全文获取类型
收费全文 | 50885篇 |
免费 | 2227篇 |
国内免费 | 92篇 |
专业分类
化学 | 32846篇 |
晶体学 | 243篇 |
力学 | 1668篇 |
综合类 | 41篇 |
数学 | 7891篇 |
物理学 | 10515篇 |
出版年
2023年 | 399篇 |
2022年 | 307篇 |
2021年 | 515篇 |
2020年 | 704篇 |
2019年 | 666篇 |
2018年 | 1049篇 |
2017年 | 845篇 |
2016年 | 1593篇 |
2015年 | 1453篇 |
2014年 | 1460篇 |
2013年 | 2987篇 |
2012年 | 3507篇 |
2011年 | 3615篇 |
2010年 | 2130篇 |
2009年 | 1779篇 |
2008年 | 3057篇 |
2007年 | 3099篇 |
2006年 | 2750篇 |
2005年 | 2672篇 |
2004年 | 2289篇 |
2003年 | 1975篇 |
2002年 | 1735篇 |
2001年 | 1054篇 |
2000年 | 831篇 |
1999年 | 592篇 |
1998年 | 436篇 |
1997年 | 393篇 |
1996年 | 486篇 |
1995年 | 310篇 |
1994年 | 361篇 |
1993年 | 373篇 |
1992年 | 400篇 |
1991年 | 342篇 |
1990年 | 324篇 |
1989年 | 321篇 |
1988年 | 260篇 |
1987年 | 263篇 |
1986年 | 227篇 |
1985年 | 396篇 |
1984年 | 364篇 |
1983年 | 253篇 |
1982年 | 344篇 |
1981年 | 331篇 |
1980年 | 287篇 |
1979年 | 275篇 |
1978年 | 264篇 |
1976年 | 273篇 |
1975年 | 249篇 |
1974年 | 252篇 |
1973年 | 249篇 |
排序方式: 共有10000条查询结果,搜索用时 268 毫秒
991.
Massue J Bellec N Chopin S Levillain E Roisnel T Clérac R Lorcy D 《Inorganic chemistry》2005,44(24):8740-8748
The reaction of tris(alkylthio)tetrathiafulvalene thiolates with 3-chloro-2,4-pentanedione affords tetrathiafulvalene (TTF) moieties substituted by the acetylacetone function (TTFSacacH), precursors of novel redox-active ligands: the acetylacetonate ions (TTFSacac). These TTFSacacHs have been characterized by X-ray diffraction analyses, and similar trends have been observed, such as a TTF core almost planar and the acetylacetone substituent located in a plane almost perpendicular to the plane formed by the TTF core. Their chelating ability has been demonstrated by the formation of the corresponding M(TTFSacac)2(pyridine)2 complexes in the presence of M(II)(OAc)2.H2O (M = Ni2+, Zn2+). These complexes with TTFSacac moieties, Ni(TTFSacac)2(pyridine)2, 6b, and Zn(TTFSacac)2(pyridine)2, 7b, have been characterized by X-ray diffraction analyses, showing in all structures the metal(II) center chelated by two TTFacac units in the equatorial plane and the octahedral coordination geometry around the metal completed by two axial pyridine ligands. Cyclic voltammetry and UV-visible-near infrared spectroscopic measurements have evidenced a sizable interaction between the two electroactive ligands and the stabilization of a mixed-valence state in the one-electron oxidized complexes. 相似文献
992.
993.
Fernández-G Juan M. Ruíz-Ramírez Oscar L. Toscano Rubén A. Macías-Ruvalcaba Norma Aguilar-Martínez Martha 《Transition Metal Chemistry》2000,25(5):511-517
Three Schiff base copper(II) complexes have been prepared and characterized by elemental analysis, mass spectra, i.r., electronic spectra, eff and X-ray crystal structures. Cyclic voltammetry studies on the complexes indicate a dependence of the cathodic potentials upon electronic effects, but independence on the solid state structure. 相似文献
994.
Determination of rotenone residues in raw honey by solid-phase extraction and high-performance liquid chromatography 总被引:8,自引:0,他引:8
Jiménez JJ Bernal JL del Nozal MJ Novo M Higes M Llorente J 《Journal of chromatography. A》2000,871(1-2):67-73
A method for determining residues of the insecticide rotenone in raw-honey by high-performance liquid chromatography (HPLC) is described. To extract the residues, organic solvents such as ethyl acetate, n-hexane/dichloromethane and solid-phase extraction with octadecylsilane cartridges or Florisil packed columns were tested. Determination was carried out by reversed-phase HPLC using acetonitrile-buffer phosphate (pH 7) (60:40, v/v) as mobile phase and detection at 210 nm. Although the data showed that the two extraction methods were able to isolate the pesticide residues, the extraction on octadecylsilane cartridges was preferred due to its simplicity and higher recovery. Recoveries depended strongly on the fortification level for the two extraction procedures. Practical determination limits of 0.015 mg/kg were obtained. In the analysis of honeys, from beehives treated with rotenone at therapeutical doses for 1 month, residual amounts below 0.2 mg/kg were found. 相似文献
995.
Van T.H. Nguyen Joachim T. Anders Qingjun Ma Regine Herbst-Irmer Peter Langer 《Tetrahedron》2007,63(52):12975-12985
Acid-mediated ring-transformations of 5-alkylidene-2,5-dihydropyrrol-2-ones, available by cyclization of 1,3-diketone dianions with bis(imidoyl) dichlorides of oxalic acid, resulted in formation of functionalized pyran-4-ones, such as 5,6-dihydrobenzo[h]chromones and 5,6,7,8-tetrahydrochromones. 相似文献
996.
J. Marton S. Hosztafi S. Berényi C. Simon S. Makleit 《Monatshefte für Chemie / Chemical Monthly》1994,125(11):1229-1239
Zusammenfassung Dihydronorthevinon (2b) wurde aus Dihydrothevinon (2a) mit Azodicarbonsäurediethylester (DEAD) hergestellt und zu einigen neuen N-substituierten Dihydronorthevinon-Derivaten (2c–2g) umgesetzt. Es wurdenGrignard-Reaktionen dieser Verbindungen mit Methylmagnesiumiodid bzw.tert-Butylmagnesiumchlorid durchgeführt. O-Demethylierungen von3a–3j ergaben die entsprechenden N-substituierten Buprenorphin- bzw. Diprenorphin-Analoga4a–4j.
Preparation of 6,14-ethenomorphinan derivatives
Summary Dihydronorthevinone (2b) was prepared from dihydrothevinone (2a) with diethyl azodicarboxylate (DEAD) and transformed into a number of new N-substituted dihydronorthevinone derivatives (2c–2g).Grignard reactions of the new compounds with methylmagnesium iodide andtert-butyl-magnesium chloride were studied. O-Demethylation of3a–3j resulted in the corresponding N-substituted buprenorphine and diprenorphine analogs4a–4j.相似文献
997.
V. Ščasnár M. Zemánek L. Šoltés M. Lukácsová 《Journal of Radioanalytical and Nuclear Chemistry》1989,134(2):433-436
An HPLC method was developed for studying the stability of [3H] stobadin stored in physiological saline or buffers at various pH. Optimum conditions for the separation of degradation products were elaborated. A high-performance liquid chromatograph equipped with a radioactivity detector was used. 相似文献
998.
Let Ω be an open subset of , N ? 3, containing 0. We consider the solutions of ?Δu(x) + g(u(x)) = f(x) in Ω-{0}, where g is nondecreasing and f is bounded and we study the possible singularities at 0: when u(x) = o(|x|1 ? N) we prove that u is isotropic near 0 and show that either it is a C1 function in Ω (removable singularity) or |x|N ? 2u(x) → c, c ≠ 0 (weak singularity) or |x|N ? 2 |u(x) |→ + ∞ (strong singularity). We also characterize the g's for which solutions with a weak singularity exist and improve a previous removability result of H. Brézis and L. Véron (Arch. Rational Mech. Anal.23 (1979), 153–166). 相似文献
999.
G. I. Burdé 《Fluid Dynamics》1985,20(6):934-941
This paper studies the stability of the adiabatic unsteady motions of a perfect gas for which the velocity is proportional to the distance to the center of symmetry. The solutions of the gas dynamic equations of such a form and their physical interpretation are considered in [1]. The stability of one of the solutions, which corresponds to the pulsations of uniform gravitating spheres, was studied in paper [2]. The stability of the other solutions (gas motions in the absence of Newtonian gravitation, nonperiodic motions of gravitating gas) has not previously been investigated. The stability of such solutions is investigated in this paper and it is shown that the motions in which gas compression occurs are, as a rule, unstable.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 6, pp. 120–127, November–December, 1985. 相似文献
1000.
In connection with an optimization problem, all functions ?: In → with continuous nonzero partial derivatives and satisfying for all xi, xj ≠ I, i, j = 1,2,…, n (n > 2) are determined (I is an interval of positive real numbers). 相似文献