全文获取类型
收费全文 | 146990篇 |
免费 | 1294篇 |
国内免费 | 397篇 |
专业分类
化学 | 80178篇 |
晶体学 | 2387篇 |
力学 | 6063篇 |
综合类 | 3篇 |
数学 | 13576篇 |
物理学 | 46474篇 |
出版年
2021年 | 1031篇 |
2020年 | 1181篇 |
2019年 | 1250篇 |
2018年 | 1638篇 |
2017年 | 1689篇 |
2016年 | 2540篇 |
2015年 | 1559篇 |
2014年 | 2350篇 |
2013年 | 6036篇 |
2012年 | 4843篇 |
2011年 | 5934篇 |
2010年 | 4183篇 |
2009年 | 4206篇 |
2008年 | 5726篇 |
2007年 | 5506篇 |
2006年 | 5377篇 |
2005年 | 4944篇 |
2004年 | 4447篇 |
2003年 | 4115篇 |
2002年 | 4007篇 |
2001年 | 5543篇 |
2000年 | 3971篇 |
1999年 | 3072篇 |
1998年 | 2110篇 |
1997年 | 2207篇 |
1996年 | 2069篇 |
1995年 | 1915篇 |
1994年 | 1912篇 |
1993年 | 1726篇 |
1992年 | 2213篇 |
1991年 | 2299篇 |
1990年 | 2136篇 |
1989年 | 2099篇 |
1988年 | 2077篇 |
1987年 | 2085篇 |
1986年 | 1909篇 |
1985年 | 2370篇 |
1984年 | 2392篇 |
1983年 | 1910篇 |
1982年 | 1914篇 |
1981年 | 1751篇 |
1980年 | 1738篇 |
1979年 | 2027篇 |
1978年 | 2101篇 |
1977年 | 2048篇 |
1976年 | 1976篇 |
1975年 | 1859篇 |
1974年 | 1842篇 |
1973年 | 1874篇 |
1972年 | 1235篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
881.
O. V. Statsenko S. N. Bolotin V. T. Panyushkin 《Russian Journal of General Chemistry》2004,74(8):1286-1288
Complexation of Cu(II) ions with L- and DL-threonine in aqueous solution was studied. Treatment of the ESR spectra of solutions containing Cu(II) and threonine in various ratios and having various pH, involving total line shape analysis, revealed formation of geometric isomers of the complex ions [Cu(thrH–1)2]2– differing in the mutual arrangement of the N and O atoms in the equatorial plane; the complexation and isomerization equilibrium constants were determined.Translated from Zhurnal Obshchei Khimii, Vol. 74, No. 8, 2004, pp. 1388–1391.Original Russian Text Copyright © 2004 by Statsenko, Bolotin, Panyushkin.This revised version was published online in April 2005 with a corrected cover date. 相似文献
882.
Pre-resonance Raman spectra have been obtained for TCNQ and LiTCNQ in acetonitrile solution using an Ar+—Kr+ laser and a tunable rhodamine 6G dye laser. Using the theory of Albrecht and Hutley, we have calculated frequency factors for the intensity variations for several symmetric vibrational modes of each molecule. The observed spectra for TCNQ and LiTCNQ with violet, blue, and green excitation give evidence for B-type resonance enhancement due to vibronic mixing between at least two violet and ultraviolet transitions. The Raman spectra for LiTCNQ with yellow, orange, and red excitation show A-type enhancement due to a single electronic excitation in the near infrared. 相似文献
883.
The α,β-unsaturated γ,δ-epoxyketone 7 is isomerized almost exclusively to the δ-diketone 9 both upon irradiation in the n → π* absorption band with light of wavelengths above 310 nm (in anhydrous dioxane or benzene solutions) and upon triplet sensitization using acetophenone in benzene. The reaction may be formulated by the cleavage of the Cγ? O oxide bond and the shift of the δ-hydrogen to the γ-position, and thus bears a formal “double bond homology” to the photochemical α,β-epoxyketone rearrangement. Excitation in the π → π* absorption band of 7 with light of wavelength 253,7 nm (in anhydrous dioxane solution) leads to the formation of product 10 as well as to the triplet rearrangement to 9 . With this result a novel partial synthesis of O-acetyl-B-nortestosterone has been accomplished, which has the advantages of fewer steps and higher product yield ( 7 → 10 : ~30% yield) than previously published syntheses. On the basis of the presently available experiments, the mechanism of the transformation 7 → 10 , which constitutes one of the still few examples of enone photoreactions induced selectively from the π,π* excited singlet, remains unknown. 相似文献
884.
V. E. Kuzmin N. S. Novikova T. A. Sidelnikova L. P. Trigub 《Journal of Structural Chemistry》1994,35(4):471-477
The use of topological indices to reveal the relationship between molecular structure and mesomorphous properties is discussed.
Mathematical models for classification of mesophase types are proposed. The effect of molecular structure and attractive ability
on phase transition temperatures is described. Examples of using correlation equations to predict the properties of 2,5-disubstituted
1,3-dioxanes are given.
A. V. Bogatskii Physicochemical Institute, Ukrainian Academy of Sciences. Translated fromZhurnal Struktumoi Khimii, Vol. 35, No. 4, pp. 53–60, July–August, 1994.
Translated by L. Smolina 相似文献
885.
L. N. Koikov E. A. Lisitsa N. A. Alekseeva K. F. Turchin T. Ya. Filipenko 《Chemistry of Heterocyclic Compounds》1992,28(11):1289-1292
3-Ethoxycarbonylquinuclidine obtained by the Grob method is a mixture of 3-ethoxycarbonyl-1-azabicylo-[2.2.2]- and, according to 13C NMR data, 5-ethoxycarbonyl-5-azatricyclo[3.2.1.02,7]octanes (1621). 3-Ethoxy-carbonylquinuclidine was purified by recrystallization of the hydrochloride, hydrolyzed by water to 3-quinuclidinecarboxylic acid, and reduced by LiAlH4 to 3-quinuclidinylmethanol.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, 1509–1512, November, 1992. 相似文献
886.
L. S. Sologub S. D. Moshchitskii T. Ya. Lavrenyuk 《Chemistry of Heterocyclic Compounds》1975,11(1):65-67
The reaction of excess primary aliphatic amines and mercaptans with 2-methyl-3,5,6-trichloro-4-pyridyl vinyl sulfone gives products of addition to the double bond. 相似文献
887.
N. N. Khimich Yu. L. Zub L. A. Koptelova T. S. Mashchenko E. P. Troshina M. G. Voronkov 《Russian Journal of Applied Chemistry》2006,79(11):1769-1774
Sol-gel process of hydrolytic polycondensation of tetramethoxysilane with trimethoxysilyl derivatives of lanthanide phosphoramide
complexes was studied in order to obtain photoluminescent organic-inorganic hybrid materials.
Original Russian Text N.N. Khimich, Yu.L. Zub, L.A. Koptelova, T.S. Mashchenko, E.P. Troshina, M.G. Voronkov, 2006, published
in Zhurnal Prikladnoi Khimii, 2006, Vol. 79, No. 11, pp. 1789–1794. 相似文献
888.
The cyclization of 2-(1-alkynyl)-2-alken-1-ones 1 proceeded very smoothly in the presence of alcohols 2 with a catalytic amount of Cu(I)Br in DMF at 80 degrees C, leading to the formation of highly substituted furans 3. The catalytic system reported herein is easy to handle, compared to the previously known system wherein the reaction between 1 and 2 needed to use moisture sensitive gold(III) chloride. 相似文献
889.
The acceleration of the electrodeposition and anodic dissolution of copper in acid sulfate solutions, caused by sodium pyrophosphate, is found to be due to a chemical heterogeneous stage, which involves an adsorbed pyrophosphate ion (probably, H2P2O7
2–). The assumption about the presence of a surface complex on the surface of copper is substantiated. As follows from adsorption isotherms, obtained for strongly acid (to 2 M H2SO4) solutions from anodic Tafel plots, the effect of the acid concentration on the rate of the discharge–ionization of copper is caused by its effect on the surface coverage by the adsorbate. Measurements of the quasi-equilibrium potential of the freshly renewed surface of copper confirm the assumption about the inhibiting of the surface diffusion of adsorbed atoms by adsorbed pyrophosphate. 相似文献
890.
T. I. Vakul'skaya L. I. Larina O. B. Nefedova V. A. Lopyrev 《Chemistry of Heterocyclic Compounds》1982,18(4):400-404
The results of an EPR investigation are presented with polarographs of electrochemical excitation of nitropyrazoles in acetonitrile. It is shown that 3- and 4-nitropyrazoles are excited in two single-electron stages with formation of dianion radicals. Upon excitation, N-alkylnitropyrazoles form stable anion radicals. 1-Nitro- and 1,4-dinitropyrazoles are excited upon splitting off of the NO2 anion. Excitation potentials of the nitropyrazoles and hyperfine interaction constants for the corresponding ion radicals are given.See [1] for Communication 2.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 523–527, April, 1982. 相似文献