全文获取类型
收费全文 | 2923篇 |
免费 | 141篇 |
国内免费 | 26篇 |
专业分类
化学 | 2133篇 |
晶体学 | 16篇 |
力学 | 55篇 |
数学 | 509篇 |
物理学 | 377篇 |
出版年
2023年 | 19篇 |
2022年 | 68篇 |
2021年 | 95篇 |
2020年 | 67篇 |
2019年 | 90篇 |
2018年 | 42篇 |
2017年 | 44篇 |
2016年 | 133篇 |
2015年 | 102篇 |
2014年 | 121篇 |
2013年 | 144篇 |
2012年 | 235篇 |
2011年 | 245篇 |
2010年 | 152篇 |
2009年 | 126篇 |
2008年 | 215篇 |
2007年 | 198篇 |
2006年 | 185篇 |
2005年 | 191篇 |
2004年 | 127篇 |
2003年 | 102篇 |
2002年 | 86篇 |
2001年 | 33篇 |
2000年 | 22篇 |
1999年 | 26篇 |
1998年 | 15篇 |
1997年 | 23篇 |
1996年 | 31篇 |
1995年 | 18篇 |
1994年 | 15篇 |
1993年 | 14篇 |
1992年 | 8篇 |
1991年 | 16篇 |
1990年 | 13篇 |
1989年 | 5篇 |
1988年 | 8篇 |
1987年 | 5篇 |
1986年 | 3篇 |
1985年 | 11篇 |
1984年 | 9篇 |
1983年 | 5篇 |
1982年 | 5篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1978年 | 2篇 |
1977年 | 4篇 |
1975年 | 1篇 |
1974年 | 2篇 |
1970年 | 1篇 |
1926年 | 1篇 |
排序方式: 共有3090条查询结果,搜索用时 0 毫秒
101.
Dr. Chandra M. R. Volla Dr. Javier Mazuela Prof. Dr. Jan‐E. Bäckvall 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(25):7608-7612
A highly efficient oxidative carbocyclization–carbonylation reaction cascade of allenynes and enallenes has been developed using a PdII salt in low catalytic amounts under ambient temperature and pressure (1 atm of carbon monoxide). The use of DMSO as an additive was found to be important for an efficient reaction. A wide range of alcohols as trapping reagents were used to give the corresponding esters in good yields. 相似文献
102.
Coordination Chemistry of Cyclopropenylidene‐Stabilized Phosphenium Cations: Synthesis and Reactivity of Pd and Pt Complexes 下载免费PDF全文
Ágnes Kozma Tobias Deden Dr. Javier Carreras Christian Wille Dr. Jekaterina Petuškova Jörg Rust Dr. Manuel Alcarazo 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(8):2208-2214
A straightforward synthesis of cyclopropenylidene‐stabilized phosphenium cations 1 a – g through the reaction of [(iPr2N)2C3+Cl]BF4 with secondary phosphines is described. Their donor ability was evaluated by analysis of the CO stretching frequency in Rh complexes [RhCl(CO)L2](BF4)2 and electrochemical methods. The cyclopropenium ring induces a phosphite‐type behavior that can be tuned by the other two substituents attached to the phosphorus atom. Despite of the positive charge that they bear, phosphenium cations 1 a – g still act as two‐electron donor ligands, forming adducts with PdII and PtII precursors. Conversely, in the presence of Pd0 species, an oxidative insertion of the Pd atom into the Ccarbene–phosphorus bond takes place, providing dimeric structures in which each Pd atom is bonded to a cyclopropenyl carbene while two dialkyl/diaryl phosphide ligands serve as bridges between the two Pd centers. The catalytic performance of the resulting library of PtII complexes was tested; all of the cationic phosphines accelerated the prototype 6‐endo‐dig cyclization of 2‐ethynyl‐1,1′‐biphenyl to afford pentahelicene. The best ligand 1 g was used in the synthesis of two natural products, chrysotoxene and epimedoicarisoside A. 相似文献
103.
Javier A. Cabeza Prof. Ignacio del Río Prof. Enrique Pérez‐Carreño Prof. Vanessa Pruneda 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(18):5425-5436
The cationic cluster complexes [Ru3(μ‐H)(μ‐κ2N,C‐L1 Me)(CO)10]+ ( 1 +; HL1 Me=N‐methylpyrazinium), [Ru3(μ‐H)(μ‐κ2N,C‐L2 Me)(CO)10]+ ( 2 +; HL2 Me=N‐methylquinoxalinium), and [Ru3(μ‐H)(μ‐κ2‐N,C‐L3 Me)(CO)10]+ ( 3 +; HL3 Me=N‐methyl‐1,5‐naphthyridinium), which contain cationic N‐heterocyclic ligands, undergo one‐electron reduction processes to become short lived, ligand‐centered, trinuclear, radical species ( 1 – 3 ) that end in the formation of an intermolecular C? C bond between the ligands of two such radicals, thus leading to neutral hexanuclear derivatives. These dimerization processes are selective, in the sense that they only occur through the exo face of the bridging ligands of trinuclear enantiomers of the same configuration, as they only afford hexanuclear dimers with rac structures (C2 symmetry). The following are the dimeric products that have been isolated by using cobaltocene as reducing agent: [Ru6(μ‐H)2{μ6‐κ4N2,C2‐(L1 Me)2}(CO)18] ( 5 ; from 1 +), [Ru6(μ‐H)2{μ6‐κ4N2,C2‐(L2 Me)2}(CO)18] ( 6 ; from 2 +), and [Ru6(μ‐H)2{μ4‐κ8N2,C6‐(L3 Me)2}(CO)18] ( 7 ; from 3 +). The structures of the final hexanuclear products depend on the N‐heterocyclic ligand attached to the starting materials. Thus, although both trinuclear subunits of 5 and 6 are face‐capped by their bridging ligands, the coordination mode of the ligand of 5 is different from that of the ligand of 6 . The trinuclear subunits of 7 are edge‐bridged by its bridging ligand. In the presence of moisture, the reduction of 3 + with cobaltocene also affords a trinuclear derivative, [Ru3(μ‐H)(μ‐κ2N,C‐L3′ Me)(CO)10] ( 8 ), whose bridging ligand (L3′ Me) results from the formal substitution of an oxygen atom for the hydrogen atom (as a proton) that in 3 + is attached to the C6 carbon atom of its heterocyclic ligand. The results have been rationalized with the help of electrochemical measurements and DFT calculations, which have also shed light on the nature of the odd‐electron species, 1 – 3 , and on the regioselectivity of their dimerization processes. It seems that the sort of coupling reactions described herein requires cationic complexes with ligand‐based LUMOs. 相似文献
104.
Javier Mazuela Alexander Paptchikhine Oscar Pàmies Pher G. Andersson Montserrat Diéguez 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(15):4567-4576
A library of readily available phosphite–oxazole/thiazole ligands ( L1 a – g – L7 a – g ) was applied in the Ir‐catalyzed asymmetric hydrogenation of several largely unfunctionalized E‐ and Z‐trisubstituted and 1,1‐disubstituted terminal alkenes. The ability of the catalysts to transfer chiral information to the product could be tuned by choosing suitable ligand components (bridge length, the substituents in the heterocyclic ring and the alkyl backbone chain, the configuration of the ligand backbone, and the substituents/configurations in the biaryl phosphite moiety), so that enantioselectivities could be maximized for each substrate as required. Enantioselectivities were therefore excellent (enantiomeric excess (ee) values up to >99 %) for a wide range of E‐ and Z‐trisubstituted and 1,1‐disubstituted terminal alkenes. The biaryl phosphite moiety was a very advantageous ligand component in terms of substrate versatility. 相似文献
105.
106.
Dry Reforming of Methane on a Highly‐Active Ni‐CeO2 Catalyst: Effects of Metal‐Support Interactions on C−H Bond Breaking 下载免费PDF全文
B. Sc. Zongyuan Liu Dr. David C. Grinter Dr. Pablo G. Lustemberg Dr. Thuy‐Duong Nguyen‐Phan Dr. Yinghui Zhou B. Sc. Si Luo Dr. Iradwikanari Waluyo Dr. Ethan J. Crumlin Dr. Dario J. Stacchiola Prof. Jing Zhou Dr. Javier Carrasco Prof. H. Fabio Busnengo Dr. M. Verónica Ganduglia‐Pirovano Dr. Sanjaya D. Senanayake Prof. José A. Rodriguez 《Angewandte Chemie (International ed. in English)》2016,55(26):7455-7459
Ni‐CeO2 is a highly efficient, stable and non‐expensive catalyst for methane dry reforming at relative low temperatures (700 K). The active phase of the catalyst consists of small nanoparticles of nickel dispersed on partially reduced ceria. Experiments of ambient pressure XPS indicate that methane dissociates on Ni/CeO2 at temperatures as low as 300 K, generating CHx and COx species on the surface of the catalyst. Strong metal–support interactions activate Ni for the dissociation of methane. The results of density‐functional calculations show a drop in the effective barrier for methane activation from 0.9 eV on Ni(111) to only 0.15 eV on Ni/CeO2?x(111). At 700 K, under methane dry reforming conditions, no signals for adsorbed CHx or C species are detected in the C 1s XPS region. The reforming of methane proceeds in a clean and efficient way. 相似文献
107.
Selective Production of Carbon Monoxide via Methane Oxychlorination over Vanadyl Pyrophosphate 下载免费PDF全文
Vladimir Paunović Guido Zichittella Dr. Réne Verel Dr. Amol P. Amrute Prof. Javier Pérez‐Ramírez 《Angewandte Chemie (International ed. in English)》2016,55(50):15619-15623
A catalytic process is demonstrated for the selective conversion of methane into carbon monoxide via oxychlorination chemistry. The process involves addition of HCl to a CH4–O2 feed to facilitate C?H bond activation under mild conditions, leading to the formation of chloromethanes, CH3Cl and CH2Cl2. The latter are oxidized in situ over the same catalyst, yielding CO and recycling HCl. A material exhibiting chlorine evolution by HCl oxidation, high activity to oxidize chloromethanes into CO, and no ability to oxidize CO, is therefore essential to accomplish this target. Following these design criteria, vanadyl pyrophosphate (VPO) was identified as an outstanding catalyst, exhibiting a CO yield up to approximately 35 % at 96 % selectivity and stable behavior. These findings constitute a basis for the development of a process enabling the on‐site valorization of stranded natural‐gas reserves using CO as a highly versatile platform molecule. 相似文献
108.
Sánchez-Méndez A Benito JM de Jesús E de la Mata FJ Flores JC Gómez R Gómez-Sal P 《Dalton transactions (Cambridge, England : 2003)》2006,(45):5379-5389
The substituted bis(pyrazolyl)methane ligands RCH(3,5-Me2pz)2(R=SiMe3, CH2Ph, G1, G2, and G3; Gn=Fréchet-type dendritic wedges of generation n) have been prepared starting from H2C(3,5-Me2pz)2. Reaction of these didentate ligands with [NiBr2(DME)] is a straightforward procedure that allows the synthesis of the nickel(II) complexes [NiBr2{RCH(3,5-Me2pz)2}]. The molecular structure of compound (R=CH2Ph) has been determined by X-ray diffraction studies. The nickel centre coordinates two bromine and two nitrogen atoms in a tetrahedral environment, and the metallacycle Ni(NN)2C adopts a boat conformation with the benzyl group in an axial position. 1H NMR studies have been carried out to characterize these paramagnetic nickel compounds in solution. Valuable information about the disposition of the ligands and dendritic wedges in solution has been obtained thanks to the influence of the paramagnetic centre on the proton resonances. 相似文献
109.