全文获取类型
收费全文 | 1927篇 |
免费 | 60篇 |
国内免费 | 2篇 |
专业分类
化学 | 1135篇 |
晶体学 | 20篇 |
力学 | 44篇 |
数学 | 173篇 |
物理学 | 617篇 |
出版年
2021年 | 19篇 |
2020年 | 36篇 |
2019年 | 30篇 |
2018年 | 14篇 |
2017年 | 14篇 |
2016年 | 45篇 |
2015年 | 49篇 |
2014年 | 61篇 |
2013年 | 91篇 |
2012年 | 109篇 |
2011年 | 87篇 |
2010年 | 65篇 |
2009年 | 63篇 |
2008年 | 79篇 |
2007年 | 66篇 |
2006年 | 69篇 |
2005年 | 55篇 |
2004年 | 69篇 |
2003年 | 46篇 |
2002年 | 43篇 |
2001年 | 55篇 |
2000年 | 47篇 |
1999年 | 33篇 |
1998年 | 18篇 |
1997年 | 32篇 |
1996年 | 41篇 |
1995年 | 15篇 |
1994年 | 22篇 |
1993年 | 31篇 |
1992年 | 18篇 |
1991年 | 27篇 |
1989年 | 33篇 |
1988年 | 26篇 |
1987年 | 22篇 |
1986年 | 24篇 |
1985年 | 22篇 |
1984年 | 23篇 |
1983年 | 12篇 |
1982年 | 23篇 |
1981年 | 24篇 |
1980年 | 16篇 |
1979年 | 19篇 |
1978年 | 22篇 |
1977年 | 22篇 |
1976年 | 21篇 |
1974年 | 19篇 |
1973年 | 15篇 |
1972年 | 12篇 |
1971年 | 14篇 |
1970年 | 12篇 |
排序方式: 共有1989条查询结果,搜索用时 218 毫秒
71.
E. Hartmann und H. Röpke 《Fresenius' Journal of Analytical Chemistry》1967,232(4):268-274
Zusammenfassung Für die Lagerfähigkeit von jodhaltigen Röntgenkontrastmitteln der Aminobenzoesäurereihe ist der Gehalt an Verbindungen mit freier Aminogruppe von besonderer Bedeutung. Zur Erfassung dieser Verunreinigungen wurden zwei sehr empfindliche Methoden entwickelt, die sich auch für die Untersuchung der Reaktionskinetik eignen und darüber hinaus bessere Reinheitskriterien als die bisher etwas überbewertete Prüfung auf Jodidverunreinigungen darstellen. Die Methoden beruhen auf der Bestimmung des mit elementarem Brom abspaltbaren Jods bzw. auf einer Diazotierung und anschließender Kupplungsreaktion.
Herrn Prof. Dr. med. Dr. rer. nat. h. c. Karl Junkmann zum 70. Geburtstag gewidmet. 相似文献
Summary Two methods have been worked out for the purity control of iodine containing X-ray contrast media of the aminobenzoic acid series. The methods apply to the determination of compounds containing a free amino group, which are especially important with regard to storage life. They are based on the determination of the iodine splitted off by means of elementary bromine and on diazotization and subsequent coupling reaction, respectively. The methods are very sensitive and can also be used for an investigation of the reaction kinetics.
Herrn Prof. Dr. med. Dr. rer. nat. h. c. Karl Junkmann zum 70. Geburtstag gewidmet. 相似文献
72.
Lukas Fischer Alexander K. Strzelczyk Nils Wedler Christian Kropf Stephan Schmidt Laura Hartmann 《Chemical science》2020,11(36):9919
Catechol and amine residues, both abundantly present in mussel adhesion proteins, are known to act cooperatively by displacing hydration barriers before binding to mineral surfaces. In spite of synthetic efforts toward mussel-inspired adhesives, the effect of positioning of the involved functional groups along a polymer chain is not well understood. By using sequence-defined oligomers grafted to soft hydrogel particles as adhesion probes, we study the effect of catechol–amine spacing, as well as positioning relative to the oligomer terminus. We demonstrate that the catechol–amine spacing has a significant effect on adhesion, while shifting their position has a small effect. Notably, combinations of non-charged amides and catechols can achieve similar cooperative effects on adhesion when compared to amine and catechol residues. Thus, these findings provide a blueprint for the design of next generation mussel-inspired adhesives.The catechol driven adhesion of precision macromolecules on glass surfaces is quantified by soft colloidal probe readout. Catechol moieties are shown to synergize with amine and amide residues depending on residue spacing and residue order. 相似文献
73.
A new methodology based on pressurized liquid extraction (PLE) followed by LC-MS is presented for the simultaneous and unequivocal determination of alkylphenol ethoxylates (APEOs) and their degradation products, alkylphenols (APs) and alkylphenoxy carboxylates (APECs), in sediment samples. The protocol, applicable to a full range of APEO oligomers and degradation products, permits the sensitive and selective determination of APEOs (nEO = 1-15), APECs (nEO = 0-1) and APs at low ppb levels (LODs = 1-5 microg/kg) in sediment samples. Optimization of the operational parameters of PLE clearly demonstrates that significant thermal losses of APs occur during extraction at elevated temperatures. The loss of octylphenol (OP) at 100 degrees C was 61.2% and of nonylphenol (NP) 40.0%, whereas other compounds were completely recovered. Thus, to avoid losses due to the volatility of alkylphenols, a low extraction temperature should be applied. The conditions that gave the best results for all target compounds were as follows: extraction solvent mixture, methanol-acetone (1:1, v/v); temperature, 50 degrees C; pressure, 1500 p.s.i.; two static cycles. Using PLE and a subsequent clean-up with solid-phase extraction (SPE), the simultaneous extraction of APEOs, APs and APECs from sediment samples was achieved yielding recoveries >70% and producing low MS background noise. The developed methodology was applied on a routine basis to the analysis of alkylphenolic compounds in sediment samples. APEOs and their persistent degradation products were detected in significant concentrations in sediments from Portuguese rivers, especially at sites situated in the proximity of industrial plants (mainly the textile industry). The total concentration of alkylphenolic compounds (APEOs+APs+APECs) ranged from 155 to 2400 microg/kg. Of all the alkylphenolic compounds, NP comprised 40 to 50% with concentrations up to 1172 microg/kg. 相似文献
74.
Herbert Schumann Sebastian Dechert Frank Girgsdies Bernd Heymer Markus Hummert Ji‐Young Hyeon Jens Kaufmann Stefan Schutte Sonja Wernik Birgit C. Wassermann 《无机化学与普通化学杂志》2006,632(2):251-263
Synthesis and Characterization of New Intramolecularly Nitrogen‐stabilized Organoaluminium‐ and Organogallium Alkoxides The intramolecularly nitrogen stabilized organoaluminium alkoxides [Me2Al{μ‐O(CH2)3NMe2}]2 ( 1a ), Me2AlOC6H2(CH2NMe2)3‐2,4,6 ( 2a ), [(S)‐Me2Al{μ‐OCH2CH(i‐Pr)NH‐i‐Pr}]2 ( 3a ) and [(S)‐Me2Al{μ‐OCH2CH(i‐Pr)NHCH2Ph}]2 ( 4 ) are formed by reacting equimolar amounts of AlMe3 and Me2N(CH2)3OH, C6H2[(CH2NMe2)3‐2,4,6]OH, (S)‐i‐PrNHCH(i‐Pr)CH2OH, or (S)‐PhCH2NHCH(i‐Pr)CH2OH, respectively. An excess of AlMe3 reacts with Me2N(CH2)2OH, Me2N(CH2)3OH, C6H2[(CH2NMe2)3‐2,4,6]OH, and (S)‐i‐PrNHCH(i‐Pr)CH2OH producing the “pick‐a‐back” complexes [Me2AlO(CH2)2NMe2](AlMe3) ( 5 ), [Me2AlO(CH2)3NMe2](AlMe3) ( 1b ), [Me2AlOC6H2(CH2NMe2)3‐2,4,6](AlMe3)2 ( 2b ), and [(S)‐Me2AlOCH2CH(i‐Pr)NH‐i‐Pr](AlMe3) ( 3b ), respectively. The mixed alkyl‐ or alkenylchloroaluminium alkoxides [Me(Cl)Al{μ‐O(CH2)2NMe2}]2 ( 6 ) and [{CH2=C(CH3)}(Cl)Al{μ‐O(CH2)2NMe2}]2 ( 8 ) are to obtain from Me2AlCl and Me2N(CH2)2OH and from [Cl2Al{μ‐O(CH2)2NMe2}]2 ( 7 ) and CH2=C(CH3)MgBr, respectively. The analogous dimethylgallium alkoxides [Me2Ga{μ‐O(CH2)3NMe2}]2 ( 9 ), [(S)‐Me2Ga{μ‐OCH2CH(i‐Pr)NH‐i‐Pr}]n ( 10 ), [(S)‐Me2Ga{μ‐OCH2CH(i‐Pr)NHCH2Ph}]n ( 11 ), [(S)‐Me2Ga{μ‐OCH2CH(i‐Pr)N(Me)CH2Ph}]n ( 12 ) and [(S)‐Me2Ga{μ‐OCH2(C4H7NHCH2Ph)}]n ( 13 ) result from the equimolar reactions of GaMe3 with the corresponding alcohols. The new compounds were characterized by elemental analyses, 1H‐, 13C‐ and 27Al‐NMR spectroscopy, and mass spectrometry. Additionally, the structures of 1a , 1b , 2a , 2b , 3a , 5 , 6 and 8 were determined by single crystal X‐ray diffraction. 相似文献
75.
Metal Derivatives of Molecular Compounds. III. Molecular and Crystal Structure of Lithium bis(trimethylsilyl)phosphide · DME and of Lithium dihydrogenphosphide · DME Lithium bis(trimethylsilyl)phosphide · DME 1 prepared from tris(trimethylsilyl)-phosphine and lithium methanide [2, 4] in 1,2-dimethoxyethane 1 1,2-Dimethoxyethan (DME); Tetrahydrofuran (THF); Bis[2-(dimethylamino)ethyl]methyl-amin (PMDETA). , crystallizes in the orthorhombic space group Pnnn {a = 881.1(9); b = 1308.5(9); c = 1563.4(9) pm at ?120 ± 3°C; Z = 4 formula units}, lithium dihydrogenphosphide · DME 2 [10] prepared from phosphine and lithium- n -butanide in the same solvent, in P2 1 2 1 2 1 {a = 671.8(1); b = 878.6(1); c = 1332.2(2) pm at ?120 ± 3°C; Z = 4 formula units}. X-ray structure determinations (R w = 0.036/0.045) show the bis(trimethylsilyl) derivative 1 to be dimeric with a planar P? Li? P? Li ring (P? Li 256 pm; Li? P? Li 76°; P? Li? P 104°), and the dihydrogenphosphide 2 to be polymeric with a linear Li? P? Li fragment (P? Li 254 to 260 pm; Li? P? Li 177°; P? Li? P 118°). The shortened P? Si distance (221 pm) of compound 1 and the structure of the PH 2 group in 2 are discussed in detail. Lithium obtains its preferred coordination number 4 by a chelation with one molecule of 1,2-dimethoxyethane (Li? O 202 to 204 pm). 相似文献
76.
Sonja Strah Branko Stanovnik Simona Goli
Grdadolnik 《Journal of heterocyclic chemistry》1997,34(1):263-267
Alkyl 2-(2-benzoyl-2-ethoxycarbonyl-1-ethenyl)amino-3-dimethylaminopropenoates 4a,b were prepared. They react with C-nucleophiles such a 2-pyridinylacetonitrile 5 and methyl-2-quinolinylacetate 8, cyclohexane-1,3-dione 10 and its derivatives 12 and 14, resorcinol derivative 16 , 2-naphtol 18, 2-pyranone derivatives 20 and 22, and 4-hydroxypyridin-2(1H)-one 24, to form substituted amino derivatives of quinolizine 6, benzo[c]quinolizine 9, tetrahydrobenzopyran-2-one 11, 13, 15 , naphto[2,1-b]pyran-3-one 19 , pyranopyranones 21, 23, and pyrano[3,2-c]pyridine 25. 相似文献
77.
78.
79.
80.
Ester derivatives of N-phosphorylated 3(5)-methylpyrazole and 3-amino-1,2,4-triazole, respectively, have been synthesized. The results of hydrolysis studies are described. 相似文献