全文获取类型
收费全文 | 5950篇 |
免费 | 153篇 |
国内免费 | 46篇 |
专业分类
化学 | 3718篇 |
晶体学 | 35篇 |
力学 | 208篇 |
数学 | 1274篇 |
物理学 | 914篇 |
出版年
2022年 | 54篇 |
2021年 | 57篇 |
2020年 | 65篇 |
2019年 | 67篇 |
2018年 | 58篇 |
2017年 | 49篇 |
2016年 | 96篇 |
2015年 | 116篇 |
2014年 | 135篇 |
2013年 | 310篇 |
2012年 | 294篇 |
2011年 | 325篇 |
2010年 | 222篇 |
2009年 | 207篇 |
2008年 | 305篇 |
2007年 | 315篇 |
2006年 | 305篇 |
2005年 | 272篇 |
2004年 | 267篇 |
2003年 | 216篇 |
2002年 | 209篇 |
2001年 | 99篇 |
2000年 | 91篇 |
1999年 | 98篇 |
1998年 | 73篇 |
1997年 | 66篇 |
1996年 | 90篇 |
1995年 | 66篇 |
1994年 | 73篇 |
1993年 | 81篇 |
1992年 | 65篇 |
1991年 | 43篇 |
1990年 | 75篇 |
1989年 | 59篇 |
1988年 | 47篇 |
1987年 | 53篇 |
1986年 | 47篇 |
1985年 | 69篇 |
1984年 | 81篇 |
1983年 | 51篇 |
1982年 | 78篇 |
1981年 | 79篇 |
1980年 | 94篇 |
1979年 | 75篇 |
1978年 | 80篇 |
1977年 | 76篇 |
1976年 | 71篇 |
1975年 | 51篇 |
1974年 | 38篇 |
1973年 | 58篇 |
排序方式: 共有6149条查询结果,搜索用时 15 毫秒
61.
Claude Villiers Pierre Thury Michel Ephritikhine 《Acta Crystallographica. Section C, Structural Chemistry》2006,62(6):m243-m245
In the title compound, (1,4,7,10,13,16‐hexaoxacyclooctadecane‐1κ6O)‐μ‐oxo‐1:2κ2O:O‐hexakis(tetrahydroborato)‐1κ3H;2κ2H;2κ2H;2κ3H;2κ3H;2κ3H‐diuranium(IV), [U2(BH4)6O(C12H24O6)], one of the U atoms (U1), located at the centre of the crown ether moiety, is bound to the six ether O atoms, and also to a tridentate tetrahydroborate group and a μ‐oxo atom in axial positions. The other U atom (U2) is bound to the same oxo group and to five tetrahydroborate moieties, three of them tridentate and the other two bidentate. The two metal centres are bridged by the μ‐oxo atom in an asymmetric fashion, thus giving the species (18‐crown‐6)(κ3‐BH4)U=(μ‐O)—U(κ3‐BH4)3(κ2‐BH4)2, in which the U1=O and U2—O bond lengths to the μ‐O atom [1.979 (5) and 2.187 (5) Å, respectively] are indicative of the presence of positive and negative partial charges on U1 and U2, respectively. 相似文献
62.
Claude Peker Michel Herlem et Janine Badoz-Lambling 《Fresenius' Journal of Analytical Chemistry》1966,224(1):302-309
Résumé L'évolution de la rapidité de la réduction électrochimique des ions Co2+, Ni2+, Pb2+, In3+ et Zn2+ dans plusieurs milieux complexants a d'abord été étudiée: l'élongation et l'étalement des pics obtenus sont donnés pour une concentration 10–5 M pour chaque ion. Ensuite la résolution des couples Zn2+/Co2+, Zn2+/Ni2+, In3+/Cd2+ et Pb2+/Tl+ a été considérée dans les mêmes milieux complexants. Pour chaque couple on donne la différence E entre les potentiels de demi-vague des éléments et l'allure des polarogrammes.
En hommage au Prof. Dr. M. von Stackelberg à l'occasion de son 70ème anniversaire.
Associé au Centre National de la Recherche Scientifique. 相似文献
Summary The electrochemical reduction rate of the ions Co2+, Ni2+, Pb2+, In3+ and Zn2+ is studied in solutions of various complexing agents. The width and the shape of the peaks for each ion in 10–5 M concentration are given. The resolution of the couples Zn2+/Co2+, Zn2+/Ni2+, In3+/Cd2+ and Pb2+/Tl+ in the same electrolytes is shown hereafter. The difference E of the half-wave potentials and the behaviour of the polarograms for each couple is given.
Zusammenfassung Die Abhängigkeit der elektrochemischen Reduktionsgeschwindigkeit von verschiedenen Komplexbildnern wird am Beispiel der Ionen Co2+, Ni2+, Pb2+, In3+ und Zn2+ untersucht. Ausdehnung und Form der Polarogramme werden für jedes Element in 10–5 m Lösung angegeben. In den gleichen komplexbildenden Elektrolyten wird sodann die Trennung der Ionen Zn2+/Co2+, Zn2+/Ni2+, In3+/Cd2+ und Pb2+/Tl+ untersucht. Für jede Ionenkombination wird die Halbstufenpotentialdifferenz E angegeben und das Polarogramm erläutert.
En hommage au Prof. Dr. M. von Stackelberg à l'occasion de son 70ème anniversaire.
Associé au Centre National de la Recherche Scientifique. 相似文献
63.
64.
Marie-Madeleine Rohmer Ren Ernenwein Michel Ulmschneider Roland Wiest Marc Benard 《International journal of quantum chemistry》1991,40(6):723-743
A vector efficient implementation of the McMurchie and Davidson algorithm for the calculation of one- and two-electron molecular integrals is presented, as available in the Cray version of the ASTERIX program system. The implementation and performance of a vector-oriented strategy for the generation and processing of the P supermatrix is also discussed. This program system has been applied to the ab initio SCF computation of the ground-state wave function for the [V10O28]6? ion, with a basis set of triple-zeta quality for the valence shell of oxygen generating 1404 GTOS and 574 CGTOS for the complete system. The performance and the bottlenecks of the integral calculation are discussed as a function of the integral classes. Two-dimensional maps of the electrostatic potential are presented for this molecule and compared to experimental information about proton fixation. 相似文献
65.
Treatment of 2,2'-bis(bromomethyl)-1,1'-binaphthyl [(R,S)-2] with 1,1'-binaphthalene-2,2'-diol (+)-(R)-1 and cesium or potassium carbonate in refluxing acetone, gave the diastereoisomeric dioxacyclophanes (-)-(R,S)-3a and (+)-(R,R)-3b, both obtained in high yield, and the cyclic tetraether (+)-(R,R,R,S)-4 as isolated side product. Boron tribomide-promoted ether cleavage of 3a and 3b gave optically pure (-)-(S)-2 and (+)-(R)-2, respectively, and the recovered diol (+)-(R)-1. Alternatively, the same reaction sequence furnished the resolved diols (-)-(S)-1 and (+)-(R)-1 from (R,S)-1 and (+)-(R)-2, as well as optically pure 2,2'-bis(chloromethyl)-1,1'-binaphthyl (+)-(R)-5 from the racemic dibromide (R,S)-2 by using boron trichloride for ether cleavage. 相似文献
66.
Osipov SN Golubev AS Sewald N Michel T Kolomiets AF Fokin AV Burger K 《The Journal of organic chemistry》1996,61(21):7521-7528
A new method for the preparation of alpha-chlorodifluoromethyl-, alpha-bromodifluoromethyl-, and alpha-difluoromethyl-substituted alpha-hydroxy and alpha-amino acid esters 11, 19-21 is described. The key step of the synthesis is the regioselective alkylation of ketones 5, 7-9 and imines 16-18 with C-nucleophiles. The ketones 7-9 are readily available from 3,3,3-trifluorolactate 1 by a five-step procedure. Subsequent removal of the protecting groups from 19-21 provides the corresponding free amino acids 25, 26, 28. 相似文献
67.
Abstract— Intact leaves, previously adapted to darkness for a prolonged period of time, were suddenly illuminated with a strong, photosynthetically saturating, white light (ca 1500 μmol m−2 s_1 ), resulting in the rapid establishment of a large energy-dependent chlorophyll fluorescence quenching (qE ) as shown by in vivo fluorescence measurements with a pulse amplitude modulation technique. Two different photothermal methods, photoacoustic spectroscopy and photothermal deflection spectroscopy, were used to monitor thermal deactivation of excited pigments during the dark-light transitions. The in vivo photothermal signals measured with both techniques were shown to remain constant during induction of photosynthesis under high light conditions, suggesting that, in contrast to current hypotheses, energy-dependent quenching qE is not associated with significant changes in thermal dissipation of absorbed light energy in the chloroplasts. When photosynthesis was induced with a low-intensity modulated light, a noticeable decrease in the heat emission yield was observed resulting from the progressive activation of the competing photochemical processes. 相似文献
68.
Marguerite Dols Magali Remaud-Simeon René-Marc Willemot Michel Vignon Pierre F. Monsan 《Applied biochemistry and biotechnology》1997,62(1):47-59
Leuconostoc mesenteroides NRRL B-1299 dextransucrase was fractionated into soluble (SGT) and insoluble (IGT) enzyme preparations differing by their dextran content. In spite of this, they displayed the same Km for sucrose (10 g/L) and the same activation energy (35 kJ/mol). But the presence of cells and insoluble dextran led to the IGT behaving like an immobilized enzyme: stabilization against thermal denaturation and diffusional limitations at low substrate concentrations were observed. On the other hand, the behavior of SGT was influenced by the presence, in the preparation, of soluble dextran that reduced enzyme inhibition by excess substrate. SGT and IGT present very different pH profiles. In the presence of 4 g/L of soluble dextran, IGT was activated and displayed the same susceptibility to pH as SGT. The activation of IGT was highly dependent on the nature of the acceptor added but also on the pH of the reaction medium. IGT and SGT synthesize both soluble and insoluble polymer containing α(l → 2), α(l → 3), and α(l → 6) linkages. A larger amount of insoluble dextran is elaborated by SGT. The polymer structures, examined by13C NMR spectrometry, revealed that they differ mainly by their α(l → 3) linkage content (from 0 to 11%). This linkage seems to be partly responsible for the dextran insolubility and can be completely eliminated by carrying out the synthesis of soluble polymer at pH 7.4 with SGT. 相似文献
69.
The structures of Beccapoline and Beccapolinium, two new isoquinoline alkaloids from , Annonaceae, have been elucidated by spectroscopic analysis. They are the first examples of natural bisaporphines. 相似文献
70.
Bruno Hasiak Guy Ricart Didier Barbry Daniel Couturier Michel Hardy 《Journal of mass spectrometry : JMS》1981,16(1):17-20
The ammonia chemical ionization desorption spectra of N,N-dimethyl quaternary ammonium iodides in addition to high protonated molecular ion [M + H]+ intensity, show signals for an ion radical composed of N-methyl abstracted salt cation and ammonia [C + NH3? CH3]+˙. These ions corresponding to the cation +2 show increased importance in the chemical ionization mode, using the same reagent gas. The technique of chemical ionization desorption appears suitable for the analysis of salts, and thus for the determination of the molecular weight of both anion and cation. 相似文献