首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   450篇
  免费   107篇
  国内免费   26篇
化学   559篇
晶体学   2篇
综合类   2篇
物理学   20篇
  2023年   6篇
  2022年   9篇
  2021年   12篇
  2020年   46篇
  2019年   24篇
  2018年   17篇
  2017年   20篇
  2016年   41篇
  2015年   25篇
  2014年   32篇
  2013年   45篇
  2012年   31篇
  2011年   22篇
  2010年   19篇
  2009年   25篇
  2008年   26篇
  2007年   23篇
  2006年   27篇
  2005年   19篇
  2004年   26篇
  2003年   20篇
  2002年   5篇
  2001年   8篇
  2000年   5篇
  1999年   5篇
  1998年   8篇
  1997年   2篇
  1996年   6篇
  1995年   4篇
  1994年   3篇
  1993年   6篇
  1992年   3篇
  1990年   3篇
  1988年   2篇
  1987年   4篇
  1985年   1篇
  1982年   1篇
  1981年   1篇
  1973年   1篇
排序方式: 共有583条查询结果,搜索用时 31 毫秒
1.
At present, the reactivity of cyclic alkanes is estimated by comparison with acyclic hydrocarbons. Due to the difference in the structure of cycloalkanes and acycloalkanes, the thermodynamic data obtained by analogy are not applicable. In this study, a molecular beam sampling vacuum ultraviolet photoionization time-of-flight mass spectrometer (MB-VUV-PI-TOFMS) was applied to study the low-temperature oxidation of cyclopentane (CPT) at a total pressure range from 1–3 atm and low-temperature range between 500 and 800 K. Low-temperature reaction products including cyclic olefins, cyclic ethers, and highly oxygenated intermediates (e. g., ketohydroperoxide KHP, keto-dihydroperoxide KDHP, olefinic hydroperoxides OHP and ketone structure products) were observed. Further investigation of the oxidation of CPT – electronic structure calculations – were carried out at the UCCSD(T)-F12a/aug-cc-pVDZ//B3LYP/6-31+ G(d,p) level to explore the reactivity of O2 molecules adding sequentially to cyclopentyl radicals. Experimental and theoretical observations showed that the dominant product channel in the reaction of CPT radicals with O2 is HO2 elimination yielding cyclopentene. The pathways of second and third O2 addition – the dissociation of hydroperoxide – were further confirmed. The results of this study will develop the low-temperature oxidation mechanism of CPT, which can be used for future research on accurately simulating the combustion process of CPT.  相似文献   
2.
We report a single-molecule mechanistic investigation into 2-cyanobenzothiazole (CBT) chemistry within a protein nanoreactor. When simple thiols reacted reversibly with CBT, the thioimidate monoadduct was approximately 80-fold longer-lived than the tetrahedral bisadduct, with important implications for the design of molecular walkers. Irreversible condensation between CBT derivatives and N-terminal cysteine residues has been established as a biocompatible reaction for site-selective biomolecular labeling and imaging. During the reaction between CBT and aminothiols, we resolved two transient intermediates, the thioimidate and the cyclic precursor of the thiazoline product, and determined the rate constants associated with the stepwise condensation, thereby providing critical information for a variety of applications, including the covalent inhibition of protein targets and dynamic combinatorial chemistry.  相似文献   
3.
《中国化学快报》2020,31(7):1911-1913
An efficient protocol for the synthesis of asymmetric 1,3-diketones was reported through diacylation of 1,1-diborylalkanes using two different acyl sources. In this transformation, an enolate boron species was initially formed by introducing an acyl group, then it was trapped by another acyl group to form 1,3-diketone. This method not only provided the gateway to obtain a series of 1,3-diketones, but also afforded an operationally simple and efficient access to pyrazoles and isoxazoles.  相似文献   
4.
In the field of organic synthesis, the advent of flow chemistry and flow microreactor technology represented a tremendous novelty in the way of thinking and performing chemical reactions, opening the doors to poorly explored or even impossible transformations using batch methods. In this Concept article, we would like to highlight the impact of flow chemistry for exploiting highly reactive organometallic reagents, and how, alongside the well-known advantages concerning safety, scalability, and productivity, flow chemistry makes possible processes that are impossible to control by using the traditional batch approach.  相似文献   
5.
6.
Redox‐inactive metal ions play important roles in tuning chemical properties of metal–oxygen intermediates. Herein we report the effect of water molecules on the redox properties of a nonheme iron(III)–peroxo complex binding redox‐inactive metal ions. The coordination of two water molecules to a Zn2+ ion in (TMC)FeIII‐(O2)‐Zn(CF3SO3)2 ( 1 ‐Zn2+) decreases the Lewis acidity of the Zn2+ ion, resulting in the decrease of the one‐electron oxidation and reduction potentials of 1 ‐Zn2+. This further changes the reactivities of 1 ‐Zn2+ in oxidation and reduction reactions; no reaction occurred upon addition of an oxidant (e.g., cerium(IV) ammonium nitrate (CAN)) to 1 ‐Zn2+, whereas 1 ‐Zn2+ coordinating two water molecules, (TMC)FeIII‐(O2)‐Zn(CF3SO3)2‐(OH2)2 [ 1 ‐Zn2+‐(OH2)2], releases the O2 unit in the oxidation reaction. In the reduction reactions, 1 ‐Zn2+ was converted to its corresponding iron(IV)–oxo species upon addition of a reductant (e.g., a ferrocene derivative), whereas such a reaction occurred at a much slower rate in the case of 1 ‐Zn2+‐(OH2)2. The present results provide the first biomimetic example showing that water molecules at the active sites of metalloenzymes may participate in tuning the redox properties of metal–oxygen intermediates.  相似文献   
7.
Phenyl groups were introduced at the β-positions proximal to the meso-hydroxy moiety in hydroxyporphyrin NiII complexes by oxidized BINAP PdII complexes. Ion-pairing assemblies of deprotonated π-electronic anions, anionic site of which was stabilized by the introduced phenyl moieties with a bulky cation, were formed. They showed charge-by-charge assemblies, assembly modes of which were modulated by the anionic building units.  相似文献   
8.
Although the chemistry of elusive tricyanomethane (cyanoform) has been studied during a period of more than 150 years, this compound has very rarely been utilized in the synthesis or modification of heterocycles. Three-membered heterocycles, such as epoxides, thiirane, aziridines, or 2H-azirines, are now treated with tricyanomethane, which is generated in situ by heating azidomethylidene-malonodinitrile in tetrahydrofuran at 45 °C or by adding sulfuric acid to potassium tricyanomethanide. This leads to ring expansion with formation of 2-(dicyanomethylidene)oxazolidine derivatives or creation of the corresponding thiazolidine, imidazolidine, or imidazoline compounds and opens up a new access to these push–pull-substituted olefinic products. The regio- and stereochemistry of the ring-enlargement processes are discussed, and the proposed reaction mechanisms were confirmed by using 15N-labeled substrates. It turns out that different mechanisms are operating; however, tricyanomethanide is always acting as a nitrogen-centered nucleophile, which is quite unusual if compared to other reactions of this species.  相似文献   
9.
The electron-poor palladium(0) complex L3Pd (L=tris[3,5-bis(trifluoromethyl)phenyl]phosphine) reacts with Grignard reagents RMgX and organolithium compounds RLi via transmetalation to furnish the anionic organopalladates [L2PdR], as shown by negative-ion mode electrospray-ionization mass spectrometry. These palladates undergo oxidative additions of organyl halides R′X (or related SN2-type reactions) followed by further transmetalation. Gas-phase fragmentation of the resulting heteroleptic palladate(II) complexes results in the reductive elimination of the cross-coupling products RR′. This reaction sequence corresponds to a catalytic cycle, in which the order of the elementary steps of transmetalation and oxidative addition is switched relative to that of palladium-catalyzed cross-coupling reactions proceeding via neutral intermediates. An attractive feature of the palladate-based catalytic system is its ability to mediate challenging alkyl–alkyl coupling reactions. However, the poor stability of the phosphine ligand L against decomposition reactions has so far prevented its successful use in practical applications.  相似文献   
10.
The design of biomimetic model complexes for the cysteine dioxygenase (CDO) and cysteamine dioxygenase (ADO) is reported, where the 3-His coordination of the iron ion is simulated by three pyrazole donors of a trispyrazolyl borate ligand (Tp) and protected cysteine and cysteamine represent substrate ligands. It is found that the replacement of phenyl groups—attached at the 3-positions of the pyrazole units in a previous model—by mesityl residues has massive consequences, as the latter arrange to a more spacious reaction pocket. Thus, the reaction with O2 proceeds much faster and afterwards the first structural characterization of an iron(II) η2-O,O-sulfinate product became possible. If one of the three Tp-mesityl groups is placed in the 5-position, an even larger reaction pocket results, which leads to yet faster rates and accumulation of a reaction intermediate at low temperatures, as shown by UV/Vis and Mössbauer spectroscopy. After comparison with the results of investigations on the cobalt analogues this intermediate is tentatively assigned to an iron(III) superoxide species.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号