全文获取类型
收费全文 | 159篇 |
免费 | 8篇 |
专业分类
化学 | 136篇 |
力学 | 2篇 |
数学 | 3篇 |
物理学 | 26篇 |
出版年
2023年 | 2篇 |
2021年 | 2篇 |
2020年 | 5篇 |
2019年 | 4篇 |
2018年 | 4篇 |
2017年 | 1篇 |
2016年 | 6篇 |
2015年 | 2篇 |
2014年 | 9篇 |
2013年 | 10篇 |
2012年 | 11篇 |
2011年 | 10篇 |
2010年 | 7篇 |
2009年 | 2篇 |
2008年 | 16篇 |
2007年 | 11篇 |
2006年 | 4篇 |
2005年 | 13篇 |
2004年 | 6篇 |
2003年 | 5篇 |
2002年 | 4篇 |
2001年 | 1篇 |
2000年 | 4篇 |
1999年 | 4篇 |
1997年 | 3篇 |
1995年 | 3篇 |
1994年 | 1篇 |
1991年 | 3篇 |
1990年 | 3篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1977年 | 1篇 |
1976年 | 2篇 |
1975年 | 1篇 |
1974年 | 1篇 |
排序方式: 共有167条查询结果,搜索用时 15 毫秒
81.
Sasikala TS Nair BP Pavithran C Sebastian MT 《Langmuir : the ACS journal of surfaces and colloids》2012,28(25):9742-9747
Hybrid silica spheres (HS) of size 270-350 nm with vinyl and aminopropyl surface groups were incorporated in polystyrene (PS), and its effect on dielectric properties, coefficient of thermal expansion (CTE), and strength of PS-HS composite was studied. Incorporation of HS in PS followed a decrease in the dielectric constant from 3.2 for PS to 2.6 for composite with 7.5 vol % HS. The decrease in the dielectric constant was attributed to (i) increased interfacial porosity, (ii) formation of anhydrous HS having low dielectric constant, during hot processing of the composites, and (iii) dispersion and preservation of the anhydrous HS in the hydrophobic matrix. The dielectric constant of the composites with HS content up to 7.5 vol % does not vary much with temperature in the range from -20 to 65 °C. These composites also exhibited reduced CTE and improved flexural strength/stiffness due to good interfacial bonding through HS vinyl groups and dispersion of the filler in the matrix. The dielectric loss increased with HS content, and the loss measured for 7.5 vol % PS-HS composite was 6 × 10(-3), as compared to 10(-4) for PS. At HS loading above 7.5 vol %, the tendency of HS to agglomerate and form percolated structure lead to an increase in the dielectric constant and decrease in the mechanical properties of the composites. 相似文献
82.
Bindu V. Meprathu 《Tetrahedron》2010,66(31):5768-10541
The synthesis and characterization of new hypervalent iodine reagents ArINTs (2a), ArIO (3a), and ArIO2 (4a) (Ar=2-tert-butylsulfonyl-5-tert-butylphenyl) are described. These reagents are compared to previously reported analogous set of reagents Ar=2-tert-butylsulfonylphenyl and found to have significantly enhanced solubility and similar chemical reactivity. The X-ray crystal structures of 4a and of ArI (1a) (Ar=2-tert-butylsulfonyl-5-tert-butylphenyl) are discussed and compared. These reagents find use in atom and group transfer reactions. 相似文献
83.
Collision-induced dissociation (CID) of deprotonated hexose-containing disaccharides (m/z 341) with 1–2, 1–4, and 1–6 linkages yields product ions at m/z 221, which have been identified as glycosyl-glycolaldehyde anions. From disaccharides with these linkages, CID of m/z 221 ions produces distinct fragmentation patterns that enable the stereochemistries and anomeric configurations of the non-reducing
sugar units to be determined. However, only trace quantities of m/z 221 ions can be generated for 1–3 linkages in Paul or linear ion traps, preventing further CID analysis. Here we demonstrate
that high intensities of m/z 221 ions can be built up in the linear ion trap (Q3) from beam-type CID of a series of 1–3 linked disaccharides conducted
on a triple quadrupole/linear ion trap mass spectrometer. 18O-labeling at the carbonyl position of the reducing sugar allowed mass-discrimination of the “sidedness” of dissociation events
to either side of the glycosidic linkage. Under relatively low energy beam-type CID and ion trap CID, an m/z 223 product ion containing 18O predominated. It was a structural isomer that fragmented quite differently than the glycosyl-glycolaldehydes and did not
provide structural information about the non-reducing sugar. Under higher collision energy beam-type CID conditions, the formation
of m/z 221 ions, which have the glycosyl-glycolaldehyde structures, were favored. Characteristic fragmentation patterns were observed
for each m/z 221 ion from higher energy beam-type CID of 1–3 linked disaccharides and the stereochemistry of the non-reducing sugar, together
with the anomeric configuration, were successfully identified both with and without 18O-labeling of the reducing sugar carbonyl group. 相似文献
84.
Chiharu Konda Frank A. Londry Brad Bendiak Yu Xia 《Journal of the American Society for Mass Spectrometry》2014,25(8):1441-1450
A systematic approach is described that can pinpoint the stereo-structures (sugar identity, anomeric configuration, and location) of individual sugar units within linear oligosaccharides. Using a highly modified mass spectrometer, dissociation of linear oligosaccharides in the gas phase was optimized along multiple-stage tandem dissociation pathways (MSn, n = 4 or 5). The instrument was a hybrid triple quadrupole/linear ion trap mass spectrometer capable of high-efficiency bidirectional ion transfer between quadrupole arrays. Different types of collision-induced dissociation (CID), either on-resonance ion trap or beam-type CID could be utilized at any given stage of dissociation, enabling either glycosidic bond cleavages or cross-ring cleavages to be maximized when wanted. The approach first involves optimizing the isolation of disaccharide units as an ordered set of overlapping substructures via glycosidic bond cleavages during early stages of MSn, with explicit intent to minimize cross-ring cleavages. Subsequently, cross-ring cleavages were optimized for individual disaccharides to yield key diagnostic product ions (m/z 221). Finally, fingerprint patterns that establish stereochemistry and anomeric configuration were obtained from the diagnostic ions via CID. Model linear oligosaccharides were derivatized at the reducing end, allowing overlapping ladders of disaccharides to be isolated from MSn. High confidence stereo-structural determination was achieved by matching MSn CID of the diagnostic ions to synthetic standards via a spectral matching algorithm. Using this MSn (n = 4 or 5) approach, the stereo-structures, anomeric configurations, and locations of three individual sugar units within two pentasaccharides were successfully determined. Fig. a
? 相似文献
85.
TFAA/H3PO4‐Mediated C‐2 Acylation of Thiophene: A Direct Synthesis of Known and Novel Thiophene Derivatives of Pharmacological Interest
下载免费PDF全文
![点击此处可从《Journal of heterocyclic chemistry》网站下载免费的PDF全文](/ch/ext_images/free.gif)
P. Bindu Shravan Reddy Naini K. Shanmukha Rao P. K. Dubey Sarbani Pal 《Journal of heterocyclic chemistry》2014,51(3):586-593
A number of aliphatic and aromatic carboxylic acids were reacted with thiophene in the presence of trifluoroacetic anhydride and H3PO4 to give a variety of acylated thiophenes in good to excellent yields. The methodology was used to prepare known nonsteroidal anti‐inflammatory drug‐based novel compounds of potential pharmacological significances. Molecular modeling studies were carried out by using nonsteroidal anti‐inflammatory drug‐based thiophene derivatives to assess their cyclooxygenase inhibiting potential in silico. On the basis of docking studies followed by subsequent in vitro assay, indomethacin‐based thiophene derivative was identified as a novel cyclooxygenase‐2 inhibitor with balanced selectivity. 相似文献
86.
Vijai N. Pathak Ragini Gupta Ranjana Tiwari Rekha Gupta Vineeta Sareen Bindu Varshney 《合成通讯》2013,43(6):977-984
In this communication, a simple and straightforward procedure for the heterocyclization of 1H‐4,5‐dihydro‐3‐(4‐haloaryl)‐5‐substituted phenylpyrazoles (4) with 1‐bromo‐3‐chloropropane and 2,3‐dibromo‐1‐(4‐fluoro‐3‐methylphenyl)‐3‐phe‐ nylpropanone affording 2,3,4,8,9‐pentahydro‐7‐(4‐haloaryl)pyrazolo[5,1‐e]benzo[1,5] oxazocines 5 and regioselective synthesis of 2,3(erythro),7,8‐tetrahydro‐2‐ aryl‐3‐(4‐fluoro‐3‐methylbenzoyl)‐6‐(4‐halophenyl)pyrazolo[5,1‐d]benzo[1,4]oxa‐ zepines 6, respectively, via solid–liquid PTC is reported. All the synthesized compounds have been characterized on the basis of their spectral studies (IR, PMR, and MS) and analytical data. 相似文献
87.
88.
89.
Mahender Khatravath Naveen Kumar Mallurwar Saidulu Konda Jagan Gaddam Pallavi Rao Javed Iqbal Prabhat Arya 《Tetrahedron letters》2019,60(37):150915
A practical stereoselective synthesis of the central C1–C10 fragment of eribulin and its two diastereomeric analogues is developed. Our approach relied on the use of l-ascorbic acid as the starting material which allowed accessing a key intermediate with a syn diol moiety (C9 and C10 of eribulin) and a carboxylic ester group. A functionalized six membered lactone having several required hydroxyl groups was then obtained. In a number of steps, the lactone was converted to an intermediate for our key oxa-Michael reaction. A regio- and stereocontrolled intramolecular oxa-Michael reaction completed the synthesis of the C1–11 fragment having a trans-fused tetrahydropyrans with the exact stereochemistry of various hydroxyl groups, as in eribulin. 相似文献
90.
High-quality Er(3+):ZnO films were grown by the pulsed-laser deposition technique for 0.5 and 2 wt. % Er doping. Two peaks were observed at approximately 1.54 microm in the photoluminescence spectra of samples with 2 wt. % doping contrary to only one peak in the 0.5 wt. % doped sample. Both peaks were found to be strongly temperature dependent. The microscopic studies clearly illustrate that the appearance of the additional peak is attributed to the environment of Er(3+) ions in the form of ErO(6) clusters, which are optically active centers in the ZnO matrix. These results are very important for designing waveguides for telecommunications. 相似文献