全文获取类型
收费全文 | 10483篇 |
免费 | 713篇 |
国内免费 | 576篇 |
专业分类
化学 | 8032篇 |
晶体学 | 34篇 |
力学 | 224篇 |
综合类 | 61篇 |
数学 | 1794篇 |
物理学 | 1627篇 |
出版年
2023年 | 76篇 |
2022年 | 113篇 |
2021年 | 165篇 |
2020年 | 189篇 |
2019年 | 262篇 |
2018年 | 206篇 |
2017年 | 227篇 |
2016年 | 276篇 |
2015年 | 240篇 |
2014年 | 426篇 |
2013年 | 665篇 |
2012年 | 453篇 |
2011年 | 659篇 |
2010年 | 577篇 |
2009年 | 738篇 |
2008年 | 664篇 |
2007年 | 613篇 |
2006年 | 563篇 |
2005年 | 415篇 |
2004年 | 353篇 |
2003年 | 335篇 |
2002年 | 269篇 |
2001年 | 156篇 |
2000年 | 155篇 |
1999年 | 133篇 |
1998年 | 113篇 |
1997年 | 91篇 |
1996年 | 93篇 |
1995年 | 67篇 |
1994年 | 66篇 |
1993年 | 56篇 |
1992年 | 58篇 |
1991年 | 46篇 |
1990年 | 28篇 |
1989年 | 26篇 |
1988年 | 22篇 |
1986年 | 17篇 |
1985年 | 417篇 |
1984年 | 283篇 |
1983年 | 238篇 |
1982年 | 213篇 |
1981年 | 152篇 |
1980年 | 117篇 |
1979年 | 131篇 |
1978年 | 75篇 |
1977年 | 86篇 |
1976年 | 87篇 |
1975年 | 89篇 |
1974年 | 115篇 |
1973年 | 130篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
101.
A new reaction of organolithium compounds with phosphonium salts is described; reaction of t-butyllithium with dibenzylaminophosphonium or tetraphenylphosphonium bromides takes place through addition of the t-butyl group on a benzene ring at the position para to the phosphorus atom, the ylid formed reacts in a characteristic Wittig reaction with p-tolylaldehyde. 相似文献
102.
The 17-electron species [M(CO)5χLχ] (M Mn, Re, χ 0; M Mn, Re; L Ph3P, χ 1, 2; M Mn, Re; L (o-MeC6H4O)3P, χ 2; M Mn; L (p-ClC6H4O)3P, (PhO)3P, χ 2; M Mn; L P(OMe)3, χ 3) have been generated by one electron oxidation of the corresponding anions and show typical radical reactivity, undergoing dimerisation or hydride abstraction in reactions controlled by steric effects. Evidence is presented for the source of the hydrogen atom. The 19-electron species [M(CO)3(η7-C7H7)]? (M Cr, Mo) and [Fe(CO)3(η5-C6H7)]?, generated by reduction of the corresponding cations, undergo dimerisation at the organic ligand. Similar treatment of [Fe(CO)2-L(η-cp)]+ (L CO, PPh3, P(OPh)3, Me2CO) yields [Fe2(CO)4(η-cp)2] and these reduction reactions are rationalised in terms of the nature of the HOMO in the intermediate radical. Similar reduction of [Rh(diphos)2]+ yield the 17-electron intermediate [Rh(diphos)2] and this also undergoes hydrogen abstraction. 相似文献
103.
Antonio García Martínez Santiago de la Moya Cerero Beatriz Lora Maroto 《Tetrahedron》2005,61(12):3055-3064
An empiric first approach to the knowledge about the structural factors influencing the catalytic behavior of conformationally flexible δ-amino-alcohol-based ligands, for the enantioselective addition of dialkylzincs to prochiral carbonyl groups, has been applied using the 1-(2-aminoethyl)norbornan-2-ol moiety as the model chiral system, and the asymmetrically catalyzed addition of diethylzinc to benzaldehyde as the test reaction. For this purpose, a selected small library of seven norbornane-based chiral ligands, bearing well-defined structural variations to allow a comparative study, that is, variation of the relative configuration and steric hindrance at the C(2), C(3) and/or C(7) norbornane positions, has been synthesized and probed in the mentioned test reaction. The experimental results obtained have been rationalized empirically using diastereomeric Noyori-like transition states, demonstrating that the conformational flexibility of the δ-amino-alcohol ligands, contrary to the more studied and rigid β-amino-alcohols, plays a crucial role on the catalytic behavior of such ligands (stereochemical sense and degree of the stereodifferentiation in the asymmetric process), which makes such structural factors, important for the improved design of new related chiral catalysts. In this sense, a robust crude empirical model for the prediction of the catalytic behavior of such δ-amino-alcohol-based ligands is proposed. 相似文献
104.
A. M. Churakov A. Yu. Tyurin E. L. Goncharova S. L. Ioffe Yu. A. Strelenko V. A. Tartakovsky 《Russian Chemical Bulletin》1995,44(5):890-896
The addition of 1-aryl-2-bromodiazene 1-oxides to olefins yields 1-aryl-2-(2-bromoalkyl)diazene 1-oxides (4). A radical mechanism of the reaction has been suggested. Compounds4 decompose to give bromohydrazones of formaldehyde and aldehydes. The structural factors that affect the rate of this process are discussed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 917–923, May, 1995.The work was carried out in the Scientific Educational Center of the Institute of Organic Chemistry of the RAS with financial support of the Russian Foundation for Basic Research (Project No. 93-03-18461). 相似文献
105.
3,4-Dihydro-5(1H)-pyrromethenones are easier attacked at themeso-position by electrophiles than 5(1H)-pyrromethenones. This is demonstrated both by aMannich-type-substitution or deuterium-exchange-experiments and by the addition of O-, S-, and N-Nucleophiles to the exocyclic double bond of the model-dihydropyrromethenone (Z)-1 under very mild reaction conditions. Applying these results to the chemistry of 2,3-dihydro-bilatrienes-abc, their chemical characteristics—especially their tautomeric behavior and their dominant C-5-selectivity towards electrophiles—become better understandable. 相似文献
106.
叠加法是由内标法衍生而来的一种气相色谱定量方法。对于较复杂混合物的分析,当选择内标物困难时,用叠加法定量更为方便。本文在大量实验的基础上,提出了叠加法定量过程中应注意的一些问题。 相似文献
107.
trans-PtH2[As(t-Bu)3]2 was prepared in very good yield by afacile reaction of K2PtC14 with As(t-Bu)3 in alkaline ethanol. Treatment of trans-PtH2[As(t-Bu)3]2 with CF3CO2H or HCI afforded trans-PtH(O2CCF3)[As(t-Bu)3]2 or trans-PtHCl[As(t-Bu)3]2. respectively, in almost quantitative yield. 相似文献
108.
The coalescence temperature for the C-methyl peaks in the NMR spectra of hydrazones of acetone are above 200°, indicating an energy barrier for syn-anti isomerization greater than 28 kcal mole?1. the presence of halogenated compounds decreases the energy barrier, due to the formation of a charge transfer complex between the hydrazone acting as a donor and the halogenated compound acting as the electron acceptor. 相似文献
109.
The reaction of tert-butylamine, isoxazolidine and hydrazine with N-[(3-chloropropyl)oxy]phthalimide is investigated. A convenient synthesis of oxazolidine is described. 相似文献
110.
The phase diagram for the binary system K2CrO4CaCrO4 has been determined for CaCrO4 concentrations up to 60 mole%, using the techniques of differential thermal analysis, X-ray diffraction, and drop calorimetry. Essential features of the phase diagram are: the solid-solid phase transition for pure K2CrO4 at 670°C, β-K2CrO4 ? α-K2CrO4; a eutectoid reaction at 14 mole% CaCrO4 and 548°C, β-K2CrO4 ? α-K2CrO4 + K2CrO4 · CaCrO4; a peritectoid event at 50 mole% CaCrO4 and 640°C, β-K2CrO4 + CaCrO4 ? K2CrO4 · CaCrO4; and a eutectic reaction at 51 mole% CaCrO4 and 678°C, L ? β-K2CrO4 + CaCrO4. X-ray diffraction studies lead to the determination of the unit cell dimensions for the K2CrO4 · CaCrO4 double salt, a C-centered monoclinic form with a0 = 7.615(6) Å, b0 = 22.797(15) Å, c0 = 9.777(9) Å, β = 115.45(5)°. 相似文献