全文获取类型
收费全文 | 20155篇 |
免费 | 175篇 |
国内免费 | 17篇 |
专业分类
化学 | 19184篇 |
晶体学 | 22篇 |
力学 | 68篇 |
数学 | 607篇 |
物理学 | 466篇 |
出版年
2021年 | 204篇 |
2019年 | 151篇 |
2016年 | 177篇 |
2015年 | 168篇 |
2013年 | 319篇 |
2012年 | 353篇 |
2011年 | 357篇 |
2010年 | 233篇 |
2009年 | 179篇 |
2008年 | 288篇 |
2007年 | 322篇 |
2006年 | 271篇 |
2005年 | 230篇 |
2004年 | 197篇 |
2003年 | 163篇 |
1987年 | 139篇 |
1985年 | 185篇 |
1984年 | 198篇 |
1980年 | 312篇 |
1979年 | 234篇 |
1978年 | 1039篇 |
1977年 | 797篇 |
1976年 | 224篇 |
1968年 | 208篇 |
1967年 | 174篇 |
1966年 | 221篇 |
1965年 | 255篇 |
1964年 | 228篇 |
1963年 | 175篇 |
1962年 | 347篇 |
1961年 | 169篇 |
1960年 | 220篇 |
1959年 | 208篇 |
1958年 | 206篇 |
1957年 | 152篇 |
1956年 | 236篇 |
1955年 | 226篇 |
1954年 | 164篇 |
1943年 | 162篇 |
1939年 | 163篇 |
1938年 | 174篇 |
1937年 | 231篇 |
1936年 | 214篇 |
1935年 | 231篇 |
1934年 | 184篇 |
1933年 | 250篇 |
1932年 | 136篇 |
1930年 | 216篇 |
1928年 | 154篇 |
1927年 | 153篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
Ohne Zusammenfassung 相似文献
112.
113.
R. Hönigschmid-Grossich und Margot Zimmermann 《Fresenius' Journal of Analytical Chemistry》1963,192(2):329-330
Ohne Zusammenfassung 相似文献
114.
Zusammenfassung Für Elementbestimmungen in Abwasser wird ein Röntgenfluorescenzverfahren beschrieben. Zur Probenvorbereitung werden die Elemente durch Gefriertrocknung auf einem inerten Träger angereichert. Diese Anreicherung ist nicht selektiv und erfaßt praktisch alle in Abwasser gelösten und suspendierten Verunreinigungen. Für die Berechnung der Elementkonzentrationen aus den gemessenen Fluorescenzintensitäten wird ein Matrixkorrekturverfahren angegeben, welches dem unterschiedlichen Absorptionsverhalten der Probenmatrix Rechnung trägt. Die Massenabsorptionskoeffizienten werden durch Messung der diffus gestreuten Bremsstrahlung bzw. der Comptonstreustrahlung ermittelt. Die Selbstabsorption des zu bestimmenden Elementes wird rechnerisch berücksichtigt. Das Korrekturverfahren kann auf alle mit Röntgenfluorescenzanalyse bestimmbaren Elemente angewendet werden.
Determination of elements in waste water by x-ray fluorescence analysis: Sample preparation and matrix correction method
Summary For sample preparation the elements are enriched upon an inert carrier by freeze-drying. This enriching procedure is not selective and collects all dissolved and suspended contaminations from the waste water. In order to calculate the element concentrations from the measured fluorescence intensities a matrix correction method is given which considers the different absorption behaviour of the matrix. The mass absorption coefficients are obtained by measuring the scattered X-ray tube spectrum or a Compton scattered X-ray tube line, respectively. The self-absorption of the element to be determined is taken into account. The correction method is applicable to all elements detectable by X-ray fluorescence.
Herrn Dr. H. Wagner danken wir für das stete Interesse, die zahlreichen konstruktiven Diskussionen und die Hilfe beim Abfassen des Manuskripts. 相似文献
115.
Quantum mechanical and quantum mechanical/molecular mechanical calculations in conjunction with continuum solvation models have been used to analyze CH-pi interactions in model systems of aryl- and alkyl-aromatic interactions, as well as in a model folding system designed to study those interactions. High level calculations reproduced accurately the interaction of CH-pi interactions in both alkyl- and aryl-based model systems. Dispersion effects dominate the interaction, but the electrostatics term is also relevant for aryl CH-pi interactions. Theoretical calculations were also used to examine the influence of CH-pi interactions in determining the conformational flexibility of folding models. Finally, a critical comparison of the results obtained from high level calculations on model systems and the experimental data derived for folding models in apolar solvents was carried out, which allowed us to reconcile the apparent discrepancy found between both data. 相似文献
116.
Attilio Citterio Massimo Ramperti Elena Vismara 《Journal of heterocyclic chemistry》1981,18(4):763-766
Free-radical decomposition of diazonium salts catalyzed by titanous or titanous and ferrous salts in the presence of β-substituted α,β-unsaturated carbonyl compounds leads to 1,4-diarylpyrazole derivatives. The reaction occurs via an intermediate azo compound ( 1 ), which can be reduced by the metal salt or can be isolated and hydrogenated to pyrazole derivatives. 相似文献
117.
Intramolecular cyclization of 1-amino-3-phenacyl-4-carbohydrazido-1,2,3-triazolium-5-olates has been shown to take place via selective interaction of the carbonyl group with the terminal amino function of the hydrazido group to form a 1,2,5-triazepine ring. Minor products, resulting from the interaction of the α-nitrogen atom of the hydrazido group with the carbonyl function, having a N-amino-pyridazine structure were also detected and isolated. A general method for the synthesis of novel mesoionic 2-amino-7-aryl-4-oxo-2,4,5,8-tetrahydro[1,2,3]triazolo[5,1-d][1,2,5]triazepin-9-ium-3-olates was developed. 相似文献
118.
Andrey Shatskiy Anton Axelsson Elena V. Stepanova Jian-Quan Liu Azamat Z. Temerdashev Bhushan P. Kore Bjrn Blomkvist James M. Gardner Peter Dinr Markus D. Krks 《Chemical science》2021,12(15):5430
A protocol for stereoselective C-radical addition to a chiral glyoxylate-derived N-sulfinyl imine was developed through visible light-promoted photoredox catalysis, providing a convenient method for the synthesis of unnatural α-amino acids. The developed protocol allows the use of ubiquitous carboxylic acids as radical precursors without prior derivatization. The protocol utilizes near-stoichiometric amounts of the imine and the acid radical precursor in combination with a catalytic amount of an organic acridinium-based photocatalyst. Alternative mechanisms for the developed transformation are discussed and corroborated by experimental and computational studies.A protocol for stereoselective C-radical addition to a chiral glyoxylate-derived N-sulfinyl imine was developed through visible light-promoted photoredox catalysis, providing a convenient method for the synthesis of unnatural α-amino acids. 相似文献
119.
Elena B Lesage A Steuernagel S Böckmann A Emsley L 《Journal of the American Chemical Society》2005,127(49):17296-17302
A refocused INEPT through-bond coherence transfer technique is demonstrated for NMR of rigid organic solids and is shown to provide a valuable building block for the development of NMR correlation experiments in biological solids. The use of efficient proton homonuclear dipolar decoupling in combination with a direct spectral optimization procedure provides minimization of the transverse dephasing of coherences and leads to very efficient through-bond (1)H-(13)C INEPT transfer for crystalline organic compounds. Application of this technique to 2D heteronuclear correlation spectroscopy leads to up to a factor of 3 increase in sensitivity for a carbon-13 enriched sample in comparison to standard through-bond experiments and provides excellent selectivity for one-bond transfer. The method is demonstrated on a microcrystalline sample of the protein Crh (2 x 10.4 kDa). 相似文献
120.