全文获取类型
收费全文 | 2344篇 |
免费 | 55篇 |
国内免费 | 10篇 |
专业分类
化学 | 1724篇 |
晶体学 | 9篇 |
力学 | 61篇 |
数学 | 318篇 |
物理学 | 297篇 |
出版年
2021年 | 15篇 |
2020年 | 13篇 |
2019年 | 25篇 |
2018年 | 24篇 |
2016年 | 34篇 |
2015年 | 36篇 |
2014年 | 38篇 |
2013年 | 141篇 |
2012年 | 115篇 |
2011年 | 108篇 |
2010年 | 87篇 |
2009年 | 89篇 |
2008年 | 103篇 |
2007年 | 111篇 |
2006年 | 117篇 |
2005年 | 122篇 |
2004年 | 105篇 |
2003年 | 92篇 |
2002年 | 69篇 |
2001年 | 25篇 |
2000年 | 27篇 |
1999年 | 21篇 |
1998年 | 17篇 |
1997年 | 23篇 |
1996年 | 31篇 |
1995年 | 24篇 |
1994年 | 31篇 |
1993年 | 28篇 |
1992年 | 20篇 |
1991年 | 30篇 |
1990年 | 25篇 |
1989年 | 16篇 |
1988年 | 15篇 |
1987年 | 19篇 |
1985年 | 34篇 |
1984年 | 41篇 |
1983年 | 25篇 |
1982年 | 39篇 |
1981年 | 34篇 |
1980年 | 46篇 |
1979年 | 42篇 |
1978年 | 52篇 |
1977年 | 38篇 |
1976年 | 32篇 |
1975年 | 36篇 |
1974年 | 36篇 |
1973年 | 37篇 |
1972年 | 15篇 |
1970年 | 11篇 |
1966年 | 13篇 |
排序方式: 共有2409条查询结果,搜索用时 31 毫秒
991.
Background
In Drosophila, cuticular sex pheromones are long-chain unsaturated hydrocarbons synthesized from fatty acid precursors in epidermal cells called oenocytes. The species D. melanogaster shows sex pheromone dimorphism, with high levels of monoenes in males, and of dienes in females. Some biosynthesis enzymes are expressed both in fat body and oenocytes, rendering it difficult to estimate the exact role of oenocytes and of the transport of fatty acids from fat body to oenocytes in pheromone elaboration. To address this question, we RNAi silenced two main genes of the biosynthesis pathway, desat1 and desatF, in the oenocytes of D. melanogaster, without modifying their fat body expression. 相似文献992.
Med Abdellatif Bensegueni Aouatef Cherouana Lamia Bendjeddou Claude Lecomte Slimane Dahaoui 《Acta Crystallographica. Section C, Structural Chemistry》2009,65(12):o607-o611
In the two title compounds, cytosinium hydrogen sulfate, C4H6N3O+·HSO4−, (I), and cytosinium perchlorate, C4H6N3O+·ClO4−, (II), the asymmetric units comprise a cytosinium cation with hydrogen sulfate and perchlorate anions, respectively. The crystal structures of (I) and (II) are similar; that of (I) is characterized by a three‐dimensional N—H...O, O—H...O and C—H...O hydrogen‐bonded network. In (I) and (II), two‐dimensional layers are formed by N—H...O and C—H...O hydrogen bonds and, in the case of (I), they are linked by O—H...O hydrogen bonds where the anion acts as a donor and the cation as an acceptor. The hydrogen‐bonded sheets in (II) form an angle of 87.1°. 相似文献
993.
Comparison of MTBSTFA and BSTFA in derivatization reactions of polar compounds prior to GC/MS analysis 总被引:2,自引:0,他引:2
In this study, MTBSTFA and BSTFA, which are among the preferred derivatization reagents for silylation were both tested on derivatization of six different groups of polar chemicals to get information about usefulness in terms of sensitivity and specificity of both reagents. Tested compound groups were nitrophenols and methoxyphenols, sterols and sugars, dicarboxylic acids and hydroxylated polycyclic aromatic hydrocarbons.It was found that MTBSTFA-derivates produce characteristic fragmentation patterns presenting mainly the fragments [M]+, [M−57]+ and [M−131]+, of which [M−57]+ is generally dominant on the mass spectrogram. BSTFA-derivates mainly show the fragments [M]+, [M−15]+ and [M−89]+ whereof the molecular ion [M]+ is generally dominant. It was also found that steric hindrance and molecular mass play a very important role in the choice of the best suited derivatization reagent: compounds with sterically hindered sites derivatized with MTBSTFA produce very small analytical responses or no signal at all, and compounds with high molecular mass produce no characteristic fragmentation pattern when derivatization is performed with BSTFA.It was also found that MTBSTFA-derivatization facilitates separation of isomer analytes, suggesting its choice in combination to semi-polar columns, whilst BSTFA seems better for sterically hindered compounds.Findings were confirmed with applications of both reagents to biological and environmental matrices (urine and atmospheric aerosols). 相似文献
994.
Emmanuel Deiters Dr. Bo Song Dr. Anne‐Sophie Chauvin Dr. Caroline D. B. Vandevyver Dr. Frédéric Gumy Jean‐Claude G. Bünzli Prof. Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(4):885-900
A series of homoditopic ligands H2LCX (X=4–6) has been designed to self‐assemble with lanthanide ions (LnIII), resulting in neutral bimetallic helicates of overall composition [Ln2(LCX)3] with the aim of testing the influence of substituents on the photophysical properties, particularly the excitation wavelength. The complex species are thermodynamically stable in water (log β23 in the range 26–28 at pH 7.4) and display a metal‐ion environment with pseudo‐D3 symmetry and devoid of coordinated water molecules. The emission of EuIII, TbIII, and YbIII is sensitised to various extents, depending on the properties of the ligand donor levels. The best helicate is [Eu2(LC5)3] with excitation maxima at 350 and 365 nm and a quantum yield of 9 %. The viability of cervix cancer HeLa cells is unaffected when incubated with up to 500 μm of the chelate during 24 h. The helicate permeates into the cells by endocytosis and locates into lysosomes, which co‐localise with the endoplasmatic reticulum, as demonstrated by counterstaining experiments. The relatively long excitation wavelength allows easy recording of bright luminescent images on a confocal microscope (λexc=405 nm). The new lanthanide bioprobe remains undissociated in the cell medium, and is amenable to facile derivatisation. Examination of data for seven EuIII and TbIII bimetallic helicates point to shortcomings in the phenomenological rules of thumb between the energy gap ΔE(3ππ*–5DJ) and the sensitisation efficiency of the ligands. 相似文献
995.
Markus Albrecht Prof. Dr. Olga Osetska Dr. Jean‐Claude G. Bünzli Prof. Dr. Frédéric Gumy Roland Fröhlich Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(35):8791-8799
Homonuclear helicates with rare‐earth‐metal(III) ions or heteronuclear derivatives with rare‐earth‐metal and aluminium or zinc centres are obtained in alkali‐metal‐templated self‐assembly processes from isobutenylidene‐bridged homoditopic bis(2‐carbamido‐8‐hydroxyquinoline)‐derived ligands 1 ? H2 and 2 ? H2 or heteroditopic (8‐hydroxyquinoline)(2‐carbamido‐8‐hydroxyquinoline)‐derived ligands 3 ? H2 and 4 ? H2. Diamagnetic coordination compounds possess a high stability in organic solvents such as CDCl3, [D4]MeOH or [D6]DMSO and can be well characterised by 1H NMR spectroscopy by using methylene protons and the protons of the vinylic units of the ligand as stereochemical or symmetry probes, respectively. Some of the homonuclear complexes could be crystallised and were characterised by using X‐ray diffraction studies. The complexes adopt a triple‐stranded helical structure with a central templating cation encapsulated in their interior. An unusual orientation of the double bond of one spacer towards this cation is observed. The homo‐ and heterodinuclear helicates with ytterbium(III), neodymium(III) or erbium(III) of ligands 2 and 4 were of special interest owing to their near‐infrared (NIR) emitting properties, which were investigated depending on the lanthanide and on the encapsulated alkali‐metal cation. 相似文献
996.
The synthesis of a new ionic liquid-type monomer has been performed by association of a methacrylate polymerizable group, a polar tri(ethylene oxide) (TEO) spacer, a trifluoromethane sulfonic (TFSI−) anion and a free imidazolium (EMIm+) cation. The ionic liquid monomer (ILM) has demonstrated a good thermal stability and a high ionic conductivity around 2.1 × 10−3 S cm−1 at 20 °C. The corresponding homopolymer has shown an ionic conductivity closely related to the monomer (6.5 × 10−4 S cm−1 at 20 °C), which confirms the ILM as a valuable monomer for the formation of polymeric ionic liquid (PIL) materials. 相似文献
997.
Elena L. Gubanova Vladislav A. Sadykov Andre C. van Veen Claude Mirodatos 《Reaction Kinetics and Catalysis Letters》2009,97(2):349-354
The temporal analysis of products (TAP) technique coupled with the oxygen TPD was used to elucidate the effects of the Pt-supported
fluorite-like doped ceria–zirconia oxide chemical composition and the type of pretreatment on their oxygen bonding strength,
mobility, and reactivity as related to catalytic properties in the partial oxidation of methane into synthesis gas. A rapid
evolution of hydrogen under CH4 pulse observed for oxidized catalysts agrees with the direct route of the methane selective oxidation into syngas. This route
is favored by the Pt-support interaction and a moderate bonding strength of surface oxygen species along with a high lattice
oxygen mobility. 相似文献
998.
999.
Dr. Fanny A. Laporte Colette Lebrun Dr. Claude Vidaud Dr. Pascale Delangle 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(36):8570-8578
Some phosphoproteins such as osteopontin (OPN) have been identified as high-affinity uranyl targets. However, the binding sites required for interaction with uranyl and therefore involved in its toxicity have not been identified in the whole protein. The biomimetic approach proposed here aimed to decipher the nature of these sites and should help to understand the role of the multiple phosphorylations in UO22+ binding. Two hyperphosphorylated cyclic peptides, pS168 and pS1368 containing up to four phosphoserine (pSer) residues over the ten amino acids present in the sequences, were synthesized with all reactions performed in the solid phase, including post-phosphorylation. These β-sheet-structured peptides present four coordinating residues from four amino acid side chains pointing to the metal ion, either three pSer and one glutamate in pS168 or four pSer in pS1368 . Significantly, increasing the number of pSer residues up to four in the cyclodecapeptide scaffolds produced molecules with an affinity constant for UO22+ that is as large as that reported for osteopontin at physiological pH. The phosphate-rich pS1368 can thus be considered a relevant model of UO22+ coordination in this intrinsically disordered protein, which wraps around the metal ion to gather four phosphate groups in the UO22+ coordination sphere. These model hyperphosphorylated peptides are highly selective for UO22+ with respect to endogenous Ca2+, which makes them good starting structures for selective UO22+ complexation. 相似文献
1000.