全文获取类型
收费全文 | 772篇 |
免费 | 5篇 |
国内免费 | 2篇 |
专业分类
化学 | 470篇 |
晶体学 | 3篇 |
力学 | 25篇 |
数学 | 129篇 |
物理学 | 152篇 |
出版年
2021年 | 3篇 |
2020年 | 5篇 |
2019年 | 4篇 |
2018年 | 8篇 |
2017年 | 6篇 |
2016年 | 17篇 |
2015年 | 7篇 |
2014年 | 9篇 |
2013年 | 24篇 |
2012年 | 48篇 |
2011年 | 58篇 |
2010年 | 26篇 |
2009年 | 24篇 |
2008年 | 52篇 |
2007年 | 46篇 |
2006年 | 53篇 |
2005年 | 49篇 |
2004年 | 44篇 |
2003年 | 33篇 |
2002年 | 34篇 |
2001年 | 12篇 |
2000年 | 8篇 |
1999年 | 17篇 |
1998年 | 4篇 |
1997年 | 10篇 |
1996年 | 11篇 |
1995年 | 5篇 |
1994年 | 6篇 |
1993年 | 11篇 |
1992年 | 9篇 |
1991年 | 11篇 |
1990年 | 6篇 |
1989年 | 11篇 |
1988年 | 4篇 |
1987年 | 5篇 |
1986年 | 6篇 |
1985年 | 15篇 |
1984年 | 7篇 |
1983年 | 7篇 |
1982年 | 6篇 |
1981年 | 8篇 |
1980年 | 3篇 |
1979年 | 7篇 |
1977年 | 6篇 |
1976年 | 5篇 |
1975年 | 3篇 |
1974年 | 4篇 |
1973年 | 4篇 |
1972年 | 4篇 |
1969年 | 3篇 |
排序方式: 共有779条查询结果,搜索用时 312 毫秒
11.
Guilard R Gryko DT Canard G Barbe JM Koszarna B Brandès S Tasior M 《Organic letters》2002,4(25):4491-4494
[reaction: see text] We have developed a new method that affords regioisomerically pure corroles possessing up to three different substituents at the meso positions. The corrole formation reaction involves the acid-catalyzed condensation of a dipyrromethane-dicarbinol with pyrrole followed by oxidation with DDQ. ABC-Type corroles were synthesized for the first time according to this procedure. 相似文献
12.
Diekert G Gugova D Limoges B Robert M Savéant JM 《Journal of the American Chemical Society》2005,127(39):13583-13588
As an illustration of how cyclic voltammetry can be used to unravel the mechanisms and kinetics of redox enzymes, the reductive dechlorination of trichloroethylene and tetrachloroethylene by a typical reductive dehalogenase, the tetrachloroethene reductive dehalogenase of Sulfurospirillum multivorans (formerly called Dehalospirillum multivorans), was investigated by means of several electrochemically generated cosubstrates. They comprised the monocation and the neutral form of methylviologen, the neutral form of benzylviologen, and cobaltocene. Cyclic voltammetry is used to produce the active form of the cosubstrate under controlled potential conditions. It shows large plateau-shaped catalytic responses, which are used to measure the kinetics of the enzymatic reaction as a function of the substrate and cosubstrate concentrations. The variation of the rate constant for the cosubstrate reaction with its standard potential shows the transition between two asymptotic behaviors, one in which the reaction is under diffusion control and the other in which it is under counter-diffusion control. Simple fitting of this plot allows an estimation of the standard potential of the electron acceptor center in the enzyme (E degrees = -0.57 V vs NHE). 相似文献
13.
B. Robrieux G. Desrousseaux A. Renou M. Gillet 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1989,12(1-4):27-29
In this paper, we show that it is possible to deduce the actual morphology of small particle condensed onto an insulator by combining the granularity analysis from electron micrographs and the electrical sheet conductance of the deposit on its substrate. Assuming the particles are truncated ellipsoids, we determine the excentricity and the contact angle with the substrate for Au on amorphous carbon and MgO substrates. 相似文献
14.
Gilbert Dana Feiga Weisbuch Brigitte Lo Cicero Jean-Michel Drancourt Jacqueline Mercier 《Tetrahedron》1985,41(7):1241-1249
We show that the four studied 4-hydrindenones (with or without a methyl substituant at C3 or C7a) give by hydrogenation on Raney nickel or on Pd/C, in various solvants, the cis 4-hydrindanones, by a kinetically controlled process. During the hydrogenation, the double bond can migrate only in the positions conjugated to the carbonyl group.In contrast, when the carbonyle is protected as a dioxolanne, the double bond is free to migrate around the five membered ring. In spite of this mobility the optical purity of the saturated dioxolannes formed is high. In the case of the (+)3-methyl Δ3(3a) 4-hydrindenone, the enantiomeric hydrindanones obtained by direct hydrogenation or with isomerization of the double bond are not the same. Therefore, the optical purity of the product is low, but its value allows an estimation of the relative importance of the two reaction pathways. 相似文献
15.
Yvette Mettey Jean-Michel Vierfond Claude Thai Marcel Miocque 《Journal of heterocyclic chemistry》1983,20(1):133-137
The action of organolithium reagents such as phenyllithium or n-bulyllithium on 2-methylquinoxaline gave lithiation of the methyl group which upon reaction with electtropholesphiles produce side chain alkenyl derivatives. On the other hand organolithium reagents react with the quinoxaline azomethine bond to give I-lithio-2-alkyl)or ary-1)-3 methylquinoxalines which can be further loithiated on the methyl group to give 2-alkyl(or aryl)-3-alkenylquinoxaline derivatives. The adducts can be condensed with clectrophiles such as benzonitrile or methlyl benzoate but only methyl benzoate leads to N condensed derivatives. Furthermore substituted 1,2,3,4-terahydroqinoxalines are available via the above lithio intermediates. 相似文献
16.
Hétérocycles à fonction quinone. 7. Benzo[f]quinazolinediones-7,10 à action antitumorale potentielle
Hlne Parrot-Lopez Jean-Michel Delacotte Jean Renault Suzanne Cros 《Journal of heterocyclic chemistry》1986,23(4):1039-1043
Few benzo[f]quinazoline-7,10-diones 4a-c are prepared by oxidation, with potassium nitrosodisulfonate, of the corresponding benzo[f]quinazolinamines, made by reduction of the nitro analogues which were obtained from 8-methoxybenzo[f]quinazolin-1(2H)-one. The compound 4a shows an interesting cytotoxicity. 相似文献
17.
Paul Watson Peter Willett Valerie J. Gillet Marcel L. Verdonk 《Journal of computer-aided molecular design》2001,15(9):835-857
A knowledge-based method for calculating the similarity of functional groups is described and validated. The method is based on experimental information derived from small molecule crystal structures. These data are used in the form of scatterplots that show the likelihood of a non-bonded interaction being formed between functional group A (the `central group') and functional group B (the `contact group' or `probe'). The scatterplots are converted into three-dimensional maps that show the propensity of the probe at different positions around the central group. Here we describe how to calculate the similarity of a pair of central groups based on these maps. The similarity method is validated using bioisosteric functional group pairs identified in the Bioster database and Relibase. The Bioster database is a critical compilation of thousands of bioisosteric molecule pairs, including drugs, enzyme inhibitors and agrochemicals. Relibase is an object-oriented database containing structural data about protein-ligand interactions. The distributions of the similarities of the bioisosteric functional group pairs are compared with similarities for all the possible pairs in IsoStar, and are found to be significantly different. Enrichment factors are also calculated showing the similarity method is statistically significantly better than random in predicting bioisosteric functional group pairs. 相似文献
18.
Solvent effects on the electrochemistry and spectroscopic properties of alkyl- and aryl-substituted corroles in nonaqueous media are reported. The oxidation and reduction of six compounds containing zero to seven phenyl or substituted phenyl groups on the macrocycle were studied in four different nonaqueous solvents (CH(2)Cl(2), PhCN, THF, and pyridine) containing 0.1 M tetra-n-butylammonium perchlorate. Dimers were formed upon oxidation of all corroles in CH(2)Cl(2), but this was not the case in the other three solvents, where either monomers or dimers were formed upon oxidation depending upon the solvent Gutmann donor number and the number or location of aryl substituents on the macrocycle. The half-wave potentials were analyzed as a function of the number of aryl substituents on the macrocycle as well as the concentration of added pyridine to PhCN solutions of the compound, and these data were combined with data from the spectroelectrochemistry experiments to determine the stoichiometry of the species actually in solution after the first oxidation or first reduction of each compound. The results of these experiments indicate that reduction of the bispyridine adduct (Cor)Co(III)(py)(2) proceeds via the monopyridine complex (Cor)Co(III)(py) to give in each case the unligated cobalt(II) corrole [(Cor)Co(II)](-). In contrast, pyridine remains coordinated after electrooxidation, and the final product was characterized as [(Cor)Co(III)(py)(2)](+). 相似文献
19.
20.
Dolain C Jiang H Léger JM Guionneau P Huc I 《Journal of the American Chemical Society》2005,127(37):12943-12951
Chiral groups attached to the end of quinoline-derived oligoamide foldamers give rise to chiral helical induction in solution. Using various chiral groups, diastereomeric excesses ranging from 9% to 83% could be measured by NMR and circular dichroism. Despite these relatively weak values and the fact that diastereomeric helices coexist and interconvert in solution, the right-handed or left-handed helical sense favored by the terminal chiral group could be determined unambiguously using X-ray crystallography. Assignment of chiral induction was performed in an original way using the strong tendency of racemates to cocrystallize, and taking advantage of slow helix inversion rates, which allowed one to establish that the stereomers observed in the crystals do correspond to the major stereomers in solution. The sense of chiral helical induction was rationalized on the basis of sterics. Upon assigning an Rs or Ss chirality to the stereogenic center using a nomenclature where the four substituents are ranked according to decreasing sizes, it is observed that Rs chirality always favors left-handed helicity and Ss chirality favors right-handed helicity (P). X-ray structures shed some light on the role of sterics in the mechanism of chiral induction. The preferred conformation at the stereocenter is apparently one where the bulkiest group should preferentially point away from the helix, the second largest group should be aligned with the helix backbone, and the smallest should point to the helix. 相似文献