全文获取类型
收费全文 | 636篇 |
免费 | 31篇 |
国内免费 | 3篇 |
专业分类
化学 | 449篇 |
力学 | 10篇 |
数学 | 26篇 |
物理学 | 185篇 |
出版年
2022年 | 4篇 |
2021年 | 10篇 |
2020年 | 11篇 |
2019年 | 13篇 |
2018年 | 6篇 |
2017年 | 9篇 |
2016年 | 18篇 |
2015年 | 15篇 |
2014年 | 15篇 |
2013年 | 30篇 |
2012年 | 49篇 |
2011年 | 43篇 |
2010年 | 23篇 |
2009年 | 17篇 |
2008年 | 48篇 |
2007年 | 39篇 |
2006年 | 36篇 |
2005年 | 33篇 |
2004年 | 25篇 |
2003年 | 30篇 |
2002年 | 16篇 |
2001年 | 18篇 |
2000年 | 13篇 |
1999年 | 6篇 |
1998年 | 7篇 |
1996年 | 9篇 |
1995年 | 4篇 |
1994年 | 11篇 |
1993年 | 6篇 |
1992年 | 13篇 |
1991年 | 6篇 |
1990年 | 9篇 |
1989年 | 4篇 |
1988年 | 5篇 |
1987年 | 9篇 |
1986年 | 10篇 |
1985年 | 13篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1981年 | 4篇 |
1980年 | 4篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1976年 | 3篇 |
1974年 | 3篇 |
1973年 | 2篇 |
1967年 | 3篇 |
1966年 | 1篇 |
1962年 | 1篇 |
排序方式: 共有670条查询结果,搜索用时 15 毫秒
61.
Young-Kil Chang Jaeheon Lee Moonsub Lee Han Kyong Kim Bo-Young Lee Kwan Soo Kim 《Tetrahedron》2010,66(30):5687-5691
An efficient large-scale synthesis of gemcitabine was achieved without chromatography or fractional crystallization. The key steps include stereospecific conversion of a novel β-ribofuranosyl phosphate into a highly crystalline α-ribofuranosyl bromide and coupling of the α-ribofuranosyl bromide and trimethylsilyl cytosine to produce a β-nucleoside. p-Phenylbenzoyl group was introduced for the protection of one of hydroxy groups in order to enhance the crystallinity of intermediates. Continuous removal of trimethylsilyl bromide, generated during the coupling reaction, by distillation from the reaction medium substantially enhanced the β-selectivity of the crucial coupling reaction. 相似文献
62.
63.
64.
65.
Chloroform extract of bark of Artabotrys crassifolius Hook.f. & Thomson exhibited antibacterial activities against both American Type Culture Collection and clinical bacterial strains in vitro with zones of inhibition ranging from 7 to 14 mm. Further analysis of this extract yielded artabotrine, liridine, lysicamine and atherospermidine. Artabotrine displayed a broad array of antibacterial activity mostly against Gram-positive bacteria with minimum inhibitory concentration (MIC) values ranging from 1.25 μg/mL to 5 μg/mL. Of note, artabotrine, liridine and lysicamine are bactericidal against Gram-negative extended-spectrum beta-lactamase-producing Klebsiella with MIC values equal 2.5, 2.5 and 10 μg/mL, respectively, and minimum bactericidal concentrations values equal to 2.5, 5 and 20 μg/mL. 相似文献
66.
Nung Kwan Yip 《Archive for Rational Mechanics and Analysis》1998,144(4):313-355
We prove the existence of a continuously time‐varying subset K(t) of R
n
such that its boundary ∂K(t), which is a hypersurface, has normal velocity formally equal to the (weighted) mean curvature plus a random driving force.
This is the first result in such generality combining curvature motion and stochastic perturbations. Our result holds for
any C
2 convex surface energy. The K(t) can have topological changes. The randomness is introduced by means of stochastic flows of diffeomorphisms generated by
Brownian vector fields which are white in time but smooth in space.
We work in the context of geometric measure theory, using sets of finite perimeter to represent K(t). The evolution is obtained as a limit of a time‐stepping scheme. Variational minimizations are employed to approximate the
curvature motion. Stochastic calculus is used to prove global energy estimates, which in turn give a tightness statement of
the approximating evolutions.
(Accepted December 22, 1997) 相似文献
67.
Enantioselective CE with sulfated cyclodextrins as chiral selectors was used to determine the CYP3A4-catalyzed N-demethylation kinetics of ketamine to norketamine and its inhibition in the presence of ketoconazole in vitro. Ketamine, a chiral phencyclidine derivative, was incubated with recombinant human CYP3A4 from a baculovirus expression system as racemic mixture and as single enantiomer. Alkaline liquid/liquid extracts of the samples were analyzed with a pH 2.5 buffer comprising 50 mM Tris and phosphoric acid together with either multiple isomer sulfated β-cyclodextrin (10 mg/mL) or highly sulfated γ-cyclodextrin (2%, w/v). Data obtained in the absence of ketoconazole revealed that the N-demethylation occurred stereoselectively with Michaelis-Menten (incubation of racemic ketamine) and Hill (separate incubation of single enantiomers) kinetics. Data generated in the presence of ketoconazole as the inhibitor could best be fitted to a one-site competitive model and inhibition constants were calculated using the equation of Cheng and Prusoff. No stereoselective difference was observed, but inhibition constants for the incubation of racemic ketamine were found to be larger compared with those obtained with the incubation of single ketamine enantiomers. 相似文献
68.
A massively parallel nanofluidic concentration device array for multiplexed and high-throughput biomolecule detection is demonstrated. By optimizing the microchannel/nanojunction design and channel conductivity, an array of up to 128 nanofluidic concentration devices were fabricated. Operation of the entire array requires only one inlet and one outlet reservoir, with the application of a single operational voltage bias across them. Concentration efficiencies of the devices were found to be uniform within the array, within 5% error. Alternatively, concentration speed in each channel can be individually tuned by controlling the length of the inlet microchannel and thus controlling the flow rate based on change of the tangential electric field. This allows immuno-binding reactions at different concentration ranges to be performed in parallel. Using multiplexed, successive-concentration enhanced detection in the device, we have shown that the dynamic range and reliability of the immunoassay can be significantly increased. 相似文献
69.
Kwan DH Tosin M Schläger N Schulz F Leadlay PF 《Organic & biomolecular chemistry》2011,9(7):2053-2056
Ketoreductase enzymes are responsible for the generation of hydroxyl stereocentres during the biosynthesis of complex polyketide natural products. Previous studies of isolated polyketide ketoreductases have shown that the stereospecificity of ketoreduction can be switched by mutagenesis of selected active site amino acids. We show here that in the context of the intact polyketide synthase multienzyme the same changes do not alter the stereochemical outcome in the same way. These findings point towards additional factors that govern ketoreductase stereospecificity on intact multienzymes in vivo. 相似文献
70.
We have studied the roles and behavior of typical resin- and silica-bound thiol scavengers in the removal of palladium (Pd) residues and in the determination of the true catalytic species in the Heck coupling of bromobenzene and styrene. The results of Pd scavenging and catalyst poisoning by elemental analysis (EA) and transmission electron microscopy (TEM) indicate that silica-bound thiols have an advantage over resin-bound thiols in residual Pd removal from a Heck reaction solution and that all of these scavengers poison effectively the catalytic species but hardly scavenge Pd clusters, even as small as 1 nm from solution presumably because of the steric barrier. Because of a smaller proportion of soluble Pd clusters, using a molecular Pd precatalyst results in a much higher Pd scavenging efficiency than using a supported Pd particle precatalyst. With the aid of catalyst poisoning by the scavengers, filtration testing and TEM studies further illustrate that Pd clusters are inactive for the Heck reaction over Pd(0)/SiO(2), with molecular Pd(0) being solely active. Studies through EA and thermogravimetric analysis suggest that the bound thiols are leached from the scavengers to a different extent at reaction temperatures of 90-135 °C, probably owing to base-catalyzed decomposition or based-promoted dissociation of functional groups from the surfaces, leading to interaction between leached thiols and a solid Pd precatalyst. Meanwhile, the effect of solid-bound thiol binding to soluble Pd on the efficiency of Pd scavenging and the impact of a scavenger on the Pd leaching from supported Pd particles are discussed. 相似文献