全文获取类型
收费全文 | 5459篇 |
免费 | 921篇 |
国内免费 | 1217篇 |
专业分类
化学 | 3357篇 |
晶体学 | 40篇 |
力学 | 359篇 |
综合类 | 25篇 |
数学 | 1363篇 |
物理学 | 2453篇 |
出版年
2024年 | 22篇 |
2023年 | 155篇 |
2022年 | 258篇 |
2021年 | 294篇 |
2020年 | 299篇 |
2019年 | 218篇 |
2018年 | 229篇 |
2017年 | 254篇 |
2016年 | 283篇 |
2015年 | 289篇 |
2014年 | 359篇 |
2013年 | 488篇 |
2012年 | 540篇 |
2011年 | 546篇 |
2010年 | 401篇 |
2009年 | 366篇 |
2008年 | 261篇 |
2007年 | 322篇 |
2006年 | 301篇 |
2005年 | 208篇 |
2004年 | 146篇 |
2003年 | 131篇 |
2002年 | 115篇 |
2001年 | 131篇 |
2000年 | 92篇 |
1999年 | 153篇 |
1998年 | 96篇 |
1997年 | 53篇 |
1996年 | 76篇 |
1995年 | 48篇 |
1994年 | 32篇 |
1993年 | 44篇 |
1992年 | 28篇 |
1991年 | 29篇 |
1990年 | 19篇 |
1989年 | 14篇 |
1988年 | 76篇 |
1987年 | 109篇 |
1986年 | 63篇 |
1985年 | 13篇 |
1984年 | 15篇 |
1983年 | 13篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1979年 | 2篇 |
1959年 | 1篇 |
1957年 | 1篇 |
1936年 | 1篇 |
排序方式: 共有7597条查询结果,搜索用时 31 毫秒
961.
As an oral hypoglycemic agent, metformin (Met) has become a best-selling inexpensive drug worldwide. In this thesis, [Cr(metformin)3] (CrMet) complex was synthesized and characterized by elemental analysis (EA), electrospray ionization mass spectrometry (ESI-MS), nuclear magnetic resonance (NMR), infrared (IR), UV-visible (UV-vis), and molar conductivity. Meanwhile, the molecule structure of CrMet complex was optimized using Gaussian 09. Considering the therapeutic effect of Met and Met/Cr(III) complex on type 2 diabetes mellitus (T2DM), the biological activities of CrMet in streptozocin (STZ)-induced diabetic mice were evaluated in detail from the aspects of fasting blood glucose (FBG), fasting serum insulin (FINS), triglyceride (TG), total cholesterol (TC), low-density lipoprotein cholesterol (LDL-c), and high-density lipoprotein cholesterol (HDL-c) levels. These results indicated that CrMet had beneficial function on blood glucose (BG) and lipid metabolism for diabetes. Additionally, the results of cytotoxicity and toxicity experiments showed that CrMet had no damage to cells and relatively high safety in mice. It maybe a potential candidate as a therapeutic agent in T2DM. 相似文献
962.
双矿化剂对合成ZSM分子筛酸性和织构特征及其甲醇转化制丙烯催化性能的影响 《燃料化学学报》2018,46(8):967-976
采用静态水热法在F--OH-体系中,以四丙基氢氧化铵为模板剂、偏铝酸钠为铝源、正硅酸乙酯为硅源,合成了纳米SiO2-ZSM-5分子筛,考察了F-/Al2O3物质的量比对所合成的ZSM-5分子筛织构性质和甲醇转化制丙烯催化性能的影响。结果发现,随着初始溶胶F-/Al2O3物质的量比的增大,产物中SiO2的含量增大,ZSM-5分子筛的相对结晶度有所降低;同时,分子筛的比表面积和孔容减小、酸强度降低、酸量减少。对于甲醇转化制丙烯,最佳F-/Al2O3物质的量比为12;此时,丙烯选择性高于45%,丙烯/乙烯(P/E)比值大于10。反应机理分析表明,过渡态择形选择性是控制烯烃选择性的重要因素。 相似文献
963.
964.
Mechanistic Insights into the Ni‐Catalyzed Reductive Carboxylation of C−O Bonds in Aromatic Esters with CO2: Understanding Remarkable Ligand and Traceless‐Directing‐Group Effects
下载免费PDF全文
![点击此处可从《化学:亚洲杂志》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Yan‐Li Han Bing‐Yuan Zhao Kun‐Yao Jiang Hui‐Min Yan Dr. Zhu‐Xia Zhang Dr. Wen‐Jing Yang Prof. Dr. Zhen Guo Prof. Dr. Yan‐Rong Li 《化学:亚洲杂志》2018,13(12):1570-1581
The mechanism of the Ni0‐catalyzed reductive carboxylation reaction of C(sp2)?O and C(sp3)?O bonds in aromatic esters with CO2 to access valuable carboxylic acids was comprehensively studied by using DFT calculations. Computational results revealed that this transformation was composed of several key steps: C?O bond cleavage, reductive elimination, and/or CO2 insertion. Of these steps, C?O bond cleavage was found to be rate‐determining, and it occurred through either oxidative addition to form a NiII intermediate, or a radical pathway that involved a bimetallic species to generate two NiI species through homolytic dissociation of the C?O bond. DFT calculations revealed that the oxidative addition step was preferred in the reductive carboxylation reactions of C(sp2)?O and C(sp3)?O bonds in substrates with extended π systems. In contrast, oxidative addition was highly disfavored when traceless directing groups were involved in the reductive coupling of substrates without extended π systems. In such cases, the presence of traceless directing groups allowed for docking of a second Ni0 catalyst, and the reactions proceed through a bimetallic radical pathway, rather than through concerted oxidative addition, to afford two NiI species both kinetically and thermodynamically. These theoretical mechanistic insights into the reductive carboxylation reactions of C?O bonds were also employed to investigate several experimentally observed phenomena, including ligand‐dependent reactivity and site‐selectivity. 相似文献
965.
By simply changing pH value of reaction solution, three low-dimensional iodoargentate hybrids, [HDABCO]2[(DABCO)2(Ag2I4)] (1), [HDABCO]0.5[(HDABCO)0.5(AgI2)] (2) and [(Me2NH2)(H2DABCO)][AgI4] (3) (DABCO = 1,4-diazabicyclo-[2.2.2]octane), have been synthesized. Noteworthy, variation in protonated degree and aggregating forms of DABCO at different pH value, the structure of inorganic moieties vary regularly from 0D [(DABCO)2(Ag2I4)]2? anion for 1 to 1D [(HDABCO)0.5(AgI2)]0.5? chain for 2 and 0D [AgI4]3? anion for 3, exhibiting unique structural directing effects. In addition, three compounds display structure-dependent semiconductor natures and thermal stabilities. 相似文献
966.
Synthesis of Higher Alcohols via Syngas on Cu/Zn/Si Catalysts. Effect of Polyethylene Glycol Content
Rong-Ji Cui Xing Yan Jin-Chuan Fan Wei Huang 《Russian Journal of Physical Chemistry A, Focus on Chemistry》2018,92(5):884-888
Cu/Zn/Si catalysts with different polyethylene glycol (PEG) content were prepared by a complete liquid-phase method, and characterized by XRD, H2-TPR, N2-adsorption, and XPS. The influence of PEG content on the higher alcohols synthesis from syngas was investigated. The results showed that addition of PEG can influence the texture and surface properties of the catalysts, and therefore affect their activity and product distribution. With an increase in PEG content, BET surface area, Cu crystallite size and surface active ingredient content of the catalysts first increased and then decreased, the CO conversion had similar variation tendency. However, the pore volume and pore diameter of the catalyst increased, and the binding energy of the active component and the content of Cu2O decreased, which resulted in higher catalyst selectivity towards higher alcohols. The highest C2+OH selectivity in total alcohols was 60.6 wt %. 相似文献
967.
968.
969.
Wang X Feng L Chen Z 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2008,71(4):1433-1437
A novel luminescent dye metal complex, (CZHQ)3Al, with 8-hydroxyquinoline aluminum and hole-transporting carbazole units was designed and synthesized. The (CZHQ)3Al optical properties were carefully investigated by UV-vis absorption and fluorescence spectra in diluent solution. The results showed that the luminescent quantum yield of (CZHQ)3Al was 0.62 in DMSO and it emitted red-light with the band gap of 2.89 eV estimated from the onset absorption. In addition, the light-emission of (CZHQ)3Al can be quenched by electron acceptor (dimethylterephalate), where the process followed the Stern-Volmer equation. However, the fluorescent intensities of (CZHQ)3Al were slowly increased with the addition of electron donor (N,N-dimethylaniline). Furthermore, the molecular interactions of (CZHQ)3Al with fullerene (C60) and carbon nanotubes (CNTs) were also respectively investigated, which indicated the metal dye can be used as new fluorescent probe. 相似文献
970.
Cryptophane-A was synthesized from vanillin by a three-step method and its spectroscopic properties in different organic solvents were determined. Two absorption bands at about 240-250 and 280-290 nm were observed for cryptophane-A. A fluorescence emission peak was obtained at 320-330 nm using a solution of approximately 10(-5)M cryptophane-A. The interaction of cryptophane-A with chlorinated compounds CH(n)Cl(4 - n) (n=0, 1, 2) in dioxane and ethyl acetate solvents were studied in detail by fluorescence spectroscopy, respectively. The results show that cryptophane-A is well suited for inclusion of CH(2)Cl(2) to form a stable 1:1 complex and the binding constant was estimated to be 19+/-2M(-1). These results were also confirmed by (1)H NMR and CPK models. Larger similar molecules such as CHCl(3) and CCl(4) are unable to enter the cavity of cryptophane-A because of their bigger sizes. However, the fluorescence emission of cryptophane-A can be efficiently quenched by CHCl(3) and CCl(4), following the Stern-Volmer relationship. 相似文献