全文获取类型
收费全文 | 1002篇 |
免费 | 36篇 |
国内免费 | 9篇 |
专业分类
化学 | 729篇 |
晶体学 | 17篇 |
力学 | 20篇 |
数学 | 49篇 |
物理学 | 232篇 |
出版年
2024年 | 2篇 |
2023年 | 10篇 |
2022年 | 9篇 |
2021年 | 32篇 |
2020年 | 20篇 |
2019年 | 22篇 |
2018年 | 15篇 |
2017年 | 16篇 |
2016年 | 32篇 |
2015年 | 31篇 |
2014年 | 40篇 |
2013年 | 76篇 |
2012年 | 92篇 |
2011年 | 90篇 |
2010年 | 63篇 |
2009年 | 38篇 |
2008年 | 77篇 |
2007年 | 58篇 |
2006年 | 56篇 |
2005年 | 62篇 |
2004年 | 41篇 |
2003年 | 36篇 |
2002年 | 37篇 |
2001年 | 20篇 |
2000年 | 17篇 |
1999年 | 12篇 |
1998年 | 9篇 |
1997年 | 4篇 |
1996年 | 3篇 |
1995年 | 7篇 |
1994年 | 1篇 |
1993年 | 3篇 |
1992年 | 6篇 |
1991年 | 1篇 |
1989年 | 5篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1984年 | 1篇 |
1981年 | 1篇 |
排序方式: 共有1047条查询结果,搜索用时 296 毫秒
151.
Kim KS Fulse DB Baek JY Lee BY Jeon HB 《Journal of the American Chemical Society》2008,130(26):8537-8547
An efficient direct one-pot glycosylation method with anomeric hydroxy sugars as glycosyl donors employing phthalic anhydride and triflic anhydride as activating agents has been developed. Thus, highly stereoselective beta-mannopyranosylations were achieved by the reaction of 2,3-di-O-benzyl-4,6-O-benzylidene-D-mannopyranose (2) with phthalic anhydride in the presence of DBU at room temperature followed by sequential addition of DTBMP and Tf2O and glycosyl acceptors to the reaction mixture at -78 degrees C in one-pot. Stereoselective alpha-glucopyranosylations with 2,3-di-O-benzyl-4,6-O-benzylidene-D-glucopyranose (25) and other glycosylations with glucopyranoses and mannopyranoses having tetra-O-benzyl- and tetra-O-benzoyl protecting groups were also possible by utilizing the present one-pot glycosylation protocol. The possible mechanism for the beta-mannosylation with 2 was proposed based on the NMR study, in which alpha-mannosyl phthalate 55alpha and alpha-mannosyl triflate 59 were detected as intermediates. The versatility and efficiency of the present glycosylation methodology, especially those of the beta-mannopyranosylation protocol, were readily demonstrated by the efficient synthesis of protected beta-(1-->4)-D-mannotriose 62 and beta-(1-->4)-D-mannotetraose 67 with perfect beta-stereoselectivity. 相似文献
152.
Isotactic 6-armed star-shaped poly(vinyl alcohol) (PVA) with a narrow molecular weight distribution was successfully prepared by the living cationic polymerization of 6-armed star-shaped poly(tert-butyl vinyl ether) (PTBVE) and subsequent acidic ether cleavage. The PTBVE was synthesized using hexa(chloromethyl) melamine (HCMM) as a hexafunctional initiator and ZnI2 or ZnCl2 as an activator in toluene/MC (1/1 v/v) at −70 °C. A better living stability of PTBVE was obtained in the ZnCl2 activator system. The number average molecular weight and the polydispersity index of the 6-armed star-shaped PTBVE polymerized with ZnCl2 at −70 °C for 24 h were 156,000 g/mol and 1.47, respectively. The fraction of the mm sequence of the resulting PVA was 52%. 相似文献
153.
Cho MH Kim CY Moon K Chung KB Yim CJ Ko DH Sohn HC Jeon H 《The Journal of chemical physics》2008,129(3):034705
Medium energy ion scattering and high-resolution transmission electron microscopy are used to investigate the depth of the interfacial reaction of Hf-silicate film. The interfacial reaction is critically affected by the film thickness and the mole fraction of HfO(2) in silicate film. The interfacial compressive strain generated at the surface of the Si substrate is dependent on the film thickness during the postannealing process in film with a thickness of approximately 4 nm. Finally, the phase separation phenomenon demonstrates critically different behaviors at different film thicknesses and stoichiometries because the diffusion of Si from interface to surface is dependent on these factors. Moreover, the oxidation by oxygen impurity in the inert ambient causes SiO(2) top formation. 相似文献
154.
155.
Open‐Pore Two‐Dimensional MFI Zeolite Nanosheets for the Fabrication of Hydrocarbon‐Isomer‐Selective Membranes on Porous Polymer Supports 下载免费PDF全文
Han Zhang Prof. Qiang Xiao Prof. Xianghai Guo Prof. Najun Li Prashant Kumar Neel Rangnekar Mi Young Jeon Prof. Shaeel Al‐Thabaiti Prof. Katabathini Narasimharao Prof. Sulaiman Nasir Basahel Prof. Berna Topuz Frank J. Onorato Prof. Christopher W. Macosko Prof. K. Andre Mkhoyan Prof. Michael Tsapatsis 《Angewandte Chemie (International ed. in English)》2016,55(25):7184-7187
Two‐dimensional zeolite nanosheets that do not contain any organic structure‐directing agents were prepared from a multilamellar MFI (ML‐MFI) zeolite. ML‐MFI was first exfoliated by melt compounding and then detemplated by treatment with a mixture of H2SO4 and H2O2 (piranha solution). The obtained OSDA‐free MFI nanosheets disperse well in water and can be used for coating applications. Deposits made on porous polybenzimidazole (PBI) supports by simple filtration of these suspensions exhibit an n‐butane/isobutane selectivity of 5.4, with an n‐butane permeance of 3.5×10?7 mol m?2 s?1 Pa?1 (ca. 1000 GPU). 相似文献
156.
Jae‐Sung Bae Eunkyung Jeon Su‐Young Moon Wangsuk Oh Sun‐Young Han Jeong Hun Lee Prof. Sung Yun Yang Prof. Dong‐Myung Kim Prof. Ji‐Woong Park 《Angewandte Chemie (International ed. in English)》2016,55(38):11495-11498
The preparation of bicontinuous nanoporous covalent frameworks, which are promising for caging active enzymes, is demonstrated. The frameworks have three‐ dimensionally continuous, hydrophilic pores with widths varying between 5 and 30 nm. Enzymes were infiltrated into the bicontinuous pore by applying a pressured enzyme solution. The new materials and methods allowed the amount of caged proteins to be controlled precisely. The resulting enzyme‐loaded framework films could be recycled many times with nearly no loss of catalytic activity. Entropic trapping of proteins by a bicontinuous pore with the right size distribution is an unprecedented strategy toward facile in vitro utilization of biocatalysts. 相似文献
157.
Dielectrophoresis switching with vertical sidewall electrodes for microfluidic flow cytometry 总被引:5,自引:0,他引:5
A novel dielectrophoresis switching with vertical electrodes in the sidewall of microchannels for multiplexed switching of objects has been designed, fabricated and tested. With appropriate electrode design, lateral DEP force can be generated so that one can dynamically position particulates along the width of the channel. A set of interdigitated electrodes in the sidewall of the microchannels is used for the generation of non-uniform electrical fields to generate negative DEP forces that repel beads/cells from the sidewalls. A countering DEP force is generated from another set of electrodes patterned on the opposing sidewall. These lateral negative DEP forces can be adjusted by the voltage and frequency applied. By manipulating the coupled DEP forces, the particles flowing through the microchannel can be positioned at different equilibrium points along the width direction and continue to flow into different outlet channels. Experimental results for switching biological cells and polystyrene microbeads to multiple outlets (up to 5) have been achieved. This novel particle switching technique can be integrated with other particle detection components to enable microfluidic flow cytometry systems. 相似文献
158.
Hyang Hwa Jeon 《Tetrahedron letters》2007,48(4):627-631
Trifluoromethylated enynyl sulfones 3 were reacted with 2-4 equiv of phenyl, n-hexyl, trimethylsilyl, or triisopropylsilyl substituted ethynyllithium reagents in THF or ether at 0 °C to give trifluoromethylated enediynes 6 (Z)-stereoselectively in 41-96% yields. The reactions of β-fluoro-β-trifluoromethylvinyl sulfone 5 with same ethynyllithium reagents (4 equiv) afforded the corresponding enediynes 6 in 41-90% yields. The cross-coupling reactions of 6 bearing TMS group with aryl iodides in the presence of Pd(PPh3)2Cl2, Ag2CO3, and n-Bu4NBr provided the corresponding enediynes 6 in 20-71% yields. Dimerization of (Z)-6 bearing TMS group in the presence of CuBr2 and K2CO3 yielded dimer (Z,Z)-7 in good yield. 相似文献
159.
trans-Bis(ferrocenecarboxylato)(5,10,15,20-tetraphenylporphyrinato)tin(IV) complex Sn(TPP)(FcCOO)2 has been synthesized and fully characterized. The X-ray structural analysis of Sn(TPP)(FcCOO)2 reveals that the tin(IV) center is octahedrally coordinated by the porphyrin occupying the square base and axial coordination of two ferrocenecarboxylato ligands in an anti orientation with respect to each other. The Fe(II) center of the ferrocenecarboxylato ligand lies 5.7 Å from the tin(IV) center of the porphyrin ring. The cyclic voltammogram of Sn(TPP)(FcCOO)2 exhibits three distinctive redox couples consisting of one oxidative wave and two reductive waves due to the ferrocenecarboxylato ligands and the porphyrin ring, respectively. 相似文献
160.
Salvi L Jeon SJ Fisher EL Carroll PJ Walsh PJ 《Journal of the American Chemical Society》2007,129(51):16119-16125
A one-pot method for the direct preparation of enantioenriched (Z)-disubstituted allylic alcohols is introduced. Hydroboration of 1-halo-1-alkynes with dicyclohexylborane, reaction with t-BuLi, and transmetalation with dialkylzinc reagents generate (Z)-disubstituted vinylzinc intermediates. In situ reaction of these reagents with aldehydes in the presence of a catalyst derived from (-)-MIB generates (Z)-disubstituted allylic alcohols. It was found that the resulting allylic alcohols were racemic, most likely due to a rapid addition reaction promoted by LiX (X = Br and Cl). To suppress the LiX-promoted reaction, a series of inhibitors were screened. It was found that 20-30 mol % tetraethylethylenediamine inhibited LiCl without inhibiting the chiral zinc-based Lewis acid. In this fashion, (Z)-disubstituted allylic alcohols were obtained with up to 98% ee. The asymmetric (Z)-vinylation could be coupled with tandem diastereoselective epoxidation reactions to provide epoxy alcohols and allylic epoxy alcohols with up to three contiguous stereogenic centers, enabling the rapid construction of complex building blocks with high levels of enantio- and diastereoselectivity. 相似文献