全文获取类型
收费全文 | 53841篇 |
免费 | 15798篇 |
国内免费 | 73篇 |
专业分类
化学 | 60731篇 |
晶体学 | 114篇 |
力学 | 2363篇 |
数学 | 3993篇 |
物理学 | 2511篇 |
出版年
2024年 | 381篇 |
2023年 | 4113篇 |
2022年 | 1497篇 |
2021年 | 2568篇 |
2020年 | 4736篇 |
2019年 | 2431篇 |
2018年 | 2429篇 |
2017年 | 734篇 |
2016年 | 5792篇 |
2015年 | 5707篇 |
2014年 | 5140篇 |
2013年 | 5628篇 |
2012年 | 3710篇 |
2011年 | 1646篇 |
2010年 | 3711篇 |
2009年 | 3623篇 |
2008年 | 1568篇 |
2007年 | 1324篇 |
2006年 | 632篇 |
2005年 | 590篇 |
2004年 | 445篇 |
2003年 | 409篇 |
2002年 | 317篇 |
2001年 | 179篇 |
2000年 | 148篇 |
1999年 | 143篇 |
1997年 | 149篇 |
1996年 | 188篇 |
1995年 | 233篇 |
1994年 | 165篇 |
1993年 | 277篇 |
1992年 | 161篇 |
1988年 | 141篇 |
1985年 | 157篇 |
1984年 | 147篇 |
1982年 | 179篇 |
1981年 | 197篇 |
1980年 | 248篇 |
1979年 | 225篇 |
1978年 | 228篇 |
1977年 | 345篇 |
1976年 | 408篇 |
1975年 | 502篇 |
1974年 | 500篇 |
1973年 | 312篇 |
1972年 | 384篇 |
1971年 | 365篇 |
1970年 | 546篇 |
1969年 | 415篇 |
1968年 | 462篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Preparation and physical and chemical properties of 3-ethinyl-2,4-diformyl-pentandial (3 a) and its salts are described.3 a contains 2 malonaldehyde groups. Starting from3 a, 4H-pyranes, 4H-dihydropyridines, dipyrazoles, diisoxazoles, bisdiazepines and vinamidine derivates are obtained.
Mit 1 Abbildung 相似文献
Mit 1 Abbildung 相似文献
92.
K. Schweiger A. Fuchsgruber Prof. Dr. G. Zigeuner 《Monatshefte für Chemie / Chemical Monthly》1977,108(1):243-255
Tetrahydro-6-hydroxy-trimethyl-1,3-thiazine-2-thione (1) reacts with secondary amines via the dialkylammonium-3-oxoalkyldithiocarbamate3, either via isothiocyanates6 to 4-dialkylaminodihydro-2(1H)-pyridinethiones7 or to dialkylammonium dithiocarbamates (13), depending on the amine used and the reaction conditions. Subsequently, 6-dialkylaminotetrahydro-1,3-thiazine-2-thiones11 or tetrahydro-6-mercapto-1,3-thiazine-2-thione10 are formed. On being heated to reflux,11 reacts to pyridinethione7 and 4-dialkylaminodihydrothiopyranthione19. With secondary amines only13 is formed from tetrahydro-6-hydroxytetramethyl-1,3-thiazine-2-thione20. The reaction of dihydrotrimethyl-1,3-thiazine-2-thione21 with secondary amines leads to N,N-dialkylthioureas16 or dialkylammonium thiocyanates17 and with dialkylformamides 4-dialkylaminodihydropyridinethiones7 are formed. Dihydrotetramethyl-1,3-thiazine-2-thione24 reacts neither with secondary amines nor with dialkylformamides. 相似文献
93.
Dr. Mamdouh S. Masoud T. M. Salem M. Elessawi 《Monatshefte für Chemie / Chemical Monthly》1978,109(1):221-227
The reactions of the bidentate dinitrosoresorcinol (DNR) with copper, cobalt, nickel, iron and zinc salts were investigated. This ligand was found to react with these metal salts in aqueous media where hydrogen ion was confirmed to be liberated except in case of iron. The solid complexes were prepared in alcoholic media. Chemical analyses, magnetic and spectral data were compatible to determine the structure of these complexes and their mode of chelation.
Mit 2 Abbildungen 相似文献
Mit 2 Abbildungen 相似文献
94.
Prof. Dr. Heinz Falk Alfred Leodolter Georg Schade 《Monatshefte für Chemie / Chemical Monthly》1978,109(1):183-192
The electrochemical oxidation of arylmethylene-pyrrolinones, pyrromethenones and pyrromethenes as representative bile pigment partial structure models was investigated by means of a rotating disc platinum electrode using acetonitrile as the solvent. Two different oxidation reactions were found. The first reaction being a reversible one-electron oxidation with compounds of the arylmethylene-pyrrolinone series and pyrromethenones which are unsubstituted in position 5 of the pyrrole ring. A two step reaction (the first one reversible, the second irreversible) on the other hand was found to be typical for pyrromethenones bearing a methyl group in this position.Through protonation the first step is at a higher potential, whereas the second one is lowered and becomes reversible. The resulting oxidation pattern can be interpreted analogous to the oxidation of hydroquinones in aprotic solvents.The geometrical isomers of a pyrromethenone were oxidized at appr. the same potential, but there is a strong dependence of the potential of the first oxidation step on the substitution: a higher degree of alkylation favours oxidation by lowering the oxidation potential.
Mit 1 Abbildung 相似文献
Mit 1 Abbildung 相似文献
95.
Dr. W. Gottardi 《Monatshefte für Chemie / Chemical Monthly》1975,106(3):611-623
By reaction of primary carboxamides with “dibromoisocyanuric acid” (DBI) N-monobromoamides can be readily obtained as well as the N,N-dibromoamides described in an earlier paper1. Reactions, some of them new, and properties of these compounds are described and compared with those of the N,N-dibromoamides. Like other compounds bearing the NHBr group2, 3 the N-monobromocarboxamides disproportionate at room temperature according to: 2 RCONHBr ? ? RCONH2+RCONBr2. For CH3CONHBr the equilibrium constant was found to beK=0.02. In aqueous solution they behave as weak acids. The dissociation constants of eight compounds [R=?CH3, ?C2H5, ?CH2Cl, ?CHCl2, ?CCl3, ?CF3, ?C(CH3)3 and ?C6H5] were measured: they differ from those of the corresponding carboxylic acids by about three powers of ten. 相似文献
96.
Dipl.-Ing. Dr. O. Hinterhofer 《Monatshefte für Chemie / Chemical Monthly》1974,105(2):279-290
The selective chlorination of a m/p-xylene mixture, followed by distillation of the unreacted p-xylene, leaves a residue containing up to 90% of monochlorinated m-xylenes. m-Xylene is recovered from the latter by heterogeneous catalytic hydrogenolysis in the gas-phase. It was found that the hydrogenolysis on certain noble metal catalysts proceeds according to an ionic reaction mechanism at temperatures below a definite temperature range. At temperatures above this range hydrogenolysis follows a radical reaction mechanism. 相似文献
97.
98.
A. Küçükönder B. G. Durdu Ö. Sö?üt E. Büyükkasap 《Journal of Radioanalytical and Nuclear Chemistry》2004,260(1):89-97
L X-ray fluorescence cross sections, and intensity ratios were measured for elements in the 70£Z£92 atomic range at the excitation energy 59.5 keV using a Si(Li) detector. Furthermore, L X-ray fluorescence cross sections
and intensity ratios were calculated for elements in the same range. The average L shell fluorescence yields were derived
using experimental L X-ray fluorescence cross sections and theoretical photoionization cross sections. The obtained results
were compared with other experimental and theoretical values.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
99.
The low vapor pressure and the versatility of the physico-chemical properties of ionic liquids make them really attractive as an alternative for conventional molecular solvents. The knowledge of their physico-chemical properties (viscosity, conductivity, miscibility with organic solvents and anion-cation interactions) has appeared mandatory for better targeting their applications, although it is generally still lacking or incomplete.This work promotes capillary electrophoresis instrumentation as an integrated apparatus for measurement of viscosity, conductivity and absorbance of pure ionic liquids and ionic liquid-molecular solvent mixtures. Compared to current conventional techniques, the assets of this instrumentation for this purpose are the combined availability of a pressure delivery system, power supply, diode array absorbance detector and thermoregulation device, allowing unattended, automatic and easy operation, involving minimum sample handling. Most importantly, the required sample volume can be reduced to about 50 μL, making this protocol very cost-effective. A protocol was optimized with respect to time, sample consumption and data reliability for the determination of these physico-chemical parameters. Ionic liquids selected for method development and validation differed in the nature of their cation (butyl- and ethyl-methylimidazolium) and anion (trifluoromethanesulfonate and bis(trifluoromethanesulfonyl)imide). Various molecular solvents were mixed with these ionic liquids (acetonitrile, methanol, dimethylformamide and trifluoroethanol) and the same physico-chemical properties were determined by optimized methods. The knowledge of these data should be of great support in various application areas, including the development of new separation media for capillary electrophoresis and chromatographic techniques. 相似文献
100.
Henri Magnin Firmin Rodriguez Marc Abadie Franois Schu 《Journal of polymer science. Part A, Polymer chemistry》1977,15(4):875-895
A kinetic study of ethylene oligomerization in hexane, in the presence of n-BuLi–TMEDA complexes, allowed us to suggest a new mechanism for anionic ethylene oligomerization. n-BuLi and n-Bu(CH2CH2)Li species have the same reactivity. The RLi–TMEDA complex in a 1-to-1 stoichiometry is the active species. The following kinetic equation has been established: It reflects the intervention of associated species (n-BuLi–TMEDA)2 as well as the influence of the concentration of the complexing agent on the kinetics of oligomerization. 相似文献