全文获取类型
收费全文 | 535篇 |
免费 | 106篇 |
国内免费 | 82篇 |
专业分类
化学 | 698篇 |
晶体学 | 3篇 |
力学 | 11篇 |
综合类 | 1篇 |
物理学 | 10篇 |
出版年
2024年 | 2篇 |
2023年 | 12篇 |
2022年 | 12篇 |
2021年 | 21篇 |
2020年 | 34篇 |
2019年 | 28篇 |
2018年 | 17篇 |
2017年 | 24篇 |
2016年 | 39篇 |
2015年 | 40篇 |
2014年 | 32篇 |
2013年 | 58篇 |
2012年 | 69篇 |
2011年 | 38篇 |
2010年 | 20篇 |
2009年 | 42篇 |
2008年 | 48篇 |
2007年 | 30篇 |
2006年 | 36篇 |
2005年 | 25篇 |
2004年 | 27篇 |
2003年 | 27篇 |
2002年 | 13篇 |
2001年 | 3篇 |
2000年 | 4篇 |
1999年 | 4篇 |
1998年 | 3篇 |
1997年 | 7篇 |
1995年 | 4篇 |
1994年 | 1篇 |
1991年 | 2篇 |
1989年 | 1篇 |
排序方式: 共有723条查询结果,搜索用时 31 毫秒
621.
622.
623.
624.
喜树碱是从中国植物喜树(Camptotheca acuminata)中分离得到的抗癌生物碱 ,由于其独特的抗癌机理而成为抗癌药研究中的热门课题.喜树碱属于单萜吲哚生 物碱类,由色胺和secologanin缩合、衍生而来.它的仿生合成始于1972年,先后 有多人参加,直到1997年在经历了四分之一世纪之后才取得了成功.介绍喜树碱仿 生合成背后鲜为人知的故事. 相似文献
625.
以模拟软体动物珍珠层的周期性基质控制形成过程制备仿生层状复合材料. 将聚苯乙烯磺酸钠(PSS)与聚二烯二甲基氯化铵(PDAC)用逐层浸渍的方法使其组装成多层膜, 用于诱导过饱和溶液中CaCO3的结晶, 详细研究了膜紫外吸收随组装层数增加的线性变化. 扫描电镜和X射线衍射表征了晶体的形貌和结构. (PDAC/PSS)15PDAC膜诱导获得的CaCO3晶体为六面体结构, 晶体尺寸为30~40 μm; (PDAC/PSS)15膜诱导CaCO3结晶, 可以在膜表面获得形貌与珍珠层非常相似的CaCO3晶体, 结晶10 h获得的晶片结构呈规则的六边形, 片尺寸约为10~20 μm. X射线衍射结果表明两种晶体的晶格结构与天然珍珠层差异明显, 说明静电作用为晶体形貌的主控因素之一, 但不是晶格结构的决定因素. 复合材料断面电镜照片表明其为层状结构. 相似文献
626.
In 1926 Hans Fischer and Bruno Walach synthesized the first porphyrins. 1 H. Fischer, B. Walach, Justus Liebigs Ann. Chem. 1926 , 450, 164–181. Currently more than 1400 new articles concerning the synthesis and uses of porphyrins are published every year. 2 CAS Online search for 1994 . However, the strong interest in these compounds indicated by this is in sharp contrast to their restricted availability. This is reflected in the current price of up to 500 DM for 5 mg of the most important porphyrins used in research and other applications (see Scheme 3). Biomimetic syntheses offer possibilities for an improved approach to porphyrins. By following the example set by nature it is also possible to obtain novel porphyrinoids which are different from naturally occurring porphyrins. This is exemplified by N,N′-bridged porphyrinogens, which have cage structures, inverted porphyrinoids (N atoms in the outer periphery) and, in particular, porphyrins with expanded systems. Among the family of expanded porphyrins are superarenes with up to 34 π electrons. Their pronounced aromaticity is indicated by 1H NMR spectra, bond length equivalence, planar structures, and electrophilic substitution. With their strong absorption bands, the strongest of which have ? values of > 1000000, a value which exceeds the absorption intensity of all other organic pigments observed until now, and their ability to act as efficient photosensitizers, the expanded porphyrins open interesting perspectives in the fields of photochemistry and photomedicine. 相似文献
627.
Isabel Toucet Maria A. Aponte 《Journal of polymer science. Part A, Polymer chemistry》1991,29(13):1883-1888
The synthesis of four new monomers containing guanine or a guanine precursor was achieved. These were two isomeric acid ester derivatives of guanine and two isomeric vinyl ether derivatives of N2-acetylguanine. In the case of the synthesis of the guanine acid esters IV and V, it was necessary to prepare first the guanine alcohol derivatives II and III. These N-7 and N-9 isomeric alcohols of guanine were separated by fractional crystallization. Subsequent esterification of these alcohols with maleic anhydride gave the desired products. In the other case, N2-acetylguanine was alkylated with 2-chloroethyl vinyl ether to yield the N-7 and N-9 isomer VI and VII, respectively. These were separated using flash column chromatography. 相似文献
628.
The preparation of a new class of tricyclic models 1 based on a Friedländer reaction between chiral piperidine-2,4-diones 2 and azomethine 3 is reported. Alkylation of the lactam allowed to install various pendant arms on the chiral cyclic inducer. The so-obtained mimics 1a,d,f,g,h,k were involved in the reduction of methyl benzoylformate to furnish methyl mandelate in 4-87% ee (R). The presence of a coordinating pendant arm proved to be essential to reach optimum results in terms of enantioinduction. Asymmetric reduction of 2-benzoylpyridine with mimics 1d,f,g produced α-phenyl-2-pyridinemethanol in 30-84% ee (R). 相似文献
629.
Cis-dioxo-catecholatotungsten(Ⅴ) complex (NH2CH2CH2NH3)4[WⅤO2(OC6H4O)2]2(NH3CH2CH2NH3)·H2O (1) was synthesized at room temperature by the reaction of tetrabutyl ammonium decatungstate with catechol in the mixed solvent of CH3OH, CH3CN and NH2CH2CH2NH2. The crystal structure of complex was determined by X-ray diffraction structural analysis. The results show that complex belongs to monoclinic system with space group P21/c,a=0.712 8(3) nm, b=3.082 3(11) nm, c=0.982 8(4) nm, β=102.639(6)°, V=2.106 8(14) nm3, Z=2, R1=0.062 8, wR2=0.183 7. Compared the complex with its analogous complexes (NH2CH2CH2NH3)3[MoⅤO2(OC6H4O)2], it is found that the coordination structure of W have no changes in the processing of electron transfer of tungsten-containing enzymes from the result of the similarity of the EPR spectra of the complexes and flavoenzyme from milk. CCDC: 272937. 相似文献
630.
Biomimetic Synthesis of CdSe Quantum Dots through Emulsion Liquid Membrane System of Gas-Liquid Transport 总被引:2,自引:0,他引:2
The cadmium selenide quantum dots (QD) have been synthesized by template-control in an emulsion fiquid membrane system. The system consisted of kerosene as solvent, L152 (dialkylene succinimide) as surfactant,N7301 (trialiphatic amine, R3N, R=C8-C10) as carrier, 0.1mol/L CdCl2 solution as internal-aqueous phase and H2Se gas as external phase. Additive organic template agent in internal-aqueous phase was necessary to form CdSe QD. The influence of the nature of template and its concentration on sizes of the formed CdSe QD has also been studied. Transmission electron microscopy showed that the sizes of the products could be controlled down to 3-4nm. X-ray diffraction analysis revealed that the crystals had cubic structure. The formation process and the optical properties of CdSe QD have also been presented. 相似文献