 |
2002年 | 61篇 |
2001年 | 18篇 |
2000年 | 11篇 |
1999年 | 11篇 |
1998年 | 15篇 |
1997年 | 8篇 |
1996年 | 14篇 |
1995年 | 10篇 |
1994年 | 7篇 |
1993年 | 8篇 |
1992年 | 9篇 |
1991年 | 7篇 |
1990年 | 6篇 |
1989年 | 3篇 |
1988年 | 4篇 |
1986年 | 5篇 |
1985年 | 9篇 |
1984年 | 17篇 |
1983年 | 6篇 |
1982年 | 5篇 |
1981年 | 3篇 |
1978年 | 5篇 |
1976年 | 3篇 |
1974年 | 4篇 |
1939年 | 2篇 |
1938年 | 2篇 |
1937年 | 2篇 |
1921年 | 2篇 |
排序方式: 共有1319条查询结果,搜索用时 15 毫秒
971.
972.
Attanasi OA De Crescentini L Favi G Filippone P Giorgi G Mantellini F Santeusanio S 《The Journal of organic chemistry》2003,68(5):1947-1953
Alpha-substituted hydrazones obtained from 1,2-diaza-1,3-butadienes and methylenic or methinic activated substrates gave rise to a wide range of cyclic compounds. In particular, in the presence of thionyl chloride as solvent-reagent, they were transformed into 1,2,3-thiadiazoles,(1) with selenium oxychloride in new 4-substituted 2,3-dihydro-1,2,3-selenadiazoles, while with selenium dioxide, they were transformed into 4-substituted 1,2,3-selenadiazoles. We have also examined the nucleophilic behavior of 1,2,3-thiadiazole 4a in the reaction with 1,2-diaza-1,3-butadienes that produced, under basic conditions, 4-hydrazono-1-(1,2,3-thiadiazolyl)pentane derivatives. This event represents an interesting example of stereoselective synthesis because it leads exclusively to the formation of the RR/SS racemic mixture. These latter compounds, treated with thionyl chloride, gave the corresponding 1,3-di-1,2,3-thiadiazolylpropane derivatives, while with sodium methoxide they afforded 1,2,3-thiadiazolyl-2-oxo-2,3-dihydro-1H-pyrrole systems. 相似文献
973.
Roberto Martínez Lucia Valle Marcos Gonzalez Claudia Romano 《Journal of heterocyclic chemistry》1997,34(3):1043-1045
Reaction of 4(7)-aminobenzimidazole with ethyl 2-alkylmalonates gave the novel 4,5,6,7-tetrahydro-5-alkyl-imidazo[1,5,4-ef][1,5]benzodiazepine-4,6-diones, and the 2-alkyl-4(7)-(2′-ethoxycarbonyl)acetamido-benzimidazoles. The structure of all the products was corroborated by ir, mass spectrometry and 1H-nmr. 相似文献
974.
A method is described for the simultaneous determination of low concentration of Ca, Cr, Cu, Fe, Mn and Ni in nuclear-grade uranium dioxide by x-ray fluroescence spectrometry, without the use of chemical treatment. The lower limits of detection range from 2 μg g?1 for nickel and manganese to 5 μg g?1 for copper. Samples are prepared in the form of double-layer pellets with boric acid as a binding agent. Standards are prepared in a U3O8 matrix, which is more chemically stable than UO2 and has similar matrix behaviour. The correlation coefficients for calibration curves are better than 0.999. Errors range from 2.4% for chromium to 6.8% for nickel. 相似文献
975.
Summary N-Cyano-N-methyl-N(2-[(5-methyl-1H-imidazol-4-yl)-methylthio] ethyl) guanidine cimetidine (CM), complexes with CoII, NiII and CuII are described. The compounds are of stoichiometry [M(CM)2]SO4 · nH2O [M = CoII, NiII or CuII; n = 3,3 or 4, respectively], [M(CM)2](ClO4)2 [M = CoII or NiII], [M(CM)2]Cl2 · nH2O [M=CoII, NiII or CuII; n = 1, 2, or 2, respectively] and [Cu(CM)SO4] · 2H2O. The electronic spectra of the compounds in solid state, magnetic susceptibilities and i.r. and e.p.r. spectra were studied. Octahedral environments are proposed for the complexes: [M(CM)2]SO4·nH2O, [M(CM)2](ClO4)2, [Ni(CM)2]Cl2 · 2H2O, [Cu(CM)2]Cl2 · 2H2O and [Cu(CM)SO4] · 2H2O and a tetrahedral structure for [Co(CM)2]Cl2 · H2O. 相似文献
976.
Brinchi L Di Profio P Germani R Savelli G Gillitt ND Bunton CA 《Journal of colloid and interface science》2001,236(1):85-95
Reactivities of methyl naphthalene-2-sulfonate, MeONs, and its 6-alkyl derivatives, alkyl=Me, n-C(6)H(13) and n-C(12)H(25), 6-Me-MeONs, 6-Hex-MeONs, and 6-Do-MeONs, respectively, are compared for reactions in cetyl trialkylammonium bromide micelles, n-C(16)H(33)NR(3)Br, R=Me, Et, n-Pr, n-Bu, CTABr, CTEABr, CTPABr, CTBABr, respectively. Similar experiments were made on reactions of Br(-) with n-butyl and n-decyl naphthalene-2-sulfonates, BuONs, and DeONs, respectively. Reactions with OH(-) were followed in cetyl trialkylammonium hydroxide, alkyl=Me, Et, n-Pr, CTAOH, CTEAOH, and CTPAOH, solubility permitting. Some reactions with OH(-) or Br(-) were also followed in mixed-ion systems of CTAOMs or CTPAOMs (OMs=MeSO(3)). Micellar rate effects were analyzed by using pseudophase treatments including interionic competition in mixed-ion systems. Second-order rate constants in the micellar pseudophase increase systematically with increases in substrate hydrophobicity and for reactions with Br(-), but not OH(-), they also increase with bulk of the cationic head group. Copyright 2001 Academic Press. 相似文献
977.
Becucci L Moncelli MR Naumann R Guidelli R 《Journal of the American Chemical Society》2005,127(38):13316-13323
A biomimetic membrane consisting of a lipid bilayer tethered to a mercury electrode via a hydrophilic spacer was investigated in aqueous KCl by potential-step chronocoulometry and electrochemical impedance spectroscopy, both in the absence and in the presence of the ionophore valinomycin. Impedance spectra, recorded from 1 x 10(-2) to 1 x 10(5) Hz over a potential range of 0.8 V, are satisfactorily fitted to a series of four RC meshes, which are straightforwardly related to the different substructural elements of the biomimetic membrane. The frequency-independent resistances and conductances of both the lipid bilayer and the hydrophilic spacer show a maximum when plotted against the applied potential. This behavior is interpreted on the basis of a general approximate approach that applies the concepts of impedance spectroscopy to a model of the electrified interphase and to the kinetics of potassium ion transport assisted by valinomycin across the lipid bilayer. 相似文献
978.
Summary We investigate the asymptotic behaviour of
n
(n),n where
n
(x) denotes the Laguerre polynomial of degreen. Our results give a partial answer to the conjecture
n
(n)>1 forn>6, made in 1984 by van Iseghem. We also show the connection between this conjecture and the continued fraction approximants of
.Work sponsored by the Consiglio Nazionale delle Ricerche and by the Ministero dell'Università e della Ricerca Scientifica e Tecnologica of Italy 相似文献
979.
Zacchigna M Di Luca G Lassiani L Varnavas A Pitotti A Boccù E 《Applied biochemistry and biotechnology》1999,76(3):171-181
Polyethylene glycols (PEGs) of various chain length were used to crosslink lysozyme onto an insoluble support such as oxirane. A very high degree of modification and no inactivation of lysozyme were obtained with PEG 20000, but enzymatic activity increased up to 20 times at pH 3.0, at which point the activity of the native enzyme was lower when using Leuconostok oenus as a macromolecular substrate. 相似文献
980.
The first convenient synthesis of 2-S-cysteinyl-3,4-dihydroxyphenylalanine (2-S-cysteinyldopa) in 30% overall yield is reported, which capitalizes on the anomalous regiochemistry of the oxidative coupling of 3,4-dihydroxycinnamic acid derivatives with thiol compounds, leading to 2-S rather than the usual 5-S conjugates. 相似文献