全文获取类型
收费全文 | 270篇 |
免费 | 4篇 |
国内免费 | 2篇 |
专业分类
化学 | 259篇 |
晶体学 | 7篇 |
物理学 | 10篇 |
出版年
2018年 | 3篇 |
2017年 | 2篇 |
2016年 | 3篇 |
2015年 | 2篇 |
2014年 | 6篇 |
2013年 | 12篇 |
2012年 | 6篇 |
2011年 | 8篇 |
2010年 | 4篇 |
2009年 | 2篇 |
2008年 | 4篇 |
2007年 | 10篇 |
2006年 | 5篇 |
2005年 | 12篇 |
2004年 | 5篇 |
2003年 | 5篇 |
2002年 | 4篇 |
2001年 | 2篇 |
2000年 | 7篇 |
1999年 | 6篇 |
1997年 | 2篇 |
1996年 | 2篇 |
1995年 | 5篇 |
1994年 | 8篇 |
1993年 | 9篇 |
1992年 | 3篇 |
1991年 | 5篇 |
1990年 | 7篇 |
1989年 | 5篇 |
1988年 | 4篇 |
1987年 | 8篇 |
1986年 | 15篇 |
1985年 | 5篇 |
1984年 | 4篇 |
1982年 | 4篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1979年 | 8篇 |
1978年 | 15篇 |
1977年 | 9篇 |
1976年 | 4篇 |
1975年 | 2篇 |
1974年 | 6篇 |
1972年 | 2篇 |
1971年 | 3篇 |
1970年 | 3篇 |
1969年 | 2篇 |
1968年 | 5篇 |
1967年 | 10篇 |
1966年 | 2篇 |
排序方式: 共有276条查询结果,搜索用时 46 毫秒
141.
Organic-inorganic hybrid precursors for new type low-melting glasses without pollution elements such as Pb and F have been synthesized through a non-aqueous acid-base reaction process. The hybrid compounds consist of -Si-O-P- framework, in which some of the bridging oxygens of a Si tetrahedron are substituted by organic functional groups. Terminating the oxide framework by the organic functional groups lowers the network dimension. Orthophosphoric acid (H3PO4) and dialkyldichlorosilane (Me2SiCl2 and Et2SiCl2) were employed as starting materials. The formation of P-O-Si linkage was confirmed by IR spectra. The larger viscosity increase at higher Si concentrations is correlated to the formation of the linkage. It is proposed that an acid-base polycondensation reaction of P-OH+Si-Cl→P-O-Si+HCl↑ dominates the network formation. When dimethylsilane SiMe2 is substituted by divalent Sn, an organic-inorganic hybrid low-melting glass in the system of SnO-Me2SiO-P2O5 can be derived, the glass transition temperature of which is about 29 °C. So the present acid-base reactions provide a new process by which precursors for new type low-melting glass are synthesized. 相似文献
142.
The glass-forming region in the BiO1.5---BaO---CuO system was determined, and the structure and crystallization of the glasses were investigated and compared to glasses containing CaO or SrO in place of BaO. It has been found that the glass-forming region in this system is wider than those in the BiO1.5---CaO---CuO and BiO1.5---SrO---CuO systems. BaBiO2.77 was preferentially formed in the first stage of crystallization at all glass compositions. It is suggested that the Ba2+ ions may preferentially occupy the sites near BiO6 octahedra, while the Ca2+ ions in BiO1.5---CaO---CuO glasses may not have a site preference with respect to the BiO6 octahedra. 相似文献
143.
144.
Dr. Kazuki Fujii Dr. Koichi Mitsudo Dr. Hiroki Mandai Dr. Toshinobu Korenaga Prof. Dr. Seiji Suga 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(9):2208-2212
An efficient enantioselective acyl migration reaction of furanyl carbonates was developed to construct all-carbon quaternary stereogenic centers. In some cases, the reactions required only 0.05 mol % (minimum 500 ppm) of catalyst and showed a high turnover frequency value (TOF; 3640 h−1). Multigram-scale reactions (10 grams) also proceeded with high enantioselectivity (>99:1 e.r.) in quantitative yield. The catalyst was robust and easily recovered in 98 % yield. A wide range of functional groups were tolerated (15 examples, >98 % yield, up to >99:1 e.r.), and a variety of optically active 3,3′-disubstituted benzofuranone derivatives, which are useful intermediates for the synthesis of natural products and pharmaceuticals, were efficiently obtained. Control experiments on the catalyst structure (e.g., catalyst 1 a vs. 1 a ′ and 1 a ′′) and computational calculations revealed that both the catalytic activity and enantioselectivity should be enhanced by hydrogen bonding between catalyst and substrate. Moreover, this system was applied to the challenging γ-selective acyl migration reaction of furanyl carbonates with high γ-selectivity and high enantioselectivity (α:γ=10:90, 95:5 e.r.). 相似文献
145.
Ken-ichi Mimura Kosuke Hiramatsu Makoto Moriya Wataru Sakamoto Satoshi Kawado Yoshiaki Uesu Toshinobu Yogo 《Journal of nanoparticle research》2010,12(5):1933-1943
A transparent BaTiO3 particle/polymer hybrid was synthesized from a Ba–Ti double alkoxide modified with an organic ligand and its refractive index
and second harmonic generation were studied. BaTiO3 particles modified with 2-vinyloxyethanol (ethylene glycol monovinyl ether, EGMVE) were polymerized with methyl methacrylate
(MMA) yielding the transparent BaTiO3 particle/poly MMA hybrid. X-ray photoelectron spectra indicated that the formation of chemical bonds between BaTiO3 nanoparticles and the organic matrix. The refractive indexes of the hybrid films depended on the crystallite size of BaTiO3 nanoparticles and the volume fractions of BaTiO3 and polymer phases. The hybrid film synthesized at BaTiO3/8EGMVE/30H2O/PMMA revealed a refractive index of 1.65 at 589 nm with an Abbe number of 54. The hybrid film exhibited a second harmonic
wave of 532 nm on irradiation with a fundamental wave of 1,064 nm. 相似文献
146.
Hideyuki Higashimura Kiyoshi Fujisawa Masaaki Kubota Shiro Kobayashi 《Journal of polymer science. Part A, Polymer chemistry》2005,43(9):1955-1962
“Radical‐controlled” oxidative polymerization of phenol (p‐1) by (1,4,7‐triisopropyl‐1,4,7‐triazacyclononane)copper(II) catalyst was performed and compared with that of 4‐phenoxyphenol (p‐2) in detail. Although the coupling selectivity for p‐1 seemed to be controlled by the catalyst, the C? C coupling, which was excluded completely for p‐2, occurred to some extent. The initial reaction rate of p‐1 was much smaller than that of p‐2, leading to the difference of polymerization behavior between p‐1 and p‐2. The rate‐determining step would be the coupling of controlled radicals species from the ESR measurement of the reaction mixture. The polymer resulting from p‐1 consisted mainly of phenylene oxide units, but had no crystallinity in contrast to the crystalline polymer from p‐2. However, the present polymer showed the highest thermal stability in the polymers obtained by oxidative polymerization of p‐1. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1955–1962, 2005 相似文献
147.
A new method for the sensitive and selective determination of stronger acids, such as methanesulfonic acid, based on an acid-base interaction with N-methylacridone fluorophore (1), is proposed. The fluorescence spectrum of 1 was changed drastically by the addition of methanesulfonic acid in methanol. A ratiometric analysis of methanesulfonic acid by measuring the fluorescence intensities of 1 both at 426 nm and 473 nm showed good linearity with a correlation coefficient of 0.9995. 相似文献
148.
Kichisuke Nishimoto Keiko Higashimura Toshio Asada 《Theoretical chemistry accounts》1999,102(1-6):355-365
The reaction mechanism of flavin-catalyzed dehydrogenation of glycine has been studied by ab initio molecular orbital calculations
using the 6-31G* basis set. 10-Methyl isoalloxazine (10-MIA) has been used as the flavin model compound. The results showed
that when we assume a proton transport channel in amino acid oxidase, which is switched on by the substrate anion, the O12-protonated
10-MIA [10-MIAH+(O12)] is generated. The main structure of 10-MIAH+(O12) is one in which the central ring is expressed by an NAD+-like structure, which is favorable for driving the hydride-transfer reaction, i.e., the abstraction of the α-hydrogen of
glycine by the hydride-transfer mechanism. We have found that this protonation results in a dramatic lowering of the activation
energy of the reaction. The proposed mechanism is summarized as follows: the hydride transfer proceeds via two-electron transfer
and synchronous intramolecular proton transfer → intermolecular proton transfer.
Received: 10 August 1998 / Accepted: 17 September 1998 / Published online: 8 February 1999 相似文献
149.
Masanobu Mori Toshinobu Suzuki Tsuyoshi Sugita Daisuke Nagai Kazuo Hirayama Makoto Onozato Hideyuki Itabashi 《Analytica chimica acta》2014
This study aimed to evaluate the heavy metal adsorptivity of calcium-alginate-modified diethylenetriamine-silica gel (CaAD) and incorporate this biosorbent into a flow analytical system for heavy metal ions using flame atomic absorption spectrometry (FAAS). The biosorbent was synthesized by electrostatically coating calcium alginate onto diethylenetriamine (dien)-silica gel. Copper ion adsorption tests by a batch method showed that CaAD exhibited a higher adsorption rate compared with other biosorbents despite its low maximum adsorption capacity. Next, CaAD was packed into a 1 mL microcolumn, which was connected to a flow analytical system equipped with an FAAS instrument. The flow system quantitatively adsorbed heavy metals and enriched their concentrations. This quantitative adsorption was achieved for pH 3–4 solutions containing 1.0 × 10−6 M of heavy metal ions at a flow rate of 5.0 mL min−1. Furthermore, the metal ions were successfully desorbed from CaAD at low nitric acid concentrations (0.05–0.15 M) than from the polyaminecarboxylic acid chelating resin (Chelex 100). Therefore, CaAD may be considered as a biosorbent that quickly adsorbs and easily desorbs analyte metal ions. In addition, the flow system enhanced the concentrations of heavy metals such as Cu2+, Zn2+, and Pb2+ by 50-fold. This new enrichment system successfully performed the separation and determination of Cu2+ (5.0 × 10−8 M) and Zn2+ (5.7 × 10−8 M) in a river water sample and Pb2+ (3.8 × 10−9 M) in a ground water sample. 相似文献
150.
Mitsuo Sawamoto Toshiyuki Hasebe Masami Kamigaito Toshinobu Higashimura 《高分子科学杂志,A辑:纯化学与应用化学》2013,50(8):937-951
Abstract Both AB and BA block copolymers of α-methylstyrene (αMeSt) and 2-chloroethyl vinyl ether (CEVE) were synthesized by the sequential living cationic polymerization initiated with the HCl-CEVE adduct (1a)/SnBr4 system in CH2Cl2 at -78°C. αMeSt-CEVE (AB) block copolymers with narrow molecular weight distributions ([Mbar]w/[Mbar]n ~ 1.15) were obtained when αMeSt was polymerized first, followed by addition of CEVE to the resulting αMeSt living polymer solution. The reverse order of monomer addition, from CEVE to αMeSt, also led to a BA-type block copolymer. In the polymerization of a mixture of the two monomers, almost random copolymers were obtained. Living polymerizations of αMeSt were also induced with functional initiating systems, HCl-functionalized vinyl ether adducts (1b-1d)/SnBr4, to give end-function-alized poly(αMeSt)s with a methacrylate, an acetate, or a phthalimide terminal. 相似文献