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11.
Enantioselective transition metal catalysis directed by chiral cations is the amalgamation of chiral cation catalysis and organometallic catalysis. Thus far, three strategies have been revealed: ligand scaffolds incorporated on chiral cations, chiral cations paired with transition metal ‘ate’-type complexes, and ligand scaffolds incorporated on achiral anions. Chiral cation ion-pair catalysis has been successfully applied to alkylation, cycloaddition, dihydroxylation, oxohydroxylation, sulfoxidation, epoxidation and C–H borylation. This development represents an effective approach to promote the cooperation between chiral cations and transition metals, increasing the versatility and capability of both these forms of catalysts. In this review, we present current examples of the three strategies and suggest possible inclusions for the future.Enantioselective transition metal catalysis directed by chiral cations is the amalgamation of chiral cation catalysis and organometallic catalysis. 相似文献
12.
The vapor‐phase reaction of citronellal (CTN) at 220 °C and atmospheric pressure has been studied using mesoporous molecular sieves and zeolites in a fixed‐bed reactor. The primary products included isopulegol (IPG), menthone, and pulegol with subsequent reactions to form cyclic hydrocarbons. The CTN conversion and the product selectivity depend on the acidity and the textural property of catalysts. Lewis and/or Brönsted acid sites are essential for catalyzing this reaction. An increase of SiO2/Al2O3 mol ratio diminishes the acid amount of all catalysts and enhances both the surface area and the structural order of MCM‐41. The catalytic activity follows the order of MCM‐41 > HZSM‐5 > Hβ > USY, in accordance with the relative total acid amount except that of MCM‐41. Despite its low acidity, Si‐MCM‐41 exhibits the best catalytic performance due to its uniform mesopores, large surface area and good stability; the CTN conversion and the IPG yield attain 91.9% and 58.6%, respectively, after at least 25 h time‐on‐stream. 相似文献
13.
Three divalent transiton‐metal complexes of 2,3‐pyridinedicarboxylate (2,3‐pda2), [Mn(2,3‐pda)‐(H2O)3]∞ ( 7 ), [Ni(2,3‐pda)(H2O)3]∞ ( 8 ), and [Cd2(2,3‐pda)2(H2O)]∞ ( 9 ) have been hydrothermally synthesized and structurally characterized. X‐ray diffraction analyses reveal that compounds 7 and 8 are zigzag‐ and linear‐type one‐dimension (1D) coordination polymers, respectively, whereas compound 9 is a three‐dimension (3D) coordination polymer. A simple comparison of the coordination geometries with the available neutral analogues {Mx(2,3‐pda)x(H2O)y}∞ containing one or two divalent transition‐metal atoms and equal ligands in the presence or absence of coordinated water molecules is also presented. 相似文献
14.
A new dihydroberberine alkaloid, 7,8‐dihydro‐8‐methoxyberberine ( 1 ), along with six known compounds including two dihydroberberine alkaloids, 7,8‐dihydro‐8‐hydroxyberberine ( 2 ) and oxyberberine ( 3 ) and four protoberberine alkaloids, berberine ( 4 ), palmatine ( 5 ), jatrorrhizine ( 6 ) and columbamine ( 7 ), were isolated from the stems of Mahonia japonica. These compounds were characterized and identified by physical and spectral evidence. 相似文献
15.
Collagen methylation has been exploited in various applications involving living cells. We have observed correlation between the collagen methylation with the rate of cell proliferation in three-dimensional (3-D) microenvironment. To quantify the degree of collagen methylation, we have developed a capillary zone electrophoresis method. Using a polyvinyl alcohol-coated fused-silica capillary and UV detection at 200 nm, we have optimized pH and separated the native collagen into three major bands in phosphate buffer (50 mM, pH 2.5) with 0.05% hydroxypropylmethylcellulose. Under these conditions, the methylated collagens were separated into four major bands, which changed with different methylation reaction conditions. We propose an index to quantify the degree of collagen methylation that also correlates with their effects on cell proliferation. 相似文献
16.
侧链液晶聚合物的合成方法,可归纳为3类:一是烯基的单体,如甲基丙烯酸酯,或丙烯酸酯,氯代丙烯酸酯等,经自由基聚合反应,阴离子聚合反应及甲基丙烯酸酯的基团转移聚合反应,得到侧链液晶聚合物;二是变性反应,如聚硅氧烷与丙烯酸酯介 相似文献
17.
Ni-B和Ni-Ce-B超细非晶态合金的退火晶化及其催化性能 总被引:2,自引:0,他引:2
采用XAFS,XRD和DTA方法研究了Ni-B和Ni-Ce-B超细非晶态合金在退火过程中的结构变化及其结构与催化性能的关系.活性结果表明,在退火温度为623K时,Ni-B和Ni-Ce-B样品的苯加氢催化反应转化率最高,分别为63%和81%,0.3%Ce的掺入提高了Ni-Ce-B的催化活性.DTA结果表明,Ni-B超细非晶态合金在598和653K有两个晶化峰,而Ni-Ce-B样品有548,603,696和801K四个晶化峰.XAFS和XRD结果进一步说明,在573K退火时,Ni-B样品晶化生成晶态Ni3B和纳米晶Ni,此时Ni-Ce-B仅有少量晶态Ni3B生成.在673K退火时,Ni-B样品中的Ni3B开始分解生成晶态Ni,同时纳米晶Ni聚集并形成大颗粒晶态Ni,而Ni-Ce-B样品晶化生成晶态Ni3B和纳米晶Ni.在773K和更高的温度退火处理后,Ni-B样品中Ni的局域环境结构与金属Ni箔基本一致,但Ni-Ce-B样品晶化生成的Ni晶格有较大畸变,同时Ni3B并未分解.说明0.3%的Ce对提高Ni-Ce-B样品的稳定性有显著作用.本文首次报道了Ni-B和Ni-Ce-B超细非晶态合金中苯加氢催化活性中心为纳米晶Ni和类似于金属Ni的Ni-B非晶态合金. 相似文献
18.
Thienylmercury(II)chloride reacts with [Pd(PPh3)2Cl2], [Pd(PPh3)4] and [Pt(PPh3)4] to afford new compounds containing a metal-2-thienyl linkage. The compound [Pd(PPh3)2(2-C4H3S)Cl] probably has trans stereochemistry.2-Bromothiophen undergoes oxidative addition with [Pd(PPh3)4] and [Pt(PPh3)4], probably via a radical mechanism. With [Pd(CO)(PPh3)3], a carbonyl inserted product is obtained. The bromo-metal(II) complexes have trans stereochemistry. The course of the reaction between 3-methyl-2-bromothiophen and Pd(PPh3)4 is more complex. Thus, there is evidence of some cis bromopalladium(II) compounds amongst the products, also there is good evidence to support the view that some isomerisation of 3-methyl-2-thienyl to 4-methyl-2-thienyl occurs during the reaction, thus giving greater molar quantities of [Pd(PPh3)2(4-CH3-2-C4H2S)Br] than can be accounted for from any initial 4-methyl-2-bromothiophen impurity.The metallation of the thiophen ring, probably in the 4-position, with palladium(II) is described for 3-theylidene-4-methylaniline. 相似文献
19.
20.
Tandem cascade reactions of allylazides and olefinic dipolarophiles to give cis‐fused 2,3,7‐triazabicyclo [3.3.0]octenes ( 5, 6 or 7 ) are reported. Therein, an intermolecular dipolar cycloaddition of azide and alkene gave a triazoline which was followed by isomerization of the triazoline to a diazoester ( 4 ) and then an intramolecular dipolar cycloaddition from the diazo functional group and the double bond in 4 to give 5 . Compound 5 may further more undergo a Michael addition to give 7‐substituted‐ 2,3,7‐ triazabicyclo [3.3.0]oct‐2‐ene ( 6 ) or a tautomerization to give 2,3,7‐triazabicyclo[3.3.0]oct‐3‐ene ( 7 ). The reaction may be manipulated to stop at a particular stage by adopting a suit able solvent or an appropriate temperature. 相似文献