全文获取类型
收费全文 | 6376篇 |
免费 | 293篇 |
国内免费 | 22篇 |
专业分类
化学 | 5343篇 |
晶体学 | 33篇 |
力学 | 78篇 |
数学 | 619篇 |
物理学 | 618篇 |
出版年
2023年 | 62篇 |
2022年 | 156篇 |
2021年 | 288篇 |
2020年 | 178篇 |
2019年 | 162篇 |
2018年 | 119篇 |
2017年 | 122篇 |
2016年 | 241篇 |
2015年 | 246篇 |
2014年 | 238篇 |
2013年 | 411篇 |
2012年 | 493篇 |
2011年 | 547篇 |
2010年 | 364篇 |
2009年 | 326篇 |
2008年 | 434篇 |
2007年 | 441篇 |
2006年 | 342篇 |
2005年 | 340篇 |
2004年 | 255篇 |
2003年 | 212篇 |
2002年 | 194篇 |
2001年 | 51篇 |
2000年 | 42篇 |
1999年 | 44篇 |
1998年 | 31篇 |
1997年 | 36篇 |
1996年 | 35篇 |
1995年 | 29篇 |
1994年 | 15篇 |
1993年 | 18篇 |
1992年 | 18篇 |
1991年 | 21篇 |
1990年 | 10篇 |
1989年 | 9篇 |
1988年 | 5篇 |
1987年 | 15篇 |
1986年 | 9篇 |
1985年 | 10篇 |
1984年 | 16篇 |
1983年 | 13篇 |
1982年 | 12篇 |
1981年 | 5篇 |
1980年 | 7篇 |
1979年 | 5篇 |
1978年 | 8篇 |
1976年 | 4篇 |
1965年 | 4篇 |
1960年 | 3篇 |
1958年 | 3篇 |
排序方式: 共有6691条查询结果,搜索用时 31 毫秒
41.
Tuo Jiang Samuele Bordi Angus E. McMillan Kuang-Yen Chen Fumito Saito Paula L. Nichols Benedikt M. Wanner Jeffrey W. Bode 《Chemical science》2021,12(20):6977
The current laboratory practices of organic synthesis are labor intensive, impose safety and environmental hazards, and hamper the implementation of artificial intelligence guided drug discovery. Using a combination of reagent design, hardware engineering, and a simple operating system we provide an instrument capable of executing complex organic reactions with prepacked capsules. The machine conducts coupling reactions and delivers the purified products with minimal user involvement. Two desirable reaction classes – the synthesis of saturated N-heterocycles and reductive amination – were implemented, along with multi-step sequences that provide drug-like organic molecules in a fully automated manner. We envision that this system will serve as a console for developers to provide synthetic methods as integrated, user-friendly packages for conducting organic synthesis in a safe and convenient fashion.Using a combination of reagent design, hardware engineering, and a simple operating system we provide an instrument capable of executing complex organic reactions using prepacked capsules with minimal user involvement. 相似文献
42.
Ana A. C. Pacheco Arnaldo F. da Silva Filho Kristoffer Kortsen Magnus W. D. Hanson-Heine Vincenzo Taresco Jonathan D. Hirst Muriel Lansalot Franck D'Agosto Steven M. Howdle 《Chemical science》2021,12(3):1016
Reversible addition–fragmentation chain transfer (RAFT) dispersion polymerisation of methyl methacrylate (MMA) is performed in supercritical carbon dioxide (scCO2) with 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (DDMAT) present as chain transfer agent (CTA) and surprisingly shows good control over PMMA molecular weight. Kinetic studies of the polymerisation in scCO2 also confirm these data. By contrast, only poor control of MMA polymerisation is obtained in toluene solution, as would be expected for this CTA which is better suited for acrylates. In this regard, we select a range of CTAs and use them to determine the parameters that must be considered for good control in dispersion polymerisation in scCO2. A thorough investigation of the nucleation stage during the dispersion polymerisation reveals an unexpected “in situ two-stage” mechanism that strongly determines how the CTA works. Finally, using a novel computational solvation model, we identify a correlation between polymerisation control and degree of solubility of the CTAs. All of this ultimately gives rise to a simple, elegant and counterintuitive guideline to select the best CTA for RAFT dispersion polymerisation in scCO2.RAFT dispersion polymerisation of methyl methacrylate is performed in scCO2 with 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (DDMAT) present as chain transfer agent (CTA) and surprisingly shows good control over PMMA molecular weight. 相似文献
43.
An automatic system that performs two analytical procedures, allowing the evaluation of the relative antioxidant capacity of wine samples, was developed. Automation was carried out using a sequential injection analysis (SIA) system that allowed, thanks to its versatility, the development of two methodologies. One is based on the decolorization assay of the 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical, using a spectrophotometric detector. A second methodology allowed the evaluation of the hydrogen peroxide scavenging activity by measuring the oxidation of homovanylic acid (HVA) to its fluorescent dimer, using a fluorescent detector.The developed automatic methodologies were evaluated using trolox as standard and subsequently using other antioxidant substances as gallic acid, caffeic acid, ascorbic acid, catechin and taxifolin which are abundant in wine and whose antioxidant activities were compared to that shown by trolox. The spectrophotometric and fluorimetric assays showed linearity intervals between 0.001 and 0.01 mM, and 0.001 and 0.008 mM of trolox, respectively.The evaluation of the antioxidant power of 20 white and red wine samples, from different Portuguese wine producing regions, was carried out sequentially, in the automatic system. The results were expressed in trolox equivalent antioxidant capacity (TEAC) and presented, for the ABTS and hydrogen peroxide scavenging activity methodologies, detection limits of 8.4 × 10−7 and 1.4 × 10−4 mM and relative standard deviation (R.S.D. (%)) in the range 0.6-2.4 and 1-1.8, respectively. 相似文献
44.
Basurto S García S Neo AG Torroba T Marcos CF Miguel D Barberá J Ros MB de la Fuente MR 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(18):5362-5376
Several new liquid-crystalline indene and pseudoazulene systems are reported. These molecules give rise to either columnar hexagonal mesophases and/or columnar plastic phases. The unique nature of these compounds stems from their non-classical discotic structure. Although the molecules have rigid aromatic cores, they lack terminal tails and instead the polarizable atoms (S, halogens) or polar groups (CN, CO) act as unusual soft parts. On the basis of many structurally related materials, we conclude that for this type of compound molecular stacking in the solid state is a prerequisite for the appearance of a columnar mesophase, although other intermolecular interactions within the layers are also important in establishing liquid-crystalline order. The behavior reported for these mesomorphic molecules opens up new possibilities in the search for related molecular interactions that might be useful for the construction of supramolecular architectures with particular properties. 相似文献
45.
[reaction: see text] A simple and efficient stereoselective synthesis of aziridine-2-phosphonate 3, and -phosphine oxide 5 by diastereoselective addition of Grignard reagents to 2H-azirine phosphonate 1 and -phosphine oxide 4 is reported. Similarly, the addition of heterocyclic amines and benzenethiol to aziridines 1 and 4 yielded functionalized aziridines 10, 11, and 18. These aziridines are used as intermediates for the regioselective synthesis of beta-aminophosphine oxides 6 and beta-aminophosphonates 7, as well as alpha- aminophosphonates 8. Phenylsulfenyl-substituted alpha-aminophosphorus derivatives 15 and 19 are obtained directly from benzenethiol and 2H-azirine phosphonates 1 and -phosphine oxides 4. 相似文献
46.
M. Jesús Rodríguez M. Isabel Fernndez Ana M. Gonzlez‐Noya Marcelino Maneiro Rosa Pedrido Miguel Vzquez Bruno Donnadieu Manuel R. Bermejo 《无机化学与普通化学杂志》2005,631(11):2161-2166
M(HL)(H2O)n complexes have been obtained by the electrochemical reaction of Fe, Co, Ni, Cu, Zn and Cd anodes with the potentially pentadentate and trianionic asymmetrical Schiff base 3‐aza‐N‐{2‐[1‐aza‐2‐(5‐nitro‐2‐hydroxylphenyl)‐vinyl]phenyl}‐4‐(5‐nitro‐2‐hydroxyphenyl)but‐3‐enamide (H3L), containing a hard amido donor atom. The complexes have been characterized by elemental analysis, mass spectrometry, IR and 1H NMR spectroscopies, magnetic measurements and molar conductivities. Co(HL)(H2O) ( 2 ) has been found to rearrange in DMF solution into a crystallographically solved octahedral complex, CoL1(H2O)2 ( 7 ) [where H2L1 is the symmetrical Schiff base ligand N,N′‐(1,2‐phenylene)‐bis(5‐nitro‐3‐hydroxysalicylidenimine)]. A hydrolysis mechanism is discussed to explain this rearrangement. 相似文献
47.
A new purine derivative with cytokinin activity was isolated from the cuckoo-pint fruits () and identified as 6-(o-hydroxybenzylamino)-2-methylthio-9-β-D-glucofuranosylpurine . 相似文献
48.
Uranium(VI and IV) and thorium(IV) give cathodic indentations in supporting electrolytes prepared from 0.1M perchloric acid, 0.5 M ammonium thiocyanate and 5·10-3M cupferron (solution A) or from 0.1 M succinic-succinate buffer pH 4, 0.1 M sodium chloride, 10-3M cupferron and 0.05% gelatine (solution B). The uranium indentation on the dE/dt = f(E) curve (Q=0.75 and 0.73) permits its detection at the 3·10-7M level. The thorium indentation (Q=0.78) permits its detection at the 4·10-7M level in solution B. Methods for the elimination of interfering ions for the uranium determination are described. In the determination of thorium, Ga(III), Fe(III), Ti(IV) and U(VI) interfere. 相似文献
49.
Matilde Fondo Ana M. García Deibe Noelia Ocampo Manuel R. Bermejo Jesús Sanmartín 《无机化学与普通化学杂志》2005,631(11):2041-2045
The heptadentate Schiff base H3L reacts with cobalt(II) acetate in methanol to form the discrete dinuclear complex Co2L(OAc)2(OMe)(H2O)2 ( 1 ·2H2O). The reaction of 1 ·2H2O with NMe4OH·5H2O in methanol gives rise to displacement of the acetate by methanolate groups, yielding Co2L(OMe)3(H2O) ( 2 ·1H2O). Recrystallizations of the Schiff base, 1 ·2H2O and 2 ·H2O in different solvents, produce single crystals of H3L, 1 ·2.5H2O and 2 ·2MeOH, respectively. The crystal structures of 1 ·2.5H2O and 2 ·2MeOH show the cobalt atoms double bridged by and endogenous phenol oxygen atom and an exogenous methanolate oxygen donor, giving rise to Co2O2 cores with Co···Co distances of ca. 2.87 Å. 相似文献
50.
Ana M. Reverdito Liliana Orelli Mnica Dalmaso Isabel Perillo Beatriz M. Fernndez 《Journal of heterocyclic chemistry》1991,28(2):273-281
Reaction of 1,2-diaryl-3-methyl-1,4,5,6-tetrahydropyrimidinium iodides 1a-i with alkaline solutions afforded N-aroyl-N-aryl-N'-methyltrimethylenediamines 2a-i . Compounds 2 are stable under acid conditions but in neutral or alkaline media spontaneously rearrange giving N-aroyl-N'-aryl-N-methyltrimethylenediamines 3a-i . Treating compounds 3 with concentrated acids reverse reaction takes place. Kinetic studies were performed on this intramolecular N → N' aroyl transfer over the Ho-pH range -0.9 to 2.30. Compounds 3 undergo acyl transfer to give 2 by a mechanism which involves a change in the rate determining step from formation to acid-catalysed decomposition of a six-membered heterocyclic intermediate on going from Ho to pH values. The existence of maxima in the pH rate profile allow to determine apparent pKa values of the hexahydropyrimidine intermediates which gave good correlation with the Swain F substituent constants. Stability of these heterocycles was also predicted by determination of thermodinamic parameters. Comparisons are made with the behaviour of five-membered heterocyclic intermediates (imidazolidine derivatives) which were studied in an earlier paper. 相似文献