全文获取类型
收费全文 | 100192篇 |
免费 | 26428篇 |
国内免费 | 21692篇 |
专业分类
化学 | 65947篇 |
晶体学 | 1264篇 |
力学 | 6633篇 |
综合类 | 377篇 |
数学 | 13007篇 |
物理学 | 61084篇 |
出版年
2024年 | 507篇 |
2023年 | 948篇 |
2022年 | 1272篇 |
2021年 | 1270篇 |
2020年 | 1573篇 |
2019年 | 2103篇 |
2018年 | 2019篇 |
2017年 | 2586篇 |
2016年 | 3100篇 |
2015年 | 3331篇 |
2014年 | 3435篇 |
2013年 | 5471篇 |
2012年 | 5974篇 |
2011年 | 8885篇 |
2010年 | 11224篇 |
2009年 | 11487篇 |
2008年 | 5018篇 |
2007年 | 4282篇 |
2006年 | 3838篇 |
2005年 | 4091篇 |
2004年 | 5128篇 |
2003年 | 4007篇 |
2002年 | 3981篇 |
2001年 | 4054篇 |
2000年 | 3032篇 |
1999年 | 3065篇 |
1998年 | 2579篇 |
1997年 | 2256篇 |
1996年 | 2628篇 |
1995年 | 3021篇 |
1994年 | 2997篇 |
1993年 | 3213篇 |
1992年 | 2815篇 |
1991年 | 2411篇 |
1990年 | 2035篇 |
1989年 | 2126篇 |
1988年 | 1938篇 |
1987年 | 1192篇 |
1986年 | 1249篇 |
1985年 | 899篇 |
1984年 | 1010篇 |
1982年 | 912篇 |
1981年 | 749篇 |
1980年 | 777篇 |
1979年 | 540篇 |
1978年 | 540篇 |
1977年 | 639篇 |
1976年 | 1045篇 |
1973年 | 438篇 |
1972年 | 537篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
961.
A lipopolysaccharide (LPS) has been isolated by phenol-water extraction from the cells of the blue-green algaMastigocladus laminosus. It has been shown that the LPS contains polysaccharide and lipid components. The polysaccharide component includes a rhamnan fragment constructed of -1,3- and, possibly, -1,2-bound L-rhamnose residues. The lipid component is constructed of glucosamine, glucose, and fatty acid residues, among which palmitic acid predominates.Pacific Ocean Institute of Bioorganic Chemistry, Far Eastern Scientific Center, Academy of Sciences of the USSR, Vladivostok. Translated from Khimiya Prirodnykh Soedinenii, No. 6, pp. 700–703, November–December, 1984. 相似文献
962.
O. Yu. Rekhlova B. Tashkhodzhaev V. I. Andrianov A. I. Saidkhodzhaev G. V. Sagitdinova N. G. Furmanova 《Chemistry of Natural Compounds》1991,26(6):643-647
The spatial structure of the sesquiterpene humulane alcohol juniferol, which is represented in the crystal by two conformers, IA and IB, differing by the conformation of the C2...C9 section, has been established by the x-ray structural method. The strain energies of the conformers have been calculated by the method of molecular mechanics. The barrier to the interconversion of the conformers by rotation of the C2...C9 section has been calculated; it amounts to 7–8 kcal/mole. This value of the transformation barrier permits the assumption that the two conformers also exist in solution, their interconversion being hindered.Institute of Chemistry of Plant Substances, Academy of Sciences of the Uzbek SSR, Tashkent. Institute of Crystallography, Academy of Sciences of the USSR, Moscow. Translated from Khimiya Prirodnykh Soedinenii, No. 6, pp. 757–762, November–December, 1990. 相似文献
963.
V. I. Akhmedzhanova I. A. Bessonova S. Yu. Yunusov 《Chemistry of Natural Compounds》1977,12(3):282-288
Summary 1. From the epigeal part of the plantH. perforatum growing in the Dzhungarian Ala-Tau we have isolated the lignane eudesmin, the known alkaloids evoxine, flindersine, and 7-isopentenyloxy--fagarine, and the new alkaloids haplamine, glycoperine, and methylevoxine. It has been shown that haplamine has the structure of 6-methoxyflindersine, glycoperine the structure of 7-hydroxy-4,8-dimethoxyfuranoquinoline 7-O--L-rhamnopyranoside, and methylevoxine the structure of 4,8-dimethoxy-7-(2-hydroxy-3-methoxy-3-methylbutoxy)furanoquinoline.Institute of the Chemistry of Plant Substances, Academy of Sciences of the Uzbek SSR, Tashkent. Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 320–328, May–June, 1976. 相似文献
964.
N. Batbayar D. Batsurén B. Tashkhodhaev V. I. Akhmedzhanova I. A. Bessonova M. R. Yagudaev 《Chemistry of Natural Compounds》1988,23(4):466-468
The spatial structure of the alkaloid kalidine obtained from the epigeal part ofKalidium gracile Fenzl. has been determined by IR and NMR spectroscopy and x-ray structural analysis. It has been established that it is the chloride of the trimethyl derivative of glycinamide.Institute of the Chemistry of Plant Substances of the Uzbek SSR Academy of Sciences, Tashkent. Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 558–561, July–August, 1987. 相似文献
965.
Monoalkylketene acetals and monoarylketene acetals react with diphenylketene to give acylketene acetals (VII). These compounds can further react with diphenylketene to give 2,4-pyronone 4-acetals (IX), which, by mild acid hydrolysis, yields 2,4-pyronones (X).
Reaction of methylketene diethylacetal with two equivalents of diphenylketene yields 3,3-di-phenyl-5-methyl-6-diphenylmethylentetrahydropyran-2,4-dione 4-diethylacetal (XIV) in addition to small quantities of the pyronone acetal IXb. 相似文献
966.
Summary A study has been made of the separation of alkali and alkaline earth metals. With mixtures of solvents, such as ethyl cellosolve, water and hydrochloric acid and acetone, water and hydrochloric acid in the ratio of 702010 and at different temperatures, a complete separation of alkaline earth metals was effected. Of the alkali metals only lithium, sodium and potassium could be separated from each other but rubidium and caesium always accompanied potassium. 相似文献
967.
Summary Procedures have been described for the quantitative determination of thiourea and its methyl derivatives, wherein iodate, permanganate, and ceric sulphate have been used as direct titration solution, either following preoxidation by alkali hypoiodite via (ICI + KOH) or in case of thiourea, by iodine bromide method. Apart from using standard oxidants the methods have an added advantage of a high conversion factor due to oxidation of sulphur to sulphate. Alternative physical methods of detecting the end point is an added feature of these procedures.The authors wish to express their grateful thanks to Dr. S. S. Joshi, D. Sc. (London), for his kind interest in the work. 相似文献
968.
The Cu complex Cu(L)nCl (L=2-propylamine) has been found to be an advantageous catalyst for the addition of polyhalogenated compounds to chloro substituted ethenes; the 11 adducts were formed almost exclusively in up to 71% yield.
, , Cu(L)nCl (L=2-), ; 71% 11.相似文献
969.
Iodide ions—in a rather narrow concentration range—induce some of the uncatalyzed bromate oscillators to a more pronounced oscillatory activity: the number of oscillations increases considerably and the overall rate of the chemical reaction drops about an order of magnitude. Preliminary suggestion is made on the mechanism of iodide-induced oscillatory reactions.
— — : . - .相似文献
970.
S. F. Bureiko N. S. Golubev G. S. Denisov I. Ya. Lange 《Reaction Kinetics and Catalysis Letters》1977,7(2):139-144
The kinetic isotope effect in proton exchange between methanol and water in CCl4 solution was studied by1H and2H NMR and kinetic IR spectroscopy. The exchange rate increases in the series CD3OH+H2O3OD+H2O3OD+D2O. It has been concluded that the limiting step of the reaction involves the formation of an H-bonded cyclic intermediate.
1H 2H - CCl4. , CD3OH+H2O相似文献3OD+H2O 3OD+D2O. , - -.