全文获取类型
收费全文 | 248篇 |
免费 | 7篇 |
专业分类
化学 | 199篇 |
晶体学 | 5篇 |
力学 | 7篇 |
数学 | 5篇 |
物理学 | 39篇 |
出版年
2023年 | 1篇 |
2022年 | 2篇 |
2021年 | 3篇 |
2020年 | 5篇 |
2019年 | 7篇 |
2017年 | 5篇 |
2016年 | 1篇 |
2015年 | 9篇 |
2014年 | 8篇 |
2013年 | 13篇 |
2012年 | 23篇 |
2011年 | 20篇 |
2010年 | 7篇 |
2009年 | 9篇 |
2008年 | 19篇 |
2007年 | 17篇 |
2006年 | 9篇 |
2005年 | 15篇 |
2004年 | 12篇 |
2003年 | 7篇 |
2002年 | 3篇 |
2001年 | 2篇 |
2000年 | 3篇 |
1999年 | 3篇 |
1998年 | 2篇 |
1997年 | 6篇 |
1996年 | 6篇 |
1995年 | 5篇 |
1994年 | 2篇 |
1993年 | 1篇 |
1992年 | 2篇 |
1991年 | 2篇 |
1990年 | 1篇 |
1986年 | 1篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1978年 | 3篇 |
1977年 | 1篇 |
1976年 | 2篇 |
1975年 | 3篇 |
1974年 | 1篇 |
1970年 | 1篇 |
1966年 | 1篇 |
排序方式: 共有255条查询结果,搜索用时 13 毫秒
201.
[reaction: see text] Ru(OH)x/Al2O3 efficiently catalyzes the heterogeneous aerobic oxygenation or oxidative dehydrogenation of alkylarenes to give the corresponding oxygenated or dehydrogenated products. Catalyst/product separation is very easy, and the recovered catalyst is reusable with retention of the high catalytic performance. 相似文献
202.
Highly selective, recyclable epoxidation of allylic alcohols with hydrogen peroxide in water catalyzed by dinuclear peroxotungstate 总被引:2,自引:0,他引:2
Kamata K Yamaguchi K Mizuno N 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(19):4728-4734
The highly chemo-, regio-, and diastereoselective and stereospecific epoxidation of various allylic alcohols with only one equivalent of hydrogen peroxide in water can be efficiently catalyzed by the dinuclear peroxotungstate, K2[[W(=O)(O2)2(H2O)]2(mu-O)].2H2O (I). The catalyst is easily recycled while maintaining its catalytic performance. The catalytic reaction mechanism including the exchange of the water ligand to form the tungsten-alcoholate species followed by the insertion of oxygen to the carbon-carbon double bond, and the regeneration of the dinuclear peroxotungstate with hydrogen peroxide is proposed. The reaction rate shows first-order dependence on the concentrations of allylic alcohol and dinuclear peroxotungstate and zero-order dependence on the concentration of hydrogen peroxide. These results, the kinetic data, the comparison of the catalytic rates with those for the stoichiometric reactions, and kinetic isotope effects indicate that the oxygen transfer from a dinuclear peroxotungstate to the double bond is the rate-limiting step for terminal allylic alcohols such as 2-propen-1-ol (1a). 相似文献
203.
CoMFA analysis, a widely used 3D-QSAR method, has limitations to handle a set of SAR data containing diverse conformational flexibility since it does not explicitly include the conformational entropic effects into the analysis. Here, we present an attempt to incorporate the conformational entropy effects of a molecule into a 3D-QSAR analysis. Our attempt is based on the assumption that the conformational entropic loss of a ligand upon making a ligand-receptor complex is small if the ligand in an unbound state has a conformational propensity to adopt an active conformation in a complex state. For a QSAR analysis, this assumption was interpreted as follows: a potent ligand should have a higher conformational propensity to adopt an `active-conformation'-like structure in an unbound state than an inactive one. The conformational propensity value was defined as the populational ratio, Nactive/Nstable, of the number of energetically stable conformers, Nstable, to the number of `active-conformation'-like structures, Nactive. The latter number was calculated by counting the number of conformers that satisfied the structural parameters deduced from the active conformation. A set of SAR data of imidazoleglycerol phosphate dehydratase inhibitors containing 20 molecules with different conformational flexibility was used as a training set for developing a 3D structure-activity relationship by a CoMFA analysis with the conformational propensity value. This resulted in a cross-validated squared correlation coefficient of the CoMFA model with the conformational propensity value (R
2
cross = 0.640) higher than that of the standard CoMFA model (R
2
cross = 0.431). Then we evaluated the quality of the CoMFA models by predicting the inhibitory activity for a new molecule. 相似文献
204.
205.
Masahiko Okada Kouji Tachikawa Keigo Aoi 《Journal of polymer science. Part A, Polymer chemistry》1997,35(13):2729-2737
A series of polyesters were synthesized by the bulk polycondensations of the respective combinations of two difuranic diesters, i.e., bis(5-(methoxycarbonyl)-2-furyl)methane ( 4a ) and 1,1-bis(5-(methoxycarbonyl)-2-furyl)ethane ( 4b ), with two 1,4 : 3,6-dianhydrohexitols [1,4 : 3,6-dianhydro-D -glucitol ( 1 ) and 1,4 : 3,6-dianhydro-D -mannitol ( 2 )], four aliphatic diols, and three oligo(ethylene glycol)s. The polycondensations were carried out at 220–230°C in the presence of titanium isopropoxide as a catalyst, giving polyesters having number average molecular weight up to 2.4 × 104. These polyesters are soluble in a variety of solvents including chlorinated hydrocarbons, 1,4-dioxane, dimethyl sulfoxide, dimethylformamide, and sulfolane. Soil-burial tests along with enzymatic degradation experiments showed that these polyesters are potentially biodegradable. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 2729–2737, 1997 相似文献
206.
Poly(vinyl alcohol)-Congo Red (PVA-CR) complexes in aqueous solutions have a unique sol-gel transition behaviour. At an appropriate PVA concentration, a sol-gel-sol-gel type reentrant transition takes place with increasing CR concentration. The microscopic structure and Theological properties of these complexes were investigated by small-angle neutron scattering (SANS) and viscometry. 相似文献
207.
Hirano T Uehara K Kamata K Mizuno N 《Journal of the American Chemical Society》2012,134(14):6425-6433
A mixture of Pd(OAc)(2) and TBA(4)[γ-SiW(10)O(34)(H(2)O)(2)] (TBA-SiW10, TBA = [(n-C(4)H(9))(4)N](+)) showed high catalytic activities for hydration of various kinds of structurally diverse nitriles including aromatic, aliphatic, heteroaromatic, and double bond-containing ones. For hydration of 3-cyanopyridine, the turnover frequency was 860 h(-1), and the turnover number reached up to 670. A dipalladium-substituted γ-Keggin silicodecatungstate, [γ-H(2)SiW(10)O(36)Pd(2)(OAc)(2)](4-) (I), was successfully synthesized by the reaction of [γ-SiW(10)O(34)(H(2)O)(2)](4-) with Pd(OAc)(2) in a mixed solvent of acetone and water. The crystal structure of I was a monomeric, dipalladium-substituted, γ-Keggin silicodecatungstate with bidentate acetate ligands. Compound I showed similar activities and selectivities to those of a simple mixture of Pd(OAc)(2) and TBA-SiW10. The kinetic, mechanistic, and density functional theory calculation studies show that the dipalladium site plays an important role in the present hydration, and the nucleophilic attack of a hydroxide or water to the nitrile carbon atom is included in the rate-determining step. 相似文献
208.
Kazuya Matsuo Dr. Rui Kamada Dr. Keigo Mizusawa Dr. Hirohiko Imai Dr. Yuki Takayama Dr. Michiko Narazaki Prof. Dr. Tetsuya Matsuda Dr. Yousuke Takaoka Prof. Dr. Itaru Hamachi 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(38):12875-12883
Specific turn‐on detection of enzyme activities is of fundamental importance in drug discovery research, as well as medical diagnostics. Although magnetic resonance imaging (MRI) is one of the most powerful techniques for noninvasive visualization of enzyme activity, both in vivo and ex vivo, promising strategies for imaging specific enzymes with high contrast have been very limited to date. We report herein a novel signal‐amplifiable self‐assembling 19F NMR/MRI probe for turn‐on detection and imaging of specific enzymatic activity. In NMR spectroscopy, these designed probes are “silent” when aggregated, but exhibit a disassembly driven turn‐on signal change upon cleavage of the substrate part by the catalytic enzyme. Using these 19F probes, nanomolar levels of two different target enzymes, nitroreductase (NTR) and matrix metalloproteinase (MMP), could be detected and visualized by 19F NMR spectroscopy and MRI. Furthermore, we have succeeded in imaging the activity of endogenously secreted MMP in cultured media of tumor cells by 19F MRI, depending on the cell lines and the cellular conditions. These results clearly demonstrate that our turn‐on 19F probes may serve as a screening platform for the activity of MMPs. 相似文献
209.
Kamata K Kotani M Yamaguchi K Hikichi S Mizuno N 《Chemistry (Weinheim an der Bergstrasse, Germany)》2007,13(2):639-648
The tetra-n-butylammonium (TBA) salt of the divacant Keggin-type polyoxometalate [TBA](4)[gamma-SiW(10)O(34)(H(2)O)(2)] (I) catalyzes the oxygen-transfer reactions of olefins, allylic alcohols, and sulfides with 30 % aqueous hydrogen peroxide. The negative Hammett rho(+) (-0.99) for the competitive oxidation of p-substituted styrenes and the low value of (nucleophilic oxidation)/(total oxidation), X(SO)=0.04, for I-catalyzed oxidation of thianthrene 5-oxide (SSO) reveals that a strongly electrophilic oxidant species is formed on I. The preferential formation of trans-epoxide during epoxidation of 3-methyl-1-cyclohexene demonstrates the steric constraints of the active site of I. The I-catalyzed epoxidation proceeds with an induction period that disappears upon treatment of I with hydrogen peroxide. (29)Si and (183)W NMR spectroscopy and CSI mass spectrometry show that reaction of I with excess hydrogen peroxide leads to fast formation of a diperoxo species, [TBA](4)[gamma-SiW(10)O(32)(O(2))(2)] (II), with retention of a gamma-Keggin type structure. Whereas the isolated compound II is inactive for stoichiometric epoxidation of cyclooctene, epoxidation with II does proceed in the presence of hydrogen peroxide. The reaction of II with hydrogen peroxide would form a reactive species (III), and this step corresponds to the induction period observed in the catalytic epoxidation. The steric and electronic characters of III are the same as those for the catalytic epoxidation by I. Kinetic, spectroscopic, and mechanistic investigations show that the present epoxidation proceeds via III. 相似文献
210.
Motozaki T Sawamura K Suzuki A Yoshida K Ueki T Ohara A Munakata R Takao K Tadano K 《Organic letters》2005,7(11):2261-2264
[reaction: see text]. Starting from diethyl (R)-malate, synthesis of the lower-half segment of (+)-tubelactomicin A, a 16-membered macrolide antibiotic, has been achieved. The synthesis involved the highly endo- and pi-facial selective intramolecular Diels-Alder reaction achieved using a trisubstituted methacrolein derivative tethering a 10-carbon dienyne unit at the beta-carbon, which in turn was prepared from a known allylated malic acid derivative. 相似文献