全文获取类型
收费全文 | 2257篇 |
免费 | 21篇 |
国内免费 | 19篇 |
专业分类
化学 | 1255篇 |
晶体学 | 81篇 |
力学 | 66篇 |
数学 | 351篇 |
物理学 | 544篇 |
出版年
2021年 | 13篇 |
2020年 | 25篇 |
2019年 | 12篇 |
2018年 | 14篇 |
2017年 | 13篇 |
2016年 | 16篇 |
2015年 | 23篇 |
2014年 | 22篇 |
2013年 | 83篇 |
2012年 | 83篇 |
2011年 | 103篇 |
2010年 | 42篇 |
2009年 | 48篇 |
2008年 | 122篇 |
2007年 | 121篇 |
2006年 | 122篇 |
2005年 | 114篇 |
2004年 | 134篇 |
2003年 | 107篇 |
2002年 | 77篇 |
2001年 | 59篇 |
2000年 | 39篇 |
1999年 | 40篇 |
1998年 | 24篇 |
1997年 | 45篇 |
1996年 | 43篇 |
1995年 | 29篇 |
1994年 | 35篇 |
1993年 | 27篇 |
1992年 | 33篇 |
1991年 | 31篇 |
1990年 | 37篇 |
1989年 | 35篇 |
1988年 | 28篇 |
1987年 | 29篇 |
1986年 | 18篇 |
1985年 | 33篇 |
1984年 | 37篇 |
1983年 | 26篇 |
1982年 | 22篇 |
1981年 | 32篇 |
1980年 | 29篇 |
1979年 | 28篇 |
1978年 | 35篇 |
1977年 | 22篇 |
1976年 | 26篇 |
1975年 | 20篇 |
1974年 | 17篇 |
1973年 | 19篇 |
1971年 | 11篇 |
排序方式: 共有2297条查询结果,搜索用时 15 毫秒
31.
32.
Using flow microcalorimetry, the ion association reaction M2+(aq)+Fe(CN)
6
4–
(aq)=MFe(CN)
6
2–
(aq) (M=Ca, Mg) has been studied at 25°C over the ionic strength range 0.02 to 0.08 mol-dm–3. Analyses of the data to obtain Ho, the enthalpy change at infinite dilution, are described. The value obtained for Ho is sensitive to the kind of functions used to correct for non-ideal behavior. 相似文献
33.
Monot Frédéric Benoit Yves Ballerini Daniel Vandecasteele Jean-Paul 《Applied biochemistry and biotechnology》1990,(1):375-386
Utilization of lipases for synthesis of esters of hydrophilic polyols has been investigated. The choice of a suitable solvent
is crucial in this type of reaction. An interesting case is fatty acid esters from neopentylpolyols, such as trimethylolpropane,
which are of great interest as high temperature lubricants. Enzymatic synthesis of trimethylolpropane tricaprylate was studied
as an alternative to chemical manufacturing. Triester production occurred only if the water produced by esterification was
continuously removed from the medium. In these condition, kinetics of appearance and transformation of mono-, di- and triesters
were determined in order to define optimal conditions. 相似文献
34.
35.
36.
37.
Poschalko A Lancelot N Marin J Larras V Limal D Elbayed K Raya J Piotto M Briand JP Guichard G Bianco A 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(18):4532-4537
A novel resin called DEUSS (perdeuterated poly(oxyethylene)-based solid support) has been prepared by anionic polymerization of deuterated [D4]ethylene oxide, followed by cross-linking with deuterated epichlorohydrin. DEUSS can be suspended in a wide range of solvents including organic and aqueous solutions, in which it displays a high swelling capacity. As measured by proton HRMAS of the swollen polymer, the signal intensity of the oxyethylene protons is reduced by a factor of 110 relative to the corresponding nondeuterated poly(oxyethylene)poly(oxypropylene) (POEPOP) resin, thus facilitating detailed HRMAS NMR studies of covalently linked molecules. This 1H NMR invisible matrix was used for the solid-phase synthesis of peptides, oligoureas, and a series of amides as well as their characterization by HRMAS NMR spectroscopy. On-bead NMR spectra of high quality and with resolution comparable to that of liquid samples were obtained and readily interpreted. The complete absence of the parasite resin signals will be of great advantage, for example, for the optimization of multistep solid-phase stereoselective reactions, and for the conformational study of resin-bound molecules in a large variety of solvents. 相似文献
38.
Reported here is an analytical method enabling the stereochemical resolution of a new antianginal compound possessing two stereogenic centers, leading to four stereoisomers. Only one of these isomers is currently under development as a novel antianginal agent and consequently, the other three isomers are considered as unwanted chiral impurities. Therefore, an enantioselective method is required in order to check its enantiomeric purity. This paper presents a method exploiting the high efficiency of capillary electrophoresis and the complexing properties of cyclodextrins to achieve the separation of the four stereoisomers of this weakly basic compound (pKa = 7.4). For this purpose, the combination of a neutral cyclodextrin, hydroxypropyl-gamma-cyclodextrin (HP-gamma-CD), and an anionic cyclodextrin, carboxymethyl-beta-cyclodextrin (CM-beta-CD), was added to the separation buffer running in an uncoated silica capillary. After selection of the suitable cyclodextrin system, satisfactorily separation conditions were as follows: 30 mM phosphate buffer (pH 6.4) containing 10 mM of HP-gamma-CD and 10 mM of CM-beta-CD, running voltage +30 kV. The resulting run time and resolutions were respectively about 17 min and between 1.95 and 2.84. Linearity curves (0.993 < r2 < 0.998) are also shown. 相似文献
39.
Jean-Franois Nierengarten Tilo Habicher Roland Kessinger Francesca Cardullo Franois Diederich Volker Gramlich Jean-Paul Gisselbrecht Corinne Boudon Maurice Gross 《Helvetica chimica acta》1997,80(7):2238-2276
The macrocyclization between buckminsterfullerene, C60, and bis-malonate derivatives in double Bingel reaction provides a versatile and simple method for the preparation of covalent bis-adducts of C60 with high regio- and diastereoselectivity. A combination of spectral analysis, stereochemical considerations, and X-ray crystallography (Fig. 2) revealed that out of the possible in-in, in-out, and out-out stereoisomers, the reaction of bis-malonates linked by o-, m-, or p-xylylene tethers afforded only the out-out ones (Scheme 1). In contrast, the use of larger tethers derived from 1,10-phenanthroline also provided a first example, (±)- 19 (Scheme 2), of an in-out product. Starting from optically pure bis-malonate derivatives, the new bis-functionalization method permitted the diastereoselective preparation of optically active fullerene derivatives (Schemes 4 and 5) and, ultimately, the enantioselective preparation (enantiomeric excess ee > 97%) of optically active cis-3 bis-adducts whose chirality results exclusively from the addition pattern (Fig. 6). The macrocyclic fixation of a bis-malonate with an optically active, 9,9′-spirobi[9H-fluorene]-derived tether to C60 under generation of 24 and ent- 24 with an achiral addition pattern (Scheme 4) was found to induce dramatic changes in the chiroptical properties of the tether chromophore such as strong enhancement and reversal of sign of the Cotton effects in the circular dichroism (CD) spectra (Figs. 4 and 5). By the same method, the functionafized bis-adducts 50 and 51 (Schemes 10 and 11) were prepared as initiator cores for the synthesis of the fullerene dendrimers 62 , 63 , and 66 (Schemes 12 and 13) by convergent growth. Finally, the new methodology was extended, to the regio- and diastereoselective construction of higher cyclopropanated adducts. Starting from mono-adduct 71 , a clipping reaction provided exclusively the all-cis-2 tris-adduct (±)- 72 (Scheme 14), whereas the similar reaction of bis-adduct 76 afforded the all-cis-2 tetrakis-adduct 77 (Scheme 15). Electrochemical investigations by steady-state voltammetry (Table 2) in CH2Cl2 (+0.1M Bu4NPF6) showed that all macroeyciic bis(methano)fullerenes underwent multiple reduction steps, and that regioisomerism was not much influencing the redox potentials, All cis-2 bis-adducts gave an instable dianion which decomposed during the electrochemical reduction. In CH2Cl2, the redox potential of the fullerene core in dendrimers 62, 63 , and 66 is not affected by differences in size and density of the surrounding poly(ether-amide) dendrons. The all-cis-2 tris- and tetrakis(meihano)fullercnes (±) -72 and 77 , respectively, are reduced at more negative potential than previously reported all-e tris- and tetrakis-adducts with methane bridges that are also located along an equatorial belt. This indicates a larger perturbation of the original fullerene π-chromophore and a larger raise in LUMO energy in the former derivatives. 相似文献
40.
Summary The problem is considered of orthogonal 1 fitting of discrete data. Local best approximations are characterized and the question of the robustness of these solutions is considered. An algorithm for the problem is presented, along with numerical results of its application to some data sets. 相似文献