全文获取类型
收费全文 | 4845篇 |
免费 | 147篇 |
国内免费 | 8篇 |
专业分类
化学 | 3173篇 |
晶体学 | 15篇 |
力学 | 103篇 |
数学 | 769篇 |
物理学 | 940篇 |
出版年
2023年 | 45篇 |
2022年 | 31篇 |
2021年 | 69篇 |
2020年 | 99篇 |
2019年 | 67篇 |
2018年 | 54篇 |
2017年 | 46篇 |
2016年 | 167篇 |
2015年 | 115篇 |
2014年 | 135篇 |
2013年 | 256篇 |
2012年 | 223篇 |
2011年 | 210篇 |
2010年 | 151篇 |
2009年 | 139篇 |
2008年 | 195篇 |
2007年 | 193篇 |
2006年 | 176篇 |
2005年 | 160篇 |
2004年 | 163篇 |
2003年 | 120篇 |
2002年 | 139篇 |
2001年 | 84篇 |
2000年 | 75篇 |
1999年 | 70篇 |
1998年 | 66篇 |
1997年 | 68篇 |
1996年 | 80篇 |
1995年 | 81篇 |
1994年 | 78篇 |
1993年 | 69篇 |
1992年 | 55篇 |
1991年 | 54篇 |
1990年 | 58篇 |
1989年 | 49篇 |
1988年 | 58篇 |
1987年 | 40篇 |
1986年 | 33篇 |
1985年 | 53篇 |
1984年 | 38篇 |
1983年 | 34篇 |
1982年 | 44篇 |
1981年 | 48篇 |
1980年 | 48篇 |
1979年 | 49篇 |
1978年 | 39篇 |
1977年 | 48篇 |
1976年 | 42篇 |
1975年 | 31篇 |
1973年 | 30篇 |
排序方式: 共有5000条查询结果,搜索用时 15 毫秒
91.
B. Stanovnik M. Tiler D. Gabrijel
i
M. Kunaver J.
mitek 《Journal of heterocyclic chemistry》1979,16(8):1567-1570
The conversion of heterocyclic amines into the corresponding azides or tetrazoloazines under phase transfer reaction conditions is described. It was found that this transformation is limited ordy to aminopyridine ( 1a-d ) aminoisoquinoline ( 1e ) and 2-aminobenzothiazole ( 1f ) derivatives, while other heterocyclic amines ( 1g-w ) could not be transformed under these conditions. 相似文献
92.
Branko Stanovnik Uroš Urleb Miha Tišler 《Monatshefte für Chemie / Chemical Monthly》1987,118(5):601-606
An unequivocal synthesis of 5-chloro-7-methyl- (8) and 7-methyl-1,2,4-triazolo[1,5-a]pyrimidine (10) from 2-amino-4-chloro-6-methylpyrimidine (5) through the corresponding amidine6 and formamide oxime7 was developed. It was unambigously shown by comparison of the chemical shifts and the magnitude of coupling constants that the compounds obtained by condensation of 3-amino-1,2,4-triazole (12) and ethyl acetoacetate (13) and some further transformations are isomeric 5-methyl substituted 1,2,4-triazolo[1,5-a]pyrimidines1,9, and11.
Eine eindeutige Synthese einiger substituierter 1,2,4-Triazolo[1,5-a]pyrimidine
Zusammenfassung Es wurde ein eindeutiger Syntheseweg für 5-Chlor-7-methyl- (8) und 7-Methyl-1,2,4-triazolo[1,5-a]pyrimidin (10) ausgehend von 2-Amino-4-chlor-6-methylpyrimidin (5) über das entsprechende Amidin6 und das Formamidoxim7 entwickelt. Durch Vergleich von chemischen Verschiebungen und Kopplungskonstanten konnte eindeutig gezeigt werden, daß die Verbindungen, die bei der Kondensation von 3-Amino-1,2,4-triazol (12) and Ethylacetoacetat (13), sowie einige weitere Transformationsprodukte, isomere 5-Methylsubstituierte 1,2,4-Triazolo[1,5-a]pyrimidine sind (1,9,11).相似文献
93.
Matthias Schüler Thomas Kovar Hans Lischka Ron Shepard Robert J. Harrison 《Theoretical chemistry accounts》1993,84(6):489-509
Summary In this work a parallel implementation of the COLUMBUS MRSDCI program system is presented. A coarse grain parallelization approach using message passing via the portable toolkit TCGMSG is used. The program is very well portable and runs on shared memory machines like the Cray Y-MP, Alliant FX/2800 or Convex C2 and on distributed memory machines like the iPSC/860. Further implementations on a network of workstations and on the Intel Touchstone Delta are in progress. Overall, results are quite satisfactory considering the complexity and the prodigious requirements, especially the I/O bandwidth, of MRCI programs in general. For our largest test case we obtain a speedup of a factor of 7.2 on an eight processor Cray Y-MP for that section of the program (hamiltonian matrix times trial vector product) which has been parallelized. The speedup for one complete diagonalization iteration amounts to 5.9. An absolute speed close to 1 GFLOPS is found. Results for the iPSC/860 show that ordinary disk I/O is certainly not sufficient in order to guarantee a satisfactory performance. As a solution for that problem, the implementation of a fully asynchronous distributed-memory model for certain data files is in preparation.
On leave from: Bereich Informatik, Universität Leipzig, Augustusplatz 10/11, O-7010 Leipzig, Germany 相似文献
94.
Uwe Klingebiel Dieter Bentmann Anton Meller 《Journal of organometallic chemistry》1978,144(3):381-388
Aminofluorsilanes are obtained by the reaction of fluorosilanes with the lithium salts of the corresponding amines. The reaction of aminofluorosilanes with butyllithium in a (2 + 2)cyclo addition reaction leads to the formation of fourmembered silicon-nitrogen ring compounds. The mechanism of the reaction is discussed, the mass, 1H and 19F NMR spectra of the compounds are reported. 相似文献
95.
Uwe Einmahl 《Probability Theory and Related Fields》1992,92(2):177-194
Summary In a previous paper we obtained upper and lower class type results refining the bounded LIL for sums of iid Hilbert space valued mean zero random variables, whose covariance operators satisfy certain regularity assumptions. We now establish precise convergence rates for the bounded LIL in the non-regular case. It turns out that the almost sure behavior in this case is entirely different from the behavior in the previous situation.Supported in part by NSF Grant DMS 90-05804 相似文献
96.
n
—n-
— ƒ()L
v
(T
n
), 1
f, - -. -. 相似文献
97.
K. Greßmann 《Mikrochimica acta》1963,51(4):782-783
Zusammenfassung Durch Tüpfeln auf Chromatographiepapier, das mit Brucin, Benzidin und Phosphorsäure imprägniert wurde, läßt sich Nitrit empfindlieh und spezifisch nachweisen. 20 ng Nitrit können neben Nitrat und anderen Anionen festgestellt werden.
Summary Nitrite can be detected sensitively and specifically by spotting on Chromatograph paper impregnated with brucine, benzidine and phosphoric acid. It is possible to detect 20g nitrite in the presence of nitrate and other anions.
Résumé On peut rechercher les nitrites d'une manière sensible et spécifique sur les taches du papier chromatographique et qui a été imprégné de brucine, de benzidine et d'acide phosphorique. On peut déceler 20g de nitrite en présence de nitrates et d'autres ions A-.相似文献
98.
Normen Peulecke Andreas Ohff Wolfgang Baumann Rhett Kempe Vladimir V. Burlakov Uwe Rosenthal 《Journal of organometallic chemistry》1996,520(1-2):235-239
The reaction of Cp2Zr(L)(η2-Me3SiC2) (L = THF, py) with equimolar amounts of H2C = CMe-CHO at room temperature depends strongly on the ligands L and the solvents that are used. With L = THF, in the THF solution the insertion product 1 was isolated, whereas by conducting the reaction in n-hexane solution an alkyne substitution with 1,4-coordination of the methacrolein takes place and the binuclear complex [ 2 was obtained. In conttrast, with L = py (a stronger ligand) only a 1:1 ratio of 1 and 2 was observed in both THF and in n-hexane. At 50°C complex 1 was converted into 2 and the alkyne was eliminated quantitatively.
Complexes 1 and 2 were characterized by IR and NMR spectroscopical measurements and 1 by an additional X-ray structure determination. 相似文献
99.
Zusammenfassung Die Herstellung eines Reagenspapiers wird beschrieben, mit dessen Hilfe die Identifizierung oxydierender Anionen [Nitrit, Hypochlorit, Chromat, Bromat, Hexacyanoferrat(III), Peroxydisulfat, Jodat, Chlorat und Nitrat ] durch einen einfachen Tüpfeltest möglich ist.
Summary The preparation of a reagent is described with whose aid it is possible to identify oxidant anions through a simple spot test. The anions are: nitrite, hypochlorite, chromate, bromate, hexacyanoferrate (III), peroxodisulfate, iodate, chlorate, nitrate.
Résumé On décrit la préparation d'un papier-réactif permettant l'identification des anions oxydants (nitrite, hypochlorite, chromate, bromate, ferricyanure, peroxydisulfurique, iodate, chlorate et nitrate) par une réaction à la touche simple.相似文献
100.
Fluoro- und aminofluoro-silanes react with the lithium salt of N,O-bis(trimethylsilyl)hydroxylamine under LiF elimination and substitution. Alkyl- and amino-fluorosilanes give O-fluorosilyl-N,N-bis(trimethylsilyl)hydroxylamines, arylfluorosilanes give N-fluorosilyl-N,O-bis(trimethylsilyl)hydroxylamines. By the further reaction of O-difluorosilyl-N,N-bis(trimethylsilyl)hydroxylamine with the lithiated hydroxylamine, O,O′-fluoromethylsilyldi[N,N-bis(trimethylsilyl)hydroxylamine] is formed. On heating N-difluorophenylsilyl-N,O-bis(trimethylsilyl)hydroxylamine di[fluorophenylsilyl(methyl)amino]pentamethylsiloxane is formed by methyl group migration. The NMR and mass spectra of the compounds are reported. 相似文献