全文获取类型
收费全文 | 2984篇 |
免费 | 64篇 |
国内免费 | 49篇 |
专业分类
化学 | 1852篇 |
晶体学 | 16篇 |
力学 | 84篇 |
数学 | 433篇 |
物理学 | 712篇 |
出版年
2021年 | 27篇 |
2020年 | 23篇 |
2019年 | 19篇 |
2018年 | 18篇 |
2017年 | 17篇 |
2016年 | 36篇 |
2015年 | 35篇 |
2014年 | 48篇 |
2013年 | 112篇 |
2012年 | 112篇 |
2011年 | 143篇 |
2010年 | 87篇 |
2009年 | 62篇 |
2008年 | 131篇 |
2007年 | 162篇 |
2006年 | 140篇 |
2005年 | 119篇 |
2004年 | 121篇 |
2003年 | 108篇 |
2002年 | 121篇 |
2001年 | 84篇 |
2000年 | 75篇 |
1999年 | 60篇 |
1998年 | 45篇 |
1997年 | 49篇 |
1996年 | 55篇 |
1995年 | 42篇 |
1994年 | 55篇 |
1993年 | 49篇 |
1992年 | 49篇 |
1991年 | 32篇 |
1990年 | 45篇 |
1989年 | 42篇 |
1988年 | 28篇 |
1987年 | 23篇 |
1986年 | 30篇 |
1985年 | 54篇 |
1984年 | 42篇 |
1983年 | 26篇 |
1982年 | 41篇 |
1981年 | 36篇 |
1980年 | 45篇 |
1979年 | 41篇 |
1978年 | 52篇 |
1977年 | 41篇 |
1976年 | 35篇 |
1975年 | 34篇 |
1974年 | 22篇 |
1973年 | 42篇 |
1972年 | 18篇 |
排序方式: 共有3097条查询结果,搜索用时 15 毫秒
81.
The differences between the far-infrared absorption of liquid (+)3-methylcyclohexanone and the racemic mixture are interpreted as supporting evidence for the direct observation of rotation-translation coupling. 相似文献
82.
Wet oxidation of BPL porous carbon by H2O2 and HNO3 changes the pore structure and also increases the concentration of surface oxides. KOH uptake is increased by oxidation and is seen to be dependent on KOH concentration and to a lesser extent on inert electrolyte concentration. Enthalpy of immersion measurements have allowed the estimation of the enthalpy of hydration of the surface oxides and the enthalpy of neutralization of the easily accessible acid oxides. 相似文献
83.
Catherine A. Evans 《Tetrahedron》2005,61(26):6309-6314
Amine-catalyzed coupling reactions of allenoate esters and α,β-unsaturated carbonyls lead to a diverse range of α,α′-disubstituted allenoates. With appropriately substituted monomers, intermolecular reactions can lead to pyrimidone products. Alternatively, with amine substituted allenoates, a 7-endo-dig cyclization can be carried out such that a divergent pathway is observed that leads to azepine scaffolds. 相似文献
84.
Dinitrogen can be reduced to the planar M2(mu-eta2:eta2-N2) structure without employing cyclopentadienyl or complicated polydentate ligands using the recently discovered divalent oxidation states of Tm(II), Dy(II), and Nd(II). Complexes of these ions with common monodentate amide and aryloxide ligands can effect N2 reduction. THF solutions of LnI2 (Ln = Tm, Dy) in the presence of 2 equiv of NaN(SiMe3)2 reduce dinitrogen to form {[(Me3Si)2N]2(THF)Ln}2(mu-eta2:eta2-N2) complexes that have planar Ln2N2 units and 1.264(7) and 1.305(6) A NN bonds consistent with (N2)2- moieties. With the stronger reductant Nd(II), aryloxides are sufficient ancillary ligands: the NdI2/2KOC6H3tBu2-2,6 (KOAr) system forms [(ArO)2(THF)2Nd]2(mu-eta2:eta2-N2), which has a 1.242(7) A NN bond. 相似文献
85.
N-[2-(Dimethylamino)ethyl]acridine-4-carboxamide is a new experimental antitumour agent which has excellent in vivo activity against the Lewis lung tumour in mice. A reversed-phase high-performance liquid chromatographic method is described for the measurement of this agent in plasma. The internal standard was N-[2-(diethylamino)ethyl]acridine-4-carboxamide. The compounds of interest were extracted from plasma (0.2 ml) with acetonitrile and further purified on C18 solid-phase extraction Bond Elut columns. After elution with acetonitrile-ammonium acetate buffer and evaporation, the final separation was carried out on a C18 muBondapak column with fluorimetric detection. Over the plasma concentration range 100-5000 nM, the intra- and inter-assay coefficients of variation were less than 4.1 and 7.7%, respectively. The accuracy of the method varied from 97 to 105% of the theoretical values. The lowest concentration which could be measured with acceptable accuracy (+/- 10%) and precision (coefficient of variation less than 10%) was 10 nM. The method was sufficiently sensitive to allow pharmacokinetic analyses of 30 mumol/kg doses for more than six half-lives (t1/2) in rabbits (t1/2 = 4) and mice (t1/2 = 1.3 h). 相似文献
86.
Diffusion coefficients of tetradecyltrimethylammonium bromide in aqueous solutions have been determined at 25, 95 and 135 °C using the Taylor dispersion technique. The diffusion coefficient exhibits a minimum at a surfactant concentration above the critical micelle concentration (CMC). The results are interpreted in terms of electrostatic coupling and rapid exchange between micelle, surfactant monomer and counterions. 相似文献
87.
Enzyme--substrate interaction by nuclear magnetic resonance 总被引:2,自引:0,他引:2
88.
Structure (I) is shown to be the correct formulation for the compound hitherto considered to be the “azine” of ethyl acetoacetate; I rearranges to IV. Both IV, 1(H)-3-methyl, and 1(H)-3-phenyl, 5-pyrazolones and related compounds are shown to exist in the enolic form (VII) in the solid state and predominantly so in the solvents in which they are soluble. Similarly the indazolones are found to be represented exclusively by the enolized structure XII. The acetates of both series are reformulated where necessary on the basis of spectroscopic data, and the high frequencies found for the carbonyl groups in the IR are shown to reflect a lack of conjugation with the π-electrons of the heterocyclic ring. 相似文献
89.
Determination of Uranium and Thorium in Zircon,Apatite, and Fluorite: Application to Laser (U-Th)/He Thermochronology 总被引:8,自引:0,他引:8
N. J. Evans J. P. Byrne J. T. Keegan L. E. Dotter 《Journal of Analytical Chemistry》2005,60(12):1159-1165
We have developed a methodology for (U-Th)/He thermochronology on a variety of mineral species. With many laboratories initiating research in the area of (U-Th)/He thermochronology, we recognize that there may be interest in a review of analytical procedures for uranium and thorium determination in single crystals of apatite, zircon, rutile, and fluorite. Uranium and thorium are both determined by inductively coupled plasma mass-spectrometry using an isotope dilution method. While standard and spike solutions can be purchased, their isotopic composition and the concentration of the standard solution need to be verified. Digestion procedures for apatite and fluorite are relatively straightforward, but zircon decomposition requires the use of pressure vessels or fusion. Matrix effects are shown to have an insignificant effect on isotope ratios, although isobaric interferences, particularly of PtAr+ on U isotopes, can be a problem. We include complete thermochronology datasets for replicate analysis of Durango apatite, Yucca Mountain fluorite, and an Australian megacryst zircon. 相似文献
90.
Samet AV Niyazymbetov ME Semenov VV Laikhter AL Evans DH 《The Journal of organic chemistry》1996,61(25):8786-8791
Regioselective Michael addition of nitro and heterocyclic compounds to levoglucosenone, 1, is effectively catalyzed by amines and also by cathodic electrolysis. In comparison to the base-catalyzed reaction, it was found that under electrochemical conditions the reaction proceeds under milder conditions and with higher yields. Cathodically-initiated Michael addition of thiols to levoglucosenone using small currents produces the previously unknown threo addition product in several instances. The normal erythro isomer, identified as the kinetic product, tends to be formed when large currents are used. In contrast, slow, low current electrolyses promote equilibration of the two forms so that erythro can be converted to threo by the retro reaction and readdition. Addition of 2-naphthalenethiol to (R)-(+)-apoverbenone is also reported. 相似文献