全文获取类型
收费全文 | 956篇 |
免费 | 24篇 |
国内免费 | 2篇 |
专业分类
化学 | 712篇 |
晶体学 | 1篇 |
力学 | 96篇 |
数学 | 85篇 |
物理学 | 88篇 |
出版年
2022年 | 7篇 |
2021年 | 10篇 |
2020年 | 11篇 |
2019年 | 11篇 |
2018年 | 6篇 |
2016年 | 16篇 |
2015年 | 21篇 |
2014年 | 19篇 |
2013年 | 40篇 |
2012年 | 48篇 |
2011年 | 45篇 |
2010年 | 34篇 |
2009年 | 34篇 |
2008年 | 54篇 |
2007年 | 65篇 |
2006年 | 58篇 |
2005年 | 56篇 |
2004年 | 42篇 |
2003年 | 37篇 |
2002年 | 25篇 |
2001年 | 14篇 |
2000年 | 7篇 |
1999年 | 10篇 |
1998年 | 6篇 |
1997年 | 10篇 |
1996年 | 20篇 |
1995年 | 13篇 |
1994年 | 7篇 |
1993年 | 19篇 |
1992年 | 11篇 |
1991年 | 5篇 |
1990年 | 9篇 |
1988年 | 11篇 |
1987年 | 6篇 |
1986年 | 10篇 |
1985年 | 11篇 |
1984年 | 12篇 |
1983年 | 10篇 |
1982年 | 11篇 |
1981年 | 11篇 |
1980年 | 17篇 |
1979年 | 16篇 |
1978年 | 11篇 |
1977年 | 17篇 |
1976年 | 13篇 |
1975年 | 10篇 |
1974年 | 8篇 |
1973年 | 5篇 |
1972年 | 4篇 |
1970年 | 4篇 |
排序方式: 共有982条查询结果,搜索用时 0 毫秒
101.
Giuseppe Cremonesi Piero Dalla Croce Alessandra Forni Maddalena Gallanti Raffaella Gandolfi Concetta La Rosa 《Tetrahedron: Asymmetry》2009,20(16):1940-1947
Enantiomerically pure (5R)-(?)-5-phenyl-4,5-dihydroisoxazole-3-carboxylic acid ethyl ester was obtained via enzymatic resolution of the corresponding racemic mixture using a lipase from hog pancreas (PPL). The following reduction of the ester group to the corresponding alcohol and the oxidation of the latter led to (5R)-(?)-5-phenyl-4,5-dihydroisoxazole-3-carbaldehyde, and the reaction between this and Schöllkopf’s reagent, (2R)-(+)-2,5-dihydro-3,6-dimethoxy-2-isopropylpyrazine, gave mixtures of adducts with a good syn/anti ratio. The steric configurations of the major diastereoisomer were assigned on the basis of spectroscopic data and X-ray analysis. The subsequent controlled hydrolysis of the pyrazine ring led to β-(5-phenyl-4,5-dihydroisoxazol-3-yl)-serine methyl esters and the corresponding dipeptides with (R)-valine. Finally, reductive cleavage of the 4,5-dihydroisoxazole ring under hydrolytic conditions made it possible to obtain the corresponding polyfunctionalised dipeptides. 相似文献
102.
103.
Carlo Banzatti Franco Heidempergher Piero Melloni 《Journal of heterocyclic chemistry》1983,20(2):259-265
The synthesis of 2H-3, 4-dihydro-3-aminomethyl-1, 4-benzoxazines substituted by one phenyl group in the 2, 3 or α-position is reported. The two diastereoisomers of the 2-phenyl-derivative were independently synthesized from ethyl trans-3-phenylglycidate by a stereospecific route. A previous attempt at the synthesis of these same compounds gave rise to an unexpected aliphatic-aromatic ketone rearrangement. 相似文献
104.
Giovanni Fronza Claudio Fuganti Piero Grasselli Stefano Servi 《Tetrahedron letters》1985,26(40):4961-4964
Baker's yeast mediated reduction of the α-acetoxy ketones (1)–(3) proceeds with high enantio- and stereoselectivity to give the carbinols (4)–(6), easily converted into the masked chiral deoxy sugars (12)–(14), from which {=D}- and {=L}-deoxysugars have been obtained. 相似文献
105.
Piero Mirti 《Analytica chimica acta》1974,69(1):69-77
N-Carboxymelhyl pyrrolidine-2-carboxylic acid (CMPCA) is suggested as an optically active titrant. The values of the acidity constants of the ligand were determined and the order of magnitude of the stability constants of the complexes formed by CMPCA with some metal ions was evaluated. In order to determine the best conditions for the spectropolarimetric titrations, the dependence of the molar rotation of CMPCA and its complexes on wavelength and pH was examined. The spectropolarimetric titrations ofcopper(II), nickel(II) and iron(III) ions were carried out successfully. 相似文献
106.
Antonella Casoli Piero Mirti Gerardo Palla 《Fresenius' Journal of Analytical Chemistry》1995,352(3-4):372-379
Proteinaceous organic materials used as ancient painting media were investigated by capillary gas chromatography (GC) and capillary gas chromatography — mass spectrometry (GC/MS). Medieval wall paintings made by the tempera technique were considered and their binding media were studied by the characterization of their main chemical components. The basic methodology is based on the determination of amino acids in samples of paint layers after hydrolysis and derivatization and on the comparison with reference proteinaceous materials. Multivariate chemometric techniques were used to facilitate the recognition of the protein source from chromatographic data. To characterize the binders further, a method was developed for the determination of fatty acids, present as minor components, by GC/MS. The use of fused-silica capillary columns coated with selected stationary phases allowed the separation of amino acid and fatty acid derivatives in a single analytical run. 相似文献
107.
Mario Bianchi Gloria Menchi Franco Francalanci Franco Piacenti Ugo Matteoli Piero Frediani Carlo Botteghi 《Journal of organometallic chemistry》1980,188(1):109-119
Saturated monocarboxylic acids up to C6, several bicarboxylic acids and some of the corresponding anhydrides are hydrogenated in the homogeneous phase with H4Ru4(CO)8(PBu3)4 as catalyst to give the corresponding alcohols (present among the reaction products as esters) or lactones at 100–200°C under a pressure of 100–200 atm of hydrogen. Anhydrides react at temperatures lower than those needed for acids. Esters are not reduced. Only δ-valerolactone is hydrogenated to 1,5-pentanediol. Ruthenium carbonyl carboxylates have been recovered at the end of the reaction and appear to be catalytically active intermediates. 相似文献
108.
A stochastic computer simulation technique has been used to solve a one-dimensional time-dependent Fokker-Planck (F-P) equation governing spacetime distribution of bacteria in a substrate (food) gradient. Because of consumption of the substrate by the bacteria the F-P equation becomes nonlinear due to coupling of the distribution with the self-generated substrate gradient. The simulation is efficient and numerically accurate for generating transient solutions. We are able not only to produce a transient solution displaying approach to the steady state solitary wave solution known from an analytical result of Keller and Segal but also to show an interesting dependence of the solitary wave propagation speed on the concentration dependence of the substrate consumption rate. 相似文献
109.
Rodolfo Nesi Stefano Chimichi Francesco De Sio Roberto Pepino Piero Tedeschi 《Tetrahedron letters》1982,23(42):4397-4400
In contrast with a previous report, permanganate oxidation of the title compound gives the nitroisoxazolone and not 3-methyl-4- nitroisoxazole-5-carboxylic acid ; a reaction pathway, involving the spirocyclisation of the carboxylate , is suggested. 相似文献
110.