全文获取类型
收费全文 | 29939篇 |
免费 | 140篇 |
国内免费 | 293篇 |
专业分类
化学 | 11135篇 |
晶体学 | 260篇 |
力学 | 1412篇 |
综合类 | 12篇 |
数学 | 9208篇 |
物理学 | 8345篇 |
出版年
2020年 | 38篇 |
2018年 | 1197篇 |
2017年 | 1459篇 |
2016年 | 743篇 |
2015年 | 578篇 |
2014年 | 487篇 |
2013年 | 688篇 |
2012年 | 3178篇 |
2011年 | 2394篇 |
2010年 | 1823篇 |
2009年 | 1567篇 |
2008年 | 606篇 |
2007年 | 674篇 |
2006年 | 683篇 |
2005年 | 4513篇 |
2004年 | 3946篇 |
2003年 | 2345篇 |
2002年 | 547篇 |
2001年 | 305篇 |
2000年 | 95篇 |
1999年 | 179篇 |
1998年 | 122篇 |
1997年 | 82篇 |
1996年 | 56篇 |
1995年 | 49篇 |
1994年 | 53篇 |
1993年 | 36篇 |
1992年 | 186篇 |
1991年 | 168篇 |
1990年 | 143篇 |
1989年 | 118篇 |
1988年 | 107篇 |
1987年 | 69篇 |
1986年 | 50篇 |
1985年 | 55篇 |
1984年 | 46篇 |
1983年 | 43篇 |
1981年 | 39篇 |
1979年 | 38篇 |
1978年 | 40篇 |
1976年 | 89篇 |
1975年 | 46篇 |
1974年 | 46篇 |
1973年 | 61篇 |
1972年 | 46篇 |
1970年 | 40篇 |
1969年 | 43篇 |
1968年 | 37篇 |
1967年 | 34篇 |
1966年 | 44篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
31.
Javad?SafariEmail author Soheila?Gandomi-Ravandi 《Journal of the Iranian Chemical Society》2015,12(1):147-154
A supported carbon material is shown to be a highly efficient, eco-friendly and recyclable solid acid catalyst for the Biginelli reaction of aldehyde, β-ketoester and urea or thiourea under microwave irradiation in the absence of solvent. This method offers significant advantages such as efficiency, the excellent yield, avoidance of the organic solvents, mild reaction conditions, easy separation and simple operation. In addition, because of employing microwave as heating source and reducing use of organic solvents, this novel method emerges as a green-approach leading to less harmful residues. Furthermore, a mechanism was proposed to rationalize the reaction and the role of NiO–MWCNTs was also investigated in these transformations. 相似文献
32.
33.
Background
Hydrazones and their metal complexes were heavily studied due to their pharmacological applications such as antimicrobial, anticonvulsant analgesic, anti-inflammatory and anti-cancer agents. This work aims to synthesize and characterize novel complexes of VO2+, Co2+, Ni2+, Cu2+, Zn2+, Zr4+and Pd2+ ions with oxalo bis(2,3-butanedione-hydrazone). Single crystals of the ligand have been grown and analyzed.Results
Oxalo bis(2,3-butanedionehydrazone) [OBH] has a monoclinic crystal with P 1 21/n 1 space group. The VO2+, Co2+, Ni2+, Cu2+, Zn2+, Zr4+ and Pd2+ complexes have the formulas: [VO(OBH–H)2]·H2O, [Co(OBH)2Cl]Cl·½EtOH, [Ni2(OBH)Cl4]·H2O·EtOH, [Cu(OBH)2Cl2]·2H2O, [Zn(OBH–H)2], [Zr(OBH)Cl4]·2H2O, and [Pd2(OBH)(H2O)2Cl4]·2H2O. All complexes are nonelectrolytes except [Co(OBH)2Cl]Cl·½EtOH. OBH ligates as: neutral tetradentate (NNOO) in the Ni2+ and Pd2+ complexes; neutral bidentate (OO) in [Co(OBH)2Cl]Cl·½EtOH, [Zr(OBH)Cl4]·2H2O and [Cu(OBH)2Cl2]·2H2O and monobasic bidentate (OO) in the Zn2+ and VO2+ complexes. The NMR (1H and 13C) spectra support these data. The results proved a tetrahedral for the Zn2+ complex; square-planar for Pd2+; mixed stereochemistry for Ni2+; square-pyramid for Co2+ and VO2+ and octahedral for Cu2+ and Zr4+ complexes. The TGA revealed the outer and inner solvents as well as the residual part. The molecular modeling of [Ni2(OBH)Cl4]·H2O·EtOH and [Co(OBH)2Cl]Cl·½EtOH are drawn and their molecular parameters proved that the presence of two metals stabilized the complex more than the mono metal. The complexes have variable activities against some bacteria and fungi. [Zr(OBH)Cl4]·2H2O has the highest activity. [Co(OBH)2Cl]Cl·½EtOH has more activity against Fusarium.Conclusion
Oxalo bis(2,3-butanedionehydrazone) structure was proved by X-ray crystallography. It coordinates with some transition metal ions as neutral bidentate; mononegative bidentate and neutral tetradentate. The complexes have tetrahedral, square-planar and/or octahedral structures. The VO2+ and Co2+ complexes have square-pyramid structure. [Cu(OBH)2Cl2]·2H2O and [Ni2(OBH)Cl4]·H2O·EtOH decomposed to their oxides while [VO(OBH–H)2]·H2O to vanadium. The energies obtained from molecular modeling calculation for [Ni2(OBH)Cl4]·H2O·EtOH are less than those for [Co(OBH)2Cl]Cl·½EtOH indicating the two metals stabilized the complex more than mono metal. The Co(II) complex is polar molecule while the Ni(II) is non-polar.Graphical abstract
34.
Tahereh?PoursaberiEmail author Z.?Falsafi M.?Hassanisadi A.?Jabbari 《Journal of the Iranian Chemical Society》2015,12(1):67-74
Cu nanoparticles@graphene oxide composites (CGC) were synthesized by a simplistic and effective chemical reduction method. The resulting CGC composites were characterized by X-ray diffraction, scanning electron microscopy with energy dispersive X-ray analysis, transmission electron microscopy, Fourier transform infrared spectroscopy and thermogravimetric analysis. The performance of the CGC composites for dechlorination of carbon tetrachloride was evaluated. The batch investigations indicated that carbon tetrachloride could be almost completely removed by CGC after 75 min. The adsorption isotherm was also discussed. The high removal capacities of CGC with respect to the bare nanoscale zerovalent copper owe to the increased adsorption sites in the composites that are resulted from aggregation inhibition, which leads to the reduced Cu nanoparticles size. 相似文献
35.
Electrochemical reduction of CO2 has the benefit of turning greenhouse gas emissions into useful resources. We performed a comparative study of the electrochemical reduction of CO2 on stepped Pb(211) and Sn(112) surfaces based on the results of density functional theory slab calculations. We mapped out the potential energy profiles for electrochemical reduction of CO2 to formate and other possible products on both surfaces. Our results show that the first step is the formation of the adsorbed formate(HCOO*) species through an Eley-Rideal mechanism. The formate species can be reduced to HCOO- through a oneelectron reduction in basic solution, which produces formic acid as the predominant product. The respective potentials of forming HCOO* are predicted to be -0.72 and -0.58 V on Pb and Sn. Higher overpotentials make other reaction pathways accessible, leading to different products. On Sn(112), CO and CH4 can be generated at -0.65 V following formate formation. In contrast, the limiting potential to access alternative reaction channels on Pb(211) is -1.33 V, significantly higher than that of Sn. 相似文献
36.
37.
38.
该文基于牛血清白蛋白模板金纳米簇(BSA@AuNCs)与羟基氧化钴(CoOOH)纳米片构建了一种激活型荧光纳米探针用于生物硫醇的检测。带负电的BSA@AuNCs能通过静电吸附作用组装到带正电的CoOOH纳米片表面,与此同时,BSA@AuNCs的荧光由于内滤效应(IFE)有效地被CoOOH纳米片猝灭,形成BSA@AuNCs-CoOOH纳米探针。当向纳米探针溶液加入生物硫醇(0.05~150 μmol/L)时,生物硫醇与纳米探针中的CoOOH纳米片发生氧化还原反应,CoOOH纳米片被降解生成Co2+,同时释放出BSA@AuNCs,BSA@AuNCs荧光信号恢复。结果表明,该纳米探针可以检测低浓度的生物硫醇,对生物硫醇(半胱氨酸、谷胱甘肽和高半胱氨酸)的检出限为30 nmol/L。相对于其他的氨基酸、金属离子及糖类化合物,该纳米探针对生物硫醇具有较高的选择性并成功应用于人血清样品中生物硫醇的检测。 相似文献
39.
根据活性亚结构拼接原理,以取代苯甲醛、盐酸羟胺、N-氯代丁二酰亚胺和2-苯并咪唑基乙腈等为原料,经肟化、氯代、环化及缩合反应,合成了一系列新型的含苯并咪唑和异噁唑结构的席夫碱化合物(5a~5h),收率60%~75%,其结构经~1H NMR,~(13)C NMR,IR和HR-MS表征。采用菌丝生长速率法测试了化合物对番茄灰霉菌和生菜菌核菌的抑菌活性。结果表明:在用药量为100μg·m ~(-1)时,苯环上含有氟原子或甲氧基的席夫碱(5c,5d和5e)对生菜菌核菌显示出较好的活性,抑制率为71.3%~76.1%。 相似文献
40.
Katharina Root Yves Wittwer Konstantin Barylyuk Ulrike Anders Renato Zenobi 《Journal of the American Society for Mass Spectrometry》2017,28(9):1863-1875
Native ESI-MS is increasingly used for quantitative analysis of biomolecular interactions. In such analyses, peak intensity ratios measured in mass spectra are treated as abundance ratios of the respective molecules in solution. While signal intensities of similar-size analytes, such as a protein and its complex with a small molecule, can be directly compared, significant distortions of the peak ratio due to unequal signal response of analytes impede the application of this approach for large oligomeric biomolecular complexes. We use a model system based on concatenated maltose binding protein units (MBPn, n = 1, 2, 3) to systematically study the behavior of protein mixtures in ESI-MS. The MBP concatamers differ from each other only by their mass while the chemical composition and other properties remain identical. We used native ESI-MS to analyze model mixtures of MBP oligomers, including equimolar mixtures of two proteins, as well as binary mixtures containing different fractions of the individual components. Pronounced deviation from a linear dependence of the signal intensity with concentration was observed for all binary mixtures investigated. While equimolar mixtures showed linear signal dependence at low concentrations, distinct ion suppression was observed above 20 μM. We systematically studied factors that are most often used in the literature to explain the origin of suppression effects. Implications of this effect for quantifying protein–protein binding affinity by native ESI-MS are discussed in general and demonstrated for an example of an anti-MBP antibody with its ligand, MBP. 相似文献