全文获取类型
收费全文 | 5415篇 |
免费 | 169篇 |
国内免费 | 38篇 |
专业分类
化学 | 3756篇 |
晶体学 | 26篇 |
力学 | 142篇 |
数学 | 876篇 |
物理学 | 822篇 |
出版年
2024年 | 6篇 |
2023年 | 33篇 |
2022年 | 145篇 |
2021年 | 175篇 |
2020年 | 122篇 |
2019年 | 118篇 |
2018年 | 123篇 |
2017年 | 112篇 |
2016年 | 220篇 |
2015年 | 158篇 |
2014年 | 205篇 |
2013年 | 308篇 |
2012年 | 363篇 |
2011年 | 435篇 |
2010年 | 283篇 |
2009年 | 241篇 |
2008年 | 349篇 |
2007年 | 318篇 |
2006年 | 291篇 |
2005年 | 260篇 |
2004年 | 245篇 |
2003年 | 154篇 |
2002年 | 148篇 |
2001年 | 86篇 |
2000年 | 46篇 |
1999年 | 64篇 |
1998年 | 49篇 |
1997年 | 50篇 |
1996年 | 57篇 |
1995年 | 54篇 |
1994年 | 38篇 |
1993年 | 39篇 |
1992年 | 24篇 |
1991年 | 37篇 |
1990年 | 26篇 |
1989年 | 27篇 |
1988年 | 16篇 |
1987年 | 16篇 |
1986年 | 24篇 |
1985年 | 30篇 |
1984年 | 29篇 |
1983年 | 15篇 |
1982年 | 29篇 |
1981年 | 11篇 |
1980年 | 5篇 |
1979年 | 4篇 |
1978年 | 8篇 |
1977年 | 5篇 |
1976年 | 7篇 |
1973年 | 3篇 |
排序方式: 共有5622条查询结果,搜索用时 15 毫秒
71.
Benniston AC Harriman A Romero FM Ziessel R 《Dalton transactions (Cambridge, England : 2003)》2004,(8):1233-1238
The photophysical properties of closely-coupled, binuclear complexes formed by connecting two ruthenium(II) tris(2,2'-bipyridine) complexes via an alkynylene group differ significantly from those of the relevant mononuclear complex. In particular, the energy of the first triplet excited state is lowered relative to the parent complex, because of the presence of the alkynylene substituent, while the triplet lifetime is prolonged, in part, because of extended electron delocalization. We now report that the triplet lifetime is also affected by the nature of the spectator 2,2'-bipyridyl ligands. Thus, replacing the parent 2,2'-bipyridine ligands with the corresponding 4,4'-dinitro-substituted ligands serves to decrease the luminescence yield and lifetime. With the corresponding carboxylate ester, the luminescence yield and lifetime are increased. Perdeuteration of the parent 2,2'-bipyridine ligands also leads to a modest increase in the luminescence yield. Such observations are indicative of electronic coupling between the various metal-to-ligand, charge-transfer excited triplet states. Temperature dependence studies confirm that these excited states are closely spaced and thermally accessible at ambient temperature. For some of the binuclear complexes, the quantum yield for formation of the lowest-energy triplet state is significantly less than unity. 相似文献
72.
Francisco J. Sayago Manuel Angulo Consolación Gasch M. Ángeles Pradera 《Tetrahedron》2007,63(22):4695-4702
Nor-tropane related bicyclic (6+5) iminocyclitols with an ether bridge and different substituents (hydroxymethyl, aminomethyl, and aminoethyl) on C-1 are prepared starting from a β-d-psicofuranosyl cyanide. The method involves an internal nucleophilic attack of a stabilized amide ion on a 5-mesyloxy derivative. The intermediate N-acetyl O-protected iminocyclitols present atropoisomerism due to restricted rotation of the N-CO amido bond. Conformational aspects of the prepared compounds are discussed. 相似文献
73.
Emilia Iglesias-Martínez Isabel Brandariz Francisco Penedo 《Journal of inclusion phenomena and macrocyclic chemistry》2007,57(1-4):573-576
The UV-vis absorption and the fluorescence emission spectra of novocaine were analysed in aqueous cyclodextrin (CD) solutions.
Either the absorbance read at λmax 290 nm or the fluorescence emission intensity at λems 346 nm increase in the presence of both α- and β-CD due to the formation of 1:1 inclusion complexes. The lower polarity of
the CD-cavity sensed by the included drug enhances the emitted fluorescence; in fact, the same effect was observed in aqueous
mixtures of acetonitrile, dioxane, or dimethylsulfoxide. The inclusion complex formation between the monocation of novocaine
and CDs diminishes the electrical conductance of aqueous solutions of novocaine hydrochloride (NoHCl). Both the nitrosation
reaction in aqueous acid medium and the ester hydrolysis in alkaline medium are retarded in the presence of CDs. The strongest
effect was observed with β-CD as a consequence of the higher stability inclusion complex. 相似文献
74.
José-francisco Rodrigues 《manuscripta mathematica》1984,46(1-3):65-82
The homogenization of the dam problem is discussed in the general weak formulation by means of the technique of correctors. In the two special cases of horizontal layers and of the isotropic coefficients with separated variables the problem is solved. However, in general, the unsaturated region is not stable for the homogenization, as a counterexample by Alt shows. 相似文献
75.
Begoña Hernández Modesto Orozco Francisco J. Luque 《Journal of computer-aided molecular design》1997,11(2):153-162
The tautomerism of 2-azaadenine and 2-hypoxanthine has been examined in the gas phase and in aqueous solution. The tautomerism in the gas phase has been studied by means of semiempirical and ab initio quantum-mechanical computations, as well as density-functional calculations. The influence of the aqueous solvent on the relative stability between tautomers has been estimated from self-consistent reaction field calculations performed with different high-level continuum models. The results provide a detailed picture of the tautomeric preference for these purine bases. The importance of tautomerism in the substrate recognition by xanthine oxidase is discussed. Finally, the rate of oxidation of 2-azaadenine and 2- hypoxanthine by xanthine oxidase is discussed in terms of the recognition model at the enzyme active site. 相似文献
76.
Francisco Sarabia 《Tetrahedron letters》2003,44(41):7671-7675
A synthetic approach to the [15]-membered stevastelins, a novel class of immunosuppressant agents, is reported based on a macrolactamization route to the 2,3-epoxy derivative 6. The synthesis of this compound was achieved via a stereoselective epoxidation of the allylic alcohol 13, followed by a coupling reaction with a variety of peptide derivatives to give the epoxy peptides 7-10. After an extensive study of cyclizations with these precursors, the best result was achieved with the macrolactamization of 8 in the presence of DEPC, to obtain the epoxy cyclic depsipeptide 6 in 42% yield. From this product, an epoxy analogue of stevastelin B, compound 27, was prepared. Finally, the synthesis of the natural product was attempted through the opening of the oxirane ring contained in 6 and 26, with a variety of methyl cuprate reagents, but without success. 相似文献
77.
Francisco Faria M. Victoria Martín M. Carmen Paredes Amelia Tito 《Journal of heterocyclic chemistry》1987,24(5):1269-1274
Cycloaddition of diazomethane to pyrrolinones 1a,b,d,e affords only one regioisomer as a mixture of the epimeric pyrrolopyrazolines 2 and 2 ′,4-Halo derivatives 1f,g react with diazomethane to give the two possible regioisomers 2 and 3. The regio- and stereochemistry of the adducts is evidenced by the 1H-nmr data. The primary adducts originated from the halopyrrolinones suffer dehydrohalogenation to give aromatized products, which by further methylation give derivatives of type 7, 8, 10 and 11. 相似文献
78.
Assessing the non-specific hydroxyl radical scavenging properties of melanoidins in a Fenton-type reaction system 总被引:1,自引:0,他引:1
Francisco J. Morales 《Analytica chimica acta》2005,534(1):171-176
Soluble high molecular weight fraction (>10 kDa, named melanoidins) was isolated from Maillard reaction model systems, coffee, beer and sweet wine by ultrafiltration. Deoxyribose method was adjusted for measuring the hydroxyl radical scavenging properties of melanoidins. The presence of competitive melanoidins with deoxyribose for OH decrease the rate of deoxyribose degradation. Possible interferences to the deoxyribose method have been evaluated. Most of isolated melanoidins exhibited a variable and measurable non-site-specific hydroxyl scavenging activity in a Fenton-type reaction system. The iron reducing properties of melanoidins at the reaction conditions were evaluated with ferrozine. It has established a kinetic approach to assess the second rate constants of hydroxyl radical scavenging reactions of melanoidins. This approach may be a valuable tool for addressing the structure-activity relationships of melanoidins in a future. There is no correlation between browning (absorbance at 420 nm) and efficiency for scavenging hydroxyl radicals in solution. 相似文献
79.
Rodrigues JA Abramovitch RA de Sousa JD Leiva GC 《The Journal of organic chemistry》2004,69(9):2920-2928
In preliminary communications, we reported the diastereoselective synthesis of cularine and sarcocapnine via the intramolecular ring closure of nitrenium and oxenium ions, a new highly diastereoselective reductive methylation with (+)-8-phenylmenthyl chloroacetate followed by reduction with sodium borohydride, and a facile entry to the isoquinoline precursors by aza-Wittig electrocyclic ring closure. We now report the full details of the syntheses of (+)-O-demethylcularine, (+)-cularine, (+)-sarcocapnidine, (+)-sarcocapnine, and (+)-crassifoline and describe different methods of synthesis of their precursors. 相似文献
80.
Lars G.J. HammarströmMichael E. Meyer David B. Smith Francisco X. Talamás 《Tetrahedron letters》2003,44(46):8361-8363
A simple synthesis of tetrasubstituted purines is disclosed based on the solution phase elaboration of 4,6-dichloro-2-methylthio-5-nitropyrimidine. One-pot sequential C4 and C6 chloride substitution by secondary and primary amines yields 4,6-diamino-2-methylthio-5-nitropyrimidines. mCPBA-mediated oxidation of the methylthio moiety to the corresponding sulfone allows facile substitution at the 2-position. CrCl2 assisted reduction of the nitro group, followed by acid catalyzed orthoester cyclization, then provides novel tetrasubstituted purines not accessible by other methods. 相似文献