全文获取类型
收费全文 | 269篇 |
免费 | 10篇 |
国内免费 | 4篇 |
专业分类
化学 | 174篇 |
力学 | 26篇 |
数学 | 51篇 |
物理学 | 32篇 |
出版年
2023年 | 4篇 |
2022年 | 4篇 |
2021年 | 11篇 |
2020年 | 2篇 |
2019年 | 11篇 |
2018年 | 4篇 |
2017年 | 4篇 |
2016年 | 9篇 |
2015年 | 9篇 |
2014年 | 16篇 |
2013年 | 25篇 |
2012年 | 16篇 |
2011年 | 18篇 |
2010年 | 10篇 |
2009年 | 11篇 |
2008年 | 14篇 |
2007年 | 9篇 |
2006年 | 17篇 |
2005年 | 11篇 |
2004年 | 8篇 |
2003年 | 11篇 |
2002年 | 5篇 |
2001年 | 1篇 |
2000年 | 3篇 |
1999年 | 2篇 |
1998年 | 2篇 |
1997年 | 2篇 |
1994年 | 3篇 |
1992年 | 2篇 |
1991年 | 4篇 |
1990年 | 2篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1987年 | 3篇 |
1986年 | 1篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1981年 | 5篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1972年 | 3篇 |
1971年 | 1篇 |
排序方式: 共有283条查询结果,搜索用时 15 毫秒
11.
1,3-Dithiolanes are rapidly cleaved by nickel boride, generating corresponding hydrocarbons in excellent yields. The hydrogenolysis is rapid at room temperature and does not require protection from the atmosphere. Mild reaction conditions, simple workup, and good yields of pure products are some of the major advantages of the procedure. 相似文献
12.
Rohan J. Williams Nathan W. McGill Jonathan M. White Spencer J. Williams 《Journal of carbohydrate chemistry》2013,32(5):236-263
Variable yields and glycosylation stereoselectivity were obtained for NIS/TfOH-medi- ated reaction of 4-methoxyphenyl 2,4,6-tetra-O-acetyl-β-D-galactopyranoside and thiogalactosides bearing acetyl, benzoyl, 2,6-dimethoxylbenzoyl, 2,4,6-trimethylbenzoyl, or 2,6-dichlorobenzoyl groups at the 2-positions and acetyl at the remainder. X-ray structures of 4-methylphenyl 2,3,4,6-tetra-O-(2,4,6-trimethylbenzoyl)-1-thio-β-D-galactopyr anoside and 4-methylphenyl 3,4-O-isopropylidene-2,6-di-O-(2,4,6-trimethylbenzoyl)-1-thio-β-D-galactopyranoside revealed slightly distorted 4 C 1 chair conformations. Variable temperature NMR revealed that activation of 4-methylphenyl 2,3,4,6-tetra-O-(2,4,6-trimethylbenzoyl)-1-thio-β-D-galactopyranoside afforded only dioxolenium ion, whereas 4-methylphenyl 3,4,6-tri-O-acetyl-2-O-(2,4,6-trimethylbenzoyl)-1-thio-β-D-galactopyranoside gave a 1:1 mixture of dioxolenium ion and glycosyl triflate. However, the reaction intermediates formed from these deactivated donors do not influence the glycosylation stereoselectivity; instead, it is influenced by steric and electronic interactions at the transition states. 相似文献
13.
E. Rohan S. Naili R. Cimrman T. Lemaire 《Journal of the mechanics and physics of solids》2012,60(5):857-881
In this paper, we develop a model of a homogenized fluid-saturated deformable porous medium. To account for the double porosity the Biot model is considered at the mesoscale with a scale-dependent permeability in compartments representing the second-level porosity. This model is treated by the homogenization procedure based on the asymptotic analysis of periodic “microstructure”. When passing to the limit, auxiliary microscopic problems are introduced, which provide the corrector basis functions that are needed to compute the effective material parameters. The macroscopic problem describes the deformation-induced Darcy flow in the primary porosities whereas the microflow in the double porosity is responsible for the fading memory effects via the macroscopic poro-visco-elastic constitutive law. For the homogenization procedure, we use the periodic unfolding method. We discuss also the stress and flow recovery at multiple scales characterizing the heterogeneous material. The model is proposed as a theoretical basis to describe compact bone behavior on multiple scales. 相似文献
14.
15.
Microcionamides A and B, bioactive peptides from the philippine sponge Clathria (Thalysias) abietina
Davis RA Mangalindan GC Bojo ZP Antemano RR Rodriguez NO Concepcion GP Samson SC de Guzman D Cruz LJ Tasdemir D Harper MK Feng X Carter GT Ireland CM 《The Journal of organic chemistry》2004,69(12):4170-4176
Microcionamides A (1) and B (2) have been isolated from the Philippine marine sponge Clathria (Thalysias) abietina. These new linear peptides are cyclized via a cystine moiety and have their C-terminus blocked by a 2-phenylethylenamine group. Their total structures, including absolute stereochemistry, were determined by a combination of spectral and chemical methods. Compound 1 was shown to slowly isomerize about the C-36/C-37 double bond when stored in DMSO. Microcionamides A (1) and B (2) exhibited significant cytotoxicity against the human breast tumor cells lines MCF-7 and SKBR-3 and displayed inhibitory activity against Mycobacterium tuberculosis H(37)Ra. 相似文献
16.
Facile reductive coupling of benzylic halides is reported with ferrous oxalate dihydrate in DMF or HMPA under nitrogen atmosphere at 155-160 degrees C. The coupling is proposed to proceed by two successive oxidative additions of benzylic halides to ferrous oxalate to give an intermediate organoiron complex which undergoes concerted dimerization to give the corresponding reductively coupled dimers in high yields. 相似文献
17.
Chemical investigations of the Australian marine sponge Ecionemia geodides resulted in the isolation of two new pyridoacridine alkaloids, ecionines A (1) and B (2), along with the previously isolated marine natural products, biemnadin (3) and meridine (4). Compounds 1 and 2 both contain an imine moiety, which is rare for the pyridoacridine structure class. The chemical structures of 1 and 2 were determined by extensive 1D and 2D NMR and MS data analyses. All compounds were tested against a panel of human bladder cancer cell lines, the increasingly metastatic TSU-Pr1 series (TSU-Pr1, TSU-Pr1-B1 and TSU-Pr1-B2) and the superficial bladder cancer cell line 5637. Ecionine A (1) displayed cytotoxicity against all cell lines, with IC50 values ranging from 3 to 7 μM. This is the first report of chemistry from the sponge genus Ecionemia. 相似文献
18.
A systematic study on the water-intake capacity of the microemulsion formed using a catanionic surfactant (synthesized by taking equimolar mixture of acid and amine) with varying hydrocarbon chain length of the acid has been carried out. A decrease in the water-intake capacity with increase in the chain length was observed. Shorter chain length of co-surfactant (1-butanol compared to 1-octanol) led to higher water-intake capacity of microemulsions which may also be attributed to the high hydrophilic-lipophilic balance (HLB) of 1-butanol. Three new microemulsions based on catanionic surfactants have been used to synthesize quantum dots of CdS. The size of CdS quantum dots decreased with increase in chain length of the acid component of the catanionic surfactant. 相似文献
19.
Abstract
A facile and environmentally benign procedure for the synthesis of octahydroquinazolinone and biscoumarin derivatives in ionic liquids is reported. Octahydroquinazolinones were synthesized in the presence of trimethylsilyl chloride (TMSCl) while the synthesis of biscoumarins required no additive. The ability to reuse the ionic liquid, the high yields, and ease of purification are the important features of this process. 相似文献20.
Anil Khurana 《Zeitschrift für Physik B Condensed Matter》1994,95(2):197-200
One-particle Green function in the paramagnetic phase of a model of interacting fermions is obtained in the planar approximation. The model is zero-dimensional, in that thermal fluctuations are the only source of kinetic energy. 相似文献