全文获取类型
收费全文 | 4281篇 |
免费 | 221篇 |
国内免费 | 27篇 |
专业分类
化学 | 2979篇 |
晶体学 | 24篇 |
力学 | 206篇 |
数学 | 677篇 |
物理学 | 643篇 |
出版年
2023年 | 35篇 |
2022年 | 42篇 |
2021年 | 86篇 |
2020年 | 108篇 |
2019年 | 101篇 |
2018年 | 73篇 |
2017年 | 49篇 |
2016年 | 137篇 |
2015年 | 140篇 |
2014年 | 177篇 |
2013年 | 228篇 |
2012年 | 369篇 |
2011年 | 392篇 |
2010年 | 233篇 |
2009年 | 197篇 |
2008年 | 301篇 |
2007年 | 274篇 |
2006年 | 236篇 |
2005年 | 204篇 |
2004年 | 196篇 |
2003年 | 167篇 |
2002年 | 170篇 |
2001年 | 52篇 |
2000年 | 53篇 |
1999年 | 43篇 |
1998年 | 29篇 |
1997年 | 35篇 |
1996年 | 33篇 |
1995年 | 27篇 |
1994年 | 23篇 |
1993年 | 25篇 |
1992年 | 20篇 |
1991年 | 11篇 |
1989年 | 13篇 |
1988年 | 8篇 |
1987年 | 11篇 |
1986年 | 9篇 |
1985年 | 20篇 |
1984年 | 17篇 |
1983年 | 11篇 |
1982年 | 11篇 |
1981年 | 12篇 |
1980年 | 8篇 |
1979年 | 11篇 |
1978年 | 10篇 |
1977年 | 12篇 |
1976年 | 13篇 |
1969年 | 8篇 |
1968年 | 16篇 |
1967年 | 10篇 |
排序方式: 共有4529条查询结果,搜索用时 46 毫秒
71.
Mathieu Lemay 《Tetrahedron》2007,63(47):11644-11655
The triflate salts of cyclic hydrazides function as asymmetric catalysts for the [3+2]cycloadditions of nitrones with α,β-unsaturated aldehydes. The camphor-derived hydrazides show a preference for the exo isomers during these reactions, providing a compliment to other organically catalyzed dipolar cycloadditions. Enantiomeric excesses as high as 93% were realized for the exo isomers, while some endo products were obtained in 94% ee. 相似文献
72.
Mathieu D Frapart YM Bartoli JF Boucher JL Battioni P Mansuy D 《Chemical communications (Cambridge, England)》2004,(1):54-55
Electron transfer from tetrahydropterins to iron porphyrins, with formation of intermediate tetrahydropterin cation radicals, is a very general reaction that was shown to occur not only with tetrahydrobiopterin, as originally found in NO-synthases, but also with another important biological cofactor, tetrahydrofolate, and various iron porphyrins, either in their ferric state, or in the Fe(II)O(2) state, as in the first model of the corresponding NO-synthase reaction described in this paper. 相似文献
73.
Mathieu Allix Philip P.C. Frampton Rocío Ruiz-Bustos 《Journal of solid state chemistry》2006,179(3):775-792
Polycrystalline samples of bilayered brownmillerite-like Ca2.5Sr0.5GaCo0.15Mn1.85O8 and Ca2.5Sr0.5Ga1.2Mn1.8O8 have been prepared and characterised by magnetometry and neutron diffraction over a wide temperature range. The structural chemistry and magnetic properties are compared to those of Ca2.5Sr0.5GaMn2O8. Ga enrichment has a significant effect on the former but not on the latter, whereas changes in both occur when paramagnetic Co3+ cations enter the parent phase on the 4-coordinate sites. The coupling between the environment around the 4-coordinate cations and the transition to an antiferromagnetic ordered state that was observed in Ca2.5Sr0.5GaMn2O8 is not apparent in the cation-substituted compositions, although both show long-range antiferromagnetic order at low temperatures. 相似文献
74.
Noble PF Cayre OJ Alargova RG Velev OD Paunov VN 《Journal of the American Chemical Society》2004,126(26):8092-8093
Novel colloidosome microcapsules with aqueous gel cores and integral shells of rodlike polymeric particles have been prepared and characterized. The synthesis is based on templating water-in-oil emulsions stabilized by rodlike particles followed by gelling of the aqueous phase, dissolution of the oil phase in ethanol, and redispersion of the obtained colloidosome capsules in water. Such capsules may find applications as vehicles for delivery and controlled release of drugs, cosmetics, and food supplements. 相似文献
75.
Michielin O Zoete V Gierasch TM Eckstein J Napper A Verdine G Karplus M 《Journal of the American Chemical Society》2002,124(37):11131-11141
A conformational analysis of a stereochemically complete set of peptide analogues based on a cis-enediol unit is presented. The cis-enediol unit, which can replace a two or a three amino acid segment of a peptide, contains two "side chains", four asymmetrical carbon atoms, and six free dihedral angles. To determine the accessible conformational space, the molecules are divided into three fragments, each containing two free dihedral angles. The energy surfaces are computed for all dihedral angle values, and the possible conformations of the cis-enediol unit analogues are built using all combinations of the surface minima. Such a "build-up" procedure, which is very fast, is able to reproduce 75% of the minima obtained from a full dihedral angle exploration of the conformational space. The cis-enediol unit minima are compared with the corresponding di- and tripeptide minima; all peptide minima can be closely matched by a cis-enediol unit minimum of low energy (less than 2.2 kcal/mol above the lowest energy conformer). However, there are low energy minima of the cis-enediol unit that have no corresponding minima in peptides. The results are shown to depend strongly on the chirality of the analogues. The ability of each of the stereoisomers to mimic natural peptides, evaluated by the present approach, is correlated with its experimental activity in a renin inhibition assay. 相似文献
76.
Hauke Harms Julien Rime Olivier Leupin Stephan J. Hug Jan Roelof van der Meer 《Mikrochimica acta》2005,151(3-4):217-222
A luminescent bacterial biosensor was used to quantify bioavailable arsenic in artificial groundwater. Its light production
above the background emission was proportional to the arsenite concentration in the toxicologically relevant range of 0 to
0.5 μM. Effects of the inorganic solutes phosphate, Fe(II) and silicate on the biosensor signal were studied. Phosphate at
a concentration of 0.25 g L−1 phosphate slightly stimulated the light emission, but much less than toxicologically relevant concentrations of the much
stronger inducer arsenite. No effect of phosphate was oberved in the presence of arsenite. Freshly prepared sodium silicate
solution at a concentration of 10 mg L−1 Si reduced the arsenite-induced light production by roughly 37%, which can be explained by transient polymerization leading
to sequestration of some arsenic. After three days of incubation, silicate did not have this effect anymore, probably because
depolymerization occurred. In the presence of 0.4 mg L−1 Fe(II), the arsenite-induced light emission was reduced by up to 90%, probably due to iron oxidation followed by arsenite
adsorption on the less soluble Fe(III) possibly along with some oxidation to the stronger adsorbing As(V). Addition of 100 μM
EDTA was capable of releasing all arsenic from the precipitate and to transform it into the biologically measurable, dissolved
state. The biosensor also proved valuable for monitoring the effectiveness of an arsenic removal procedure based on water
filtration through a mixture of sand and iron granules. 相似文献
77.
Sénéchal K Maury O Le Bozec H Ledoux I Zyss J 《Journal of the American Chemical Society》2002,124(17):4560-4561
Thermally stable dipolar and octupolar (D2d, D3) NLO-phores are readily accessible by combining one, two, or three 4,4'-bis(dialkylaminostyryl)-[2,2']-bipyridyl ligands with zinc(II) salts. The off-resonant beta0 values point out the superiority of octupoles versus dipoles in terms of nonlinearity/transparency tradeoff. The octahedral tris(bipyridyl)zinc(II) complex exhibits a very large beta0 value (241 x 10-30 esu), which is the largest ever reported for octupolar molecules. 相似文献
78.
K. P. Kotchi R. Castanet J. C. Mathieu 《Journal of Thermal Analysis and Calorimetry》1987,32(5):1477-1488
The heat effects corresponding to the dissolution of solid PbSe in liquid Pb-Se alloys starting from pure Pb were measured with a direct reaction calorimeter. The partial enthalpy of formation of PbSe in the melt and the integral enthalpy of formation of the Pb-PbSe system in the liquid state referred the liquid Pb and PbSe were deduced. Then, from a very simple change of reference state, and in the knowledge of the enthalpy of PbSe, the integral enthalpy of formation of Pb-Se liquid alloys with reference to pure liquid components could be derived. The results agreed very well with those obtained previously on the dissolution of Se in Pb. The method used can be applied whenever the component to be added to the bath has a high vapor pressure.
Zusammenfassung Die bei der Überführung von festem PbSe in flüssige Pb-Se-Legierungen auftretenden Wärmeeffekte wurden mittels direkter Reaktionskalorimetrie bestimmt. Die auf flüssiges Pb und PbSe bezogene partielle Bildungsenthalpie des Pb-PbSe-Systems im flüssigen Zustand wurde ermittelt. Durch eine geringfügige Veränderung des Referenzzustandes konnte bei Kenntnis der Enthalpie von PbSe die integrale Bildungsenthalpie von flüssigen Pb-Se-Legierungen in Bezug auf die reinen Komponenten abgeleitet werden. Die Ergebnisse stimmen gut mit denen überein, die früher für die Auflösung von Se in Pb erhalten wurden. Die Methode ist anwendbar, wenn dem Bade zuzugebende Komponenten einen hohen Dampfdruck haben.
Pb-Se Pb-Se, . Pb-Se Pb-PbSe Pb Pb-Se. PbSe, Pb-Se . , . , .相似文献
79.
Hu D Croutxé-Barghorn C Feuillade M Carré C 《The journal of physical chemistry. B》2005,109(32):15214-15220
Hybrid sol-gel materials have been prepared by hydrolytic polycondensation of an alkoxysilane. The sol-gel process of methacryloxypropyltrimethoxysilane (MAPTMS) has been followed by fluorescence spectroscopy with 2-naphthol as a probe. The experimental results showed that this photoprobe was dramatically sensitive to the microenvironment polarity. Spectroscopic studies revealed fluctuations of the maximum emission intensity and wavelength as a function of time. These fluctuations were attributed to the amphiphilic behavior of the hydrolyzed precursor. The maximum emission wavelength of the probe corresponding to its protonated form was higher than in pure water. All the results suggest that the presence of water molecules, tightly bonded to the polar head of the silanols, increased locally the sol polarity and induced a red-shifted emission. Fluorescence spectroscopy emphasized the reversibility of monomeric silanol aggregates and the changes in hydroxy group number of the silica network during the sol maturation. The behavior of this system upon shaking confirmed this statement. 相似文献
80.
An efficient preparation of N-methyl-O-tert-butylhydroxylamine hydrochloride has been settled, which allowed the synthesis of modified Weinreb amides. Nucleophilic addition of organolithium and Grignard reagents on these N-tert-butoxy-N-methylamides afforded efficiently the corresponding ketones and reduction with DIBAL furnished the corresponding aldehydes in good yields up to 97%. 相似文献