全文获取类型
收费全文 | 694篇 |
免费 | 16篇 |
国内免费 | 4篇 |
专业分类
化学 | 413篇 |
晶体学 | 3篇 |
力学 | 27篇 |
数学 | 157篇 |
物理学 | 114篇 |
出版年
2023年 | 5篇 |
2022年 | 11篇 |
2021年 | 26篇 |
2020年 | 21篇 |
2019年 | 15篇 |
2018年 | 12篇 |
2017年 | 13篇 |
2016年 | 30篇 |
2015年 | 23篇 |
2014年 | 26篇 |
2013年 | 40篇 |
2012年 | 48篇 |
2011年 | 48篇 |
2010年 | 35篇 |
2009年 | 23篇 |
2008年 | 37篇 |
2007年 | 38篇 |
2006年 | 35篇 |
2005年 | 36篇 |
2004年 | 27篇 |
2003年 | 19篇 |
2002年 | 19篇 |
2001年 | 10篇 |
1999年 | 9篇 |
1998年 | 4篇 |
1997年 | 2篇 |
1996年 | 10篇 |
1995年 | 6篇 |
1994年 | 7篇 |
1993年 | 5篇 |
1992年 | 5篇 |
1991年 | 2篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1985年 | 6篇 |
1984年 | 6篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1979年 | 4篇 |
1978年 | 4篇 |
1976年 | 3篇 |
1975年 | 2篇 |
1973年 | 2篇 |
1921年 | 2篇 |
1919年 | 2篇 |
1915年 | 4篇 |
1909年 | 2篇 |
1901年 | 2篇 |
1875年 | 1篇 |
1868年 | 2篇 |
排序方式: 共有714条查询结果,搜索用时 234 毫秒
651.
Florio S Perna FM Luisi R Barluenga J Rodríguez F Fañanás FJ 《The Journal of organic chemistry》2004,69(16):5480-5482
Regioselective addition of lithiated oxazoline 2a, easily available from 2-(1-chloroethyl)-4,4-dimethyl-2-oxazoline 1a (LDA, THF, -98 degrees C), to alpha,beta-unsaturated Fischer carbene complexes 3 afforded cyclopropylcarbene complexes 4 as sole diastereoisomers. Exposure of carbene complexes 4a-c (M = Cr) to air and sunlight gave cyclopropane carboxylate derivatives 5a-c. A plausible mechanistic explanation is proposed. Moreover, when lithiated oxazoline 2b was generated from 1b in the presence of the carbene complex 3a,b, the oxazolinylcyclopropane carboxylates 6a,b formed as a 1:1 mixture of diastereoisomers. Chiral lithiated oxazoline 2c added regioselectively and diastereoselectively to chromium complexes 3a,b and to tungsten complexes 3d,e, leading, after oxidation of the metal fragment, to esters 7a,b with good diastereoselectivity (dr = 4:1). The reaction of lithiated oxazoline 2d with chromium complex 3b and tungsten complex 3e proceeded less diastereoselectively, furnishing, in both cases, after oxidation, the ester 7c as a 3:2 diastereoselective mixture. 相似文献
652.
We have functions expressed as antisymmetrized products of strongly orthogonal geminals have been evaluated for some three membered ring molecules. GF results are compared with previously computed SCF-MO results, obtained employing the same atomic basis. Transferability features of bonds and inner shells are shown. 相似文献
653.
Filippo?De Angelis Simona?Fantacci Antonio?SgamellottiEmail author Nazzareno?Re 《Theoretical chemistry accounts》2003,110(3):196-204
The energetics and reaction mechanism of the migratory insertion of carbon monoxide and methyl isocyanide into the zirconium–carbon and titanium–carbon bonds in [calix[4](OMe)2(O)2–M–Me2], (M=Zr, Ti), have been investigated by combining static and dynamic density functional calculations. Two steps have been characterized: the coordination of the incoming nucleophilic moiety leading to relatively stable facial adducts; its subsequent insertion into the M–C bond, leading to 2-bound acyl or iminoacyl complexes, providing a rationale for the different behavior of CO and MeNC towards both insertion and deinsertion reactions. Our results indicate that the rate-determining step for the overall MeNC insertion into the M–C bond is its coordination to the electron-deficient metal center, with the titanium system featuring a higher energy barrier (12.7 versus 5.5 kcal mol–1). Ab initio molecular dynamics simulations have been performed on the Zr system by means of the Car–Parrinello method, to study the hitherto inaccessible mechanistic features of the insertion reactions.Contribution to the Björn Roos Honorary Issue 相似文献
654.
655.
The reaction of M(η6-1,3,5-Me3C6H3)2, M = Cr, Mo, with the tetrahalides of Groups 4 and 5 elements proceeds with the monoelectronic oxidation of the metal bis-arene to the [M(η6-Me3C6H3)2]+ cation. In the case of MX4, M = Ti, X = Cl, Br, M = V, X = Cl, and of Nb2Cl10 the reduction products are the titanium(III), vanadium(III) halides and the niobium(IV) chloride, isolated as the solvate anions [MCl4(THF)2]− and [NbCl4(CH3CN)]−. The reaction of the tetrachloro complexes MCl4(THF)2, M = Zr, Hf, with Cr(η6-1,3,5-Me3C6H3)2 in THF produces the ionic [Cr(η6-1,3,5-Me3C6H3)2][MCl5(THF)], which has been characterized by single-crystal X-ray diffraction in the case of hafnium. 相似文献
656.
Stefania Della Sciucca Gloria Spagnoli Maurizio Penco Salvatore Battiato Filippo Samperi Raniero Mendichi 《Journal of Polymer Science.Polymer Physics》2009,47(6):596-607
The structural relaxation of poly(methyl‐methacrylate) (PMMA)‐based copolymers with different chain flexibility has been studied by DSC with the classical procedure of the isothermal and dynamical approach. Modified PMMA with different chain flexibility have been prepared by free radical polymerization in solution using a mixture of monomers containing 10 mol % of alkyl methacrylate (i.e., ethyl, buthyl, and hexyl methacrylate). The molecular characteristics of all the prepared copolymers have been performed by a multiangle laser light scattering (MALS) photometer on‐line to a size exclusion chromatography (SEC) system (SEC‐MALS) after and before the thermal treatments, NMR (1H and 13C) and MALDI‐TOF mass spectrometry. A comparison of the apparent relaxation rate (RH) was appraised from the enthalpy loss by annealing the different samples at the same level of undercooling (Ta = Tg ? 18 °C). It was found an increase of RH increasing the chain flexibility in the copolymers. Dynamical tests, performed at different cooling rates, have been used to estimate the apparent activation energy of the relaxation process. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 596–607, 2009 相似文献
657.
Graziano Crasta Ilaria Fragalà Filippo Gazzola 《Zeitschrift für Angewandte Mathematik und Physik (ZAMP)》2005,23(4):763-782
The electrostatic capacity of a convex body is usually not simple to compute. We discuss two possible approximations of it. The first one is related to a long-standing conjecture by Pólya–Szegö. It states that, among all convex bodies, the “worst shape” for the approximation exists and is the planar disk. We prove the first part of this conjecture, and we establish some related results which give further evidence for the validity of the second part. We also suggest some complementary conjectures and open problems. The second approximation we study is based on the use of web functions. 相似文献
658.
Filippo Bracci Manuel D. Contreras Santiago Díaz-Madrigal 《Advances in Mathematics》2007,208(1):318-350
We give a complete classification up to conjugation of continuous semigroups of linear fractional self-maps of the unit ball. 相似文献
659.
Let x be a Kobayashi hyperbolic complex manifold, and assume that X does not contain compact complex submanifolds of positive dimension (e.g., X Stein). We shall prove the following generalization of Ritt's theorem: every holomorphic self-map f:X→X such that f(X) is relatively compact in X has a unique fixed point τ(f)∈X, which is attracting. Furthermore, we shall prove that τ(f) depends holomorphically on f in a suitable sense, generalizing results by Heins, Joseph-Kwack and the second author. 相似文献
660.
Gaetano Chiodelli Filippo Maglia Umberto Anselmi-Tamburini Zuhair A. Munir 《Solid State Ionics》2009,180(4-5):297-301
We investigated the conductivity of samples of bulk nanometric cubic yttria stabilized zirconia (YSZ 8%) with a grain size of about 16 nm and a relative density above 98%. In an oxygen atmosphere the material showed a large grain boundary resistivity. However, when exposed to a moist atmosphere at temperatures below 150 °C it showed a high conductivity, several orders of magnitude higher than the corresponding extrapolated ionic conductivity. A fairly high conductivity was measured even at room temperature. The conductivity was strongly dependent on the water partial pressure and showed a distinct isotopic effect, suggesting a protonic conductivity mechanism. This effect was not observed in samples with grain size above 50 nm, suggesting the possibility that nanostructure can induce drastic modification in the conduction mechanism of ceramic electrolytes. 相似文献