全文获取类型
收费全文 | 24589篇 |
免费 | 2380篇 |
国内免费 | 5168篇 |
专业分类
化学 | 26512篇 |
晶体学 | 151篇 |
力学 | 397篇 |
综合类 | 142篇 |
数学 | 1625篇 |
物理学 | 3310篇 |
出版年
2024年 | 52篇 |
2023年 | 378篇 |
2022年 | 639篇 |
2021年 | 840篇 |
2020年 | 1221篇 |
2019年 | 1070篇 |
2018年 | 1079篇 |
2017年 | 1012篇 |
2016年 | 1083篇 |
2015年 | 1014篇 |
2014年 | 1563篇 |
2013年 | 2415篇 |
2012年 | 1336篇 |
2011年 | 1674篇 |
2010年 | 1319篇 |
2009年 | 1448篇 |
2008年 | 1523篇 |
2007年 | 1628篇 |
2006年 | 1497篇 |
2005年 | 1418篇 |
2004年 | 1373篇 |
2003年 | 1034篇 |
2002年 | 660篇 |
2001年 | 530篇 |
2000年 | 541篇 |
1999年 | 461篇 |
1998年 | 416篇 |
1997年 | 372篇 |
1996年 | 332篇 |
1995年 | 321篇 |
1994年 | 272篇 |
1993年 | 209篇 |
1992年 | 206篇 |
1991年 | 145篇 |
1990年 | 125篇 |
1989年 | 117篇 |
1988年 | 90篇 |
1987年 | 57篇 |
1986年 | 52篇 |
1985年 | 67篇 |
1984年 | 57篇 |
1983年 | 33篇 |
1982年 | 53篇 |
1981年 | 62篇 |
1980年 | 49篇 |
1979年 | 39篇 |
1978年 | 38篇 |
1977年 | 42篇 |
1976年 | 52篇 |
1974年 | 42篇 |
排序方式: 共有10000条查询结果,搜索用时 390 毫秒
991.
992.
993.
The Mitsunobu reaction is a widely used and versatile method for the dehydrative oxidation–reduction condensation of an acid/pronucleophile usually with a primary or secondary alcohol that requires the combination of a reducing phosphine reagent together with an oxidizing azo reagent. The utility of this reaction stems from the fact that it is generally highly stereoselective and occurs with inversion of the stereochemical configuration of the alcohol starting material. Furthermore, as carboxylic acids, phenols, imides, sulfonamides, and other compounds can be used as the acid/pronucleophile, this reaction is useful for the preparation of a wide variety of functional groups. This Focus Review of the Mitsunobu reaction summarizes its origins, the current understanding of its mechanism, and recent improvements and applications. 相似文献
994.
Suzuki-Miyaura coupling was achieved on a variety of aryl chlorides by using di(2,6-dimethylmorpholino)phenylphosphine (L1) as a bulky electron-rich monoaryl phosphine ligand. We report the couplings of various chlorobenzenes and heteroaryl chlorides. 相似文献
995.
A palladium bipyridyl complex anchored onto nanosized mesoporous silica MCM-41 catalyzed the cross-coupling of aryl iodides or bromides with Grignard reagents to provide the corresponding biaryls in high yields. The reaction proceeded smoothly with an equal molar amount of substrate and Grignard reagent in the presence of 0.2-0.02 mol % of catalyst in THF at 50 °C or under refluxing conditions. The catalyst prepared may be used in a very low percentage, recovered after reaction, and re-used. 相似文献
996.
Stefan Motiu Daniela Dogaru Valentin Gogonea 《International journal of quantum chemistry》2007,107(5):1248-1252
This work puts forth a reaction pathway for the reactivation of exogenous ligand inhibited H‐cluster, the active site of Fe‐only hydrogenases. The H‐cluster is a dimetal complex, Fe–Fe, with the metal centers bridged by di(thiomethyl)amine. Exogenous ligands, H2O, and OH?, are bound to the distal iron (Fed). Density functional theory (DFT) calculations on the native and ruthenium‐modified H‐cluster have been performed using the B3LYP functional with 6‐31+G** and 6‐311+G** basis sets. We have ascertained that there is a thermodynamically favorable pathway for the reactivation of the OH? inhibited H‐cluster, which proceeds by an initial protonation of the Fed–OH? complex. The proposed reaction pathway has all its intermediate reactions ensue exothermically. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 相似文献
997.
The mechanism of a cycloaddition reaction between singlet dichloromethylene germylene and ethylene has been investigated with B3LYP/6-31G* method, including geometry optimization and vibrational analysis for the involved stationary points on the potential energy surface. Energies for the involved conformations were calculated by CCSD(T)//B3LYP/6-31G* method. On the basis of the surface energy profile obtained with CCSD(T)// B3LYP/6-31G* method for the cycloaddition reaction between singlet dichloromethylene germylene and ethylene, it can be predicted that the dominant reaction pathway is that an intermediate INT1 is firstly formed between the two reactants through a barrier-free exothermic reaction of 61.7 kJ/mol, and the intermediate INT1 then isomerizes to an active four-membered ring product P2.1 via a transition state TS2, an intermediate INT2 and a transition state TS2.1, in which energy barriers are 57.7 and 42.2 kJ/mol, respectively. 相似文献
998.
J. Horn H. -Ch. Semmelhack H. Börner F. Schlenkrich 《Journal of Thermal Analysis and Calorimetry》1994,41(5):995-1005
We propose a reaction model for the synthesis of YBa2Cu4O8 under normal pressure conditions, which contains 4 partial reaction steps. In a first step bariumnitrate and copperoxide react to Ba2Cu3O5+δ. This substance will be formed for each mixtures Ba:Cu=2∶3...3∶2. The following two partial reaction steps are connected to Ba2Cu3O5+δ, which reacts with Y2O3 and CuO to YBa2Cu4O8 or decomposes to BaCuO2 and CuO. In a last step parts of BaCuO2 reacts with Y2O3 and CuO to YBa2Cu4O8. 相似文献
999.
Hetero-biaryl compounds were prepared via the Suzuki-Miyaura coupling reaction of hetero-aryl moieties containing an unprotected NH2 group and arylboronic acids. D-t-BPF was found to be an efficient ligand for the cross-coupling of NH2-unprotected hetero-aryl chlorides with phenylboronic acid. 相似文献
1000.
cis,cis,cis-1,2,3,4-Tetrakis(diphenylphosphinomethyl)cyclopentane/[PdCl(C3H5)]2 efficiently catalyses the Heck reaction of alk-1-en-3-ol with a variety of aryl bromides. In the presence of hex-1-en-3-ol or oct-1-en-3-ol, the β-arylated carbonyl compounds were selectively obtained. Linalool and 2-methylbut-3-en-2-ol led to the corresponding 1-arylalk-1-en-3-ol derivatives. Turnover numbers up to 69,000 can be obtained for this reaction. A minor electronic effect of the substituents of the aryl bromide was observed. Similar reaction rates were observed in the presence of activated aryl bromides such as bromoacetophenone and deactivated aryl bromides such as bromoanisole. 相似文献