全文获取类型
收费全文 | 1099篇 |
免费 | 35篇 |
国内免费 | 7篇 |
专业分类
化学 | 861篇 |
晶体学 | 10篇 |
力学 | 8篇 |
数学 | 77篇 |
物理学 | 185篇 |
出版年
2023年 | 4篇 |
2022年 | 7篇 |
2021年 | 8篇 |
2020年 | 13篇 |
2019年 | 18篇 |
2018年 | 14篇 |
2017年 | 10篇 |
2016年 | 30篇 |
2015年 | 25篇 |
2014年 | 23篇 |
2013年 | 47篇 |
2012年 | 71篇 |
2011年 | 65篇 |
2010年 | 39篇 |
2009年 | 44篇 |
2008年 | 72篇 |
2007年 | 64篇 |
2006年 | 64篇 |
2005年 | 69篇 |
2004年 | 73篇 |
2003年 | 54篇 |
2002年 | 61篇 |
2001年 | 20篇 |
2000年 | 14篇 |
1999年 | 13篇 |
1998年 | 13篇 |
1997年 | 9篇 |
1996年 | 10篇 |
1995年 | 8篇 |
1994年 | 13篇 |
1993年 | 16篇 |
1992年 | 11篇 |
1991年 | 7篇 |
1990年 | 6篇 |
1989年 | 9篇 |
1988年 | 3篇 |
1987年 | 6篇 |
1985年 | 13篇 |
1984年 | 9篇 |
1983年 | 12篇 |
1982年 | 6篇 |
1981年 | 11篇 |
1980年 | 9篇 |
1979年 | 10篇 |
1978年 | 6篇 |
1977年 | 10篇 |
1976年 | 4篇 |
1975年 | 8篇 |
1974年 | 3篇 |
1967年 | 2篇 |
排序方式: 共有1141条查询结果,搜索用时 765 毫秒
41.
Hu W Haddad PR Tanaka K Sato S Mori M Xu Q Ikedo M Tanaka S 《Journal of chromatography. A》2004,1039(1-2):59-62
A new ion chromatographic (IC) system has been established by using micelles of 3-(N,N-dimethylmyristylammonio)propanesulfonate (Zwittergent 3-14) loaded onto a reversed-phase packed column as the separation column with an electronic rotary switching valve packed-bed suppressor for conductometric detection of inorganic anions. An aqueous H3BO3-Na2B4O7 solution has been demonstrated to be the most desirable eluent for this IC system. The relationship between retention time and the concentration of the borate eluent was determined for a series of model anionic analytes and this relationship was found to be opposite to that exhibited in a conventional anion-exchange IC system. The rapid elution and complete separation of monovalent inorganic anions were obtained by initially using a high-concentration borate solution as the eluent for a short-period, and then switching to a lower-concentration borate eluent to complete the separation. Detection limits for nitrite, bromide, nitrate, and chlorate were 0.85, 0.88, 0.95 and 4.8 microM, respectively, when a 7.0 mM Na2B4O7 eluent was used. Moreover, the ability to directly detect these monovalent anions in samples containing high concentrations of sulfate and/or chloride ions provided a major advantage of this approach. 相似文献
42.
Miki Murata Hiyoruki Yamasaki Tsukasa Ueta Masayuki Nagata Masanori Ishikura Shinji Watanabe Yuzuru Masuda 《Tetrahedron》2007,63(19):4087-4094
The general and efficient silylation of aryl halides has been developed utilizing triethoxysilane and a rhodium catalyst. The substrate scope is broad and includes ortho-, meta-, and para-substituted electron-rich and -deficient aryl iodides. In addition, the silylation of aryl bromides and fluoroalkanesulfonates proceeded in the presence of tetra-n-butylammonium iodide. 相似文献
43.
Lin Wang Shinji Takeoka Eishun Tsuchida Satoru Tokuyama Takahiro Mashiko Tadashi Satoh 《先进技术聚合物》1992,3(1):17-21
The carbonyl hemoglobin (CO-Hb), which was used to prevent denaturation (metHb) during the preparation of samples, was encapsulated into lipid vesicles constituted from unsaturated phospholipid, cholesterol and unsaturated fatty acid. Unsaturated components were polymerized by γ-irradiation to enhance the stability of bilayer membrane. An aqueous dispersion of resulting Hb vesicles was freeze-dried in the presence of saccharides (50–200 mM) to obtain a dehydrated powder of Hb vesicles. Change in the vesicle size, the leakage of encapsulated Hb and the oxidation of Hb to metHb were not observed. Therefore, the long-term storage of Hb vesicles can be realized as a dry powder. 相似文献
44.
Determination of 4-alkylphenols by novel derivatization and gas chromatography-mass spectrometry 总被引:2,自引:0,他引:2
A simple and sensitive method for the determination of alkylphenols in water samples has been developed using gas chromatography-mass spectrometry. Alkylphenols were determined after the extractive derivatization with pentafluoropyridine. The derivatization of alkylphenols efficiently proceeded to give the corresponding 4-tetrafluoropyridyl derivatives under the biphasic reaction system. The derivatization conditions including the phase-transfer catalyst, the amount of pentafluoropyridine, the reaction time, the concentration of NaOH and organic solvent were optimized. On the mass spectra of these derivatives, intense specific ion peaks were observed: m/z 256 for 4-n-alkylphenols and m/z 284 for 4-tert.-alkylphenols. Calibration curves were linear in the range of 20-1000 ng/l (200-10,000 ng/l for nonylphenol), and the detection limits varied between 6.93 and 15.7 ng/l (85.2 ng/l for nonylphenol). The average recoveries of the alkylphenols in a fortified river water sample (100 ng/l except for nonylphenol: 1000 ng/l) ranged from 91.1 to 112%. The relative standard deviations were found to be between 5.6 and 16%. This method was successfully applied to the determination of alkylphenols in river water. 相似文献
45.
Oki Michinori Toyofuku Yoshitaka Sakaue Tatsuya Hirose Takanori Asakura Mitsuhiro Morita Nobuhiro Toyota Shinji 《Russian Journal of Organic Chemistry》2003,39(4):542-553
X-ray crystallography of the ap-form of the 1-(9-fluorenyl)-2-(2-methyl-2-oxiranyl)naphthalene has revealed that the carbon atom of the oxirane which is connected to the naphthyl group in this compound is almost planar. The specific structural features of the epoxy ring in this compound are caused by sterical effects and by the presence of a -system in the immediate vicinity of the oxirane ring. Certain differences have been found also in reactivity of rotational isomers of 2-X-substituted 1-[1-(9-fluorenyl)-2-naphthyl]ethyl cations (X = O, S, Se). At X = O arose more deprotonation product from the -position of the oxygen than in reactions of sulfur and selenium-containing analogs. Reactions of epoxides with zinc chloride almost exclusively gave the corresponding aldehydes. 相似文献
46.
Chatani N Oshita M Tobisu M Ishii Y Murai S 《Journal of the American Chemical Society》2003,125(26):7812-7813
A GaCl3-catalyzed reaction of alpha,beta-unsaturated ketones with isocyanides leading to the formation of unsaturated lactone derivatives is described. This is the first example of the catalytic [4+1] cycloaddition of alpha,beta-unsaturated ketones and isocyanides. GaCl3 is an excellent catalyst due to its lower oxophilicity, which is desirable for all of the key steps, such as E/Z isomerization, cyclization, and deattachment from the products. 相似文献
47.
Bunichiro Yamada Shinji Tagashira Shuzo Aoki 《Journal of polymer science. Part A, Polymer chemistry》1994,32(14):2745-2754
The dimerization of methyl methacrylate, ethyl methacrylate, methacrylonitrile, and α-methylstyrene to 2-substituted-1-allylic compounds [CH2?C(X)CH2C(CH3)2X] (X = COOR, C6H5, or CN), and methyl α-ethylacrylate to a 3-substituted-2-allylic compound [CH3CH?C(COOCH3)CH2C(CH3)(C2H5) COOCH3] was carried out by catalytic chain transfer using benzylbis (dimethylglyoximato) (pyridine) cobalt (III). These dimers were then used as addition-fragmentation chain transfer agents in the polymerizations of methyl methacrylate and styrene at 800C or above. Cross-dimers from methacrylic ester-α-methylstyrene and methacrylonitrile-α-methylstyrene mixtures were similarly prepared. Except for those from methyl α-ethylacrylate and methacrylonitrile, all the dimers participated in the addition-fragmentation and the copolymerization to different extents. The dimer of methyl α-ethylacrylate was actually inactive during the styrene and methyl methacrylate polymerizations. The methacrylonitrile dimer was primarily incorporated in the polymer chain through copolymerization. Among the dimer and the cross-dimers from α-methylstyrene with the other monomers, those bearing the α-methylstyrene moiety in the α-substituent [CH2?C(X)CH2C(CH3)2C6H5, X?COOCH3, COOC2H5, and CN] are noted as highly reactive chain transfer agents. © 1994 John Wiley & Sons, Inc. 相似文献
48.
Shinji Suzuki Furitsu Suzuki Yasumasa Kanie Koji Tsujitani Asako Hirai Hironori Kaji Fumitaka Horii 《Cellulose (London, England)》2012,19(3):713-727
The structure and crystallization of carefully isolated sub-elementary fibrils (SEFs) of bacterial cellulose have been investigated
using TEM, WAXD, and high-resolution solid-state 13C NMR. The addition of a suitable amount of fluorescent brightener (FB) to the incubation medium of Acetobacter xylinum effectively suppressed the aggregation of the SEFs into the microfibrils, as previously reported. However, this study confirmed
for the first time that serious structural change in the SEFs occurs during the removal of excess FB by washing with buffer
solutions having pH values higher than 6 or with the alkaline aqueous solution that was frequently used in previous studies.
In contrast, the isolation of unmodified SEFs was successfully performed by utilizing a washing protocol employing pH 7 citrate–phosphate
buffer solution containing 1% sodium dodecyl sulfate. High-resolution solid-state 13C NMR and WAXD measurements revealed that the SEFs thus isolated are in the noncrystalline state in which the pyranose rings
of the almost parallel cellulose chains appear to be stacked on each other. The respective CH2OH groups of the SEFs adopt the gt conformation instead of the tg conformation found in cellulose I
α
and I
β
crystals, and undergo significantly enhanced molecular motion in the absence of intermolecular hydrogen bonding associated
with these groups. The main chains are also subject to rapid motional fluctuations while maintaining the parallel orientation
of the respective chains, indicating that the SEFs have a liquid crystal-like structure with high molecular mobility. Moreover,
the SEFs crystallize into cellulose I
β
when the FB molecules that may adhere to the surface of the SEFs are removed by extraction with boiling 70 v/v% ethanol and
0.1N NaOH aqueous solution. On the basis of these results, the crystallization of the SEFs into the I
α
and I
β
forms is discussed, including the possible formation of the crystalline-noncrystalline periodic structure in native cellulose. 相似文献
49.
Kei Fujise Dr. Eiji Tsurumaki Prof. Dr. Kan Wakamatsu Prof. Dr. Shinji Toyota 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(14):4548-4552
Polycyclic aromatic compounds consisting of four or five fused anthracene units were synthesized by PtCl2-catalyzed cycloisomerization as novel long expanded helicenes. These compounds have helical structures with significant stacking of the terminal anthracene moieties at 0.33 nm interlayer distance. In the UV-vis and fluorescence spectra, the absorption and emission bands were red-shifted as the number of fused anthracene units was increased. The characteristic broad and long-lived emission bands of the long analogues are explained by the excimer-like stabilization of the excited state. These photophysical data as well as their cyclic voltammetric data are discussed on the basis of the π-conjugation and interlayer π⋅⋅⋅π interactions in the molecular structures and the molecular orbitals. The barrier and mechanism of helical inversion are also reported. 相似文献
50.
Akira Mori Hitoshi Takeshita Ryoji Mori Shinji Takematsu Manabu Takemoto Seiji Ujiie 《Liquid crystals》2013,40(2):171-178
Two kinds of monocyclic troponoid mesogens, 2,5-dialkanoyloxytropones (4) and 5-alkanoyloxy2-alkoxytropones (5), were prepared. The former showed monotropic smectic A phases and the virtual isotropic liquid-smectic A transitions of the latter were determined by extrapolation of results in a binary phase diagram. Comparing the mesogenic properties between the tropones 4 and the 2-alkanoyloxy-5-alkoxytropones (1), the alkanoyloxy group at C-5 enhances both the melting points and the transition temperatures of the smectic A phases. From the comparison between 5 and 1, the alkanoyloxy group at C-2 lowers the melting points. 相似文献