全文获取类型
收费全文 | 5967篇 |
免费 | 154篇 |
国内免费 | 46篇 |
专业分类
化学 | 3693篇 |
晶体学 | 29篇 |
力学 | 234篇 |
数学 | 1277篇 |
物理学 | 934篇 |
出版年
2022年 | 31篇 |
2021年 | 59篇 |
2020年 | 67篇 |
2019年 | 70篇 |
2018年 | 59篇 |
2017年 | 49篇 |
2016年 | 97篇 |
2015年 | 117篇 |
2014年 | 140篇 |
2013年 | 311篇 |
2012年 | 299篇 |
2011年 | 335篇 |
2010年 | 226篇 |
2009年 | 215篇 |
2008年 | 312篇 |
2007年 | 317篇 |
2006年 | 308篇 |
2005年 | 275篇 |
2004年 | 272篇 |
2003年 | 219篇 |
2002年 | 209篇 |
2001年 | 100篇 |
2000年 | 96篇 |
1999年 | 98篇 |
1998年 | 73篇 |
1997年 | 62篇 |
1996年 | 75篇 |
1995年 | 64篇 |
1994年 | 71篇 |
1993年 | 77篇 |
1992年 | 64篇 |
1991年 | 41篇 |
1990年 | 73篇 |
1989年 | 48篇 |
1988年 | 45篇 |
1987年 | 50篇 |
1986年 | 43篇 |
1985年 | 66篇 |
1984年 | 80篇 |
1983年 | 46篇 |
1982年 | 76篇 |
1981年 | 77篇 |
1980年 | 91篇 |
1979年 | 75篇 |
1978年 | 78篇 |
1977年 | 76篇 |
1976年 | 72篇 |
1975年 | 51篇 |
1974年 | 38篇 |
1973年 | 59篇 |
排序方式: 共有6167条查询结果,搜索用时 718 毫秒
81.
Berger A Djukic JP Pfeffer M de Cian A Kyritsakas-Gruber N Lacour J Vial L 《Chemical communications (Cambridge, England)》2003,(5):658-659
In the presence of large amounts of [Me4N]Cl, the reaction of ortho-chloromercurated 2-[(eta6-phenyl)tricarbonylchromium]pyridine with mu-chloro cyclopalladated aromatic compounds yields a series of new heteroleptic heterodinuclear cis-(C--N)2Pd(II) complexes, which are valuable precursors of planar chiral cyclopalladated (eta6-arene)Cr(CO)3 complexes. 相似文献
82.
EDA-complexes of bicyclo[2,2,n]alkadienes (n = 1, 2, 3, 4) ( 1 (n)-series), 1,4-cyclohexadiene ( 2 ) and various other cyclic monoenes, dienes and trienes as donors and tetracyanoethylene (TCNE) as acceptor were investigated. Spectroscopic and thermodynamic constants of the complexes were determined and correlated with the ionisation potentials (ID) of the hydrocarbon donors obtained from PE. spectroscopy. The nature of the dominant energy contributions to the ground state and the two lowest CT-states of these weak complexes is discussed and structural conclusions are drawn. The role of the complexes in the addition reaction of the hydrocarbon components and TCNE is discussed. The homo Diels-Alder addition product of 1 (2) and TCNE, 9,9,10,10-tetracyanoquadricyclo[2,2,2,02,6,23,5]decane, and the ‘ene’-addition product of 2 and TCNE, 5-[1′,1′,2′,2′-tetracyanoethyl]-1,3-cyclohexadiene were prepared and characterized. Preliminary results for the mechanistic scheme governing the dehydrogenation of 2 by TCNE are reported. 相似文献
83.
Reaction of N-tert-butyloxycarbonylasparagine (Boc-Asn) with 2 equiv of benzyl bromide in presence of cesium carbonate led to N-benzyl-3-Boc-amino-pyrrolidin-2,5-dione 1a (N-benzyl-3-Boc-aminosuccinimide). Borane dimethylsulfide reduced 3-Boc-aminopyrrolidine-2,5-dione 1a into 3-Boc-aminopyrrolidin-2-one 2a. The same procedure could also be used to prepare derivatives 1 and 2 substituted on the aromatic ring. 相似文献
84.
In the presence of a catalytic amount of BH3·Me2S, TiCl4 or Me3SiCl, LiBH4 or NaBH4 are capable of hydroborating alkenes by following the unusual order of decreasing reactivity: tetramethylethylene > 1-methylcyclohexene > cyclohexene; the key step of the catalytic cycle is the exchange reaction between LiBH4 and the mono- or dialkylboranes resulting from hydroboration of the more substituted alkenes with BH3. 相似文献
85.
Joëlle Mayrargue Michel Duchon D'Engenires Marcel Miocque 《Journal of heterocyclic chemistry》1978,15(2):263-267
By cyclization in aqueous media, 2-aminomethyl 1-alcynyl 1-cyclohexanols lead to perhydroisoindolinium or perhydroisoquinolinium salts. The stereochemistry of isolated diastereoisomers is elucidated. A stereoselectivity favouring trans junction of the rings is pointed out. 相似文献
86.
Michel Spiesser Robert P. Gruska S.N. Subbarao Carlos A. Castro Aaron Wold 《Journal of solid state chemistry》1978,26(2):111-114
Crystals of antimony-doped In2Se3 were grown by the Bridgeman method. This compound, whose composition is In1.8Sb0.2Se3, appears to be isostructural with In1.9As0.1Se3. The refined unit cell parameters are a = 3.97(1), c = 18.87(1) Å. Orthorhombic crystals of InSbSe3 were grown from an isothermal melt. The refined unit cell parameters are a = 9.43(1), b = 14.02(5), and c = 3.96(1) Å. These parameters agree with those determined for α-InSbSe3 by other studies. The observed densities measured by a hydrostatic technique are 5.98(3) g/cm3 for In1.8Sb0.2Se3 and 6.07(2) g/cm3 for InSbSe3. The room temperature dc resistivity for In1.8Sb0.2Se3 has been found to be 4.4 × 104 Ω-cm, whereas that of InSbSe3 has been found to be 15.2(1) Ω-cm. A resistivity versus temperature study has beenn carried out for InSbSe3 between 230 and 400°K. Optical studies indicate that In1.8Sb0.2Se3 is an n-type semiconductor with a band gap of 1.1 eV and InSbSe3 is a p-type semiconductor with a band gap of 0.92 eV. 相似文献
87.
Alain Merschaert Laurent DelhayeJean-Paul Kestemont Willy BrionePieter Delbeke Vincent MancusoFreddy Napora Khalid DikerDaniel Giraud Michel Vanmarsenille 《Tetrahedron letters》2003,44(24):4531-4534
A stereoselective approach has been developed for the synthesis of cis- and trans-2-methyl-4-arylpiperidines from a common intermediate. The Ni-catalyzed hydrogenolysis of N-Boc-2-methyl-4-aryl-4-piperidinols, obtained by addition of organometallic reagents on N-Boc-2-methyl-4-piperidone, afforded the trans derivatives with up to 95% selectivity whereas the corresponding cis isomers were obtained in the presence of palladium catalysts. 相似文献
88.
Quantification of veterinary antibiotics (sulfonamides and trimethoprim) in animal manure by liquid chromatography-mass spectrometry 总被引:22,自引:0,他引:22
Haller MY Müller SR McArdell CS Alder AC Suter MJ 《Journal of chromatography. A》2002,952(1-2):111-120
A fast and cost effective method was developed to extract and quantify residues of veterinary antimicrobial agents (antibiotics) in animal manure by liquid-liquid extraction and liquid chromatography-mass spectrometry. The compounds investigated include six sulfonamides, one metabolite, and trimethoprim. The method was performed without sample clean up. Recoveries from spiked manure slurry samples (spike level = 1 mg/kg) were as follows: sulfaguanidine (52%), sulfadiazine (47%), sulfathiazole (64%), sulfamethazine (89%), its metabolite N4-acetyl-sulfamethazine (88%), sulfamethoxazole (84%), sulfadimethoxine (51%), and trimethoprim (64%). Relative standard deviations of the recoveries were less than 5% within the same day and less than 20% between days. The limit of quantification was below 0.1 mg/kg liquid manure slurry for all compounds and calibration curves obtained from extracts of spiked samples were linear up to a level of 5 mg/kg liquid manure, except for trimethoprim (0.01-0.5 mg/kg). Analysis of six grab samples taken in Switzerland from manure pits on farms where medicinal feed had been applied revealed total sulfonamide concentrations of up to 20 mg/kg liquid manure. 相似文献
89.
Thierry Le Borgne Pierre Thury Michel Ephritikhine 《Acta Crystallographica. Section C, Structural Chemistry》2002,58(1):m8-m9
The ligand 1,2,3,4‐tetramethyl‐5‐(2‐pyridyl)cyclopentadiene (cp*py) forms a dinuclear complex with UIV, i.e. di‐μ‐oxo‐bis{chloro(diethyl ether‐κO)[(η5,κN)‐1,2,3,4‐tetramethyl‐5‐(2‐pyridyl)cyclopentadienyl]uranium(IV)}, [U2Cl2O2(C14H16N)2(C4H10O)2], in which cp*py acts as a chelating ligand, being bound to the metal atom by the cyclopentadienyl unit and also by the N atom of the pyridyl ring. 相似文献
90.
Abdelaziz GHERIB Ahmed AOUISSI Alain RIVES Michel FOURNIER Robert HUBAUT 《催化学报》2007,28(12):1041-1046
制备了用于正己烷临氢异构化的氧化硅担载的Keggin型杂多酸和还原态Pt-Ce氧化物催化剂.这些混合氧化物催化剂或单独使用,或与氧化硅担载的杂多酸催化剂一起使用.用X射线衍射、傅里叶变换红外光谱、傅里叶变换拉曼光谱、热重和N2物理吸附等方法表征在氧化、还原态和催化实验后的催化剂状态.本文的目的是在最佳反应条件(反应温度、还原温度和流量等)下,筛选出具有最好异构化收率的催化剂.结果表明,还原氧化物的存在不但有利于消除积炭,而且在反应期间能有效地维持氧化硅担载杂多酸的结构.从这些催化剂样品中,可以找到具有更好异构化活性和选择性的催化剂. 相似文献