全文获取类型
收费全文 | 79584篇 |
免费 | 20150篇 |
国内免费 | 344篇 |
专业分类
化学 | 73707篇 |
晶体学 | 326篇 |
力学 | 3749篇 |
综合类 | 12篇 |
数学 | 12576篇 |
物理学 | 9708篇 |
出版年
2023年 | 4759篇 |
2022年 | 1622篇 |
2021年 | 2821篇 |
2020年 | 5177篇 |
2019年 | 2588篇 |
2018年 | 3742篇 |
2017年 | 2117篇 |
2016年 | 6877篇 |
2015年 | 6680篇 |
2014年 | 6002篇 |
2013年 | 6425篇 |
2012年 | 6694篇 |
2011年 | 3635篇 |
2010年 | 5598篇 |
2009年 | 5289篇 |
2008年 | 1865篇 |
2007年 | 1555篇 |
2006年 | 882篇 |
2005年 | 4656篇 |
2004年 | 4113篇 |
2003年 | 2463篇 |
2002年 | 629篇 |
2001年 | 407篇 |
1999年 | 247篇 |
1997年 | 225篇 |
1996年 | 204篇 |
1995年 | 299篇 |
1994年 | 236篇 |
1993年 | 334篇 |
1992年 | 334篇 |
1991年 | 301篇 |
1990年 | 248篇 |
1989年 | 246篇 |
1988年 | 292篇 |
1987年 | 216篇 |
1982年 | 202篇 |
1981年 | 237篇 |
1980年 | 251篇 |
1979年 | 264篇 |
1978年 | 267篇 |
1977年 | 376篇 |
1976年 | 506篇 |
1975年 | 544篇 |
1974年 | 563篇 |
1973年 | 393篇 |
1972年 | 501篇 |
1971年 | 460篇 |
1970年 | 671篇 |
1969年 | 505篇 |
1968年 | 544篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
991.
Dr. A. Woppmann 《Monatshefte für Chemie / Chemical Monthly》1974,105(2):419-426
The kinetics of the diazotization of o-, m-, p-chloroaniline in 0.005n- to 0.4n-methanolic HCl-solution at 25, 15, 0, ?10 ?20, and ?30°C was invertigated. It was found that the nitrosation reaction (the same as in1) $$C_6 H_4 ClNH_2 + NOCl \mathop \rightleftharpoons \limits^k C_6 H_4 ClNH_2 NO^ + + Cl^ - $$ is a proceeding advance-back-reaction. The decomposition of C6H4ClNH2NO+ by splitting off a proton is the rate determining step. The free activation enthalpies ΔG * for the nitrosation reaction, the activation entropies ΔS *, the activation enthalpies ΔH * and the activation energiesE a at the given temperatures are calculated. The experimentally found and the calculated velocities are given in Tables 1–6. The equilibrium constants of the o-, m-, p-chloroanilinium ions, and nitrosyl-chloride in methanol are indicated in Table 7, diagram 1. TheK M values (the ionic products of methanol, extrapolated at infinite dilution) together with theK A values of Table 7 give theK B values (p. 2) using the table10. The ΔG B values can be calculated using equation ΔG B = ?RTlnK B Fig 2 shows the linear dependance of the logarithmus of the ΔG * values from the logarithmus of theK B values. 相似文献
992.
The model of Onsager in which a polar molecule undergoes a reaction field due to the polarization of the molecular surroundings is used to evaluate by a S.C.F. calculation (CNDO/2 approximation) the modifications of a molecular structure in the liquid state.Application to water molecule and to three polar dimers for values of the dielectric constant varying between 3 and 78, shows that most of geometric parameters and dipoles moments vary of few per cent when the molecule is inserted in a liquid. In the liquid state dipole moments do not depend very much on the dielectric constant but energies and relative stabilities of isomers are strongly dependent on the medium.[/p] 相似文献
993.
Dr. Marita Fell 《Colloid and polymer science》1967,220(2):107-110
Zusammenfassung Aus einem durch Trypsinabbau von -Keratose erhaltenen Gemisch von Peptiden wurden 15 neutrale, 5 saure und 2 basische Peptide durch Kombination papierchromatographischer und papierelektrophoretischer Methoden getrennt und in sehr geringen Mengen isoliert. Es wurden neben der Aminosäurezusammensetzung 14N-terminale Aminosäuren sowie einige Teilsequenzen in den untersuchten Peptiden ermittelt.Die mittlere Kettenlänge der Peptide beträgt 5,8 Aminosäuren.Mit 1 Tabelle3. Mitt. über Untersuchungen an -Keratose. 2. Mitt.:M. Fell undE. Schnabel, Hoppe-Seyler's Z. physiol. Chem.333, 218 (1963). 相似文献
994.
Ralf Otte Roland Fröhlich Shuichi Nakamura Takeshi Toru Dieter Hoppe 《Tetrahedron letters》2007,48(49):8636-8642
The enantio- and diastereoselective synthesis of allenyl aryl sulfides by asymmetric lithiation of 2-alkynyl (2-hetero)aryl sulfides is described. A dynamic thermodynamic resolution by selective crystallization of the intermediate lithium complexes derived from deprotonation, applying a bis(oxazoline) ligand, was achieved to give enantioselectivities up to 85% ee. Subsequent stereospecific nickel-catalyzed cross-coupling reactions with arylzinc reagents established a versatile access to threefold carbon-substituted allenes. 相似文献
995.
Winter B Ivanco J Netzer FP Ramsey MG Salzmann I Resel R 《Langmuir : the ACS journal of surfaces and colloids》2004,20(18):7512-7516
A combination of in situ surface sensitive-techniques, UV photoemission and low energy electron diffraction, with ex situ bulk sensitive X-ray diffraction reveals the formation of epitaxial thin films of sexiphenyl on Al(111) starting from the first monolayer. For room temperature growth, highly ordered films are formed with a unique alignment of the sexiphenyl molecules with the long axes of all molecules aligned parallel to both the surface and the <10> azimuthal directions of Al(111). This is related to a densely packed highly commensurate first monolayer, which acts as a template for the unique (21) crystallite orientation observed. 相似文献
996.
Atom transfer radical polymerization conditions with copper(I) bromide/2,2′-bipyridine (Cu/2,2′-bpy) as the catalyst system were employed for the homopolymerization and random copolymerization of 1-phenoxycarbonyl ethyl methacrylate (PCMA) with methyl methacrylate (MMA). Temperature studies indicated that the polymerizations occurred smoothly in bulk at 110 °C. Poly(PCMA)(polydispersity index=1.27) homopolymer was characterized and then used as macroinitiator for increasing its molecular weight. The homopolymerization of PCMA was also carried out under free radical conditions using 2,2′-azobisisobutyronitrile as an initiator.The monomer and polymers were characterized by FT-IR and 1H and 13C-NMR techniques. The glass transition temperatures, the solubility parameters and average-molecular weights of the polymers were determined. Thermal stabilities of the polymers were given as compared with each other by using TGA curves. Thermal degradation products of poly(PCMA)s obtained by ATRP and free radical polymerization were compared with each other by using 1H-NMR technique. 相似文献
997.
Based on an analysis of previously reported structures and a potential geometry fit with substrates, a new family of chiral dioxocyclam derivatives have been designed. The synthesis of those ligands was accomplished starting from l-proline and α-d-amino acids (converted to β-amino acids) with a key step of macrocyclization reaction of amino esters. All ligands were converted into neutral copper(II) complexes (amide groups underwent deprotonation of upon treatment of ligands with copper(II) acetate). The complexes exhibit the desired shape of their active surfaces, as proved by X-ray analysis. 相似文献
998.
A method for the quantitative analysis of tricyclic antidepressants in the serum of psychiatric patients is described. The method can be used for determining amitriptyline, nortriptyline, imipramine, demethyllimipramine, clomipramine, demethylclomipramine, trimipramine and protriptyline. The method consists in a series of extraction steps followed by gas chromatography with flame-ionization detector. The drugs are determined in their native state. The internal standard method is used for the quantitation. 相似文献
999.
Dr. R. Franzmair 《Monatshefte für Chemie / Chemical Monthly》1976,107(2):511-520
The reaction of 5-oxo-A-nor-3,5-secocholestan-3-oic acid (1) with simple N-mono and N,N-disubstituted hydrazines including acetylhydrazide was studied. Derivatives of 4-amino-4-aza-cholest-5-ene-3-one (2) were obtained.
Herrn Prof. Dr.H. Bretschneider mit den besten Wünschen zum 70. Geburtstag gewidmet. 相似文献
Herrn Prof. Dr.H. Bretschneider mit den besten Wünschen zum 70. Geburtstag gewidmet. 相似文献
1000.
Agents that target bacterial virulence without detrimental effect on bacterial growth are useful chemical probes for studies of virulence and potential candidates for drug development. Several gram-negative pathogens employ type III secretion to evade the innate immune response of the host. Screening of a chemical library with a luciferase reporter gene assay in viable Yersinia pseudotuberculosis furnished several compounds that inhibit the reporter gene signal expressed from the yopE promoter and effector protein secretion at concentrations with no or modest effect on bacterial growth. The selectivity patterns observed for inhibition of various reporter gene strains indicate that the compounds target the type III secretion machinery at different levels. Identification of this set of inhibitors illustrates the approach of utilizing cell-based assays to identify compounds that affect complex bacterial virulence systems. 相似文献