全文获取类型
收费全文 | 3676篇 |
免费 | 82篇 |
国内免费 | 19篇 |
专业分类
化学 | 2263篇 |
晶体学 | 24篇 |
力学 | 46篇 |
数学 | 394篇 |
物理学 | 475篇 |
无线电 | 575篇 |
出版年
2023年 | 18篇 |
2022年 | 31篇 |
2021年 | 50篇 |
2020年 | 61篇 |
2019年 | 45篇 |
2018年 | 60篇 |
2017年 | 42篇 |
2016年 | 102篇 |
2015年 | 90篇 |
2014年 | 133篇 |
2013年 | 182篇 |
2012年 | 303篇 |
2011年 | 361篇 |
2010年 | 226篇 |
2009年 | 226篇 |
2008年 | 294篇 |
2007年 | 244篇 |
2006年 | 223篇 |
2005年 | 216篇 |
2004年 | 195篇 |
2003年 | 138篇 |
2002年 | 78篇 |
2001年 | 62篇 |
2000年 | 37篇 |
1999年 | 51篇 |
1998年 | 20篇 |
1997年 | 27篇 |
1996年 | 25篇 |
1995年 | 18篇 |
1994年 | 15篇 |
1993年 | 24篇 |
1992年 | 18篇 |
1991年 | 16篇 |
1990年 | 9篇 |
1989年 | 10篇 |
1988年 | 12篇 |
1987年 | 10篇 |
1986年 | 6篇 |
1985年 | 18篇 |
1984年 | 11篇 |
1983年 | 7篇 |
1982年 | 4篇 |
1981年 | 6篇 |
1980年 | 11篇 |
1979年 | 12篇 |
1978年 | 5篇 |
1977年 | 7篇 |
1976年 | 4篇 |
1974年 | 3篇 |
1966年 | 3篇 |
排序方式: 共有3777条查询结果,搜索用时 15 毫秒
811.
812.
813.
Remedios González-Luque Ignacio Nebot-Gil Manuela Merchán Francisco Tomás 《Theoretical chemistry accounts》1986,69(2):101-106
All-electron and pseudopotential non-empirical calculations have been performed on C
2v and C
s (syn, anti) allyl anion conformations. Using a double-zeta valence-shell basis set within the Epstein-Nesbet definition of the unperturbed Hamiltonian, a value about 19 kcal/mol is found for the barrier to rotation of the allyl anion. This value is the theoretical value obtained with greater accuracy, and the lowest one for the rotational barrier. 相似文献
814.
4-Cyano-1-(2,3,5-tri-O-acetyl-β-D-ribofuranosyl)-5-methylimidazole ( 4 ) and its corresponding 5-cyano-4-methyl substituted isomer ( 5 ) have been obtained by ribosylation of 4(5)-cyano-5(4)-methylimidazole ( 3 ) via the mercuric cyanide method or by ribosylation of the trimethylsilyl derivative of 3 . Treatment of 4 with methanolic ammonia, ammonium chloride in liquid ammonia and potassium hydrosulfide provided 4-cyano-1-β-D-ribofuranosyl-5-methylimidazole ( 6 ), 1-β-D-ribofuranosyl-5-methylimidazole-4-carboxamide ( 2 ) and 1-β-D-ribofuranosyl-5-methylimidazole-4-thiocarboxamide ( 11 ) respectively. Reaction of 6 with hydroxylamine afforded the corresponding 4-carboxamidoxime substituted nucleoside ( 13 ) which on catalytic reduction in the presence of ammonium chloride, was transformed into 1-β-D-ribofuranosyl-5-methylimidazole-4-carboxamidine ( 14 ) as hydrochloride salt. 相似文献
815.
Royo S Martínez-Máñez R Sancenón F Costero AM Parra M Gil S 《Chemical communications (Cambridge, England)》2007,(46):4839-4847
The ease of production, the extreme toxicity of organophosphorus-containing nerve agents, and their facile use in terrorism attacks underscores the need to develop accurate systems to detect these chemicals. Among different technologies we review here recent advances in the design of chromo-fluorogenic methods for the specific detection of nerve agents. Optical sensing (especially colorimetric detection) requires usually low-cost and widely used instrumentation and offers the possibility of so-called "naked eye detection". Recent reported examples suggest that the application of chromo-fluorogenic supramolecular concepts for the chromogenic or fluorogenic sensing of nerve agents might be an area of increasing interest that would allow developing systems able to overcome some of the limitations shown by classical analytical methods. 相似文献
816.
Mario E. Flores Pablo Garcs‐Jerez Daniel Fernndez Gustavo Aros‐Perez Diego Gonzlez‐Cabrera Eduardo lvarez Ignacio Caas Felipe Oyarzun‐Ampuero Ignacio Moreno‐Villoslada 《Macromolecular rapid communications》2016,37(21):1729-1734
The formation of redox‐active, totally organic nanoparticles in water is achieved following a strategy similar to that used to form metal nanoparticles. It is based on two fundamental concepts: i) complexation through aromatic–aromatic interactions of a water‐soluble precursor aromatic molecule with polyelectrolytes bearing complementary charged aromatic rings, and ii) reduction of the precursor molecule to achieve stabilized nanoparticles. Thus, formazan nanoparticles are synthesized by reduction of a tetrazolium salt with ascorbic acid using polyelectrolytes bearing benzene sulfonate residues of high linear aromatic density, but cannot be formed in the presence of nonaromatic polyelectrolytes. The red colored nanoparticles are efficiently encapsulated in calcium alginate beads, showing macroscopic homogeneity. Bleaching kinetics with chlorine show linear rates on the order of tenths of millimeters per minute. A linear behavior of the dependence of the rate of bleaching on the chlorine concentration is found, showing the potential of the nanoparticles for chlorine sensing.
817.
The monohapto neutral 2-(diphenylphosphino)aniline (PNH(2)) complexes [Au(C(6)F(5))(2)X(PNH(2))] (X = C(6)F(5) (1), Cl (2)) have been obtained from [Au(C(6)F(5))(3)(tht)] or [Au(C(6)F(5))(2)(micro-Cl)](2) and PNH(2), and the cationic [Au(C(6)F(5))(2)(PNH(2))]ClO(4) (3) has been similarly prepared from [Au(C(6)F(5))(2)(OEt(2))(2)]ClO(4) and PNH(2) or from 2 and AgClO(4). The neutral amido complex [Au(C(6)F(5))(2)(PNH)] (4) can be obtained by deprotonation of 3 with PPN(acac) (acac = acetylacetonate) or by treatment of the chloro complex 2 with Tl(acac). It reacts with [Ag(OClO(3))(PPh(3))] or [Au(OClO(3))(PPh(3))] to give the dinuclear species [Au(C(6)F(5))(2)[PNH(MPPh(3))]]ClO(4) (M = Ag (5), Au (6)). The latter can also be obtained by reaction of equimolar amounts of 3 and [Au(acac)(PPh(3))]; when the molar ratio of the same reagents is 1:2, the trinuclear cationic complex [Au(C(6)F(5))(2)[PN(AuPPh(3))(2)]]ClO(4) (7) is obtained. The crystal structures of complexes 2-4 and 7 have been established by X-ray crystallography; the last-mentioned displays an unusual Au(I)-Au(III) interaction. 相似文献
818.
A visible light mediated photocatalytic oxidative coupling reaction of N-phenyl tetrahydroisoquinoline with β-keto acids has been developed. This approach provides a mild and operationally simple access to the synthesis of C1-acylmethylated tetrahydroisoquinolines. 相似文献
819.
José Roberto Medina-Medrano María Daniela Mares-Quiñones Juan Ignacio Valiente-Banuet Monserrat Vázquez-Sánchez Dioselina Álvarez-Bernal Edgar Villar-Luna 《液相色谱法及相关技术杂志》2017,40(17):900-906
Solanum ferrugineum Jacq. is a wild species that in previous analysis reported a significant antioxidant capacity. The aim of our research was to determine total phenolic content (TPC) and total flavonoid content (TFC), and the phenolic composition by HPLC-DAD and HPLC/ESI-MS/TOF of methanolic extracts of S. ferrugineum fruits, collected from Paredones, Jiquilpan, and Fray Dominguez, Pajuacarán in the Mexican state of Michoacán. TPC and TFC were determined by the spectrophotometric Folin–Ciocalteu reagent and the AlCl3 method, respectively. TPC in S. ferrugineum fruit [31.41?±?0.91?mg gallic acid equivalent/g dry tissue (DT)] was similar to those reported for Turkey berry (Solanum torvum Sw.) and eggplant (Solanum melongena L.) fruits. The TFC values of S. ferrugineum fruits (29.14?±?4.99?mg catechin equivalent /g DT) corresponded to 80.24% of the TPC. Eight phenolic compounds (PC) were identified by HPLC analysis. The main PC identified in S. ferrugineum fruits were chlorogenic acid, caffeic acid, p-coumaric acid, gallic acid, quercetin, and kaempferol. S. ferrugineum fruits could be used as a starting material for the extraction of high-value PC with potential applications. 相似文献
820.