全文获取类型
收费全文 | 1435篇 |
免费 | 25篇 |
国内免费 | 8篇 |
专业分类
化学 | 938篇 |
晶体学 | 4篇 |
力学 | 94篇 |
数学 | 211篇 |
物理学 | 221篇 |
出版年
2022年 | 28篇 |
2021年 | 35篇 |
2020年 | 10篇 |
2019年 | 12篇 |
2018年 | 20篇 |
2017年 | 18篇 |
2016年 | 30篇 |
2015年 | 30篇 |
2014年 | 41篇 |
2013年 | 58篇 |
2012年 | 81篇 |
2011年 | 85篇 |
2010年 | 52篇 |
2009年 | 48篇 |
2008年 | 75篇 |
2007年 | 83篇 |
2006年 | 69篇 |
2005年 | 76篇 |
2004年 | 59篇 |
2003年 | 70篇 |
2002年 | 62篇 |
2001年 | 17篇 |
2000年 | 19篇 |
1999年 | 16篇 |
1998年 | 16篇 |
1997年 | 10篇 |
1996年 | 24篇 |
1995年 | 15篇 |
1994年 | 15篇 |
1993年 | 13篇 |
1992年 | 9篇 |
1991年 | 11篇 |
1990年 | 8篇 |
1988年 | 6篇 |
1987年 | 10篇 |
1986年 | 6篇 |
1985年 | 9篇 |
1984年 | 17篇 |
1983年 | 13篇 |
1982年 | 21篇 |
1981年 | 17篇 |
1980年 | 6篇 |
1979年 | 13篇 |
1978年 | 9篇 |
1977年 | 12篇 |
1976年 | 10篇 |
1975年 | 8篇 |
1974年 | 9篇 |
1967年 | 7篇 |
1954年 | 8篇 |
排序方式: 共有1468条查询结果,搜索用时 0 毫秒
41.
Francesca Benedini Giorgio Bertolini Francesco Ferrario Angelo Motti Alberto Sala Flavio Somenzi 《Journal of heterocyclic chemistry》1995,32(1):103-107
The preparation and the physico-chemical characterization of 2H-pyrido[2,3-e]-1,3-oxazine-2,4(3H)-diones, 2H-pyrido[4,3-e]-1,3-oxazine-2,4(3H)-diones, 2H-pyrido[4,3-e]-1,3-oxazin-4(3H)-ones, 2H-thieno[2,3-e]-1,3-oxazin-4(3H)-ones and 2H-thieno[3,4-e]-1,3-oxazine-2,4(3H)-diones are reported. 相似文献
42.
Alberti A Fuochi P Guerra M Macciantelli D Torri G Valerio A Vismara E 《Organic & biomolecular chemistry》2011,9(9):3199-3204
Irradiation of a number of different sutures largely employed in the clinical practice with either high energy electrons or with γ-rays followed by quenching with glycidyl methacrylate (GMA) conveniently led to derivatization through a radical-based process. The radicals involved were detected by means of ESR spectroscopy and were characterized on the basis of their ESR spectral parameters which were also found to be consistent with the hfs constants predicted by DFT calculations. Evidence of the GMA derivatization of the sutures was obtained via(13)C CP-MAS NMR spectroscopy, while its extent was evaluated gravimetrically. 相似文献
43.
Marcella Pani Angelo Mugnoli Enzo Sottofattori 《Acta Crystallographica. Section C, Structural Chemistry》2000,56(7):840-842
In the crystal structure of C12H11ClO, the (planar) molecules give rise to a parallel packing. A model crystal obtained by semi‐empirical and packing‐energy calculations is consistent with the observed structure. 相似文献
44.
Gallo E Caselli A Ragaini F Fantauzzi S Masciocchi N Sironi A Cenini S 《Inorganic chemistry》2005,44(6):2039-2049
A reproducible synthesis of a competent epoxidation catalyst, [Ru(VI)(TPP)(O)2)] (TPP = tetraphenylporphyrin dianion), starting from [Ru(II)(TPP)(CO)L] (L = none or CH3OH), is described. The molecular structure of the complex was determined by using ab initio X-ray powder diffraction (XRPD) methods, and its solution behavior was in detail investigated by NMR techniques such as PGSE (pulsed field gradient spin-echo) measurements. [Ru(IV)(TPP)(OH)]2O, a reported byproduct in the synthesis of [Ru(VI)(TPP)(O)2], was synthesized in a pure form by oxidation of [Ru(II)(TPP)(CO)L] or by a coproportionation reaction of [Ru(VI)(TPP)(O)2] and [Ru(II)(TPP)(CO)L], and its molecular structure was then determined by XRPD analysis. [Ru(VI)(TPP)(O)2] can be reduced by dimethyl sulfoxide or by carbon monoxide to yield [Ru(II)(TPP)(S-DMSO)2] or [Ru(II)(TPP)(CO)(H2O)], respectively. These two species were characterized by conventional single-crystal X-ray diffraction analysis. 相似文献
45.
Photosensitized hydrogen abstraction from 2-alkyl-1,3-dioxolanes by triplet benzophenone gives the corresponding 1,3-dioxolan-2-yl radicals and these are trapped by ,β-unsatured ketones yielding monoprotected 1,4-diketones. With open chain ketones (3-buten-2-one and 4-penten-3-one) the yields are low and competitive pathways in part consume the radicals. With cyclic enones however, yields are good as tested with cyclopentenone, cyclohexenone and 4-hydroxy-cyclopentenone. More generally, this is a viable alternative for the synthesis of 1,4-diketones via radicals while the thermal initiation gives only low yield. The reaction cannot be extended to strongly stabilized radicals, such as the 2-phenyl-1,3-dioxolanyl radical. 相似文献
46.
Palladium‐Catalyzed Cross‐Coupling of Styrenes with Aryl Methyl Ketones in Ionic Liquids: Direct Access to Cyclopropanes 下载免费PDF全文
Dr. Pietro Cotugno Dr. Antonio Monopoli Prof. Francesco Ciminale Dr. Antonella Milella Prof. Angelo Nacci 《Angewandte Chemie (International ed. in English)》2014,53(49):13563-13567
The combined use of Pd(OAc)2, Cu(OAc)2, and dioxygen in molten tetrabutylammonium acetate (TBAA) promotes an unusual cyclopropanation reaction between aryl methyl ketones and styrenes. The process is a dehydrogenative cyclizing coupling that involves a twofold C? H activation at the α‐position of the ketone. The substrate scope highlights the flexibility of the catalyst; a reaction mechanism is also proposed. 相似文献
47.
48.
Luigi Resconi Isabella Camurati Cristina Fiori Davide Balboni Pierluigi Mercandelli Angelo Sironi 《Helvetica chimica acta》2006,89(8):1497-1523
The acid‐catalyzed reaction between formaldehyde and 1H‐indene, 3‐alkyl‐ and 3‐aryl‐1H‐indenes, and six‐membered‐ring substituted 1H‐indenes, with the 1H‐indene/CH2O ratio of 2 : 1, at temperatures above 60° in hydrocarbon solvents, yields 2,2′‐methylenebis[1H‐indenes] 1 – 8 in 50–100% yield. These 2,2′‐methylenebis[1H‐indenes] are easily deprotonated by 2 equiv. of BuLi or MeLi to yield the corresponding dilithium salts, which are efficiently converted into ansa‐metallocenes of Zr and Hf. The unsubstituted dichloro{(1,1′,2,2′,3,3′,3a,3′a,7a,7′a‐η)‐2,2′‐methylenebis[1H‐inden‐1‐yl]}zirconium ([ZrCl2( 1′ )]) is the least soluble in organic solvents. Substitution of the 1H‐indenyl moieties by hydrocarbyl substituents increases the hydrocarbon solubility of the complexes, and the presence of a substituent larger than a Me group at the 1,1′ positions of the ligand imparts a high diastereoselectivity to the metallation step, since only the racemic isomers are obtained. Methylene‐bridged ‘ansa‐zirconocenes’ show a noticeable open arrangement of the bis[1H‐inden‐1‐yl] moiety, as measured by the angle between the planes defined by the two π‐ligands (the ‘bite angle’). In particular, of the ‘zirconocenes’ structurally characterized so far, the dichloro{(1,1′,2,2′,3,3′,3a,3′a,7a,7′a‐η)‐2,2′‐methylenebis[4,7‐dimethyl‐1H‐inden‐1‐yl]}zirconium ([ZrCl2( 5′ )] is the most open. The mixture [ZrCl2( 1′ )]/methylalumoxane (MAO) is inactive in the polymerization of both ethylene and propylene, while the metallocenes with substituted indenyl ligands polymerize propylene to atactic polypropylene of a molecular mass that depends on the size of the alkyl or aryl groups at the 1,1′ positions of the ligand. Ethene is polymerized by rac‐dichloro{(1,1′,2,2′,3,3′,3a,3′a,7a,7′a‐η)‐2,2′‐methylenebis[1‐methyl‐1H‐inden‐1‐yl]}zirconium ([ZrCl2( 2′ )])/MAO to polyethylene waxes (average degree of polymerization ca. 100), which are terminated almost exclusively by ethenyl end groups. Polyethylene with a high molecular mass could be obtained by increasing the size of the 1‐alkyl substituent. 相似文献
49.
50.
Gabriella Ieronimo Alessandro Mondelli Francesco Tibiletti Angelo Maspero Giovanni Palmisano Simona Galli Stefano Tollari Norberto Masciocchi Kenneth M. Nicholas Silvia Tagliapietra Giancarlo Cravotto Andrea Penoni 《Tetrahedron》2013
The thermal reaction between nitrosoarenes and alkynes under alkylating conditions produces N-alkoxyindoles as the major products in moderate to good yields and excellent regioselectivity. Various electrophiles are used affording different N–O-protected hydroxyindoles in a multi-component fashion. Privileged acetylenic substrates used in reactions with substituted nitrosoarenes are arylalkynes or propiolates. Potentially bioactive compounds and other classes of highly functionalizable indole products were prepared. Reactions between o-carbomethoxy-nitrosoarenes and arylacetylenes provided tricyclic compounds containing an acylaziridine indoline skeleton. 相似文献