全文获取类型
收费全文 | 206篇 |
免费 | 3篇 |
专业分类
化学 | 151篇 |
晶体学 | 3篇 |
力学 | 1篇 |
数学 | 28篇 |
物理学 | 26篇 |
出版年
2019年 | 3篇 |
2017年 | 3篇 |
2016年 | 3篇 |
2015年 | 2篇 |
2014年 | 4篇 |
2013年 | 4篇 |
2011年 | 4篇 |
2010年 | 4篇 |
2009年 | 3篇 |
2008年 | 5篇 |
2007年 | 5篇 |
2006年 | 5篇 |
2005年 | 4篇 |
2004年 | 3篇 |
2003年 | 6篇 |
2002年 | 10篇 |
2001年 | 2篇 |
1999年 | 6篇 |
1998年 | 4篇 |
1997年 | 5篇 |
1996年 | 6篇 |
1994年 | 2篇 |
1993年 | 2篇 |
1992年 | 5篇 |
1991年 | 5篇 |
1989年 | 2篇 |
1987年 | 3篇 |
1986年 | 3篇 |
1985年 | 3篇 |
1983年 | 5篇 |
1982年 | 2篇 |
1981年 | 5篇 |
1980年 | 4篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 7篇 |
1975年 | 3篇 |
1974年 | 5篇 |
1973年 | 7篇 |
1972年 | 2篇 |
1971年 | 2篇 |
1968年 | 2篇 |
1966年 | 3篇 |
1962年 | 2篇 |
1943年 | 2篇 |
1932年 | 2篇 |
1927年 | 2篇 |
1926年 | 4篇 |
1879年 | 2篇 |
1862年 | 2篇 |
排序方式: 共有209条查询结果,搜索用时 81 毫秒
91.
Escher BI Berg M Mühlemann J Schwarz MA Hermens JL Vaes WH Schwarzenbach RP 《The Analyst》2002,127(1):42-48
The extraction of two methylated anilines and three chlorinated phenols by solid-phase microextraction (SPME) fibers coated with polyacrylate was investigated as a function of pH. Only the neutral species of the acids and bases partitioned into the polymer. Extraction kinetics were accelerated for the hydrophobic phenols at pH values around their acidity constant. This is presumably due to a reconstitution of the neutral species in the unstirred aqueous layer adjacent to the polymer surface by the charged species through the fast acid-base equilibrium. Although the charged species is not taken up into the polymer, liposome/water distribution ratios could be measured up to a pH value, where 99% of the compounds were present as charged species. The partition coefficients of the neutral and charged species were extrapolated from the pH profiles of the liposome/water distribution ratios. The resulting values were slightly lower than those measured with equilibrium dialysis. The discrepancies are discussed with respect to differences in the experimental conditions and the possibility of matrix effects during SPME measurements. 相似文献
92.
The complexes of In(III) with thioglycolate (HO? CH2? CH2? S?) have been investigated by pH-metric methods. Only the mononuclear species In(SR)n(3-n)+ (n = 1, 2, 3, 4) are formed (Fig. 1): It was difficult to obtain an accurate value for the first stability constant K1 because of the high stability of the 1:1-complex and therefore mixtures of KCl-KNO3 were used to keep up the ionic strength of μ = 0.1. In presence of chloride, the constants Kn are smaller because of the formation of chloro complexes and mixed complexes In(SR)nCli(3-n-1)+ and the stability constants of these have also been determined. The constants for the complexes without coordinated chloride can finally be obtained by an extrapolation (Fig. 2). The problem of the influence of increasing charge on the stability of various complexes of the isoelectronic 4d10 cations is discussed (Fig. 3). 相似文献
93.
Janos Borgulya Marcel Gerold Fernand Schneider Karl Bernauer 《Helvetica chimica acta》1977,60(2):598-617
Chemical and Pharmacological Studies on Derivatives of Benzo[de]quinoline, II Spirocompounds of type V ( 12–18 ) were synthesized by a carbenium ion cyclization reaction of the intermediates 7 and 11 , respectively. 7 was prepared from 2-oxocyclopentanepropionic acid via 1, 2 , and 4–6; 11 from the same starting material via 1, 2 , and 8–10 . - The spiro[7H-benzo[de]quinolin-7,1′-cyclohexan]-2′-one 27 and other compounds of type VI could be synthesized from the (1-isoquinolinyl)ethyl-cyclohexanone 25: 25 is hydroxylated to the hydroxyketone 26 ; treatment of 26 with a mixture of sulfuric acid and oleum gives 27 . The structure of 27 has been proved by transformation into the known spirocyclohexane 33 . - Aporphine analogues of type VII ( 38, 39 ) were synthesized from the cyclic ketone 34 via either the formylketone 35 and its methyl vinyl ketone adducts 36 and 37 or the methyl ethynyl ketone adduct 40 . Compound 40 has also been cyclized to the 10-hydroxy-aporphine 42 . 9 compounds of the types mentioned have been screened for hypotensive action in rats, and partly in cats and dogs. Only compound 13 · HCl proved to be (moderately) active in rats and dogs. 相似文献
94.
Eric Meier Olivier Cherpillod Tristano Boschi Raymond Roulet Pierre Vogel Cyril Mahaim Alan A. Pinkerton Dieter Schwarzenbach Gervais Chapuis 《Journal of organometallic chemistry》1980,186(2):247-264
The reaction of 2,3,5,6-tetrakis(methylene)-7-oxabicyclo[2.2.1]heptane (I) with iron carbonyls in various solvents yields the (η4-1,3-diene)Fe(CO)3 isomers (II: exo; III: endo) and the bimetallic isomers bis[(η4-1,3-diene)Fe(CO)3] (IV: bis(exo); V: endo,exo). In weakly coordinating solvents, a parallel rearrangement of I occurs through CO bond cleavage of the allylic ether by Fe2(CO)9 yielding an unsaturated ketone (VI) bonded to two Fe(CO)3 groups through a trimethylenemethane and a 1,3-diene system, respectively. The geometries of III and VI have been ascertained by X-ray crystal structure determinations. 相似文献
95.
96.
The bulk water surface is of fundamental interest to physical as well as environmental chemistry. As there is a lack of wide-ranging adsorption data from the air to the bulk water surface, a large and diverse data set of adsorption coefficients of nonionic, organic compounds has been produced with inverse gas chromatography. The 61 compounds were chosen to cover a large range of properties, considering the intermolecular interactions between the compounds and the bulk water surface, i.e., van der Waals and electron-donor/acceptor interactions. The data set gained in this work was interpreted with a linear free energy relationship (LFER) based on these intermolecular interactions. From this LFER, a general adsorption model is derived, including compound (i) and surface (surf) properties: log K(i surf/air)(m(3)/m(2)) = 0.135(+/-0.003) log K(i hexadecane/air)(gamma(surf)(vdW))(0.5) + 5.11(+/-0.15)Sigma beta(i2)(H)EA(surf) + 3.60(+/-0.28)Sigma alpha(i2)(H)ED(surf) - 8.47. This adsorption model can be used for the characterization of adsorption to any other surface. The adsorption model for bulk water surface adsorption as well as the general adsorption model can be used as prediction tools in natural or technical systems. 相似文献
97.
A. Alan Pinkerton Dieter Schwarzenbach Raphy Gabioud Pierre Vogel 《Helvetica chimica acta》1986,69(2):305-314
The Friedel-Crafts mono and double acylations of trans-μ-[(1RS,2RS,3SR,5RS,6SR,7SR)-C,2,3,C-η:C,6,7,C-η-(2,3,6,7-tetramethylidenebicyclo[3.2.1]octane)]bis(tricarbonyliron) ( 4 ) are highly stereoselective and yield trans-μ-{(1RS,2RS,3SR,5RS,6SR,7RS)-C,2,3,C-η :C,6,7,C-η-[(Z)-1-(3,6,7-trimethylidenebicyclo[3.2.1]-oct-2-ylidene)-2-propanone]}bis(tricarbonyliron) ( 5 ) and trans-μ-{(1RS,2RS,3SR,5RS,6SR,7SR)-C,2,3,C-η :C,6,7,C-η-[(Z,Z)-1,1′-(3,7-dimethylidenebicyclo [3.2.1] octane-2,6-diylidene)di(2-propanone)]}bis(tricarbonyliron) ( 6 ) whose structure has been established by single-crystal X-ray diffraction. 相似文献
98.
A high-throughput experimental (HTE) setup using conventional powders as raw materials was developed to synthesize and characterize ceramic bulk samples avoiding the customary liquid- or vapor-phase synthesis routes. Its functionality was verified using the well-known binary material system ZrO2-Y2O3. Libraries of 0.5 mol % yttria resolution were prepared using varying boundary systems as well as a number of liquid mixing aids. Automated powder X-ray diffraction (XRD) was applied to check the monoclinic phase content retained after heat-treatment on various positions for each sample. This phase information was used for comparison with a reference library comprising conventionally prepared samples by means of a regression analysis and mean deviation of monoclinic phase percentage. Out of the eight HTE libraries, three showed a significant comparability to the reference library. 相似文献
99.
Gutzeit D Mönch S Jerz G Winterhalter P Rychlik M 《Analytical and bioanalytical chemistry》2008,391(1):211-219
A stable isotope dilution assay was adopted for quantitation of folate vitamers in sea buckthorn berries, juice, and concentrate using fourfold labeled folate isotopologues of the folate derivatives as the internal standards and reversed-phase liquid chromatography-tandem mass spectrometry with electrospray ionization (LC-ESI-MS/MS). Processing effects and storage stability were investigated during juice and concentrate production from sea buckthorn berries (Hippopha? rhamnoides). The technological processing of the berries caused a total degradation of tetrahydrofolate and 5-formyltetrahydrofolate in the generated juice. The content of the main folate vitamer 5-methyltetrahydrofolate remained approximately unchanged during the whole processing from the berries to the concentrate. Sea buckthorn juice was stored under two household storage conditions (6 degrees C, 25 degrees C), and also under accelerated aging conditions (40 degrees C) for up to 7 days to determine the effects of storage temperature on the stability of 5-methyltetrahydrofolate. The content of 5-methyltetrahydrofolate was nearly unchanged during the storage at 6 degrees C after 7 days. The juice showed almost identical degradation of 5-methyltetrahydrofolate of about 17-20% at 25 degrees C and 40 degrees C after 7 days of storage. [figure: see text] 相似文献
100.
Separation of betalains from berries of Phytolacca americana by ion-pair high-speed counter-current chromatography 总被引:1,自引:0,他引:1
Jerz G Skotzki T Fiege K Winterhalter P Wybraniec S 《Journal of chromatography. A》2008,1190(1-2):63-73
The first preparative fractionation of betalain pigments by means of ion-pair high-speed counter-current chromatography (IP-HSCCC) from berry extracts of Phytolacca americana (Phytolaccaceae) is presented. A novel HSCCC solvent system consisting of 1-butanol-acetonitrile-water (5:1:6, v/v/v) was applied using ion-pair forming trifluoroacetic acid at low concentration (0.7%, v/v). Affinity of polar betacyanins and betaxanthins to the organic stationary phase of the biphasic HSCCC solvent mixture was considerably improved. Partitioning coefficient values and influence of increasing trifluoroacetic acid additions to the biphasic solvent mixture were measured for all identified betacyanins and betaxanthins. Gentle separation by IP-HSCCC of the injected pigment extract (900 mg) yielded sufficient amounts of the principal pigments 15S-betanin/15R-isobetanin. The pure epimers separated by C18-HPLC were immediately studied by one- and two-dimensional NMR. In the recovered fractions, minor concentrated betacyanins and betaxanthins were significantly enriched by IP-HSCCC and were detected for the first time in the extracts of P. americana. IP-HSCCC and C18-HPLC were shown to be complementary techniques in the isolation procedure of recovering minor concentrated, highly polar and chemically instable betacyanins and betaxanthin from complex plant matrices. Altogether, identification of 17 betalains was achieved by HPLC-diode array detection-electrospray ionization MS/MS in the HSCCC fractions with their respective isomers, also resulting in the tentative elucidation of betacyanins with novel salicylic acid substitution pattern in the berry extracts of P. americana. 相似文献