全文获取类型
收费全文 | 404篇 |
免费 | 6篇 |
专业分类
化学 | 296篇 |
晶体学 | 5篇 |
力学 | 18篇 |
数学 | 36篇 |
物理学 | 55篇 |
出版年
2023年 | 2篇 |
2021年 | 2篇 |
2020年 | 10篇 |
2019年 | 4篇 |
2018年 | 1篇 |
2017年 | 3篇 |
2016年 | 9篇 |
2015年 | 1篇 |
2014年 | 6篇 |
2013年 | 18篇 |
2012年 | 25篇 |
2011年 | 21篇 |
2010年 | 17篇 |
2009年 | 10篇 |
2008年 | 30篇 |
2007年 | 29篇 |
2006年 | 25篇 |
2005年 | 28篇 |
2004年 | 20篇 |
2003年 | 21篇 |
2002年 | 19篇 |
2001年 | 7篇 |
2000年 | 3篇 |
1999年 | 2篇 |
1998年 | 4篇 |
1997年 | 3篇 |
1996年 | 6篇 |
1995年 | 3篇 |
1994年 | 3篇 |
1993年 | 3篇 |
1991年 | 4篇 |
1990年 | 3篇 |
1989年 | 5篇 |
1988年 | 3篇 |
1987年 | 3篇 |
1985年 | 6篇 |
1984年 | 7篇 |
1983年 | 1篇 |
1982年 | 5篇 |
1981年 | 7篇 |
1980年 | 2篇 |
1979年 | 4篇 |
1978年 | 5篇 |
1977年 | 3篇 |
1976年 | 3篇 |
1975年 | 4篇 |
1974年 | 4篇 |
1973年 | 1篇 |
1972年 | 2篇 |
1971年 | 2篇 |
排序方式: 共有410条查询结果,搜索用时 15 毫秒
101.
Andrs Strawczynski Renzo Ros Raymond Roulet Fabrizia Grepioni Dario Braga 《Helvetica chimica acta》1988,71(8):1885-1894
The two complexes [Ir4(CO)10(diarsine)] (1) and [Ir4(CO)10(1,5-cyclooctadiene)] (2) bear a bidentate ligand chelating one metal atom of the basal face of the Ir4 tetrahedron. However, they differ in fluxional behaviour as observed by 2D-exchange and variable-temperature 13C-NMR. The CO-site exchange with lowest activation energy proceeds via an unbridged intermediate in 2 , whereas that in 1 occurs via a concerted edge-bridging of CO's to an alternative face of the metal core. This difference is apparently related to different ground-state geometries: the basal CO's are symmetrically bridging in 2 , whereas two CO's are semi-bridging in 1 . The molecular structure of 2 was determined by single crystal X-ray diffractometry. The crystals are orthorhombic, space group Pbca, a = 11.651(4), b = 13.118(3), c = 28.64(1)Å. The idealized molecular symmetry is Cs. The diolefin chelates a basal Ir-atom replacing an axial and a radial CO group on the tetrahedral metal-atom framework. 相似文献
102.
[reaction: see text] The stereoselective synthesis of novel alpha-epoxy-beta-amino acids is described by a route that combines the chemistry of oxazolinyloxiranyllithiums with that of nitrones. The intermediate trioxadiazadispiro[2.0.4.3]undecanes 4 have been isolated and converted by hydrolysis into epoxy-5-isoxazolidinones 5 which can be transformed into the alpha-epoxy-beta-amino acids 8 by N-O reduction. 相似文献
103.
Readily available methyl hydrogen (R)-3-methylglutarate(2) is a useful chiral building block for the synthesis of several biologically active compounds. Enantiomerically pure (R)-2 has been employed to synthesize stereospecifically each of the two enantiomers, 1a and 1b, of 10-methyl-2-tridecanone, the sex pheromone of the southern corn rootworm, Diabrotica undecimpunctata howardi Barber. Compound (R)-2 has been also used to prepare 99% optically pure (R)-3-methyl-1-pentanol (6) and enantiomerically pure (R)-5-methyl-i-tricosyne (7). These compounds are useful building blocks suitable for the further elaboration to other chiral insect pheromones. 相似文献
104.
Marisa Belicchi-Ferrari Franco BisceglieChiara Cavalieri Giorgio PelosiPieralberto Tarasconi 《Polyhedron》2007
This paper reports the synthesis and characterization of six compounds of copper(I), stabilized in its reduced state by two triphenylphosphines, in which 4-fluorobenzaldehyde thiosemicarbazone and N-methylthiosemicarbazone act as chelating through their sulfur and imino nitrogen. The three oxoanions that have been chosen, NO3−, SO42− and CH3COO−, play an important role: their oxygens are bad competitors with the imino nitrogen of the thiosemicarbazone moiety and moreover they form strong charge assisted hydrogen bondings that stabilize the neutral form of the ligand. The overall packing is determined by intermolecular phenyl–phenyl van der Waals interactions. 相似文献
105.
[reaction: see text] A stereoselective/stereospecific synthesis of polysubstituted tetrahydronaphthols based on the Michael addition of ortho-lithiated stilbene oxides to alpha,beta-unsaturated Fischer carbene complexes followed by an unusual cyclization of the corresponding intermediate in a 6-endo-tet mode is described. 相似文献
106.
Ph3SiSH transforms amides to thioamides and Ph3SnOH performs the reverse process, with the concomitant formation of Ph3SiOH (or Ph3SiOSiPh3) and Ph3SnSSnPh3, respectively. The chemistry is a delightful illustration of the oxophilicity of silicon compared to the thiophilicity of tin and occurs under relatively mild conditions, and for amide to thioamide transformation requires no amide activation. The chemistry is in accord with available data for Si? (S)(O), Sn? (O)(S) and C?(O)(S) bond energies. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
107.
Dr. Haibo Mei Dr. Jianlin Han Dr. Santos Fustero Mercedes Medio-Simon Daniel M. Sedgwick Dr. Claudio Santi Dr. Renzo Ruzziconi Dr. Vadim A. Soloshonok 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(51):11797-11819
Over the last two decades, fluorine substitution has become one of the essential structural traits in modern pharmaceuticals. Thus, about half of the most successful drugs (blockbuster drugs) contain fluorine atoms. In this review, we profile 17 fluorine-containing drugs approved by the food and drug administration (FDA) in 2018. The newly approved pharmaceuticals feature several types of aromatic F and CF3, as well as aliphatic (CF2) substitution, offering advances in the treatment of various diseases, including cancer, HIV, malarial and smallpox infections. 相似文献
108.
Yoko Omura Renzo Okamoto Minoru Konno Motoo Shiro 《Micron (Oxford, England : 1993)》2010,41(8):974-982
To reconsider the problems arising from the use of the phase plate as a test plate inserted into a polarized light microscope system for the analysis of triclinic calcium pyrophosphate dihydrate (t-CPPD) crystals, or Ca2P2O7·2H2O in the synovial fluid of arthritis patients, we made the polarized light microscopy observations using a phase plate with a retardation of 530 nm for the synthesized t-CPPD crystals well-characterized by X-ray powder pattern indexing and single crystal X-ray diffraction measurements. The microscopy observations were made of crystals of different sizes, thicknesses and shapes. The retardation was assessed using the interference color chart at four extinction and diagonal positions both with and without the test plate. The addition and subtraction states produced by superimposing the retardations of two anisotropic materials, that is, the t-CPPD crystal and the 530 nm phase plate, were deduced from the interference color change by inserting the test plate at four diagonal positions. When the color change of a crystal at a diagonal position resulting from 90-degree rotation exhibits no clear birefringence, the interference color chart was shown to be useless. We suggested the use of a compensator whose retardation can be changed to obtain an accurate value for the retardation of the crystal. 相似文献
109.
Andrea Mazzanti Dr. Lodovico Lunazzi Prof. Renzo Ruzziconi Prof. Sara Spizzichino Manfred Schlosser Prof. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(30):9186-9192
By making use of a novel diastereotopicity probe, namely C(CF3)2OH, it has been possible to measure by very low temperature 19F NMR spectroscopy the elusive aryl–aryl rotation barriers of biphenyls bearing an OH or F group in one ortho position. The experimental values (5.4 and 4.4 kcal mol?1, respectively) are matched by those from ab initio calculations (5.3 and 4.3 kcal mol?1, respectively). 相似文献
110.
Christine Weber Andreas Krieg Renzo M. Paulus Hanneke M. L. Lambermont-Thijs C. Remzi Becer Richard Hoogenboom Ulrich S. Schubert 《Macromolecular Symposia》2011,308(1):17-24
Summary: The cationic ring opening polymerization of 2-ethyl-2-oxazoline (EtOx) was applied for the synthesis of methacrylate end-functionalized well-defined macromonomers that could be polymerized in a controlled manner using reversible addition-fragmentation chain transfer polymerization. The obtained comb polymers revealed lower critical solution temperature behavior in aqueous solution. The cloud points of these solutions could be tuned in a range from 35 °C to 85 °C by the incorporation of hydrophobic methyl methacrylate comonomer in varying amounts into the graft copolymers whereas copolymerization with methacrylic acid rendered temperature and pH sensitive copolymers. Thermo-gravimetric analysis showed a two-step decomposition of the graft copolymers and differential scanning calorimetry revealed glass transition temperatures that are significantly lowered in comparison to linear PEtOx. 相似文献