全文获取类型
收费全文 | 334647篇 |
免费 | 17592篇 |
国内免费 | 1201篇 |
专业分类
化学 | 205556篇 |
晶体学 | 4559篇 |
力学 | 14461篇 |
综合类 | 10篇 |
数学 | 33876篇 |
物理学 | 94978篇 |
出版年
2023年 | 4385篇 |
2021年 | 4319篇 |
2020年 | 6433篇 |
2019年 | 4573篇 |
2018年 | 5062篇 |
2017年 | 3376篇 |
2016年 | 9741篇 |
2015年 | 8270篇 |
2014年 | 9284篇 |
2013年 | 18052篇 |
2012年 | 13019篇 |
2011年 | 13520篇 |
2010年 | 11546篇 |
2009年 | 11475篇 |
2008年 | 12196篇 |
2007年 | 11930篇 |
2006年 | 10481篇 |
2005年 | 9683篇 |
2004年 | 8648篇 |
2003年 | 7733篇 |
2002年 | 7531篇 |
2001年 | 9693篇 |
2000年 | 7178篇 |
1999年 | 5575篇 |
1998年 | 4494篇 |
1997年 | 4549篇 |
1996年 | 4116篇 |
1995年 | 3916篇 |
1994年 | 3621篇 |
1993年 | 3690篇 |
1992年 | 4199篇 |
1991年 | 4187篇 |
1990年 | 3901篇 |
1989年 | 3814篇 |
1988年 | 3886篇 |
1987年 | 3820篇 |
1986年 | 3578篇 |
1985年 | 4839篇 |
1984年 | 4940篇 |
1983年 | 3971篇 |
1982年 | 4308篇 |
1981年 | 4217篇 |
1980年 | 4106篇 |
1979年 | 4246篇 |
1978年 | 4499篇 |
1977年 | 4460篇 |
1976年 | 4638篇 |
1975年 | 4209篇 |
1974年 | 4373篇 |
1973年 | 4439篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
821.
[formula: see text] The diastereoselective synthesis of cis-2,5-disubstituted-3-methylenetetrahydrofurans via Pd(0)-catalyzed cyclization of 2-methylene-1,4-diols is described. Investigations into the scope of the reaction and its application toward the synthesis of amphidinolide K is reported. 相似文献
822.
THE EFFECTS OF PORPHYRIN STRUCTURE AND AGGREGATION STATE ON PHOTOSENSITIZED PROCESSES IN AQUEOUS AND MICELLAR MEDIA 总被引:2,自引:0,他引:2
C. R. Lambert E. Reddi J. D. Spikes M. A. J. Rodgers G. Jori 《Photochemistry and photobiology》1986,44(5):595-601
The efficiency of several porphyrins at 10 μM and 83 μM as sensitizers of the photooxidation of 0.1 mM tryptophan and histidine via a singlet oxygen-mechanism was studied in pH 7.4-buffered aqueous solutions and in aqueous dispersions of Triton X-100 micelles. The porphyrins were either solubilized in the bulk aqueous medium or associated with the micellar phase, whereas the amino acids were always located in the aqueous phase. With those porphyrins, such as uroporphyrin I, meso-tetra (4-sulfonatophenyl)porphine, meso-tetra(4-carboxyphenyl)porphine and meso-tetra)N,N,N-trimethylanilinium)porphine, which are > 98% monomeric in both media, the efficiency of histidine photooxidation was independent of the site of O2(1Δg) generation, as shown by the closely similar values for the photooxidation rate constant and oxygen-consumption quantum yield in the presence and absence of Triton micelles; the same indications were provided by photokinetic experiments with tryptophan. Actually, laser flash photolysis studies showed that the micelle-incorporation of the above mentioned porphyrins brought about only minor changes in their photophysical properties, including the relative yield of O2(1Δg) generation. On the other hand, hematoporphyrin IX, its Zn2+-complex, and coproporphyrin III are largely aggregated in homogeneous aqueous solution; their incorporation into Triton micelles caused an increase of the triplet quantum yield and an enhancement of the oxygen-consumption quantum yield and photooxidation rate constant for both histidine and tryptophan. The lower photosensitizing efficiency of aggregated porphyrin species in comparison with the corresponding monomeric porphyrin was confirmed by measuring the initial rate and quantum yield of oxygen consumption upon irradiation of 1 mM histidine and tryptophan in the presence of different hematoporphyrin concentrations within the 0.3-100μM range. 相似文献
823.
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
824.
R. Spatz und K. H. Lieser 《Fresenius' Journal of Analytical Chemistry》1977,288(4):267-272
Zusammenfassung Bei Verwendung einer Röntgenröhre mit Sekundärtargets kann durch geeignete Wahl des Sekundärtargets eine hohe Nachweisempfindlichkeit im Bereich bestimmter Ordnungszahlen erreicht werden. Für schwere Elemente ist die Messung der L-Linien vorteilhaft. Bei Verwendung von Radionuklidquellen werden etwa die gleichen Nachweisgrenzen erreicht. Im Bereich hoher Energien sind Radionuklidquellen besonders günstig.
Critical comparison of the measuring range and detection limits in energy-dispersive X-ray fluorescence analysis with tube excitation (secondary target) and with radionuclide excitation (Cd-109 and Am-241), by means of bulk powder samples on silicagel basis as an example
Summary Using a X-ray tube in combination with secondary targets by proper choice of the secondary target high detection limits for ranges of certain atomic numbers are obtained. In the case of heavy elements measurement of L-lines is more favourable. By use of radionuclides as X-ray sources nearly the same detection limits are obtained. In the range of high energies the application of radionuclides has distinct advantages.
Wir danken dem BMFT für finanzielle Unterstützung. 相似文献
825.
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
826.
The rapid determination of chloride and inorganic phosphate in blood serum based on continuous flow spectrophotometry was adapted to Flow Injection Analysis by applying dialysis as well as differential dilution of the injected samples. In addition to the manual injection procedure, an automated microsampling unit was developed and used. By continuous dialysis, chloride could be determined reproducibly at a sampling rate of 125 samples per hour at sample volumes of 60 μl or less. A similar sampling rate and high precision were obtained for the determination of inorganic phosphate by using differential dilution. 相似文献
827.
The electrochemical oxidation of various substituted chrysoidines was studied by cyclic voltammetry, to determine which have stable oxidation products. Only 4-hydroxy-chrysoidine has a stable product; the apparent oxidation potential is 0.779 V vs. NHE. 4-Methoxy- and 4-ethoxy-chrysoidine rapidly lose methanol or ethanol, respectively, so that the 4-hydroxychrysoidine wave appears on subsequent scans. All the other chrysoidines studied are irreversible. The results indicate that a hydroxy group in the 4-position is necessary for stability; the 4-alkoxychrysoidines can achieve the stable quinoidal structure by cleavage of the alkoxy group after nucleophilic attack. 相似文献
828.
The basic principles of Flow Injection Analysis are outlined. The parameters governing the dispersion of the injected sample zone in the system are discussed, and it is demonstrated how these parameters can be manipulated in order to suit the requirements of an individual analytical procedure. A number of examples illustrating the practical application of f.i.a. are described, comprising the use of automated, stopped-flow, merging-zones, extraction techniques as well as f.i.a. scanning and methods based on intermittent pumping. Updated lists on f.i.a. procedures published and species that can be determined by f.i.a. are included. 相似文献
829.
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
830.
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