全文获取类型
收费全文 | 2072篇 |
免费 | 74篇 |
国内免费 | 3篇 |
专业分类
化学 | 1304篇 |
晶体学 | 11篇 |
力学 | 62篇 |
数学 | 145篇 |
物理学 | 627篇 |
出版年
2023年 | 25篇 |
2022年 | 42篇 |
2021年 | 64篇 |
2020年 | 57篇 |
2019年 | 48篇 |
2018年 | 45篇 |
2017年 | 37篇 |
2016年 | 74篇 |
2015年 | 42篇 |
2014年 | 99篇 |
2013年 | 137篇 |
2012年 | 119篇 |
2011年 | 158篇 |
2010年 | 109篇 |
2009年 | 94篇 |
2008年 | 127篇 |
2007年 | 82篇 |
2006年 | 80篇 |
2005年 | 88篇 |
2004年 | 61篇 |
2003年 | 47篇 |
2002年 | 31篇 |
2001年 | 24篇 |
2000年 | 19篇 |
1999年 | 27篇 |
1998年 | 13篇 |
1997年 | 18篇 |
1996年 | 21篇 |
1995年 | 16篇 |
1994年 | 19篇 |
1993年 | 21篇 |
1992年 | 15篇 |
1991年 | 16篇 |
1990年 | 12篇 |
1989年 | 11篇 |
1988年 | 9篇 |
1987年 | 11篇 |
1986年 | 18篇 |
1985年 | 18篇 |
1984年 | 24篇 |
1983年 | 23篇 |
1982年 | 18篇 |
1981年 | 11篇 |
1980年 | 10篇 |
1979年 | 16篇 |
1978年 | 12篇 |
1977年 | 13篇 |
1976年 | 14篇 |
1975年 | 7篇 |
1974年 | 9篇 |
排序方式: 共有2149条查询结果,搜索用时 0 毫秒
991.
Mesoporous Silica MCM‐41 Supported N‐Heterocyclic Carbene‐Pd Complex for Heck and Sonogashira Coupling Reactions 下载免费PDF全文
Heterocyclic carbene‐Pd complex was anchored onto the mesoporous silica MCM‐41 which exhibits high catalytic activity in Heck reaction under phosphine free reaction conditions for the reaction of iodo/bromoarenes with olefinic compounds such as butyl acrylate, isopropyl acrylate and styrene. This catalytic system also showed high activity for Sonogashira coupling reaction of various aryl halides under copper, phosphine and solvent‐free reaction conditions. The air and thermally stable catalyst were reused several times without significant loss of its activity. High efficiency of the catalyst along with its recycling ability and the rather low Pd‐loading demonstrated in both Heck and Sonogashira coupling reactions are the merits of the presented catalyst system. 相似文献
992.
Alcohol oxidation reactions catalyzed by ruthenium–carbonyl complexes of thioarylazoimidazoles 下载免费PDF全文
Shyamal Kumar Sarkar Mahendra Sekhar Jana Tapan Kumar Mondal Chittaranjan Sinha 《应用有机金属化学》2014,28(8):641-651
Alcohols are oxidized by N‐methylmorpholine‐N‐oxide (NMO), ButOOH and H2O2 to the corresponding aldehydes or ketones in the presence of catalyst, [RuH(CO)(PPh3)2(SRaaiNR′)]PF6 ( 2 ) and [RuCl(CO)(PPh3)(SκRaaiNR′)]PF6 ( 3 ) (SRaaiNR′ ( 1 ) = 1‐alkyl‐2‐{(o‐thioalkyl)phenylazo}imidazole, a bidentate N(imidazolyl) (N), N(azo) (N′) chelator and SκRaaiNR′ is a tridentate N(imidazolyl) (N), N(azo) (N′), Sκ‐R is tridentate chelator; R and R′ are Me and Et). The single‐crystal X‐ray structures of [RuH(CO)(PPh3)2(SMeaaiNMe)]PF6 ( 2a ) (SMeaaiNMe = 1‐methyl‐2‐{(o‐thioethyl)phenylazo}imidazole) and [RuH(CO)(PPh3)2(SEtaaiNEt)]PF6 ( 2b ) (SEtaaiNEt = 1‐ethyl‐2‐{(o‐thioethyl)phenylazo}imidazole) show bidentate N,N′ chelation, while in [RuCl(CO)(PPh3)(SκEtaaiNEt)]PF6 ( 3b ) the ligand SκEtaaiNEt serves as tridentate N,N′,S chelator. The cyclic voltammogram shows RuIII/RuII (~1.1 V) and RuIV/RuIII (~1.7 V) couples of the complexes 2 while RuIII/RuII (1.26 V) couple is observed only in 3 along with azo reductions in the potential window +2.0 to ?2.0 V. DFT computation has been used to explain the spectra and redox properties of the complexes. In the oxidation reaction NMO acts as best oxidant and [RuCl(CO)(PPh3)(SκRaaiNR′)](PF6) ( 3 ) is the best catalyst. The formation of high‐valent RuIV=O species as a catalytic intermediate is proposed for the oxidation process. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
993.
Peptide Conjugates of a Nonsteroidal Anti‐Inflammatory Drug as Supramolecular Gelators: Synthesis,Characterization, and Biological Studies 下载免费PDF全文
Rajdip Roy Dr. Jolly Deb Dr. Siddhartha Sankar Jana Prof. Parthasarathi Dastidar 《化学:亚洲杂志》2014,9(11):3196-3206
Indomethacin ( IND ), which is a well‐known nonsteroidal anti‐inflammatory drug (NSAID), was conjugated with various naturally occurring amino acids. Most of these bioconjugates were capable of gelling pure water, a solution of NaCl (0.9 wt %), and phosphate‐buffered saline (pH 7.4), as well as a few organic solvents. The gels were characterized by table‐top and dynamic rheology, and electron microscopy. Variable‐temperature 1H NMR spectroscopy studies on a selected gel were performed to gain insights into the self‐assembly process during gel formation. Both 1D and 2D hydrogen‐bonded networks were observed in the single‐crystal structures of two of the gelators. Plausible biological applications of the hydrogelators were evaluated with the ultimate aim of drug delivery in a self‐delivery fashion. All hydrogelators were stable in phosphate‐buffered saline at pH 7.4 at 37 °C, and biocompatible in mouse macrophage RAW 264.7 cell line (3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyltetrazolium bromide assay). Two of the most biocompatible hydrogelators displayed an anti‐inflammatory response comparable to that of the parent drug IND in prostaglandin E2 assay. Release of the bioconjugates into the bulk solvent interfaced with the corresponding hydrogels indicated their plausible future application in drug delivery. 相似文献
994.
Three‐dimensional networks containing rectangular Sr4 and Ba4 units: Synthesis,structure, bonding,and potential application for Ne gas separation 下载免费PDF全文
Subhajit Mandal Sudip Pan Dibakar Deb Santanab Giri Soma Duley Slavko Radenković David L. Cooper Patrick Bultinck Anakuthil Anoop Manish Bhattacharjee Pratim K. Chattaraj 《International journal of quantum chemistry》2015,115(20):1501-1510
New porous three‐dimensional metal‐organic frameworks are synthesized that contain infinite chains of Srn and Ban rectangles. Their structures are elucidated by means of spectroscopic techniques such as nuclear magnetic resonance and Fourier transform infrared, and the respective crystal structures are determined. The electronic structure of basic units of the crystals are computed using density functional theory at the B3LYP/6‐31G(d,p)/def2‐TZVP level, and the bonding and reactivity are analyzed using natural bond orbital analysis, the quantum theory of atoms in molecules, and conceptual density functional theory. The possibilities of noble gas (Ng) storage inside the crystal structures are explored through modeling a Ng atom inside the frozen geometry of the crystal. It was found that a neon atom can fit into a cavity in the Sr and Ba crystal structures whereas other Ngs (He, Ar, Kr) exhibit repulsive interactions with the crystal structure. Ab initio molecular dynamics simulations for up to 500 fs at 77 and 298 K suggest that the structures incorporating a neon atom are kinetically stable. © 2015 Wiley Periodicals, Inc. 相似文献
995.
Ruthenium(II)‐Catalyzed CH Activation with Isocyanates: A Versatile Route to Phthalimides 下载免费PDF全文
Dr. Suman De Sarkar Prof. Dr. Lutz Ackermann 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(43):13932-13936
A cationic ruthenium(II)‐complex was utilized in the efficient synthesis of phthalimide derivatives by C?H activation with synthetically useful amides. The reaction proceeded through a mechanistically unique insertion of a cycloruthenated species into a C?Het multiple bond of isocyanate. The novel method also proved applicable for the synthesis of heteroaromatic unsymmetric diamides as well as a potent COX‐2 enzyme inhibitor. 相似文献
996.
Ultrafast FRET to Study Spontaneous Micelle‐to‐Vesicle Transitions in an Aqueous Mixed Surface‐Active Ionic‐Liquid System 下载免费PDF全文
Sarthak Mandal Jagannath Kuchlyan Debasis Banik Surajit Ghosh Chiranjib Banerjee Vijaykant Khorwal Nilmoni Sarkar 《Chemphyschem》2014,15(16):3544-3553
The spontaneous micelle‐to‐vesicle transition in an aqueous mixture of two surface‐active ionic liquids (SAILs), namely, 1‐butyl‐3‐methylimidazolium n‐octylsulfate ([C4mim][C8SO4]) and 1‐dodecyl‐3‐methylimidazoium chloride ([C12mim]Cl) is described. In addition to detailed structural characterization obtained by using dynamic light scattering, transmission electron microscopy (TEM), and cryogenic TEM techniques, ultrafast fluorescence resonance energy transfer (FRET) from coumarin 153 (C153) as a donor (D) to rhodamine 6G (R6G) as an acceptor (A) is also used to study micelle–vesicle transitions in the present system. Structural transitions of SAIL micelles ([C4mim][C8SO4] or [C12mim]Cl micelles) to mixed SAIL vesicles resulted in significantly increased D –A distances, and therefore, increased timescale of FRET. In [C4mim][C8SO4] micelles, FRET between C153 and R6G occurs on an ultrafast timescale of 3.3 ps, which corresponds to a D –A distance of about 15 Å. As [C4mim][C8SO4] micelles are transformed into mixed micelles upon the addition of a 0.25 molar fraction of [C12mim]Cl, the timescale of FRET increases to 300 ps, which suggests an increase in the D –A distance to 31 Å. At a 0.5 molar fraction of [C12mim]Cl, unilamellar vesicles are formed in which FRET occurs on multiple timescales of about 250 and 2100 ps, which correspond to D –A distances of 33 and 47 Å. Although in micelles and mixed micelles the obtained D –A distances are well correlated with their radius, in vesicles the obtained D –A distance is within the range of the bilayer thickness. 相似文献
997.
Ursula Pachmayr Fabian Nitsche Dr. Hubertus Luetkens Sirko Kamusella Felix Brückner Dr. Rajib Sarkar Prof. Dr. Hans‐Henning Klauss Prof. Dr. Dirk Johrendt 《Angewandte Chemie (International ed. in English)》2015,54(1):293-297
Superconducting [(Li1?xFex)OH](Fe1?yLiy)Se (x≈0.2, y≈0.08) was synthesized by hydrothermal methods and characterized by single‐crystal and powder X‐ray diffraction. The structure contains alternating layers of anti‐PbO type (Fe1?yLiy)Se and (Li1?xFex)OH. Electrical resistivity and magnetic susceptibility measurements reveal superconductivity at 43 K. An anomaly in the diamagnetic shielding indicates ferromagnetic ordering near 10 K while superconductivity is retained. The ferromagnetism is from the iron atoms in the (Li1?xFex)OH layer. Isothermal magnetization measurements confirm the superposition of ferromagnetic and superconducting hysteresis. The internal ferromagnetic field is larger than the lower, but smaller than the upper critical field of the superconductor. The formation of a spontaneous vortex phase where both orders coexist is supported by 57Fe‐Mössbauer spectra, 7Li‐NMR spectra, and μSR experiments. 相似文献
998.
999.
A wide variety of 5,6-unsubstituted 1,4-dihydropyridines derivatives (4a–q) were synthesized through a one-pot three-component reaction from α,β-unsaturated aldehydes, amines, and 1,3-diketones at room temperature using hydrated ferric sulfate as a Lewis acid catalyst. The key features of the present protocol are mild and simple reaction procedure, good to excellent yields, and use of inexpensive, recyclable, and environmentally benign catalyst. 相似文献
1000.
Depanjan Sarkar Soujit Sen Gupta Rahul Narayanan Thalappil Pradeep 《Journal of the American Society for Mass Spectrometry》2014,25(3):380-387
We report in-situ production and detection of intermediates at graphenic surfaces, especially during alcohol oxidation. Alcohol oxidation to acid occurs on graphene oxide-coated paper surface, driven by an electrical potential, in a paper spray mass spectrometry experiment. As paper spray ionization is a fast process and the time scale matches with the reaction time scale, we were able to detect the intermediate, acetal. This is the first observation of acetal formed in surface oxidation. The process is not limited to alcohols and the reaction has been extended to aldehydes, amines, phosphenes, sugars, etc., where reaction products were detected instantaneously. By combining surface reactions with ambient ionization and mass spectrometry, we show that new insights into chemical reactions become feasible. We suggest that several other chemical transformations may be studied this way. This work opens up a new pathway for different industrially and energetically important reactions using different metal catalysts and modified substrate. Figure
? 相似文献