全文获取类型
收费全文 | 290862篇 |
免费 | 1882篇 |
国内免费 | 1000篇 |
专业分类
化学 | 140951篇 |
晶体学 | 4016篇 |
力学 | 14953篇 |
综合类 | 6篇 |
数学 | 54508篇 |
物理学 | 79310篇 |
出版年
2020年 | 1970篇 |
2019年 | 2264篇 |
2018年 | 13389篇 |
2017年 | 13158篇 |
2016年 | 10186篇 |
2015年 | 3181篇 |
2014年 | 4085篇 |
2013年 | 9529篇 |
2012年 | 10523篇 |
2011年 | 18603篇 |
2010年 | 11530篇 |
2009年 | 11787篇 |
2008年 | 14499篇 |
2007年 | 16663篇 |
2006年 | 7805篇 |
2005年 | 8159篇 |
2004年 | 7624篇 |
2003年 | 7535篇 |
2002年 | 6466篇 |
2001年 | 6165篇 |
2000年 | 4947篇 |
1999年 | 3595篇 |
1998年 | 3178篇 |
1997年 | 3168篇 |
1996年 | 3004篇 |
1995年 | 2432篇 |
1994年 | 2492篇 |
1993年 | 2541篇 |
1992年 | 2644篇 |
1991年 | 2718篇 |
1990年 | 2608篇 |
1989年 | 2591篇 |
1988年 | 2519篇 |
1987年 | 2490篇 |
1986年 | 2413篇 |
1985年 | 3082篇 |
1984年 | 3236篇 |
1983年 | 2595篇 |
1982年 | 2866篇 |
1981年 | 2740篇 |
1980年 | 2547篇 |
1979年 | 2761篇 |
1978年 | 2866篇 |
1977年 | 2874篇 |
1976年 | 2847篇 |
1975年 | 2703篇 |
1974年 | 2676篇 |
1973年 | 2835篇 |
1972年 | 1892篇 |
1967年 | 1840篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
971.
The reaction between ethyl o-fluorobenzoylacetate and cyclic imino ethers is described. The products, the corresponding 1,2-fused quinolines ( 13a-17a ), were isolated in good yields. In one instance the uncyclized condensation intermediate 18 was isolated and characterized. 相似文献
972.
973.
Diazadiphosphetidines. VIII. Further Diazadiphosphetidine Oxides and an Oxide Sulfide Synthesis and properties of 1,3-dimethyl-2-phenyl-4-diethylamino-1,3,2,4-diazadiphosphetidine-2-oxide ( II ), 1,3-dimethyl-2-diethylamino-4-phenyl-1,3,2,4-diazadiphosphetidine-2-sulfide-4-oxide ( III ), and 1,3-dimethyl-2,4-diphenyl-1,3,2,4-diazadiphosphetidine-2,4-dioxide ( V ) are reported. 相似文献
974.
Immobilized sulfhydryl groups were prepared by partial thiolation of NH2-glass beads. The microenvironment of the immobilized SH groups was varied by different chemical modifications of neighboring
NH2 groups. Introduction of a strong charge in the surroundings of immobilized sulfhydryls results in their dramatic stabilization
against autooxidation. This effect is due to the salting of O2 from the surface microlayer of the thiolated beads. 相似文献
975.
Heating a toluene solution of dicarhomethoxyacetylenebis(triphenylphosphine)platinum(0) at 130°C gives the ortho-metalated complex (Ph3P)(Ph2t-trans-(COOMe)CCHCOOMe. 相似文献
976.
M. E. Perel'son A. A. Kir'yanov Yu. E. Sklyar V. V. Vandyshev 《Chemistry of Natural Compounds》1973,6(6):696-698
Summary A spectrophotometric method is proposed for determining the content of aspidin in a crystalline powder and in the rhizomes of the fernsDryopteris spinulosa (M),D. phegopteris (L.), andD. fragrans (L.) Schott.All-Union Scientific-Research Institute for Medicinal Plants. Translated from Khimiya Prirodnykh Soedinenii, No. 6, pp. 684–687, November–December, 1970. 相似文献
977.
Thermal cis, trans geometrical isomerization theoretically involves a 90° twisted, singlet diradical-like transition state which may serve as a base for the examination of structural perturbations. Although thermal rearrangement of hexa-1, trans-3,5-triene (and all-trans octa-2,4,6-triene) to the cis isomer cannot be followed directly owing to subsequent cyclization and 1,5 hydrogen shifts, activation parameters for disappearance have been determined. Experimental complications and mechanistic uncertainties which make interpretation difficult are removed in the bicyclic hexatriene, cyclopentenylidenecyclopentene. These geometrical isomers undergo uncomplicated thermal cis, trans isomerization in vessels of lead-potash glass: log k1 = 12·03±0·32?41·7±0·8/(0·004575Tabs). Extraction of a value for allylic delocalization energy from the behavior of hexa-1, trans-3,5-triene [log k1 = 12·91 ± 0·47?44·3±1·2/(0·004575 Tabs)] requires corrections of the Dewar-Schmeising type for changes in hybridization of the σ bonds. Depending on whether ethylene or trans-butene is taken as standard, values of 12·2 and 13·1 kcal/mol are obtained (estimated uncertainty ± 2 kcal/mol). 相似文献
978.
Syed Laik Ali 《Analytical and bioanalytical chemistry》1973,262(4):278-282
The different aspects of Vitamin D3 determination and the data reported in the literature are discussed. The spectrophotometric determination of Vitamin D3 in cod-liver oil is carried out after the alkaline saponification, extraction of unsaponified parts, precipitation of accompanying sterols and the column- and thin-layer chromatographic purification and separation of vitamin D3 on 40 cm plates from other vitamins. The dyestuff α-naphtholbenzein is suited well as standard substance for the better location and identification of vitamin D3 zone on the thin-layer plate. The results obtained from the chemical method were checked through the simultaneous biological determination. 相似文献
979.
980.
We show that the plasmon resonances in single metallic nanoshells and multiple concentric metallic shell particles can be understood in terms of interaction between the bare plasmon modes of the individual surfaces of the metallic shells. The interaction of these elementary plasmons results in hybridized plasmons whose energy can be tuned over a wide range of optical and infrared wavelengths. The approach can easily be generalized to more complex systems, such as dimers and small nanoparticle aggregates. 相似文献