全文获取类型
收费全文 | 1262篇 |
免费 | 76篇 |
国内免费 | 4篇 |
专业分类
化学 | 1000篇 |
晶体学 | 3篇 |
力学 | 47篇 |
数学 | 134篇 |
物理学 | 158篇 |
出版年
2023年 | 10篇 |
2022年 | 21篇 |
2021年 | 29篇 |
2020年 | 32篇 |
2019年 | 26篇 |
2018年 | 29篇 |
2017年 | 22篇 |
2016年 | 42篇 |
2015年 | 54篇 |
2014年 | 45篇 |
2013年 | 82篇 |
2012年 | 78篇 |
2011年 | 94篇 |
2010年 | 60篇 |
2009年 | 57篇 |
2008年 | 92篇 |
2007年 | 83篇 |
2006年 | 74篇 |
2005年 | 53篇 |
2004年 | 62篇 |
2003年 | 56篇 |
2002年 | 45篇 |
2001年 | 16篇 |
2000年 | 11篇 |
1999年 | 10篇 |
1998年 | 11篇 |
1997年 | 7篇 |
1996年 | 11篇 |
1995年 | 5篇 |
1994年 | 9篇 |
1993年 | 12篇 |
1992年 | 8篇 |
1991年 | 4篇 |
1990年 | 6篇 |
1989年 | 6篇 |
1988年 | 9篇 |
1986年 | 2篇 |
1985年 | 8篇 |
1984年 | 15篇 |
1983年 | 6篇 |
1982年 | 9篇 |
1981年 | 3篇 |
1980年 | 8篇 |
1979年 | 7篇 |
1978年 | 1篇 |
1977年 | 2篇 |
1976年 | 3篇 |
1975年 | 2篇 |
1973年 | 2篇 |
1968年 | 1篇 |
排序方式: 共有1342条查询结果,搜索用时 0 毫秒
11.
Cavé C Desmaële D d'Angelo J Riche C Chiaroni A 《The Journal of organic chemistry》1996,61(13):4361-4368
The Michael-type addition of chiral imines, derived from racemic alpha-substituted cyclanones and optically active 1-phenylethylamine, to electrophilic alkenes, in neutral conditions, constitutes one of the most efficient methods for the stereocontrolled construction of quaternary carbon centers. In order to create an additional stereogenic center at the alpha- or beta-position to the quaternary one, the behavior of a variety of alpha- and beta-substituted alkenyl acceptors was examined. In general, these additions are highly regioselective, the alkylation taking place predominantly, if not exclusively, at the more substituted alpha-side of the imine function; however, in some cases (electrophilic alkenes 28 and 49), significant amounts (10-15%) of regioisomeric adducts were obtained. With the exception of methyl propiolate 52, a remarkable control of the absolute configuration of the adducts were always observed with these Michael acceptors. According to the general rule we have previously proposed, the alkylation process takes place preferentially on the less hindered pi-face of the more substituted secondary enamine, in tautomeric equilibrium with the starting imine. An excellent diastereocontrol was always obtained by using the present alpha- and beta-substituted alkenes. These stereochemical outcomes can be interpreted by invoking that the reaction proceeds through a compact approach of the reactants, the hydrogen atom at the nitrogen center of the enamine being transferred to the alpha-vinylic carbon atom of the acceptor, concertedly with the creation of the C-C bond. In this respect the "endo-approach" 58, in which the electron-withdrawing group of the acceptor faced to the nitrogen atom of the enamine (case of acceptors 10, methyl methacrylate, 24, 28, 43, 47, and 49) largely prevails over the "exo-approach" 59 (case of acceptor 38). This predominant "endo-preference" can be reasonably interpreted in terms of a cooperative effect between steric and stereoelectronic factors. 相似文献
12.
Supramolecular H-bonded assemblies of redox-active metallodendrimers and positive and unusual dendritic effects on the recognition of H2PO4- 总被引:1,自引:0,他引:1
The DSM polyamine dendrimers dend-DAB-(NH2)x of generations 1 (x = 4) to 4 (x = 32) form H-bonded dendritic assemblies with the phenol AB3 units p-HOC6H4C(CH2CHCH2)3 and p-HOC6H4C{(CH2)3SiCH2NHCOFc}3 (Fc = ferrocenyl), as shown by the shifts of the NH2 and OH signals giving a concentration-dependent common signal between 2.4 and 4.1 ppm in CDCl3. The supramolecular dendrimers efficiently recognize H2PO4- anions with positive and unusual dendritic effects upon electrochemical titration involving half-stoichiometry for G1, a sudden cyclovoltammetry wave change at the equivalent point, and a dramatic intensity decrease of the new wave. 相似文献
13.
This paper presents the construction of amperometric biosensors for the highly sensitive detection of carbamate insecticides based on the inhibition of acetylcholinesterase (AChE). This enzyme was immobilised by entrapment in an optimised sol-gel matrix on TCNQ-modified screen-printed electrodes. The enzyme activity was estimated by measuring the thiocholine produced by the enzymatic hydrolysis of the acetylthiocholine using TCNQ as mediator. Wild and genetically engineered AChEs from Drosophila melanogaster (Dm) were chosen for their high sensitivity towards insecticides, which substantially improves the LOD compared with cholinesterases from other sources. The wild type and three mutant enzymes were tested against three carbamate insecticides: carbaryl, carbofuran and pirimicard. The best LOD were obtained with the Y370A mutant for carbaryl (1 × 10−8 M), the E69W mutant for pirimicarb (2 × 10−8 M) and the I161V mutant for carbofuran (8 × 10−10 M). The biosensors were applied to the analysis of two potable water samples. 相似文献
14.
Synthesis of methylenedioxy-bearing 1-aryl-3-carboxylisoquinolines using a modified Ritter reaction procedure 总被引:1,自引:0,他引:1
This paper describes original approaches aimed at the preparation of electron-rich 1-aryl-3-carboxylisoquinolines. Our first attempt led to an efficient preparation of 1-hydroxyisoquinoline-3-carboxylic acid methyl ester starting from bromophthalide via a rearrangement of 2-acetylamino-2-(3-oxo-1,3-dihydroisobenzofuran-1-yl)-malonic acid dimethyl ester. However, as its eventual application to the synthesis of methylenedioxy-bearing substrates seemed rather long, a second approach involving an extension of the Ritter reaction to safrole was devised. We thus report that, under proper experimental settings, the use of 54% tetrafluoroboric acid in ether enables a Ritter reaction between safrole and 3,4,5-trimethoxybenzonitrile yielding 17% of 7-methyl-5-(3,4,5-trimethoxyphenyl)-7,8-dihydro-[1,3]dioxolo[4,5-g]isoquinoline. This acidic reagent avoids the extensive decomposition seen when using the classical Ritter reaction conditions (i.e.: concentrated sulfuric acid). Further chemical transformations of this methyl-bearing dihydroisoquinoline led to the methylenedioxy-bearing 1-aryl-3-carboxylisoquinoline. These derivatives are related to the peripheral benzodiazepine receptor ligand PK 11195 as well as falcipain-2 inhibitors and other potential antitumor agents. 相似文献
15.
Omid Ranaei Siadat Andrée Lougarre Lucille Lamouroux Caroline Ladurantie Didier Fournier 《BMC biochemistry》2006,7(1):12-7
Background
Acetylcholinesterase is irreversibly inhibited by organophosphate and carbamate insecticides allowing its use in biosensors for detection of these insecticides. Drosophila acetylcholinesterase is the most sensitive enzyme known and has been improved by in vitro mutagenesis. However, its stability has to be improved for extensive utilization. 相似文献16.
UV photolysis of [CpFeII(CO)3]+ PF66? (I) or [CpFeII(η6-toluene)]+ PF6?? (II) in CH3CN in the presence of 1 mole of a ligand L gives the new air sensitive, red complexes [CpFeII(NCCH3)2L]+PF6? (III, L = PPh3; IV; L = CO; VIII, L = cyclohexene; IX, L = dimethylthiophene) and the known air stable complex [CpFeII(PMe3)2(NCMe)]+ PF6? (V). The last product is also obtained by photolysis in the presence of 2 or 3 moles of PMe3. In the presence of dppe, the known complex [CpFeII (dppe)(NCCH3)]+ (XI) is obtained. Complex III reacts with CO under mild conditions to give the known complex [CpFe(NCCH3)(PPh3)CO]+ PF6? (X). UV photolysis of I in CH3CN in the presence of 1-phenyl-3,4-dimethylphosphole (P) gives [CpFeIIP3]+ PF6? (XII); UV photolysis of II in CH2Cl2 in the presence of 3 moles of PMe3 or I mole of tripod (CH3C(CH2Ph2)3) provides an easy synthesis of the known complexes [CpFeII(PMe3)3]+ PF6? (VII) or [CpFeIIη3-tripod]+ PF6t- (XIII). Since I and II are easily accessible from ferrocene, these photolytic syntheses provide access to a wide range of piano-stool cyclopentadienyliron(II) cations in a 2-step process from ferrocene. 相似文献
17.
Bruno Hasiak Guy Ricart Didier Barbry Daniel Couturier Michel Hardy 《Journal of mass spectrometry : JMS》1981,16(1):17-20
The ammonia chemical ionization desorption spectra of N,N-dimethyl quaternary ammonium iodides in addition to high protonated molecular ion [M + H]+ intensity, show signals for an ion radical composed of N-methyl abstracted salt cation and ammonia [C + NH3? CH3]+˙. These ions corresponding to the cation +2 show increased importance in the chemical ionization mode, using the same reagent gas. The technique of chemical ionization desorption appears suitable for the analysis of salts, and thus for the determination of the molecular weight of both anion and cation. 相似文献
18.
Städler B Falconnet D Pfeiffer I Höök F Vörös J 《Langmuir : the ACS journal of surfaces and colloids》2004,20(26):11348-11354
We present a novel concept for the creation of lipid vesicle microarrays based on a patterning approach termed Molecular Assembly Patterning by Lift-off (MAPL). A homogeneous MAPL-based single-stranded DNA microarray was converted into a vesicle array by the use of vesicles tagged with complementary DNAs, permitting sequence-specific coupling of vesicles to predefined surface regions through complementary DNA hybridization. In the multistep process utilized to fulfill this achievement, active spots consisting of PLL-g-PEGbiotin with a resistant PLL-g-PEG background, as provided by the MAPL process, was converted into a DNA array by addition of complexes of biotin-terminated DNA and NeutrAvidin. This was then followed by addition of POPC vesicles tagged with complementary cholesterol-terminated DNA, thus providing specific coupling of vesicles to the surface through complementary DNA hybridization. Quartz crystal microbalance with dissipation (QCM-D) and optical waveguide lightmode spectroscopy monitoring were used to optimize the multistep surface modification process. It was found that the amount of adsorbed biotinDNA-NeutrAvidin complexes decreases with increasing molar ratio of biotinDNA to NeutrAvidin and decreasing ionic strength of the buffer solution. Modeling of the QCM-D data showed that the shape of the immobilized vesicles depends on the amount of available anchoring groups between the vesicles and the surface. Fluorescent microscopy images confirmed the possibility to create well-defined patterns of DNA-tagged, fluorescently labeled vesicles in the micrometer range. 相似文献
19.
Dalla Serra M Coraiola M Viero G Comai M Potrich C Ferreras M Baba-Moussa L Colin DA Menestrina G Bhakdi S Prévost G 《Journal of chemical information and modeling》2005,45(6):1539-1545
Staphylococcal gamma-hemolysins are bicomponent toxins forming a protein family with leucocidins and alpha-toxin. Two active toxins (AB and CB) can be formed combining one of the class-S components, HlgA or HlgC, with the class-F component HlgB. These two gamma-hemolysins form pores with marked similarities to alpha-toxin in terms of conductance, nonlinearity of the current-voltage curve, and channel stability in the open state. AB and CB pores, however, are cation-selective, whereas alpha-toxin is anion-selective. gamma-Hemolysins' pores are hetero-oligomers formed by three or four copies of each component (indicated as 3A3B and 3C3B or 4A4B and 4C4B). Point mutants located on a beta-strand of the class-S component that forms part of the protomer-protomer contact region can prevent oligomer assembly. Interestingly, these mutants inhibit growth of pores formed not only by their natural components but also by nonstandard components. This lead to the hypothesis that mixed ABC pores could also be formed. By studying the conductance of pores, assembled in the presence of all three components (in different ratios), it was observed that the magnitudes expected for mixed pores were, indeed, present. We conclude that the gamma-hemolysin/leucocidin bicomponent toxin family may form a larger than expected number of active toxins by cross-combining various S and F components. 相似文献
20.