全文获取类型
收费全文 | 80819篇 |
免费 | 7500篇 |
国内免费 | 7731篇 |
专业分类
化学 | 28796篇 |
晶体学 | 448篇 |
力学 | 7634篇 |
综合类 | 1361篇 |
数学 | 27544篇 |
物理学 | 30267篇 |
出版年
2024年 | 109篇 |
2023年 | 710篇 |
2022年 | 1416篇 |
2021年 | 1586篇 |
2020年 | 1700篇 |
2019年 | 1917篇 |
2018年 | 1587篇 |
2017年 | 1924篇 |
2016年 | 2204篇 |
2015年 | 2018篇 |
2014年 | 2942篇 |
2013年 | 5207篇 |
2012年 | 3401篇 |
2011年 | 3960篇 |
2010年 | 3522篇 |
2009年 | 4699篇 |
2008年 | 5051篇 |
2007年 | 5148篇 |
2006年 | 4854篇 |
2005年 | 4128篇 |
2004年 | 3854篇 |
2003年 | 3785篇 |
2002年 | 3459篇 |
2001年 | 2911篇 |
2000年 | 2843篇 |
1999年 | 2542篇 |
1998年 | 2346篇 |
1997年 | 1933篇 |
1996年 | 1719篇 |
1995年 | 1518篇 |
1994年 | 1395篇 |
1993年 | 1189篇 |
1992年 | 1141篇 |
1991年 | 890篇 |
1990年 | 749篇 |
1989年 | 667篇 |
1988年 | 592篇 |
1987年 | 463篇 |
1986年 | 413篇 |
1985年 | 459篇 |
1984年 | 447篇 |
1983年 | 220篇 |
1982年 | 360篇 |
1981年 | 418篇 |
1980年 | 315篇 |
1979年 | 299篇 |
1978年 | 232篇 |
1977年 | 212篇 |
1976年 | 160篇 |
1974年 | 112篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
981.
From conductance and viscosities measurements on Pr4NCl, Et4NBr, and AgNO3 in acetonitrile and Et3NBr in nitrobenzene, the Walden products of the anion at infinite dilution were determined in presence of various concentrations of substituted benzoic acids. From these data it was possible to compute the values of the Walden products of the once complexed anions and to estimate the order of magnitude of the Walden products of twice complexed anions. Stokes' law is not obeyed, and the Walden products are not proportional to the third root of the molar volume of the complexed ions, as a consequence of their lack of sphericity. The assumption that the drag force which acts on the ions is proportional to the volume of the substituents results in a linear expression between the reciprocals of the Walden products and the molar volume of the ligands. The experimental results fit this expression within the limits of the experimental errors, and the slopes of the lines are nearly the same for all the anions and for the two solvents studied here, namely, 2.5×10–4 ohm-cm–5 mole-cP–1. 相似文献
982.
The rg structure of cyclopentene oxide has been determined by the simultaneous least squares analysis of electron diffraction and microwave spectroscopic data. The investigation has reaffirmed previous studies indicating that the molecule prefers a boat conformation. The methylene and epoxide flap angles obtained are 152.3±2.1° and 104.7±1.0° respectively. Other structural parameters determined are: rg (C-H avg.) = 1.120±0.004 Å; rg (C-C avg.) = 1.538±0.002 Å; rg (C-O) = 1.443±0.003 Å, and rg (C-C) = 1.482±0.004 Å for the carbon-carbon bond in the three membered epoxide ring. These results compare favorably with the reported structures of ethylene oxide and cyclohexene oxide. A tentative rationalization of the unusual boat conformation is also offered. 相似文献
983.
The determination of 0.4–4 μg of selenium by molecular emission cavity analysis is described. Selenium in organic compounds is determined after oxygen flask combustion. Metal ion interferences are eliminated by reduction of selenium to the element, filtration onto a glass-fibre paper, and direct incorporation of the filter into the cavity. Applications to the determination of selenium in inorganic and organic compounds are described. The determination of μg-amounts of tellurium is also outlined. 相似文献
984.
The most important geometric parameters and associated uncertainties (2σ) determined for F2POPF2 are the distances (rg) P-O = 1.631 ± 0.010 Å, P-F = 1.568 ± 0.004 Å, and angles POP = 135.2 ± 1.8°, OPF = 97.6 ± 1.2°, and FPF = 99.2 ± 2.4°. Amplitudes of vibration were also found. The large POP angle and relatively short P-O bond length are consistent with a significant degree of pπ-dπ bonding. Our structure interpretation differs from an earlier one reported by Arnold and Rankin in the relative P-O and P-F bond lengths and in the conclusion that the molecule exists in a distribution of not very rigid, probably staggered, conformers instead of one fairly rigid structure. 相似文献
985.
986.
987.
Direct titration of nitrite with cerium (IV), with ferroin as indicator, is shown to give satisfactory results if the acidity is kept between 0.033 and 0.055M at the end-point. Loss of nitrous acid owing to volatilization and decomposition is discussed. From 10 to 60 mg of sodium nitrite can be estimated with a standard deviation of 5 mug and an average error of 0.2%. 相似文献
988.
K. Jones 《Chromatographia》1988,25(7):577-581
Summary The flexibility of process scale high performance liquid chromatography is demonstrated by three examples of reversed-phase
separations (a) a standard purification (b) isolation of trace compounds, and (c) recovery from crystallisation mother liquors.
It was shown that a material, thought to be acceptably pure, contained a toxic impurity, and a remaining impurity, a previously
unseen component of potential therapeutic interest. Favourable cost data were indicated for example (3).
This work was conducted in the Chemical Technology Unit, University of Manchester Institute of Science and Technology, PO
Box 88, Manchester M6O 1OD, UK. 相似文献
989.
Spectrophotometric determination of the formation constants of iron(III)-8-hydroxyquinoline complexes in 0.1 M sodium perchlorate solution at 25° gave the values K1=[FeOx2+]/([Fe3+][Ox-])=4.9·1013, K2=[FeOx2+]/([FeOx2+][Ox-])=4.2·1012 and K3=[FeOx3]/([FeOx2+][Ox-])=3.9·1010. FeOx2OH and FeOx(OH)2 are obtainable as solid phases. FeOx3 (Ksp=3·10-44, intrinsic solubility 1.6·10-7 M) dissolves in basic solutions to form FeOx2(OH)2 -In a solution saturated with FeOx3, ([FeOx2(OH)2-][Ox-])/[OH-]2=7.7·10-5. 相似文献
990.
The reactions of tris(acetonitrile)tricarbonylchromium 1 with trimethylsilyl derivatives 2–5 of phenalene, indene, 1,2-dihydronaphthalene and trans-β-methylstyrene gave products 10-13, respectively, containing no trimethylsilyl groups. The reactions of 1 with trimethylsilyl derivatives 6–8 of benzene, toluene and cycloheptatriene gave products 14–16, respectively, containing trimethylsilyl groups. The reaction of 1 with 1,2-bis(trimethylsily-1,2-dihydro)naphthalene 9 gave product 17 in which only trimethylsilyl at the allylic position was cleaved. Compound 10 crystallizes in the orthorhombic system, space group Pbca, with a = 12.228(4), b = 14.288(1), c = 15.128(3) Å, Z = 8, RF= 0.046, and Rw = 0.047. X-ray crystallographic data confirm that the Cr(CO)3 moiety is bonded to phenalene in a η6-mold. 相似文献