全文获取类型
收费全文 | 710篇 |
免费 | 25篇 |
国内免费 | 1篇 |
专业分类
化学 | 584篇 |
晶体学 | 14篇 |
力学 | 9篇 |
数学 | 67篇 |
物理学 | 62篇 |
出版年
2023年 | 5篇 |
2022年 | 12篇 |
2021年 | 19篇 |
2020年 | 12篇 |
2019年 | 8篇 |
2018年 | 13篇 |
2017年 | 12篇 |
2016年 | 16篇 |
2015年 | 26篇 |
2014年 | 18篇 |
2013年 | 28篇 |
2012年 | 55篇 |
2011年 | 81篇 |
2010年 | 27篇 |
2009年 | 27篇 |
2008年 | 46篇 |
2007年 | 53篇 |
2006年 | 48篇 |
2005年 | 43篇 |
2004年 | 43篇 |
2003年 | 28篇 |
2002年 | 29篇 |
2001年 | 4篇 |
2000年 | 4篇 |
1999年 | 5篇 |
1998年 | 2篇 |
1997年 | 4篇 |
1996年 | 7篇 |
1995年 | 12篇 |
1994年 | 8篇 |
1993年 | 4篇 |
1992年 | 4篇 |
1991年 | 3篇 |
1988年 | 4篇 |
1986年 | 1篇 |
1985年 | 6篇 |
1984年 | 1篇 |
1982年 | 4篇 |
1981年 | 2篇 |
1980年 | 5篇 |
1979年 | 2篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1938年 | 1篇 |
1934年 | 1篇 |
1905年 | 1篇 |
排序方式: 共有736条查询结果,搜索用时 15 毫秒
711.
Mijangos MV González-Marrero J Miranda LD Vincent-Ruz P Lujan-Montelongo A Olivera-Díaz D Bautista E Ortega A de la Luz Campos-González M Gamez-Montaño R 《Organic & biomolecular chemistry》2012,10(15):2946-2949
Flavones were directly alkylated at the C-3 position in moderate yields using a xanthate-based oxidative radical addition procedure. This methodology is a suitable synthetic tool for the direct substitution of the vinylic and unactivated C-H bond of the C ring of the flavone by an alkyl functionality under neutral conditions. 相似文献
712.
Afewerki S Ibrahem I Rydfjord J Breistein P Córdova A 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(10):2972-2977
The first direct intermolecular regiospecific and highly enantioselective α-allylic alkylation of linear aldehydes by a combination of achiral bench-stable Pd(0) complexes and simple chiral amines as co-catalysts is disclosed. The co-catalytic asymmetric chemoselective and regiospecific α-allylic alkylation reaction is linked in tandem with in situ reduction to give the corresponding 2-alkyl alcohols with high enantiomeric ratios (up to 98:2 e.r.; e.r.=enantiomeric ratio). It is also an expeditious entry to valuable 2-alkyl substituted hemiacetals, 2-alkyl-butane-1,4-diols, and amines. The concise co-catalytic asymmetric total syntheses of biologically active natural products (e.g., Arundic acid) are disclosed. 相似文献
713.
Samson Afewerki Prof. Dr. Ismail Ibrahem Jonas Rydfjord Palle Breistein Prof. Dr. Armando Córdova 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(10):2972-2977
The first direct intermolecular regiospecific and highly enantioselective α‐allylic alkylation of linear aldehydes by a combination of achiral bench‐stable Pd0 complexes and simple chiral amines as co‐catalysts is disclosed. The co‐catalytic asymmetric chemoselective and regiospecific α‐allylic alkylation reaction is linked in tandem with in situ reduction to give the corresponding 2‐alkyl alcohols with high enantiomeric ratios (up to 98:2 e.r.; e.r.=enantiomeric ratio). It is also an expeditious entry to valuable 2‐alkyl substituted hemiacetals, 2‐alkyl‐butane‐1,4‐diols, and amines. The concise co‐catalytic asymmetric total syntheses of biologically active natural products (e.g., Arundic acid) are disclosed. 相似文献
714.
Kopylovich MN Karabach YY da Silva MF Figiel PJ Lasri J Pombeiro AJ 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(3):899-914
Template combination of copper acetate (Cu(AcO)2?H2O) with sodium dicyanamide (NaN(C≡N)2, 2 equiv) or cyanoguanidine (N≡CNHC(=NH)NH2, 2 equiv) and an alcohol ROH (used also as solvent) leads to the neutral copper(II)–(2,4‐alkoxy‐1,3,5‐triazapentadienato) complexes [Cu{NH?C(OR)NC(OR)?NH}2] (R=Me ( 1 ), Et ( 2 ), nPr ( 3 ), iPr ( 4 ), CH2CH2OCH3 ( 5 )) or cationic copper(II)–(2‐alkoxy‐4‐amino‐1,3,5‐triazapentadiene) complexes [Cu{NH?C(OR)NHC(NH2)?NH}2](AcO)2 (R=Me ( 6 ), Et ( 7 ), nPr ( 8 ), nBu ( 9 ), CH2CH2OCH3 ( 10 )), respectively. Several intermediates of this reaction were isolated and a pathway was proposed. The deprotonation of 6 – 10 with NaOH allows their transformation to the corresponding neutral triazapentadienates [Cu{NH?C(OR)NC(NH2)?NH}2] 11 – 15 . Reaction of 11 , 12 or 15 with acetyl acetone (MeC(?O)CH2C(?O)Me) leads to liberation of the corresponding pyrimidines NC(Me)CHC(Me)NC NHC(?NH)OR, whereas the same treatment of the cationic complexes 6 , 7 or 10 allows the corresponding metal‐free triazapentadiene salts {NH2C(OR)?NC(NH2)?NH2}(OAc) to be isolated. The alkoxy‐1,3,5‐triazapentadiene/ato copper(II) complexes have been applied as efficient catalysts for the TEMPO radical‐mediated mild aerobic oxidation of alcohols to the corresponding aldehydes (molar yields of aldehydes of up to 100 % with >99 % selectivity) and for the solvent‐free microwave‐assisted synthesis of ketones from secondary alcohols with tert‐butylhydroperoxide as oxidant (yields of up to 97 %, turnover numbers of up to 485 and turnover frequencies of up to 1170 h?1). 相似文献
715.
Caseiro A Vitorino R Barros AS Ferreira R Calheiros-Lobo MJ Carvalho D Duarte JA Amado F 《Biomedical chromatography : BMC》2012,26(5):571-582
Diabetic patients show a high susceptibility to oral diseases of inflammatory, catabolic and chronic nature with potential impact on saliva composition. In this study, our purpose was to characterize type 1 diabetes‐induced alterations in the salivary peptidome aiming to find prospective biomarkers for type 1 diabetes oral health evaluation. Peptidomic analysis of saliva from controls (n = 5) and type 1 diabetic patients (n = 5) were performed by liquid chromatography followed by mass spectrometry. The proteolytic activity and metalloproteinases expression was accessed by zymography and slot blot analysis, respectively. Data evidenced a significant increase in the percentage of peptides in diabetic patients paralleled by a higher proteolytic activity, compared with healthy individuals. The nonsalivary gland protein fragments identified in saliva were mainly derived from collagen and extracellular matrix proteins, namely collagen type I. The cleavage site frequency analysis showed significant differences between healthy and type 1 diabetic individuals, highlighting the activity of proteases such as matrix metalloproteinase‐9 and cathepsin D. Our results highlight salivary collagen fragments as potential biomarkers to follow up diabetes‐related oral damage. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
716.
Electrostatic assembly of Ag nanoparticles onto nanofibrillated cellulose for antibacterial paper products 总被引:1,自引:0,他引:1
Natércia C. T. Martins Carmen S. R. Freire Ricardo J. B. Pinto Susana C. M. Fernandes Carlos Pascoal Neto Armando J. D. Silvestre Jessica Causio Giovanni Baldi Patrizia Sadocco Tito Trindade 《Cellulose (London, England)》2012,19(4):1425-1436
Nanofibrillated cellulose offers new technological solutions for the development of paper products. Here, composites of nanofibrillated cellulose (NFC) and Ag nanoparticles (NP) were prepared for the first time via the electrostatic assembly of Ag NP (aqueous colloids) onto NFC. Distinct polyelectrolytes have been investigated as macromolecular linkers in order to evaluate their effects on the building-up of Ag modified NFC and also on the final properties of the NFC/Ag composite materials. The NFC/Ag nanocomposites were first investigated for their antibacterial properties towards S. aureus and K. pneumoniae microorganisms as compared to NFC modified by polyelectrolytes linkers without Ag. Subsequently, the antibacterial NFC/Ag nanocomposites were used as fillers in starch based coating formulations for Eucalyptus globulus-based paper sheets. The potential of this approach to produce antimicrobial paper products will be discussed on the basis of complementary optical, air barrier and mechanical data. 相似文献
717.
Di Fabio G Malgieri G Isernia C D'Onofrio J Gaglione M Messere A Zarrelli A De Napoli L 《Chemical communications (Cambridge, England)》2012,48(32):3875-3877
A first solid phase approach to obtain monosubstituted CD conjugates to different labels has been developed. A new solid support has been designed to get a variety of C-6 monofunctionalized CDs (α, β, MeβCD and HPβCD) covalently linked through a phosphodiester bridge to different labels, in highly pure form and under very mild detachment conditions. 相似文献
718.
Kirillova MV Kirillov AM Martins AN Graiff C Tiripicchio A Pombeiro AJ 《Inorganic chemistry》2012,51(9):5224-5234
The facile aqueous medium reactions of copper(II) nitrate with BES biobuffer [(HOCH(2)CH(2))(2)N(CH(2)CH(2)SO(3)H), hereinafter referred as H(3)bes] in the presence of various benzenecarboxylic acids [benzoic (Hba), 3-hydroxybenzoic (Hhba), and 3,5-dihydroxybenzoic (Hdhba) acid] and lithium hydroxide gave rise to the self-assembly generation of three new heterometallic Cu(II)/Li materials, [Li(H(2)O)(4)][Cu(4)(μ(2)-Hbes)(4)(μ(2)-ba)]·H(2)O (1) and [Cu(4)(μ(3)-Hbes)(4)(L){Li(H(2)O)(2)}](n)·3nH(2)O {L = μ(2)-hba (2) and μ(2)-dhba (3)}. They were isolated as air-stable crystalline solids and fully characterized by infrared (IR) and UV-vis spectroscopy and electrospray ionization (ESI)-MS(±), elemental, thermal, and single-crystal X-ray diffraction analyses. The latter revealed that 1-3 have comparable packing patterns and unit cell parameters, being composed of similar [Cu(4)(μ-Hbes)(4)(μ-carboxylate)](-) cores and [Li(H(2)O)(4)](+) cations (in 1) or [μ-Li(H(2)O)(2)](+) groups (in 2 and 3), which are arranged into discrete 0D aggregates in 1 or infinite 3D noninterpenetrating metal-organic networks in 2 and 3. The topological analysis of the coordination polymers 2 and 3 disclosed the trinodal 3,3,4-connected underlying nets with an unprecedented topology defined by the point symbol of (4.6.8)(4)(4(2).6)(2)(6(2).16(2).18(2)), further simplification of which resulted in the binodal 4,4-connected nets with the pts (PtS) topology. Apart from representing very rare examples of coordination compounds derived from H(3)bes, 1-3 feature solubility in water and were applied as efficient and versatile catalyst precursors for the mild (60 °C) single-pot hydrocarboxylation, by CO and H(2)O, of various gaseous, linear, and cyclic C(n) (n = 2-9) alkanes into the corresponding C(n+1) carboxylic acids, in H(2)O/MeCN medium under homogeneous conditions and in the presence of potassium peroxodisulfate. Total yields (based on alkane) of carboxylic acids up to 78% were achieved, which are remarkable in the field of alkane functionalization under mild conditions, especially for a C-C bond formation reaction in aqueous acid-solvent-free medium. 相似文献
719.
DS Nesterov EN Chygorin VN Kokozay VV Bon R Boča YN Kozlov LS Shul'pina J Jezierska A Ozarowski AJ Pombeiro GB Shul'pin 《Inorganic chemistry》2012,51(16):9110-9122
The heterometallic complex [Co(4)Fe(2)OSae(8)]·4DMF·H(2)O (1) was synthesized by one-pot reaction of cobalt powder with iron chloride in a dimethylformamide solution of salicylidene-2-ethanolamine (H(2)Sae) and characterized by single crystal X-ray diffraction analysis, magnetic measurements, high frequency electron paramagnetic resonance (HF-EPR), and M?ssbauer spectroscopies. The exchange coupling in the Fe(III)-Fe(III) pair is of antiferromagnetic behavior with J/hc = -190 cm(-1). The HF-EPR spectra reveal an unusual pattern with a hardly detectable triplet signal of the Fe(III) dimer. The magnitude of D (ca. 13.9 cm(-1)) was found to be much larger than in related dimers. The catalytic investigations disclosed an outstanding activity of 1 toward oxidation of cycloalkanes with hydrogen peroxide, under mild conditions. The most efficient system showed a turnover number (TON) of 3.57 × 10(3) with the concomitant overall yield of 26% for cyclohexane, and 2.28 × 10(3)/46%, respectively, for cyclooctane. A remarkable turnover frequency (TOF) of 1.12 × 10(4) h(-1) (the highest initial rate W(0) = 3.5 × 10(-4) M s(-1)) was achieved in oxidation of cyclohexane. Kinetic experiments and selectivity parameters led to the conclusion that hydroxyl radicals are active (attacking C-H bonds) species. Kinetic and electrospray ionization mass spectrometry (ESI-MS) data allowed us to assume that the trinuclear heterometallic particle [Co(2)Fe(Sae)(4)](+), originated from 1 in solution, could be responsible for efficient generation of hydroxyl radicals from hydrogen peroxide. 相似文献
720.
Laura Rubio-Pérez Pankaj Sharma F. Javier Pérez-Flores Luis Velasco J. Luis. Arias Armando Cabrera 《Tetrahedron》2012,68(10):2342-2348
A one-pot stibine modified Co2(CO)8 homogeneous catalytic reductive N-alkylation of primary amides using aldehydes/ketones as alkylating agents, is reported. Good to excellent yields of a wide range of secondary amides are obtained (up to 97%) under relative mild conditions. 相似文献