全文获取类型
收费全文 | 65595篇 |
免费 | 13965篇 |
国内免费 | 4609篇 |
专业分类
化学 | 61100篇 |
晶体学 | 634篇 |
力学 | 2617篇 |
综合类 | 313篇 |
数学 | 5596篇 |
物理学 | 13909篇 |
出版年
2024年 | 85篇 |
2023年 | 668篇 |
2022年 | 663篇 |
2021年 | 1113篇 |
2020年 | 2390篇 |
2019年 | 3579篇 |
2018年 | 1960篇 |
2017年 | 1508篇 |
2016年 | 4615篇 |
2015年 | 4694篇 |
2014年 | 4969篇 |
2013年 | 6113篇 |
2012年 | 5658篇 |
2011年 | 4989篇 |
2010年 | 4618篇 |
2009年 | 4537篇 |
2008年 | 4177篇 |
2007年 | 3499篇 |
2006年 | 3026篇 |
2005年 | 2908篇 |
2004年 | 2394篇 |
2003年 | 2112篇 |
2002年 | 2774篇 |
2001年 | 1981篇 |
2000年 | 1878篇 |
1999年 | 1102篇 |
1998年 | 717篇 |
1997年 | 689篇 |
1996年 | 749篇 |
1995年 | 573篇 |
1994年 | 539篇 |
1993年 | 440篇 |
1992年 | 425篇 |
1991年 | 378篇 |
1990年 | 282篇 |
1989年 | 254篇 |
1988年 | 193篇 |
1987年 | 164篇 |
1986年 | 138篇 |
1985年 | 130篇 |
1984年 | 96篇 |
1983年 | 68篇 |
1982年 | 44篇 |
1981年 | 44篇 |
1980年 | 18篇 |
1978年 | 19篇 |
1977年 | 23篇 |
1976年 | 21篇 |
1975年 | 28篇 |
1974年 | 27篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Huang X Margulis CJ Li Y Berne BJ 《Journal of the American Chemical Society》2005,127(50):17842-17851
When supercritical CO2 is dissolved in an ionic liquid, its partial molar volume is much smaller than that observed in most other solvents. In this article we explore in atomistic detail and explain in an intuitive way the peculiar volumetric behavior experimentally observed when supercritical CO2 is dissolved in 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim+] [PF6(-)]). We also provide physical insight into the structure and dynamics occurring across the boundary of the CO2 ionic liquid interface. We find that the liquid structure of [Bmim+] [PF6(-)] in the presence of CO2 is nearly identical to that in the neat ionic liquid (IL) even at fairly large mole fractions of CO2. Our simulations indicate, in agreement with experiments, that partial miscibilities of one fluid into the other are very unsymmetrical, CO2 being highly soluble in the ionic liquid phase while the ionic liquid is highly insoluble in the CO2 phase. We interpret our results in terms of the size and shape of spontaneously forming cavities in the ionic liquid phase, and we propose that CO2 occupies extremely well-defined locations in the IL. Even though our accurate prediction of cavity sizes in the neat IL indicates that these cavities are small compared with the van der Waals radius of a single carbon or oxygen atom, CO2 appears to occupy a space that was for the most part a priori "empty". 相似文献
992.
[reaction: see text] A broad range of substituted 1-amino-1,3-butadienes undergo enantioselective Diels-Alder reactions with methacrolein in the presence of 5 mol % of Cr(III)-salen complex 1. The reactions are carried out conveniently, at room temperature, and they afford the cycloadducts in high yields and excellent ee's. 相似文献
993.
The rates at which aluminum was removed from the N- and C-terminal monoaluminum ovotransferrins by pyrophosphate were evaluated by UV difference spectra in 0.01 mol/L Hepes, pH=7.4 and at 37℃. Pesudo first-order rate constants as a function of pyrophosphate concentration were measured. The results indicate that the pathways of aluminum removal are different. For the N-terminal binding site, aluminum removal follows simple saturation kinetics, while the removal of aluminum from the C-terminal binding site reverts to the combination of saturation and first-order kinetics. The saturation component is consistent with a rate-limiting conformational change in the protein as has been reported. We propose that the first-order kinetics mechanism is attributed to a pre-equilibrium process. The rate constants of saturation kinetics are accelerated from both terminals with the addition of 0.1 mol/L chloride to the monoaluminum ovotransferrin solutions, whereas the rates of the first-order kinetics are decreased for the C-terminal binding site. The effect of chloride ionic strength causes a continuing increase on kobs for the N- and C-terminal binding sites. Moreover, the kinetics behavior of the N-terminal is more easily affected by chloride than that of the C-terminal. In the experiment presumably the N-terminal site is apparently kinetically more labile than the C-terminal site. 相似文献
994.
水热合成CdS纳米晶体的形貌控制研究 总被引:4,自引:0,他引:4
研究了水热合成CdS纳米晶体形貌的化学控制,选择不同的络合试剂为模板,研究其对水热合成CdS纳米晶体形貌的影响.实验发现若以络合试剂乙二胺、甲胺为模板时,产品CdS晶体的形貌分别为(20~30) nm×(200~600) nm和(40~50) nm×(200~600) nm尺寸的纳米棒;而以络合试剂吡啶、 氨为模板时,产品CdS晶体的形貌分别为平均尺寸约30 nm和20 nm的纳米颗粒.用XRD、TEM、XPS、PL和Raman光谱等技术对所得CdS纳米棒进行了表征.同时对水热合成CdS纳米晶体形貌的模板控制机制进行了探讨,提出了一种水热合成CdS纳米棒的络合物结构诱导生长机理. 相似文献
995.
996.
997.
Controlling chemical reactivity has been the central theme in chemistry. Herein, we review the recent progress on the development of genetically encoded protein coupling reactions and their potential applications. The chemical reactivity is encoded in the protein sequences. The information is read out by folding and molecular recognition between two reactive components and subsequently translated into chemical bonding via autocatalysis. It has emerged as a unique way to tune the chemical reactivity and is regarded as one type of information‐coded reactions. Not only has it received many applications such as protein topology engineering, bioconjugation, biomaterials and synthetic biology, but also its principle may be extended beyond protein chemistry to enable new modes of supramolecular interactions that promote chemical bonding and that are simultaneously reinforced by covalent bonds. 相似文献
998.
999.
A simple and sensitive kinetic method was developed for the determination of ritodrine hydrochloride in pharmaceutical preparations. The method is based upon a kinetic investigation of the oxidation reaction of the drug with alkaline potassium permenganate at room temperature for a fixed 12.5 min; the absorbance of the colored permenganate ion was measured at 609 nm. The absorbance concentration plots were rectilinear over the range of 1.6–11.2 μg mL?1 (r = 0.9992) with a minimum detectability of 0.096 μgmL?1 (2.88 × 10?7 M). The different experimental parameters affecting the development and stability of color were carefully studied and optimized. The determination of ritodrine hydrochloride by the fixed concentration and rate constant methods is also feasible with the calibration equations obtained, but the fixed time method has been found to be more applicable. The proposed method was applied successfully to the determination of ritodrine hydrochloride in tablets and ampoules with average recoveries of 100.37 ± 0.93% and 100.42 ± 0.87%, respectively. The results obtained were in good agreement with those obtained using a reference method for comparison. A proposal of the reaction pathway is also presented. 相似文献
1000.
Bao Shan Huang Mary Jane Lauzon James C. Parham 《Journal of heterocyclic chemistry》1979,16(4):811-813
Alkylation of 4(5)-nitroimidazole-5(4)-sulfonamide with benzyl bromide occurred on both ring nitrogens. The structures of the products could be assigned by comparison of the chemical shifts of the sulfonamide hydrogens in the nmr spectra with those of the isomeric methyl derivatives, which were prepared by differing routes. Uv and nmr spectral data are reported for a number of bromo-, nitro-, mercapto-, sulfamyl- and amino- substituted imidazoles as well as for both of the isomeric methylated derivatives of the series. 相似文献