全文获取类型
收费全文 | 3712篇 |
免费 | 712篇 |
国内免费 | 640篇 |
专业分类
化学 | 2630篇 |
晶体学 | 58篇 |
力学 | 228篇 |
综合类 | 53篇 |
数学 | 259篇 |
物理学 | 1836篇 |
出版年
2024年 | 14篇 |
2023年 | 78篇 |
2022年 | 147篇 |
2021年 | 178篇 |
2020年 | 194篇 |
2019年 | 164篇 |
2018年 | 151篇 |
2017年 | 176篇 |
2016年 | 211篇 |
2015年 | 231篇 |
2014年 | 276篇 |
2013年 | 298篇 |
2012年 | 354篇 |
2011年 | 343篇 |
2010年 | 258篇 |
2009年 | 260篇 |
2008年 | 316篇 |
2007年 | 211篇 |
2006年 | 196篇 |
2005年 | 179篇 |
2004年 | 143篇 |
2003年 | 104篇 |
2002年 | 137篇 |
2001年 | 96篇 |
2000年 | 104篇 |
1999年 | 49篇 |
1998年 | 27篇 |
1997年 | 40篇 |
1996年 | 12篇 |
1995年 | 22篇 |
1994年 | 12篇 |
1993年 | 16篇 |
1992年 | 12篇 |
1991年 | 9篇 |
1990年 | 6篇 |
1989年 | 2篇 |
1988年 | 3篇 |
1986年 | 5篇 |
1985年 | 8篇 |
1984年 | 2篇 |
1981年 | 3篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1973年 | 1篇 |
1971年 | 1篇 |
1968年 | 1篇 |
1966年 | 2篇 |
1963年 | 1篇 |
1957年 | 1篇 |
1911年 | 4篇 |
排序方式: 共有5064条查询结果,搜索用时 15 毫秒
21.
Separated macromolecular fractions of in vitro synthesized poly(ADP-ribose) by liver nuclei were subjected to ion-exchange chromatography in a programmed high-performance liquid chromatographic elution system. The effects of ionic strength, pH and temperature on the separation of poly(ADP-ribose) chains were determined. Short chain oligomers (up to n = 11) were fractionated into individual components by baseline separation. Each fraction was analyzed for chain length. Trace amounts of Ado(P)Rib(P)Rib(P) found in phosphodiesterase digests were taken as indication of apparent branching. In phosphodiesterase digests of the shorter oligomers, besides traces of the above component, two other digestion products were also observed, presumably representing oligomer termini, one terminal fragment being dominant in short oligomers. Medium and long chain oligomers were partly resolved to individual components, and especially the long oligomers exhibited marked temperature dependent elution patterns. Apparent branching increased with increasing chain length up to about 3% for n = 44 and components presumably indicating termini diminished to mere traces. The adenine spectra of all fractions identified individual components. 相似文献
22.
23.
Aihua Zhang Kun Liu Lan Li Siyu Ma Zonghe Li 《International journal of quantum chemistry》2005,104(3):373-378
The ring‐closing reaction of 5′‐adenosine monophosphate (5′‐AMP) to generate cyclic 3′, 5′‐adenosine monophosphate (cAMP) and H2O was theoretically investigated at the B3LYP/6‐31G**level. It was found that the ring‐closing reaction of 5′‐AMP may proceed in a synchronous way or in a stepwise way. For the latter, the reaction is a multichannel elimination reaction including inner H transfer. The potential energy surface of Path 3 is lowest in all the ring‐closing reaction paths. In addition, H shuttling reaction with the participation of a water molecule to act as a shuttle were also studied at the same level. The calculations indicate that the participation of a water molecule facilitates hydrogen transfer reaction. Our present calculations rationalized all the possible reaction channels. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 相似文献
24.
超声波作用下的糖精相转移催化N-苄基化反应的研究嵇学林刘华黄昆(江苏镇江医学院有机化学教研室镇江212001)关键词超声波相转换催化糖精苄基化有机声化学近年来,由于超声波(Ultrasoun,缩写U.S)在有机合成中应用的不断发展,逐渐形成了一门新兴... 相似文献
25.
Qiang ZHAO Qian Kun ZHUANG 《中国化学快报》2005,16(9):1237-1240
With 1-Pymnebutyric acid (PBA) and multiwalled carbon nanotubes (MWNTs), glassy carbon electrode modified was successfully prepared. In phosphate buffer solution (pH 7,0), the direct electrochemistry of cytochrome C (Cyt C) was realized. In the cyclic voltammetry experiment two pairs of redox peaks ofCyt C were observed at 0.018 V and -0.314 V (vs. SCE), respectively. The redox reaction at 0.018 V was diffusion controlled, while the redox reaction at -0.314 V was adsorption controlled. 相似文献
26.
This paper is the study of the fluorescence enhancement of Eu3+-1-ethyl-6-fluoro-4-oxo-7-(1-piperazinyl)-1,8-naphthyridin-3-carbonic acid (enoxacin, EFLX) system by surfactants. It was
found that sodium dodecylbenzenesulfonate (SDBS) exhibits great enhancement on the fluorescence of the Eu-EFLX system. The
molar ratio is 1:2:1 for Eu:EFLX: SDBS. Under the optimum conditions, the fluorescence intensity is a linear function of europium
in the range of 1.0 × 10−8 ∼ 5.0 × 10−6 mol/L, the detection limit is 1.0 × 10−9 mol/L. The application of the Eu-EFLX-SDBS system for the determination of trace europium in rare earth samples gave satisfactory
results.
Received October 19, 2000. Revision August 10, 2001. 相似文献
27.
A new approach to isoprostanes and neuroprostanes featuring cis-dialkyl stereochemistry at the cyclopentane ring has been developed by employing an intramolecular cross-coupling reaction of an alkyl iodide and a tethered alkenylsiloxane for stereoselective installation of a functionalized omega-side chain. 相似文献
28.
Compound [Zn(phen)3][BDA] (1) (BDA=6,6′-dibromo-2,2′-dimethoxy-1,1′-binaphthylene-4,4′-disulfonate, phen= 1,10-phenanthroline) composes of the anion part (racemic-(R,S)-6,6′-dibromo-2,2′-dimethoxy-1,1′-binaphthylene-4,4′-disulfonate ) and the cation part which consists of a racemic octahedrally coordinated zinc center defined six nitrogen atoms from three phen rings to form an inorganic chirality that can be resolution by chiral organic ligand, the 3D framework was formed through the strong H-bonding interaction between sulfonate and water. CCDC: 277924. 相似文献
29.
Bromoiodomethane photodissociation in the low-lying excited states has been characterized using unrestricted Hartree-Fock, configuration-interaction-singles, and complete active space self-consistent field calculations with the SDB-aug-cc-pVTZ, aug-cc-pVTZ, and 3-21g** basis sets. According to the results of the vertical excited energies and oscillator strengths of these low-lying excited states, bond selectivity is predicted. Subsequently, the minimum energy paths of the first excited singlet state and the third excited state for the dissociation reactions were calculated using the complete active space self-consistent field method with 3-21g** basis set. Good agreement is found between the calculations and experimental data. The relationships of excitations, the electronic structures at Franck-Condon points, and bond selectivity are discussed. 相似文献
30.
Regioselective and stereospecific intramolecular alkylative arylation of unsaturated oxabicyclic diol 6, mediated by Lewis acid or strong protic acid to give the tetracyclic products 7a and 7b, as shown above, represents the first example of an electrophilic (cationic in character) ring-opening-cyclization of oxabicyclic alkene. This constitutes the key cyclization step for a long-standing and potentially useful diene approach for the synthesis of estrone and analogous steroid structures. [structure: see text] 相似文献