全文获取类型
收费全文 | 3401篇 |
免费 | 56篇 |
国内免费 | 8篇 |
专业分类
化学 | 2294篇 |
晶体学 | 19篇 |
力学 | 60篇 |
数学 | 656篇 |
物理学 | 436篇 |
出版年
2020年 | 26篇 |
2019年 | 27篇 |
2017年 | 28篇 |
2016年 | 72篇 |
2015年 | 49篇 |
2014年 | 62篇 |
2013年 | 153篇 |
2012年 | 117篇 |
2011年 | 113篇 |
2010年 | 93篇 |
2009年 | 86篇 |
2008年 | 143篇 |
2007年 | 117篇 |
2006年 | 122篇 |
2005年 | 114篇 |
2004年 | 120篇 |
2003年 | 81篇 |
2002年 | 79篇 |
2001年 | 62篇 |
2000年 | 51篇 |
1999年 | 81篇 |
1998年 | 55篇 |
1997年 | 38篇 |
1996年 | 40篇 |
1995年 | 38篇 |
1994年 | 43篇 |
1993年 | 38篇 |
1992年 | 43篇 |
1991年 | 52篇 |
1990年 | 51篇 |
1989年 | 53篇 |
1988年 | 57篇 |
1987年 | 56篇 |
1986年 | 46篇 |
1985年 | 69篇 |
1984年 | 71篇 |
1983年 | 59篇 |
1982年 | 74篇 |
1981年 | 56篇 |
1980年 | 57篇 |
1979年 | 54篇 |
1978年 | 63篇 |
1977年 | 60篇 |
1976年 | 48篇 |
1975年 | 59篇 |
1974年 | 45篇 |
1973年 | 30篇 |
1972年 | 34篇 |
1971年 | 29篇 |
1970年 | 31篇 |
排序方式: 共有3465条查询结果,搜索用时 31 毫秒
71.
Quantification of phosphorylated sugar constituents of lipopolysaccharides has been performed by the following sequence: dephosphorylation by treatment with hydrofluoric acid, cleavage to monomeric constituents by methanolysis and analysis of the released sugars by capillary gas chromatography. Lipopolysaccharides of Salmonella minnesota Rd1P+, Bordetella pertussis NIH 114 and Vibrio cholerae, NAG and 95R strains, were used as model substances. Comparison of the chromatographic data obtained from hydrofluoric acid-treated and untreated lipopolysaccharide preparations indicated that all lipopolysaccharides examined contained one moiety of glucosamine bound to phosphate in a stable linkage. 2-Keto-3-deoxyoctonic acid appeared phosphorylated to a variable extent. Lipopolysaccharides of the two V. cholerae strains contained one moiety of fully phosphorylated 2-keto-3-deoxyoctonic acid, whereas in that of S. minnesota Rd1P+ only one of the three moieties was phosphorylated. Lipopolysaccharide of B. pertussis had one moiety of 2-keto-3-deoxyoctonic acid, ca. 70% phosphorylated. All four of the preparations examined contained L-glycero-D-manno-heptose in amounts varying from 2.6 to 5.2 moieties. In the lipopolysaccharides of B. pertussis and strain 95R of V. cholerae this sugar was unphosphorylated, whereas the two remaining strains contained one phosphorylated moiety of this sugar. Phosphorylated lipopolysaccharide constituents can be analysed by this approach on a 50-100 micrograms scale. 相似文献
72.
Veronika Gemmer-Colos und Rolf Neeb 《Fresenius' Journal of Analytical Chemistry》1978,293(4):290-294
Zusammenfassung Kleine Mengen Ni, Cu und Zn in wäßrigen Lösungen können nach Ausschütteln ihrer Dipropyldithiocarbamatochelate gas-chromatographisch bestimmt werden. Arbeitsbedingungen für den 0,01–1 ppm-Bereich werden angegeben. Die Reproduzierbarkeit der Bestimmung wird durch Zusatz eines inneren Standards verbessert.
Dialkyldithiocarbamates as reagents for the gas-chromatographic determination of metalsII. Determination of zinc, copper and nickel as dipropyldithiocarbamato chelates after extraction from aqueous solution
Summary Small amounts of Ni, Cu and Zn in aqueous solutions can be determined by gas chromatography after extraction of their dipropyldithiocarbamatochelates. Working details for the 0.01–1 ppm range are reported. Reproducibility is markedly improved by addition of an inner standard.
Die Untersuchungen wurden in dankenswerter Weise durch Mittel der Deutschen Forschungsgemeinschaft und des Verbands der chemischen Industrie unterstützt. 相似文献
73.
R. Zerne L. Caiyan J. Zhankui J. Larsson 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1996,37(3):259-261
The Landé g J factors in the 6snd 1,3 D 2 sequences as well as states from the perturbing configurations of neutral Ytterbium have been measured by timeresolved fluorescence spectroscopy. Strong singlet-triplet mixing effects were observed. The g J factors were measured using the Zeeman quantum-beat technique following step-wise laser excitation. The experiment provided new data for checking the Multi-channel Quantum Defect Theory (MQDT) used in energy-level and lifetime analysis. 相似文献
74.
Schmidt EM Gleiter R Rominger F 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(8):1814-1822
The synthesis of 1-alkyl and 1-aryl-1-azacyclotetradeca-3,5,10,12-tetraynes was achieved in a stepwise approach. The key intermediate was 1,13-dibromotrideca-2,4,9,11-tetrayne (18). Reaction with methyl- (19 a), ethyl- (19 b), isopropyl- (19 c), n-butyl- (19 d), and tert-butylamine (19 e) as well as aniline (19 f) and p-methoxyaniline (19 g) gave the corresponding 14-membered tetraynes 20 a-20 g. The ring inversion process of 20 b was studied by variable temperature (1)H NMR spectroscopy. From these measurements a value of 10.6 kcal mol(-1) was calculated for DeltaG(not equal). X-ray investigations on single crystals of 20 b, 20 c, and 20 f revealed the axial position for the substituent at each nitrogen atom. For 20 b we encountered the chair conformation, for 20 c both chair and boat conformations, and for 20 f the boat conformation in the solid state. The reaction of 20 c with concentrated HCl in ethanol yielded 2,10-dichloro-6-isopropyl-6-azatricyclo[9.3.0.0(4,8)]tetradeca-1(11),2,4(8),9-tetraene (25 c). Compound 25 c was oxidized by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) to 27 c. The structure of the latter was confirmed by X-ray investigations. The reaction of 20 c in aqueous HCl lead to the formation of 10-chloro-2-isopropyl-1,3,4,6,7,8-hexahydro-2H-benzo[g]isoquinolin-9-one (37 c). The structure of 37 c was verified by X-ray studies on single crystals. 相似文献
75.
Ernst Schaumann Erwin Kausch Karl-Heinz Klaska Rolf Klaska Otto Jarchow 《Journal of heterocyclic chemistry》1977,14(5):857-860
The crystal and molecular structures of the cycloadduct from isopropyl[α-(dimethylthio-carbamoyl)isopropyl]carbodiimide and methyl isothiocyanate were determined by single-crystal X-ray methods. The product was established to be 2-(1-isopropyl-3,5-dimethyl-4,6-dithioxo-hexahydro-1,3,5-triazine-2-ylidenamino)-N,N-dimethylthioisoburyramide, in which contrary to expectations based on hybridisation the heterocyclic ring adopts a boat conformation. 相似文献
76.
Gies P Roy C McLennan A Pailthorpe M Hilfiker R Osterwalder U Monard B Moseley H Sliney D Wengraitis S Wong J Human S Bilimis Z Holmes G 《Photochemistry and photobiology》2003,77(1):58-67
In recent years the need to standardize measurement protocols for quantifying the degree of ultraviolet radiation (UVR) protection provided by clothing has led to the introduction of a number of standards around the world. To date, these standards have specified spectral measurements of UVR transmission by clothing and fabrics. Development of a standard test method has become an important part of the testing process, and this article presents results from an intercomparison involving 10 independent testing laboratories and 11 different UVR transmission measurement instruments. In addition to comparing the measured ultraviolet protection factors (UPF), this intercomparison also incorporates detailed scan results from all 10 laboratories and highlights differences in performance of the various instruments in different wavelength regions. Careful examination of these differences can indicate where changes to the systems could be made to allow improvements both in equipment performance and in agreement of the final results. The variability in the measurements of UPF in this study suggest that the protection categories in standards may need to be broadened. 相似文献
77.
78.
On the Preparation of Arsenic Dichloride Trifluoride, AsCl2F3 AsCl2F3 is prepared by pyrolysis of [AsCl4]+ [AsF6]?. The vibrational spectra of the new low temperature compound are reported and the valence force constants are calculated. The formation of AsCl2F3 by ligand exchange reactions is discussed. 相似文献
79.
Erik Larsson 《Fresenius' Journal of Analytical Chemistry》1930,79(5-6):170-175
Zusammenfassung Auf Grund der Dissoziatio nskonstanten der Thioglykolsäure,- und-Thiomilchsäure,-Mercaptoisobuttersäure,-Thiomalamidsäure und Thioäpfelsäure wurde gezeigt, dass von diesen Säuren nur die-Mercaptoisobuttersäure und Thioäpfelsäure gut mit Phenolphthalein alkalimetrisch titriert werden können, aber auch nur dann, wenn man viel Indikator benutzt. Bei den anderen Säuren hat man Indikatoren mit saurerem Umschlag zu wählen. Für die ersten drei Säuren dürften Neutralrot und Phenolrot geeignete Indikatoren sein. Dies wird auch bei der Thioglykolsäure experimentell erwiesen. Für die alkalimetrische Titration von-Thiomalamidsäure soll man Bromthymolblau als Indikator verwenden. 相似文献
80.
Reto Stahl Renato Galli Rolf Güller Hans-Jürg Borschberg 《Helvetica chimica acta》1994,77(8):2125-2132
The oxidative transformation of synthetic (+)-aristoteline ((+)- 6 ) into other metabolites which had been isolated from Aristotelia species was investigated. Thus, treatment of (+)- 6 with I2 as the single oxidant furnished the naturally occurring indole alkaloids (+)-makonine ((+)- 9 ),(+)-aristotelinone ((+)- 11 ), or (+)-11, 12-didehydroaristoteline ((+)- 7 ) in good yields, the selectivity of the oxidation process depending on the chosen reaction conditions. 相似文献