全文获取类型
收费全文 | 6270篇 |
免费 | 278篇 |
国内免费 | 55篇 |
专业分类
化学 | 4251篇 |
晶体学 | 42篇 |
力学 | 217篇 |
综合类 | 2篇 |
数学 | 777篇 |
物理学 | 662篇 |
无线电 | 652篇 |
出版年
2024年 | 10篇 |
2023年 | 60篇 |
2022年 | 232篇 |
2021年 | 342篇 |
2020年 | 250篇 |
2019年 | 271篇 |
2018年 | 249篇 |
2017年 | 196篇 |
2016年 | 330篇 |
2015年 | 230篇 |
2014年 | 243篇 |
2013年 | 596篇 |
2012年 | 383篇 |
2011年 | 407篇 |
2010年 | 256篇 |
2009年 | 221篇 |
2008年 | 274篇 |
2007年 | 284篇 |
2006年 | 236篇 |
2005年 | 201篇 |
2004年 | 182篇 |
2003年 | 153篇 |
2002年 | 155篇 |
2001年 | 63篇 |
2000年 | 63篇 |
1999年 | 48篇 |
1998年 | 34篇 |
1997年 | 46篇 |
1996年 | 42篇 |
1995年 | 39篇 |
1994年 | 45篇 |
1993年 | 33篇 |
1992年 | 36篇 |
1991年 | 30篇 |
1990年 | 47篇 |
1989年 | 26篇 |
1988年 | 25篇 |
1987年 | 29篇 |
1986年 | 27篇 |
1985年 | 33篇 |
1984年 | 20篇 |
1983年 | 18篇 |
1982年 | 25篇 |
1981年 | 20篇 |
1980年 | 14篇 |
1979年 | 17篇 |
1978年 | 11篇 |
1977年 | 12篇 |
1976年 | 9篇 |
1975年 | 7篇 |
排序方式: 共有6603条查询结果,搜索用时 125 毫秒
991.
Mohamed R. Shaaban 《Journal of fluorine chemistry》2008,129(12):1156-1161
4,4,4-Trifluoro-1-(thien-2-yl)butane-1,3-dione (1) reacts with 5-aminopyrazole, 1,2,4-aminotriazole and 2-aminobenzimidazole derivatives, in the presence of triethylorthoformate under pressurized microwave irradiation to afford the corresponding trifluoromethyl derivatives of pyrazolo[1,5-a]pyrimidine, 1,2,4-triazolo[1,5-a]pyrimidine, and pyrimido[1,2-a]benzimidazoles. Also, compound 1 couples readily with azole diazonium salts to give pyrazolo[5,1-c]triazine, benzimidazo[5,1-c]1,2,4-triazine, and triazolo[3,4-c]1,2,4-triazine derivatives incorporating trifluoromethyl group. 相似文献
992.
Saleh M. Al‐Mousawi Morsy A. El‐Apasery Najat Al‐Kandery Mohamed H. Elnagdi 《Journal of heterocyclic chemistry》2008,45(2):359-364
993.
Mohamed Ghazzali Vratislav Langer Lars Öhrström 《Journal of solid state chemistry》2008,181(9):2191-2198
Two new dual-metal assemblies: 2[Ru(phen)3]2+·[Fe(SCN)4]2−·2SCN−·4H2O 1 and [Ru(phen)3]2+·[Co(SCN)4]2−2, (phen:1,10-phenanthroline), have been prepared and their structures were characterized by X-ray diffraction. In 1, the cationic octahedral enantiomers are arranged with a ΛΔΛΔΛ sequence supported by π-π stacking and the anionic inorganic tetrahedral units are oriented between these stacks by interacting with the nearby water molecules through strong O-H?O and O-H?S hydrogen bonds. In 2, homochiral double helices in the b-direction are revealed, with tetrakis-isothiocyanate CoII anions arranged in the crystal to furnish one-dimensional (1D)-helical chains with S?S intermolecular interactions at 3.512(2) and 3.966(2) Å supporting [Ru(phen)3]2+Λ- and Δ-helices with Ru?Ru shortest distance of 8.676(7) Å. In both 1 and 2, the supramolecular assembly is maintained by C-H?S hydrogen bonds extending between the phenanthroline aromatic carbons in the cationic nodes and the sulphur atoms of the isothiocyanates anions. Analysis of S?S interactions in isothiocyanate containing compounds using Cambridge structural database (CSD) showed an angle dependence categorizing these interactions into “type-I” and “type-II”. 相似文献
994.
Somnath Roy Tarak Nath Mandal Anil Kumar Barik Samik Gupta Ray J. Butcher Mohamed Salah El Fallah Javier Tercero Susanta Kumar Kar 《Polyhedron》2008
The synthesis and structure of a pyrazole-based orthogonal ferromagnetically coupled tetracopper(II) 2 × 2 homoleptic grid complex [Cu4(PzOAPyz)4(ClO4)2](ClO4)2 · 6H2O (1), formed by the reaction between the ditopic ligand PzOAPyz and Cu(ClO4)2 · 6H2O, are described. The ligand contains terminal pyrazole and pyrazine residues bound to a central flexible diazine subunit (N–N) as well as one potentially bridging alkoxo group. The two adjacent metal centers are linked by an alkoxo oxygen forming essentially a square Cu4(μ-O4) cluster. In the Cu4(μ-O4) core, out of the four copper centers, two copper centers are penta-coordinated and the remaining two are hexa-coordinated. In each case of hexa-coordination, the sixth position is occupied by one of the oxygen atoms of a coordinated perchlorate ion. Complex 1 has been characterized structurally and magnetically. Although the large Cu–O–Cu bridge angles (137–138°) and short Cu–Cu distances (3.964–3.970 Å) are suitable for the transmission of the expected antiferromagnetic coupling, the square-based Cu4(μ-O4) cluster exhibits an intramolecular ferromagnetic exchange (J = 7.47 cm−1) between the metal centers with an S = 2 magnetic ground state associated with the quasi orthogonal arrangement of the magnetic orbitals (dx2-y2). The exchange pathway parameters have been evaluated from density functional calculations. 相似文献
995.
This work presents quantification of proanthocyanidins (PAs) isolated from grapevine using disposable screen-printed carbon electrodes (SPCE). Procyanidin B2 (B2) used as a model to investigate the electrochemical characteristics of complicated PAs structures in Britton Robinson buffer solution using cyclic voltammetry and square wave voltammetry. B2 exhibits a well-defined reversible redox wave at +0.49 V vs. Ag/AgCl. Significantly, the B2 was determined over a linear concentration range of 3.45–34.6 μM with a detection limit of 2.07 μM. The SPCE was used to analyze PAs in grapevine samples, and the results were consistent with those obtained using Folin-Ciocalteu standard method. 相似文献
996.
EISSA Abd EI-Monem Mohamed Farag 《中国化学》2008,26(8):1509-1514
A variety of pyrimidine derivatives 2—4 and annulated pyrimidine derivatives 5—17 have been synthesized via a sequence of heterocyclization reaction of readily available 6-naphthyl-4-oxo-2-thioxo-1,2,3,4-tetrahydroprimi- 相似文献
997.
Ahmed A. Mohamed Hanan E. Abdou Andrew Mayer John P. Fackler Jr. 《Journal of Cluster Science》2008,19(4):551-559
Abstract Attempts to remove the halide atoms from [Au2(hpp)2Cl2], 1, Hhpp = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine, with Ag(I) benzoate lead to the formation of the Au(I)–Ag(I) product, [(PhCOO)2Au4(hpp)4Ag2(PhCOO)4], 2. This material is stable to air and light at room temperature and shows a UV–vis spectrum in THF with absorbances at 575,
440, 345, and 273 nm. The mixed metal product crystallizes as green crystals in the monoclinic space group P21/n. The Au–Au distances of 2.4473(19) ? are the shortest gold–gold distances reported to date. The gold···silver distance is
3.344(3) ? and the silver···silver distance is 2.771(6) ?. This latter distance is short compared with the Ag···Ag distance
of 2.902(3) ? in the eight-membered silver benzoate dimer starting material. The Au(II) hpp and Ag(I) benzoate components
are linked by carboxylate groups and two gold-silver interactions. This result stands in structural contrast to terminal carboxylate
products observed with Au(II) ylides and amidinates wherein the carboxylate is not bridging to another metal atom.
Index Abstract Three equivalents of silver benzoate react with [Au2(hpp)2Cl2], 1, Hhpp = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine, to form the gold(II)-silver(I) product, 2, [(PhCOO)2Au4(hpp)4Ag2(PhCOO)4]. The gold–gold distance of 2.4473(19) ? is the shortest gold–gold distance reported to date. The gold–silver distance is
3.344(3) ? and the silver–silver distance is 2.771(6) ?.
Dedicated to the memory of F. Albert Cotton (1930–2007). 相似文献
998.
A capillary electrophoresis (CE) method has been developed that allows the separation and estimation of primaquine enantiomers using hydroxypropyl-gamma-cyclodextrin (HP-gamma -CD) as a chiral selector. The influence of chemical and instrumental parameters on the separation, such as type and concentration of CD, buffer concentration, buffer pH, applied voltage, capillary temperature, and injection time, were investigated. Good separation of the racemic mixture of primaquine was achieved using a fused-silica capillary (52.5 cm effective length x 50 microm id) and a background electrolyte composed of tris-phosphate buffer solution (50 mM, pH 2.5) containing 15 mM HP-gamma-CD as a chiral selector. The recommended applied voltage, capillary temperature, and injection time were 15 kV, 25 degrees C, and 6 s, respectively. Within-day and interday reproducibility of peak area and migration time gave relative standard deviation values ranging from 1.05-3.30%. Good recoveries (range of 96.8-104.9%) were obtained from the determination of placebos that were spiked with 0.25-1.00 mg/L primaquine. The proposed CE method was successfully applied to the assay of primaquine diphosphate in pharmaceutical formulations (tablets). 相似文献
999.
Rishard MZ Brown EA Ausman LK Drucker S Choo J Laane J 《The journal of physical chemistry. A》2008,112(1):38-44
The cavity ringdown spectra of 2-cyclohexen-1-one (2CHO) and its 2,6,6-d3 isotopomer (2CHO-d3) have been recorded in the spectral region near their S1(n,pi)<--S0 band origins which are at 26,081.3 and 26,075.3 cm-1, respectively. The data allow several of the quantum states of nu39, the ring inversion, to be determined for both the ground and excited electronic states. These were utilized to calculate the one-dimensional potential energy functions which best fit the data. The barriers to inversion for the S0 and S1(n,pi) states were found to be 1,900 +/- 300 and 3,550 +/- 500 cm-1, respectively. Density functional theory calculations predict values of 2,090 and 2,265 cm-1, respectively. 相似文献
1000.
Microsomal prostaglandin E synthase-1 (mPGES-1) is a promising target for development of next-generation anti-inflammatory drugs. It is crucial for rational design of the next-generation anti-inflammatory drugs to know the three-dimensional (3D) structure of mPGES-1 trimer and to understand how mPGES-1 binds with substrates and inhibitors. In the current work, a 3D structural model of human mPGES-1 trimer has been developed, for the first time, by performing combined homology modeling, molecular docking, and molecular dynamics simulation. The 3D structural model enables us to understand how mPGES-1 binds with its substrates/inhibitors, and the key amino acid residues for the mPGES-1 binding with ligands have been identified. The detailed 3D structures and calculated binding free energies for mPGES-1's binding with substrates and inhibitors are all consistent with available experimental data, suggesting that the 3D model of the mPGES-1 trimer and the enzyme-ligand binding modes are reasonable. The new structural insights obtained from this study should be valuable for rational design of next-generation anti-inflammatory drugs. 相似文献