全文获取类型
收费全文 | 28819篇 |
免费 | 1813篇 |
国内免费 | 4390篇 |
专业分类
化学 | 28861篇 |
晶体学 | 199篇 |
力学 | 367篇 |
综合类 | 155篇 |
数学 | 545篇 |
物理学 | 4895篇 |
出版年
2025年 | 45篇 |
2024年 | 378篇 |
2023年 | 463篇 |
2022年 | 932篇 |
2021年 | 1152篇 |
2020年 | 1598篇 |
2019年 | 1448篇 |
2018年 | 1178篇 |
2017年 | 1093篇 |
2016年 | 1122篇 |
2015年 | 1056篇 |
2014年 | 1563篇 |
2013年 | 2516篇 |
2012年 | 1391篇 |
2011年 | 1716篇 |
2010年 | 1275篇 |
2009年 | 1422篇 |
2008年 | 1520篇 |
2007年 | 1640篇 |
2006年 | 1568篇 |
2005年 | 1436篇 |
2004年 | 1393篇 |
2003年 | 1092篇 |
2002年 | 774篇 |
2001年 | 662篇 |
2000年 | 640篇 |
1999年 | 539篇 |
1998年 | 470篇 |
1997年 | 396篇 |
1996年 | 342篇 |
1995年 | 345篇 |
1994年 | 283篇 |
1993年 | 218篇 |
1992年 | 216篇 |
1991年 | 155篇 |
1990年 | 121篇 |
1989年 | 118篇 |
1988年 | 94篇 |
1987年 | 62篇 |
1986年 | 54篇 |
1985年 | 66篇 |
1984年 | 63篇 |
1983年 | 31篇 |
1982年 | 44篇 |
1981年 | 40篇 |
1979年 | 30篇 |
1978年 | 35篇 |
1977年 | 36篇 |
1976年 | 45篇 |
1974年 | 41篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
K. C. Kumara Swamy E. Balaraman M. Phani Pavan N. N. Bhuvan Kumar K. Praveen Kumar N. Satish Kumar 《Journal of Chemical Sciences》2006,118(6):495-501
The diversity of products in the reaction of diethyl azodicarboxylate (DEAD)/diisopropyl azodicarboxylate (DIAD) and activated
acetylenes with PIII compounds bearing oxygen or nitrogen substituents is discussed. New findings that are useful in understanding the nature
of intermediates involved in the Mitsunobu reaction are highlighted. X-ray structures of two new compounds (2-t-Bu-4-MeC6H3O)P (μ-N-t-Bu)2P+[(NH-t-Bu)N[(CO2]-i-Pr)(HNCO2-i-Pr)]](Cl-)(2-t-Bu-4-MeC6H3OH)(23)and [CH2(6-t-Bu-4-Me-C6H2O)2P(O)C(CO2Me)C-(CO2Me)CClNC(O)Cl] (33) are also reported. The structure of23 is close to one of the intermediates proposed in the Mitsunobu reaction. 相似文献
2.
3.
The paper deals with a singularly perturbed reaction diffusionmodel problem. The focus is on reliable a posteriori error estimatorsfor the H1 seminorm that can be applied to anisotropic finiteelement meshes. A residual error estimator and a local problemerror estimator are proposed and rigorously analysed. They arelocally equivalent, and both bound the error reliably. Threemodifications of these estimators are introduced and discussed. Much attention is given to the performance of the error estimatorin numerical experiments. This helps to identify those estimatorsthat are suitable for practical applications. 相似文献
4.
P. Tekely V. G rardy P. Palmas D. Canet A. Retournard 《Solid state nuclear magnetic resonance》1995,4(6):361-367
A simple modification of the standard cross-polarization method designed for quenching the proton T1ρ dependence when studying polarization transfer is presented. It is demonstrated that by using this simple procedure, new and subtle details of cross-polarization dynamics, previously hidden by the T1ρ(1H) effect, can be observed in dipolar-coupled spin systems. 相似文献
5.
The synthesis and characterization of metal poly-yne polymers containing disilane, disiloxane and phosphine groups in the main chain are described. The platinum and palladium poly-yne polymers were synthesized by polycondensation reactions between a metal chloride and an α, ω-bisethynyl complex in amines in the presence of cuprous iodide as a catalyst. The nickel poly-yne polymers were synthesized by an alkynyl ligand exchange reaction between a nickel acetylide and an α, ω-bisethynyl complex in diethylamine in the presence of cuprous iodide as a catalyst. The reaction of the platinum poly-yne polymer, containing disiloxane groups in the main chain, with copper (I) salts afforded adducts of η-2-bonded σ-acetylide polymer complexes. The reactions of the palladium poly-yne polymer, containing phosphine groups in the main chain, with transition-metal carbonyl complexes afforded polymer complexes which have phosphorus in the main chain-transition-metal bonds. A concentrated solution of the platinum poly-yne polymer containing disiloxane groups in the main chain forms a lyotropic liquid crystal in dichloromethane or 1, 2-dichloroethane. 相似文献
6.
The solid state reaction of 3-methyl-1-phenyl-5-pyrazolone (MPP) with aromatic aldehydes and ke-tones benzil derivatives and imides,and the solid state Michael addition reaction of MPP with 4-arylidene-3-methyl-1-phenyl-5-pyrnzolone 2 were investigated.Some new solid state reactions between the reactants were found,from which a series of new compounds were obtained The structures of the products were identified by IR,1H NMR,MS,elemental analyses and also by X-ray crystal analysis,and the reaction mechanism of MPP with aromatic aldehydes and ketones was proposed 相似文献
7.
The first MCM-41-supported bidentate phosphine palladium(0) complex has been prepared. This complex is a highly efficient catalyst for Sonogashira reaction and can be reused at least 10 times without any decrease in activity. 相似文献
8.
A new technique of searching the conformational space of transition states was used to explore the cause of the rate acceleration in the gem-dialkyl effect in intramolecular cyclization reactions. Several previous hypotheses were discarded and a new hypothesis was advanced based on this new data. This hypothesis, the “Facilitated Transition” hypothesis, states that increased steric hindrance reduces the overall activation energy by facilitating rotation through the transition state. The older “Reactive Rotamer” hypothesis was eliminated by generating all conformations of the starting materials using the
and
computer programs and demonstrating that no relationship is found between rate increase and a change in the concentration of “reactive rotamers”. 相似文献
9.
V. Z. Shirinian M. M. Krayushkin D. M. Nikalin A. A. Shimkin 《Russian Chemical Bulletin》2005,54(3):738-742
The reaction of alkyl 4-(2-acetylhydrazino)-2-methyl-3-thiophenecarboxylates with 3-methylbutan-2-one was carried out under the Fischer reaction conditions. The influence of Lewis acids and solvents on the process was studied. A convenient method for the synthesis of thieno [3.2-b]pyrrolenine derivatives was proposed.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 725–729, March, 2005. 相似文献
10.
An increasing number of biologically active compounds in the pharma and agro-chemical sector contain carbon fluorine bonds. One of the most common methods to introduce fluorine into intermediates is the well-investigated halogen-exchange reaction, in which chloro- and bromoaromatics activated towards nucleophilic substitution, react with a fluoride source to yield the corresponding fluoroarenes. In general, the reaction is supported by phase-transfer catalysts. The use of a new class of very active phase-transfer catalysts gives the possibility of substituting even halogens with weak activation giving a convenient access to interesting compounds that are not available so far and opening up new synthetic routes in Halex chemistry. Our new classes of catalysts, CNC+ (1a), PNC+ (2a) and several different approaches presented by other groups are described and experimental results discussed. 相似文献