全文获取类型
收费全文 | 979篇 |
免费 | 27篇 |
国内免费 | 5篇 |
专业分类
化学 | 719篇 |
晶体学 | 13篇 |
力学 | 9篇 |
数学 | 92篇 |
物理学 | 178篇 |
出版年
2020年 | 9篇 |
2019年 | 7篇 |
2018年 | 8篇 |
2017年 | 3篇 |
2016年 | 12篇 |
2015年 | 16篇 |
2014年 | 15篇 |
2013年 | 59篇 |
2012年 | 40篇 |
2011年 | 48篇 |
2010年 | 29篇 |
2009年 | 37篇 |
2008年 | 68篇 |
2007年 | 77篇 |
2006年 | 56篇 |
2005年 | 54篇 |
2004年 | 59篇 |
2003年 | 50篇 |
2002年 | 44篇 |
2001年 | 28篇 |
2000年 | 28篇 |
1999年 | 7篇 |
1998年 | 10篇 |
1997年 | 7篇 |
1996年 | 10篇 |
1995年 | 6篇 |
1994年 | 6篇 |
1993年 | 13篇 |
1992年 | 18篇 |
1991年 | 14篇 |
1990年 | 7篇 |
1989年 | 15篇 |
1988年 | 11篇 |
1986年 | 6篇 |
1985年 | 6篇 |
1984年 | 13篇 |
1983年 | 13篇 |
1982年 | 8篇 |
1981年 | 11篇 |
1980年 | 3篇 |
1979年 | 3篇 |
1978年 | 14篇 |
1977年 | 10篇 |
1976年 | 6篇 |
1975年 | 9篇 |
1974年 | 10篇 |
1973年 | 5篇 |
1972年 | 3篇 |
1970年 | 3篇 |
1966年 | 3篇 |
排序方式: 共有1011条查询结果,搜索用时 625 毫秒
11.
Ken-Ichi Ito Masamichi Nishimura Minoru Sashio Masahiro Tsunooka 《Journal of polymer science. Part A, Polymer chemistry》1994,32(11):2177-2185
It was found that amines were formed efficiently by the photolysis of O-acyloximes followed by hydrolysis in polystyrene films and the relationship between structures of O-acyloximes and yields of amines were investigated. O-phenylacetyl acetophenone oxime (PaApO), O-pivaloyl acetophenone oxime (PApO), and O-benzoyl acetophenone oxime (BApO) as monofunctional O-acyloximes and O,O′-succinyl diacetophenone oxime (SDApO) and O,O′-glutaryl diacetophenone oxime (GDApO) as bifunctional O-acyloximes were examined. The yields of amines for PaApO and SDApO under N2 were ca. 70%, which was the highest among O-acyloximes examined in this experiment. On the other hand, the yields for PApO, BApO, and GDApO were less than 15% and it was verified that the hydrogen abstraction by imino radicals via 6-membered cyclic intermediates resulted in the lowering of yields. Although the effect of oxygen under photolysis on the yields of amine for PaApO was negligible under 50% conversion of PaApO, the yield decreased with further increase in the conversion and was 50% at 90% conversion. © 1994 John Wiley & Sons, Inc. 相似文献
12.
Motozaki T Sawamura K Suzuki A Yoshida K Ueki T Ohara A Munakata R Takao K Tadano K 《Organic letters》2005,7(11):2265-2267
[reaction: see text]. We have completed the total synthesis of natural (+)-tubelactomicin A (1), a 16-membered macrolide antibiotic. This Letter presents a highly efficient synthesis of the upper-half segment (C14-C24) and the completion of the total synthesis featuring a high-yielding Stille coupling for the connection of the upper-half and lower-half segments and Mukaiyama macrolactonization for the construction of the entire structure of 1. 相似文献
13.
Shin-ichi Hirano Toshinobu Yogo Wataru Sakamoto Seiji Yamada Tomoyuki Nakamura Tomoe Yamammto Hiroyuki Ukai Kouichi Banno Tomoko Nakafuku Yukari Ando 《Journal of Sol-Gel Science and Technology》2003,26(1-3):35-41
Nano sized crystalline particles/polymer hybrids were synthesized form designed metal-organic precursors. The newly developed method is composed of the synthesis of organic matrix by polymerization and the in situ nucleation and growth of crystalline oxide particles in the organic matrix below 100°C. The design of metal-organic precursor modified with polymerizable ligand and the selection of reaction conditions does influence the size and crystallinity of ceramic particles in organic matrix. The nano-sized magnetic particle/polymer hybrid exhibits the interesting feature of superparamagnetism and quantum size effect. The crystalline particles of BaTiO3/, PbTiO3/, and KNbO3/polymer hybrids behave to be dielectric and show the typical electro-rheological behavior. 相似文献
14.
Fujii H Hirano N Uchiro H Kawamura K Nagase H 《Chemical & pharmaceutical bulletin》2004,52(6):747-750
7beta-Carbamoyl-4,5alpha-epoxymorphinans 5 were stereoselectively synthesized from the 7alpha-carboxylate intermediate 3 in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and amines under reflux conditions in mesitylene via a novel and reactive gamma-lactone 7. These were the first examples of the stereoselective syntheses of 7beta-substituted 4,5alpha-epoxymorphinans. The mechanism of the reaction process was elucidated as follows: 1) epimerization of 7alpha-carboxylate 3, 2) intramolecular lactonization of 7beta-carboxylate 6, and 3) aminolysis of the resultant gamma-lactone 7. The aminolysis of the isolated reactive gamma-lactone 7 with allylamine and the alcoholysis with MeOH in the presence of NaBH(4) proceeded at room temperature. The gamma-lactone 7 can be a useful intermediate for the preparation of 7beta-substituted 4,5alpha-epoxymorphinans that would be potent selective delta opioid receptor ligands. The stereoselective syntheses of the 7alpha-carbamoyl-4,5alpha-epoxymorphinans 9 from 7alpha-carboxylate 3 via 7alpha-carboxylic acid were also successful. 相似文献
15.
Three-component reactions of aldehydes, ammonia, and allylboronates were found to provide homoallylic primary amines in high yields with high chemo- and stereoselectivities. A two-step, one-pot, stereoselective synthesis of an uncommon alpha-amino acid, alloisoleucine, was achieved utilizing this reaction. 相似文献
16.
Sutasinee Kaewpuang-Ngam Koji Inazu Ken-Ichi Aika 《Research on Chemical Intermediates》2002,28(5):471-477
Catalytic wet air oxidation of ammonia over a co-precipitated transition metal-aluminium catalyst was investigated. Copper-aluminium (Cu-Al-O) catalyst exhibited the highest activity and N2 selectivity among those prepared from Co, Fe, Mn, and Ni. 50% of 1500 ppm of ammonia could be removed from wastewater of pH 12 at 503 K under 2.0 MPa of air by using 4.0 g of catalyst without formation of toxic nitrogen containing compounds. Cu and Al ions were not found in solution after the reaction. It has been found that the catalytic performance of Cu-Al-O catalyst was strongly dependent on the preparation methods. The co-precipitated Cu-Al-O catalysts showed high N2 selectivity. The presence of CuO is concluded to promote the reaction and CuAl2O4 in bulk phase is needed to stabilize the catalyst. 相似文献
17.
The 26S proteasome complex, consisting of two multisubunit complexes, a 20S proteasome and a pair of 19S regulatory particles, plays a major role in the nonlysosomal degradation of intracellular proteins. The 20S proteasome was purified from yeast and separated by two-dimensional gel electrophoresis (2-DE). A total of 18 spots separated by 2-DE were identified as the 20S proteasome subunits by peptide mass fingerprinting with matrix assisted laser desorption/ionization-time of flight-mass spectrometry (MALDI-TOF-MS). The alpha2-, alpha4- and alpha7-subunits gave multiple spots, which converged into one spot for each subunit when treated with alkaline phosphatase. The difference of pI between phosphorylated and dephosphorylated spots and their reaction against anti-phosphotyrosine antibody suggested that the alpha2- and alpha4-subunits are phosphorylated either at Ser or at Thr residue, and the alpha7-subunit is phosphorylated at Tyr residue(s). These phosphorylated subunits were analyzed by electrospray ionization-quadrupole time of flight-tandem MS (ESI-QTOF-MS/MS) to deduce the phosphorylation sites. The 20S proteasome has three different protease activities: chymotrypsin-like, trypsin-like and peptidylglutamyl peptide-hydrolyzing activities. The phosphatase treatment increased K(m) value for chymotrypsin-like activity of the 20S proteasome, indicating that phosphorylation may play an important role in regulating the proteasome activity. 相似文献
18.
19.
Hirano T Li W Abrams L Krusic PJ Ottaviani MF Turro NJ 《The Journal of organic chemistry》2000,65(5):1319-1330
The photochemistry of tetraphenylacetone (1) adsorbed on the external surface of a MFI zeolite (the sodium form of LZ-105) has been investigated in combination with computational chemistry, surface area measurements, EPR analysis, and classical adsorption isotherms. All of the methods are consistent with a supramolecular structural model in which 1 is first adsorbed strongly through intercalation of a single benzene ring into a hole on the LZ-105 external surface (site I) followed by a weaker binding to the external framework between the holes (site II) until a monolayer of 1 is formed. From both computational and surface area measurements, it is estimated that the site I holes on the external surface will be filled at ca. 0.3-0.5 wt %/wt loading of 1/LZ-105, which corresponds to 6.5 x 10(18) (ca. 10(-)(5) mol) of holes or molecules of 1 adsorbed in holes per gram of zeolite. The supramolecular composition of ca. 0.3-0.5% of 1 on LZ-105 characterizes a "break point" for the photochemistry and the EPR measurements, since it represents the value for saturation of the site I holes with 1. These conclusions are supported quantitatively by experimental isotherms of the adsorption of 1 on LZ-105. Photolysis of 1 intercalated in the site I holes causes fragmentation into two isomeric supramolecular diphenylmethyl (DPM) radicals, one (DMP)(in) which is adsorbed into the internal surface and becomes strongly persistent (half-life of many weeks) and the other (DMP)(ex) which diffuses on the external surface and rapidly dimerizes (less than a few minutes) to produce the radical-radical combination product tetraphenylethane (2). Photolysis of 1 adsorbed on the solid external surface produces two supramolecularly equivalent DPM radicals (DMP)(ex) that diffuse on the external surface and rapidly dimerize to produce 2, and do not produce persistent DPM radicals. 相似文献
20.
Michio Horiike Gu Yuan Chul-Sa Kim Chisato Hirano Katsuhiko Shibuya 《Journal of mass spectrometry : JMS》1992,27(9):944-948
Fourteen isomers of chemically unmodified hexadecenol were analysed by two types of quadrupole mass spectrometer, coupled with a gas chromatograph, for the location of the double-bond position. A series of spectra were interpreted in terms of mass spectral patterns on a fuzzy classification, in which the intensity ratios of six diagnostic pairs of the predominant ions were preferred in devising similarity indices. The accuracy of the method in the location of double-bond position was confirmed by testing with other series of spectra. 相似文献