全文获取类型
收费全文 | 3849篇 |
免费 | 97篇 |
国内免费 | 12篇 |
专业分类
化学 | 2900篇 |
晶体学 | 12篇 |
力学 | 65篇 |
数学 | 535篇 |
物理学 | 446篇 |
出版年
2023年 | 24篇 |
2022年 | 34篇 |
2021年 | 28篇 |
2020年 | 46篇 |
2019年 | 38篇 |
2018年 | 24篇 |
2017年 | 22篇 |
2016年 | 102篇 |
2015年 | 78篇 |
2014年 | 73篇 |
2013年 | 147篇 |
2012年 | 168篇 |
2011年 | 255篇 |
2010年 | 128篇 |
2009年 | 128篇 |
2008年 | 239篇 |
2007年 | 194篇 |
2006年 | 201篇 |
2005年 | 178篇 |
2004年 | 182篇 |
2003年 | 125篇 |
2002年 | 118篇 |
2001年 | 69篇 |
2000年 | 69篇 |
1999年 | 48篇 |
1998年 | 45篇 |
1997年 | 65篇 |
1996年 | 55篇 |
1995年 | 60篇 |
1994年 | 72篇 |
1993年 | 66篇 |
1992年 | 37篇 |
1991年 | 52篇 |
1990年 | 43篇 |
1989年 | 46篇 |
1988年 | 54篇 |
1987年 | 39篇 |
1986年 | 40篇 |
1985年 | 53篇 |
1984年 | 38篇 |
1983年 | 40篇 |
1982年 | 41篇 |
1981年 | 49篇 |
1980年 | 31篇 |
1979年 | 28篇 |
1978年 | 21篇 |
1977年 | 24篇 |
1976年 | 32篇 |
1975年 | 29篇 |
1973年 | 22篇 |
排序方式: 共有3958条查询结果,搜索用时 15 毫秒
61.
Dilithiated N,N′‐dimethyl‐piperazine, LiCH2N(CH2CH2)2 NCH2Li ( 2 ) was prepared by transmetallation of N,N′‐bis(trimethylstannylmethyl)‐piperazine ( 1 ) with nBuLi and was isolated as a highly pyrophoric yellowish powder in high yield. Compound 2 was characterized by elemental analysis and was reacted as difunctional aminomethylating reagent with dialkyl‐earth metal chlorides, R2MCl (M = Al, Ga; R = Me, tBu) which resulted in the formation of spirocyclic adducts of N,N′‐bis(dialkylmetallamethyl)‐piperazine and unreacted dialkylmetal chlorides, [(Me2AlCl)Me2AlCH2N(CH2CH2)2NCH2AlMe2(ClAlMe2)] ( 3 ) and [(tBu2GaCl)tBu2GaCH2N(CH2CH2)2NCH2GatBu2(ClGatBu2)] ( 4 ) with five‐membered rings. Compounds 1 , 3 and 4 were identified by NMR‐spectroscopy (1H, 13C, 119Sn for 1 , 27Al for 3 ), mass spectra (EI, for 1 ) and by crystal structure determinations. 相似文献
62.
Roland Wilberger Holger Piotrowski Marcus Warchhold Ingo‐Peter Lorenz 《无机化学与普通化学杂志》2003,629(14):2485-2492
Novel Neutral and Cationic Mono‐Aziridine Complexes of the Type [CpMn(CO)2Az], [CpCr(NO)2Az]+, and [(Ph3P)(CO)4ReAz]+ via CO‐, Hydride‐, and Chloride‐Elimination Reactions The monoaziridine complexes 1 — 5 are obtained by three differently induced substitution reactions. The photolytically induced CO substitution reaction of [CpMn(CO)3] with 2, 2‐dimethylaziridine leads to the neutral N‐coordinate aziridine complex [Cp(CO)2Mn{$\overline{N(H)CMe2C}$ H2}] ( 1 ). The protonation of [(Ph3P)(CO)4ReH] with CF3SO3H and consecutive treatment with 2, 2‐dimethylaziridine or 2‐ethylaziridine gives the salt‐like aziridine complexes [(Ph3P)(CO)4Re{$\overline{N(H)CMe2C}$ H2}](CF3SO3) ( 2 ) or [(Ph3P)(CO)4Re{ H2}](CF3SO3) ( 3 ) by hydride elimination reactions. The like‐wise salt‐like complexes [Cp(NO)2Cr{$\overline{N(H)CMe2C}$ H2}](BF4) ( 4 ) and [Cp(NO)2Cr{ H2}](CF3SO3) ( 5 ) are synthesized from [CpCr(NO)2Cl] by chloride elimination with AgX (X = BF4, CF3SO3) in the presence of 2, 2‐dimethylaziridine or 2‐ethylaziridine, respectively. As a result of X‐ray structure analyses, the metal atoms are trigonal pyramidally ( 1, 4, 5 ) or octahedrally ( 2, 3 , cis‐position) configurated; the intact three‐membered rings coordinate through the distorted tetrahedrally configurated N atoms. All compounds 1‐5 are stable with respect to the directed thermal alkene elimination to give the corresponding nitrene complexes; the IR, 1H‐ and 13C{1H}‐NMR, and MS spectra are reported and discussed. 相似文献
63.
X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), rotating disc electrode-electrochemical impedance spectroscopy (RDE-EIS) and synchrotron radiation-grazing incidence X-ray diffraction (SR-GIXRD) have been used to study the response mechanism of the mercury(II) chalcogenide ion-selective electrode (ISE) in saline media. XPS and SIMS have shown that the chalcogenide surface is poisoned by silver chloride, or a mixture of silver halides, on continuous exposure to synthetic and real seawater. Significantly, the in-situ SR-GIXRD study demonstrated that electrode fouling in synthetic seawater is linked to the formation of poorly crystalline or amorphous silver chloride, and that the low level of free mercury(II) in a calibration buffer (i.e., 10(-14) M) is able to undergo metathesis with silver(II) sulfide in the membrane generating mercury(II) sulfide. Significantly, the results of this detailed surface study have shown that silver chloride fouling of the electrode is ameliorated in real seawater comprising natural organic ligands, and this has been attributed to the peptization of silver chloride by the surfactant-like nature of seawater ligands at pH 8. RDE-EIS aging studies have revealed that the chalcogenide membrane experiences a sluggish charge transfer reaction in seawater, and contrary to a previous report for a static electrode, the seawater matrix does not passivate the RDE. The results of this XPS, SIMS, RDE-EIS and SR-GIXRD study have elucidated the response mechanism of the mercury(II) ISE in saline media. 相似文献
64.
In the presence of triethylamine, 2-mercaptobenzoic acid readily adds onto acylhydrazonoyl chlorides (1a-c) (precursors of the reactive nitrile imine 1,3-dipolar species) to afford good yields of the corresponding 2-[(2-oxo-1-arylhydrazonopropan-1-yl)mercapto]benzoic acids (2a-c). The latter acyclic adducts, in THF in the presence of 1,1'-carbonyldiimidazole, undergo intramolecular cyclization involving the activated carboxy and the enol functionality to deliver the respective 2-(N-arylhydrazono)-3-oxobenzothiophenes (3a-c). In the solid state, the latter compounds adopt the (Z)-geometry around the C=N double bond as evidenced by single crystal X-ray structure determination for 3b. 相似文献
65.
The synthesis of 4-methylumbelliferyl α-D -glycoside 13 of N-acetyl-4-deoxyneuraminic acid and its behaviour towards bacterial sialidases is described. N-Acetyl-4-deoxyneuraminic acid ( 1 ) was transformed into its methyl ester 2 and then acetylated to give the anomeric pentaacetates 3 and 4 of methyl 4-deoxyneuraminate and the enolacetate 5 (Scheme). A mixture 3/4 was treated with HCl/AcCl to give the glycosyl chloride, which was directly converted into the 4-methylumbelliferyl α-D -glycoside 9 of methyl 7-O,8-O,9-O,N-tetraacetylneuraminate and into the 2,3-dehydrosialic acid 11 . The ketoside 9 was de-O-acetylated to 12 with NaOMe in MeOH. Saponification (NaOH) of the methyl ester 12 followed by acidification gave the free 13 , which was also converted into the sodium salt 14 by passage through Dowex 50 (Na+). The 4-deoxy α-D -glycoside 13 is not hydrolyzed at significant rates by Vibrio cholerae and Arthrobacter ureafaciens sialidase. Neither the free N-acetyl-4-deoxyneuraminic acid ( 1 ), nor the α-D -glycoside 13 inhibit the activity of these sialidases. 相似文献
66.
67.
The chemical synthesis and NMR characterization of the benzo ring tetrahydro- and diol epoxides of the carcinogen benz[c]acridine are described. 相似文献
68.
The quenching of excited singlet oxonine by EDTA in aqueous solution leads mainly to deactivation of the dye to the ground state and, to a lesser extent, to electron abstraction. The rate constants for these processes have been measured and compared to those for the same reactions involving the oxonine triplet state. The rate constant of electron abstraction is about ten times greater via the singlet state than via the triplet state. However, the rate constant of deactivation to the ground state is 103-104 times greater for the excited singlet state than for the triplet state, so that the efficiency of electron transfer is much smaller for the singlet state. 相似文献
69.
Roland Herrmann 《Fresenius' Journal of Analytical Chemistry》1965,212(1):1-18
Zusammenfassung Es wird gezeigt, daß man durch Einsatz von reduzierenden Flammen, durch Einsatz von Chemiluminescenzvorgängen in Flammen, durch Atomabsorptionsverfahren, durch Modulations- und Differentiations-operationen eine Reihe von weiteren Elementen mit den schnellen und spezifischen Flammenmethoden analytisch gut erfassen kann. Hierbei wird insbesondere auf die Bestimmung von Spurenelementen in Mikroproben mit möglichst einfachen Mitteln eingegangen.
Unterstützt mit Mitteln der Deutschen Forschungsgemeinschaft.
Herrn Prof. Dr. Bohnstedt zum 65. Geburtstag gewidmet.
Meinen Mitarbeitern, insbesondere Herrn Dipl. Phys. W. Lang und Frau B. Gutsche, die an den zuletzt besprochenen Ergebnissen einen maßgeblichen Anteil hatten, als auch der Deutschen Forschungsgemeinschaft sei an dieser Stelle herzlichst gedankt. 相似文献
Summary It is demonstrated that one can grasp analytically well a series of further elements with the quick and specific flame methods by use of chemiluminescence processes in flames, by atomic absorption methods, by operations of modulation and differentiation. Hereby the determination of trace elements is specifically treated in microprobes with the most possible simple means.
Unterstützt mit Mitteln der Deutschen Forschungsgemeinschaft.
Herrn Prof. Dr. Bohnstedt zum 65. Geburtstag gewidmet.
Meinen Mitarbeitern, insbesondere Herrn Dipl. Phys. W. Lang und Frau B. Gutsche, die an den zuletzt besprochenen Ergebnissen einen maßgeblichen Anteil hatten, als auch der Deutschen Forschungsgemeinschaft sei an dieser Stelle herzlichst gedankt. 相似文献
70.
Hill M Erker G Kehr G Fröhlich R Kataeva O 《Journal of the American Chemical Society》2004,126(35):11046-11057
The dimethylsilanediyl-bridged ansa-zirconocene dichloride 1, that contains a pendent allyl substituent at a Cp-ring, adds HB(C(6)F(5))(2) to the vinyl group to yield the bifunctional zirconocene/borane complex 2. Substituted benzimidazoles were added to the strongly electrophilic borane moiety as protective groups, which allowed subsequent chloride versus -CH(2)SiMe(3) exchange at zirconium to take place by treatment with the respective alkyllithium reagent. Alternatively, the introduction of active sigma-ligands at zirconium is carried out first, followed by the hydroboration reaction. This route was followed for the synthesis of the diphenyl-ansa-zirconocene/borane complex 12. Complex 12 reacts slowly in solution by intramolecular electrophilic attack of the borane at its adjacent Cp-ring, followed by deprotonation using a [Zr]-Ph group to yield the zwitterionic complex 14 featuring a borata-tetrahydroindenyl moiety as part of the ansa-metallocene framework. Complex 14 was characterized by X-ray diffraction. It adds PMe(3) at zirconium to yield 15. Thermolysis of 12 with excess PMe(3) leads to the formation of the (aryne)zirconocene complex 18, which is stabilized by PMe(3) coordination to zirconium and PMe(3) addition to boron. N-Methylbenzimidazole adds to the -B(C(6)F(5))(2) unit of 12 to give the 1:1 adduct 19. Thermolysis of 19 at 80 degrees C in benzene solution in the presence of one additional equivalent of N-methylbenzimidazole results in deprotonation of the substrate to yield the sigma-N-methylbenzimidazolyl zirconium complex 20 (as a mixture of two diastereoisomers). An additional N-methylbenzimidazole ligand is bonded to the B(C(6)F(5))(2) unit in this product. 相似文献