全文获取类型
收费全文 | 3608篇 |
免费 | 227篇 |
国内免费 | 405篇 |
专业分类
化学 | 4031篇 |
晶体学 | 16篇 |
力学 | 4篇 |
综合类 | 17篇 |
物理学 | 172篇 |
出版年
2024年 | 8篇 |
2023年 | 24篇 |
2022年 | 93篇 |
2021年 | 94篇 |
2020年 | 146篇 |
2019年 | 126篇 |
2018年 | 100篇 |
2017年 | 122篇 |
2016年 | 123篇 |
2015年 | 143篇 |
2014年 | 154篇 |
2013年 | 305篇 |
2012年 | 204篇 |
2011年 | 180篇 |
2010年 | 171篇 |
2009年 | 188篇 |
2008年 | 215篇 |
2007年 | 191篇 |
2006年 | 168篇 |
2005年 | 175篇 |
2004年 | 190篇 |
2003年 | 176篇 |
2002年 | 132篇 |
2001年 | 78篇 |
2000年 | 66篇 |
1999年 | 73篇 |
1998年 | 83篇 |
1997年 | 55篇 |
1996年 | 71篇 |
1995年 | 54篇 |
1994年 | 57篇 |
1993年 | 65篇 |
1992年 | 46篇 |
1991年 | 29篇 |
1990年 | 19篇 |
1989年 | 17篇 |
1988年 | 13篇 |
1987年 | 9篇 |
1986年 | 15篇 |
1985年 | 10篇 |
1984年 | 15篇 |
1983年 | 8篇 |
1982年 | 9篇 |
1981年 | 6篇 |
1980年 | 5篇 |
1979年 | 5篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1973年 | 1篇 |
1972年 | 1篇 |
排序方式: 共有4240条查询结果,搜索用时 15 毫秒
41.
C. Lavilla A. Alla A. Martínez de Ilarduya E. Benito M.G. García‐Martín J.A. Galbis S. Muñoz‐Guerra 《Journal of polymer science. Part A, Polymer chemistry》2012,50(16):3393-3406
Random poly(hexamethylene terephthalate‐co‐galactarate)s and poly(dodecamethylene terephthalate‐co‐galactarate)s copolyesters covering the whole range of compositions were obtained with weight‐average molecular weights of ~30,000–50,000 g mol?1 by melt polycondensation. They were thermally stable above 300 °C, and displayed Tg in the +20 to ?20 °C range with values steadily decreasing with the content in galactarate units. All the copolyesters were semicrystalline with Tm between 50 and 150 °C and those made from dodecanediol were able to crystallize from the melt at a crystallization rate depending on composition. Copolyesters containing up to 50% of galactaric units retained the crystal structure of their respective polyterephthalate homopolyesters, whereas they adopted the structure of the respective polygalactarates when the content in Galx units reached 70%. Stress‐strain essays revealed decay in the mechanical parameters as the aromatic units were replaced by Galx. Incubation in aqueous buffer revealed that hydrolysis of the polyesters were largely enhanced by copolymerization and evidenced the capacity of the Galx unit for making aromatic polyesters susceptible to biodegradation. A detailed NMR analysis complemented by SEM observations indicated that hydrolysis took place by preferred splitting of galactarate ester bonds with releasing of alkanediol and Galx‐diacid. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012 相似文献
42.
《Journal of Coordination Chemistry》2012,65(18):3188-3204
Four organic–inorganic crystals, [(HL1)2(ZnCl4)]·H2O (1) (L1?=?2-methylquinoline), [(HL1)2(CuCl4)] (2), [(HL2)2SnCl6] (3) (L2?=?6-bromobenzo[d]thiazol-2-amine), and [(HL3)FeCl4] (4) (L3?=?5,7-dimethyl-1,8-naphthyridine-2-amine), derived from N-containing aromatic Brønsted bases and metal(II) chlorides (zinc(II) chloride, copper(II) chloride dihydrate, tin(II) chloride dihydrate, and iron(III) chloride hexahydrate) were prepared at room temperature and characterized by IR, X-ray structure analysis, elemental analysis, and TG analysis. The crystals are built up by perchlorometallates (Zn, Cu, Sn, and Fe) associated with organic cations through multiple non-covalent associations. X-ray diffraction analysis reveals that 1 and 2 have 3-D network structures built from hydrogen bonds between the cations and chlorometallates. Water molecules play an important role in structure extension in 1. Anhydrous 3 and 4 produced from 2-aminoheterocyclic derivatives display 2-D sheet structures. Arrangements of anions and cations are dominated by shape and size of cations, and also by the different structures of the chlorometallates as well as non-bonding interactions in the crystal structures. Except for 1, the other compounds are thermally stable below 240°C. 相似文献
43.
E. Vessally 《Phosphorus, sulfur, and silicon and the related elements》2013,188(10):2636-2644
The molecular structures of S-phenyl-thiabenzene 1, S-phenyl-1-thianaphthalene 2, S-phenyl-2-thianaphthalene 3, and S-phenyl-9-thiaanthracene 4 are studied by ab initio calculations using HF as well as DFT methods at the 6–311+G? level of theory. The non-planar boat conformers of 1–4 with 6π electrons in the heterocyclic ring appear to be more stable than the corresponding planar conformers with 8π electrons in the ring. The activation energy for the inversion at the sulfur atom is compared for 1–4. Conformational flexibility of 1–4 is studied. 相似文献
44.
Neila A. Caplan G. M. Blackburn 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1-4)
Abstract Novel, aromatic bisphosphonates have been synthesised as non-systematic analogues of 1,3-bisphosphoglyceric acid (1,3-BPG). These incorporate non-scissile α-halo and α-methylene phosphonates and have submicromolar K i values for 3 -PGK. 相似文献
45.
Tetrabutylammonium fluoride(TBAF) effectively facilitated a denitrative substitution reaction of electron-deficient nitroarenes with phenylthiotrimethylsilane(PhSTMS) under mild and base-free neutral conditions at room temperature,providing a practical and efficient synthesis of useful unsymmetrical diaryl thioethers.Nitroarenes bearing ortho-and para-positioned electron-withdrawing groups are the most reactive substrates,indicating that this reaction most possibly proceeded via the nucleophilic aromatic substitution(S_NAr) mechanism. 相似文献
46.
Electrochemical bromination of alkyl aromatic compounds by two-phase electrolysis yields the corresponding α, α-dibrominated products. The reaction has been carried out in a single-compartment electrochemical cell using aqueous sodium bromide (40–50%), containing a catalytic amount of HBr as electrolyte, and chloroform, containing an alkyl aromatic compound, as the organic phase with a Pt plate as anode at 10–15°C. Two-phase electrolysis results in high yields (70–90%) of dibromomethyl, bis(bromomethyl), and bis(dibromomethyl) arenes, depending upon the charge passed. 相似文献
47.
A novel protocol for intramolecular ketyl-olefin radical cyclization with low-valent titanium reagent is outlined. It allows the formation of the benzopyran nucleus from ortho-allyloxy propiophenones as the sole product in moderate yields via intramolecular radical cyclization. 相似文献
48.
49.
Regioselectivity of nucleophilic aromatic photosubstitution has been shown experimentally to depend upon activation energies of the competing transition states. Computational means of determining relative activation energies were sought, therefore, in order to predict regioselectivity. Optimization of the three triplet transition states of 2-chloro-4-nitroanisole with hydroxide ion gave energies of insufficient accuracy to predict regioselectivity. Computed enthalpy changes from the first triplet transition state to the triplet σ-complexes correlated precisely with the experimental activation energies. This exemplifies the Bell–Evans–Polanyi Principle, and it provides an accurate means of assessing regioselectivity. 相似文献
50.
Jiang Zhao Meng Lian Yue Yu Xiaogang Yan Xianbin Xu Xiaolong Yang Guijiang Zhou Zhaoxin Wu 《Macromolecular rapid communications》2015,36(1):71-78
A series of novel red phosphorescent polymers is successfully developed through Suzuki cross‐coupling among ambipolar units, functionalized IrIII phosphorescent blocks, and fluorene‐based silane moieties. The photophysical and electrochemical investigations indicate not only highly efficient energy‐transfer from the organic segments to the phosphorescent units in the polymer backbone but also the ambipolar character of the copolymers. Benefiting from all these merits, the phosphorescent polymers can furnish organic light‐emitting diodes (OLEDs) with exceptional high electroluminescent (EL) efficiencies with a current efficiency (η L) of 8.31 cd A−1, external quantum efficiency (η ext) of 16.07%, and power efficiency (η P) of 2.95 lm W−1, representing the state‐of‐the‐art electroluminescent performances ever achieved by red phosphorescent polymers. This work here might represent a new pathway to design and synthesize highly efficient phosphorescent polymers.