全文获取类型
收费全文 | 2863篇 |
免费 | 132篇 |
国内免费 | 13篇 |
专业分类
化学 | 2170篇 |
晶体学 | 10篇 |
力学 | 126篇 |
数学 | 327篇 |
物理学 | 375篇 |
出版年
2023年 | 31篇 |
2022年 | 59篇 |
2021年 | 59篇 |
2020年 | 67篇 |
2019年 | 66篇 |
2018年 | 44篇 |
2017年 | 36篇 |
2016年 | 89篇 |
2015年 | 81篇 |
2014年 | 94篇 |
2013年 | 147篇 |
2012年 | 220篇 |
2011年 | 256篇 |
2010年 | 126篇 |
2009年 | 142篇 |
2008年 | 220篇 |
2007年 | 204篇 |
2006年 | 208篇 |
2005年 | 196篇 |
2004年 | 170篇 |
2003年 | 136篇 |
2002年 | 117篇 |
2001年 | 53篇 |
2000年 | 28篇 |
1999年 | 20篇 |
1998年 | 20篇 |
1997年 | 20篇 |
1996年 | 22篇 |
1995年 | 6篇 |
1994年 | 8篇 |
1993年 | 8篇 |
1992年 | 14篇 |
1991年 | 4篇 |
1990年 | 1篇 |
1989年 | 2篇 |
1988年 | 6篇 |
1987年 | 3篇 |
1986年 | 1篇 |
1985年 | 2篇 |
1984年 | 3篇 |
1982年 | 3篇 |
1981年 | 3篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1977年 | 3篇 |
1976年 | 2篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1971年 | 1篇 |
排序方式: 共有3008条查询结果,搜索用时 15 毫秒
91.
92.
Lafaye A Junot C Ramounet-Le Gall B Fritsch P Tabet JC Ezan E 《Rapid communications in mass spectrometry : RCM》2003,17(22):2541-2549
This work reports the use of reverse-phase liquid chromatography coupled to electrospray ion trap (QIT) mass spectrometry for the analysis of the metabolome in rat urine. An injection of 20 microL of urine into the chromatographic system is followed by a slow gradient elution and mass spectrometric detection in the scanning mode from m/z 100-1000 in both positive and negative modes. Over a time scale of 90 min, 30 and 20 resolved peaks were observed in the positive and the negative modes, respectively, corresponding to the presence of a few hundred m/z ratios. By using a QIT analyzer, data-dependent tandem mass spectrometry of selected m/z ratios identified several compounds in rat urine and characterized various chemical families, including carboxylic acids, amines, sulfated compounds, glucuronides and glycosides, by the observation of characteristic fragment ions or neutral losses. The method has been applied to the investigation of the chronic toxicity of heavy metals in rat urine. A few tens of m/z ratios, differing in intensity more than threefold from control values, were observed in both positive and negative modes. The time variations for some selected ions suggest that LC/ESI-MS could allow selective characterization of biomarkers in response to specific toxic compounds. 相似文献
93.
Behnaz Ghaffari Jorge Mendes-Burak Ka Wing Chan Prof. Dr. Christophe Copéret 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(61):13869-13873
Manganese, the third most abundant transition-metal element after iron and titanium, has recently been demonstrated to be an effective homogeneous catalyst in numerous reactions. Herein, the preparation of silica-supported MnII sites is reported using Surface Organometallic Chemistry (SOMC), combined with tailored thermolytic molecular precursors approach based on Mn2[OSi(OtBu)3]4 and Mn{N(SiMe3)2}2⋅THF. These supported MnII sites, free of organic ligands, efficiently catalyze numerous reactions: hydroboration and hydrosilylation of ketones and aldehydes as well as the transesterification of industrially relevant substrates. 相似文献
94.
Metal olefin complexes that are ubiquitous intermediates in catalysis are investigated by a detailed analysis of their 13C‐NMR chemical shift tensors. This analysis allows evidencing specific electronic features, namely the olefin‐to‐metal σ‐donation and the metal‐to‐olefin π‐backdonation as proposed in the Dewar?Chatt?Duncanson model. Apart from these interactions, the chemical shift tensor analysis reveals an additional ligand‐to‐metal π‐donation of the olefin σ(C=C) orbital in systems with suitably oriented vacant d‐orbitals. This interaction which is not accounted for in the Dewar?Chatt?Duncanson model explains the reactivity of this type of metal olefin complexes towards oxidative cyclization (olefin insertion) and protonolysis. 相似文献
95.
Won Seok Ham Julius Hillenbrand Jrme Jacq Christophe Genicot Tobias Ritter 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(2):542-546
(Hetero)arylamines constitute some of the most prevalent functional molecules, especially as pharmaceuticals. However, structurally complex aromatics currently cannot be converted into arylamines, so instead, each product isomer must be assembled through a multistep synthesis from simpler building blocks. Herein, we describe a late‐stage aryl C?H amination reaction for the synthesis of complex primary arylamines that other reactions cannot access directly. We show and rationalize through a mechanistic analysis the reasons for the wide substrate scope and the constitutional diversity of the reaction, which gives access to molecules that would not have been readily available otherwise. 相似文献
96.
Fei Ye Florian Berger Hao Jia Joseph Ford Alan Wortman Jonas Brgel Christophe Genicot Tobias Ritter 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(41):14757-14761
Incorporation of the CF3 group into arenes has found increasing importance in drug discovery. Herein, we report the first photoredox‐catalyzed cross‐coupling of aryl thianthrenium salts with a copper‐based trifluoromethyl reagent, which enables a site‐selective late‐stage trifluoromethylation of arenes. The reaction proceeds with broad functional group tolerance, even for complex small molecules on gram scale. The method was further extended to produce pentafluoroethylated derivatives. 相似文献
97.
Christophe Raynaud Iker del Rosal Franck Jolibois Laurent Maron Romuald Poteau 《Theoretical chemistry accounts》2010,126(3-4):151-163
A new multicentered effective group potential (EGP) is obtained for η6-benzene. Applications on $[\hbox{Ru}_{4}(\hbox{H})_{4}(\hbox{C}_{6}\hbox{H}_{6})_{4}]^{n+}$ clusters (n = 0 or 2) are in excellent agreement with reference DFT studies in terms of geometries, energies and electronic structures. In particular, the small singlet–triplet energy difference (3.8 kcal mol?1) in [Ru4(H)4(C6H6)4]2+ is very well reproduced. This new EGP is nevertheless not free from the limitations associated to this first generation of molecular pseudopotentials. A cautious analysis of the nature and exact role of this EGP is made, which provides new directions for the elaboration of the next generation of EGPs. In addition, the η5-cyclopentadienyl EGP has been used to perform a constrained dynamical simulation for the reaction of Cp2LaH with H2. The energy conservation during the simulation as well as the activation barrier extracted from the simulation clearly demonstrate the good behavior of this EGP in the context of molecular dynamics. Anharmonic effects on this reaction are underlined, further demonstrating the high accuracy of the potential energy surface obtained with EGPs. From a more general point of view, such EGPs are expected to provide accurate albeit low-cost ligand-field effects in organometallic clusters or nanoparticles and to allow dynamical studies at the surface of such compounds. 相似文献
98.
Total synthesis of amaryllidaceae alkaloid buflavine 总被引:1,自引:0,他引:1
A concise synthesis of the amaryllidaceae alkaloid buflavine (1) and its regioisomer (2) involving sequential Meyers' biaryl coupling, enecarbamate formation, and hydrogenation followed by ultimate intramolecular reductive amination is presented. 相似文献
99.
Odin C 《Magnetic resonance in chemistry : MRC》2004,42(4):381-388
A variable-temperature high-resolution 13C and 87Rb solid-state NMR study of powder rubidium hydrogencarbonate, RbHCO3, is presented for the first time. At ambient temperature, RbHCO3 is formed by centrosymmetric dimers linked by hydrogen bonds, but almost no information is available on this compound concerning proton disorder and the low-temperature phase. However, potassium hydrogencarbonate, KHCO3, which has an isomorphic structure for the high temperature phase, was well studied: it undergoes a non-ferroic, non-ferroelectric phase transition at Tc = 318 K between two monoclinic structures. The protons are disordered in an asymmetric double-well potential in the low-temperature phase, and the double-well potential becomes symmetric in the high-temperature phase. By comparison with recent solid-state NMR experimental results on KHCO3, we show that RbHCO3 undergoes a phase transition at Tc approximately 245 K, and give evidence that the proton dynamic disorder in both compounds is very similar. 相似文献
100.
A photosensitive precursor (1) to phenylsulfanylcarbene 2 has been synthesized. Laser flash photolysis (308 nm) of 1 and chemical trapping studies of 2 are reported. [structure: see text] 相似文献