全文获取类型
收费全文 | 1847篇 |
免费 | 58篇 |
国内免费 | 4篇 |
专业分类
化学 | 1570篇 |
晶体学 | 18篇 |
力学 | 16篇 |
数学 | 20篇 |
物理学 | 285篇 |
出版年
2021年 | 11篇 |
2020年 | 16篇 |
2019年 | 20篇 |
2018年 | 15篇 |
2017年 | 14篇 |
2016年 | 25篇 |
2015年 | 37篇 |
2014年 | 41篇 |
2013年 | 95篇 |
2012年 | 93篇 |
2011年 | 89篇 |
2010年 | 66篇 |
2009年 | 43篇 |
2008年 | 83篇 |
2007年 | 93篇 |
2006年 | 105篇 |
2005年 | 113篇 |
2004年 | 93篇 |
2003年 | 88篇 |
2002年 | 76篇 |
2001年 | 22篇 |
2000年 | 31篇 |
1999年 | 25篇 |
1998年 | 18篇 |
1997年 | 23篇 |
1996年 | 28篇 |
1995年 | 14篇 |
1994年 | 14篇 |
1993年 | 17篇 |
1992年 | 21篇 |
1991年 | 17篇 |
1990年 | 18篇 |
1989年 | 16篇 |
1988年 | 21篇 |
1987年 | 19篇 |
1986年 | 21篇 |
1985年 | 31篇 |
1984年 | 33篇 |
1983年 | 17篇 |
1982年 | 29篇 |
1981年 | 30篇 |
1980年 | 40篇 |
1979年 | 17篇 |
1978年 | 15篇 |
1977年 | 15篇 |
1976年 | 17篇 |
1974年 | 18篇 |
1973年 | 17篇 |
1968年 | 17篇 |
1967年 | 14篇 |
排序方式: 共有1909条查询结果,搜索用时 15 毫秒
31.
Reactions of the extremely labile molybdenocene olefin complexe Mo(η5-C5H5)2[(Z)-C6H5CH=CHC6H5] with heteroallenes X=C=Y (X=C=Y = CS2, (p-tolyl)NCN(p-tolyl), (C6H5)2CCO) gives the corresponding heteroallene complexes of molybdenocene Mo(η5-C5H5)2(X=C=Y) in high yields. Spectroscopic data clearly indicate a dihapto-coordination of the heteroallenes via the C=X bond (X = O, S, N). 相似文献
32.
Yoshio Kato Katsuo Komiya Yuuzi Sawada Hiroo Sasaki Tsutomu Hashimoto 《Journal of chromatography. A》1980,190(2)
The purification of enzymes was investigated by high-speed gel filtration on TSK-GEL G3000SWG columns packed with porous silica gel deactivated by chemically bonded hydrophilic compounds. Crude β-galactosidase from bacterial cells and commercial urease were purified ca. 15-fold in a single gel filtration. These enzymes were eluted within an hour from the column and the recoveries of enzymatic activity were almost 100% although the operation was carried out at room temperature (22°). Samples up to 100 mg could be applied to the column without loss of separation efficiency. 相似文献
33.
Yoshinori Tominaga Hiroto Okuda Shinya Kohra Hisako Mazume 《Journal of heterocyclic chemistry》1991,28(5):1245-1255
Reaction of various types of enaminones, which are prepared by the condensation of 1,3-dicarbonyl compounds with aromatic amines, with carbon disulfide in the presence of sodium hydroxide as the base in dimethyl sulfoxide to give the corresponding enamino dithiocarboxylates, 1,3-thiazines and trithiones. Enamino dithiocarboxylates are cyclized under refluxing in diphenyl ether to give the fused quinoline derivatives. The reaction of 6-arylamino-1,3-dimethyluracils with excess carbon disulfide in the presence of sodium hydroxide and subsequent methylation with dimethyl sulfate gave directly the corresponding 1,3-dimethyl-5-methylthiopyrimido[4,5-b]quinoline-2,4(1H,3H)-diones. 相似文献
34.
M Koizumi T Akao S Kadota T Kikuchi T Okuda K Kobashi 《Chemical & pharmaceutical bulletin》1991,39(10):2638-2643
This report discusses a novel type of arylsulfotransferase (AST) which was derived from human intestinal bacterium sulfated polyphenolic compounds when p-nitrophenyl sulfate (PNS) was taken as a donor substrate. (+)-Catechin, (+/-)-catechin, (-)-epicatechin and (-)-epicatechin gallate were better substrates than tyramine. (-)-Epigallocatechin and (-)-epigallocatechin gallate were slightly worse substrates than tyramine. Although gallic acid was a bad substrate, alkyl gallate esters were better substrates than tyramine. The degree of acceptor specificity increased in proportion to the length of the alkyl group up to the carbon number of five. Pedunculagin, geraniin and corilagin were less effective than tyramine. Rosmarinic acid and penta-O-galloyl-beta-D-glucose were similarly well sulfated. Two products, 4'-monosulfate and 4',5-disulfate of (+)-catechin, were detected at a two-fold molar excess of PNS over (+)-catechin. When (+)-catechin-4'-monosulfate as an acceptor was enzymatically sulfated with PNS as a donor, only the 4',5-disulfate was produced. Thus, arylsulfotransferase was useful for the convenient preparation of sulfate esters of polyphenols at their specific hydroxyl groups. 相似文献
35.
Toru Takagishi Sadatsugu Okuda Nobuhiko Kuroki Hiroshi Kozuka 《Journal of polymer science. Part A, Polymer chemistry》1985,23(8):2109-2116
Measurements have been made of the binding of divalent metal ions, Cu2+, Ni2+, Co2+, and Zn2+ ions, by polyethylenimine (PEI) and its acetyl or alkyl derivatives by the equilibriumdialysis technique. These metal ions, in particular the Cu2+ ion, exhibited tremendously remarkable binding affinity toward PEI. The extent of complexation of the polymer with the metal ions was decreased markedly by acetylation or alkylation of the polymer. PEI with no primary amine showed an appreciable decrease in its affinity for the metal ion. These results indicate the participation of the primary amine of the polymer in the formation of the complex. A cooperative binding isotherm was observed in PEI–metal ion complex formation, suggesting swelling or conformational change of the polymer induced by this coordination process. Binding of the Cu2+ ion by PEI was found to be essentially independent of temperature over the range 5–35°C. 相似文献
36.
Tsutomu Takeichi Nobuyuki Takahashi Rikio Yokota 《Journal of polymer science. Part A, Polymer chemistry》1994,32(1):167-174
Polyimide/polyimide molecular composite (MC) films comprised of a rigid polyimide derived from biphenyltetracarboxylic dianhydride (BPDA) and p-phenylenediamine (PDA) and a flexible polyimide derived from BPDA and bis (3,3'-diaminodiphenyl) acetylene (intA) and/or oxydianiline (ODA) were prepared by blending the polyamic acid solutions in 7 : 3 weight ratio, and then imidizing the blend films. Acetylene content in the flexible polyimide backbone was controlled by the ratio of intA and ODA. Cold-drawing of the blend polyamic acid films, followed by imidization, gives high modulus polyimide/polyimide MC films. The modulus of the MC films increased almost linearly with the draw ratio, reaching 25.5 GPa for the 40% drawn film. Acetylene groups in the flexible polyimide can be thermally cured to crosslink. The onset of exotherm appeared at 340°C on DSC, reaching maximum at 398°C. After the thermal crosslinking, the MC films maintained the high modulus, though elongation became small. Taking advantage of the crosslinkable acetylene units, two MC films were laminated and processed at 400°C for 20 min under 100 kg/cm2 to give a good-quality laminate film. The interface of the two films was strongly bonded through the crosslinking of acetylene groups. Laminate films maintained the high modulus afforded by the cold-drawing. © 1994 John Wiley & Sons, Inc. 相似文献
37.
Tsutomu Konno Jungha Chae Tomoo Tanaka Takashi Ishihara Hiroki Yamanaka 《Journal of fluorine chemistry》2006,127(1):36-43
Hydroalumination, hydrocupration, and hydroboration reactions of various fluorine-containing alkynes were investigated. The alkyne reacted smoothly with 2.0 equiv. of Red-Al at −78 °C to give the hydroaluminated adduct in a highly regio- and stereoselective manner, which was treated with iodine, the corresponding vinyliodide being produced in moderate yield. Hydrocupration of the alkynes also took place, but the resulting vinylmetal reacted with various electrophiles sluggishly. In sharp contrast, the reaction with dicyclohexylborane proceeded smoothly to afford the cis-addition products preferentially, which were subjected to Suzuki-Miyaura cross-coupling reaction, leading to trisubstituted alkenes in high yields. 相似文献
38.
Woodfordin C, a macro-ring hydrolyzable tannin dimer with antitumor activity, and accompanying dimers from Woodfordia fruticosa flowers 总被引:1,自引:0,他引:1
T Yoshida T Chou A Nitta K Miyamoto R Koshiura T Okuda 《Chemical & pharmaceutical bulletin》1990,38(5):1211-1217
Three new dimeric hydrolyzable tannins, woodfordins A, B and C, along with seven known hydrolyzable tannins, including oenothein B, a dimer exhibiting marked host-mediated antitumor activity, were isolated from an Indonesian crude drug, Sidowayah [dried flowers of Woodfordia fruticosa (L.) Kurz (Lythraceae)]. The structures of the new tannins were elucidated based on chemical and spectral evidence. Woodfordin C, having a macro-ring structure, was also found to exhibit a significant antitumor activity. 相似文献
39.
Highly Selective Separation of Rhodopsin from Bovine Rod Outer Segment Membranes Using Combination of Divalent Cation and Alkyl(thio)glucoside 总被引:1,自引:0,他引:1
The micellization process of bovine rod outer segment (ROS) membranes is investigated utilizing a series of neutral detergents. It is found that when alkyl(thio)glucosides with an appropriate hydrophilic–lipophilic balance (e.g. octylthioglucoside) are used in combination with a divalent cation, rhodopsin is selectively extracted from ROS membranes at a specific detergent-to-membrane ratio. This allows remarkable purification of rhodopsin by a single-step solubilization, because the residual membranes are heavily aggregated in the presence of divalent cation and are therefore easily sedimented by low-speed centrifugation. The absorption spectrum of the supernatant reproducibly exhibits an A280 /A500 value of 1.6, an excellent value that could rarely be obtained by chromatographic purification. The degree of purification also depends on the type of divalent cation included in the solubilization solution; specific binding of IIB-series cations (Zn2+ and Cd2+ ) to ROS membranes is suggested to play an important role in the solubilization process. The present result represents a unique example of selective solubilization of a specific membrane protein from highly aggregated membranes. 相似文献
40.
Masakazu Uramoto Kimie Kobinata Kiyoshi Isono Tsutomu Higashijima Tatsuo Miyazawa Ellen E. Jenkins James A. McCloskey 《Tetrahedron》1982,38(11):1599-1608
New chitin synthetase inhibitors, neopolyoxins A, B, and C were isolated from the culture filtrate of Streptomyces cacaoi subsp. asoensis. Their absolute structures have been established on the basis of chemical and spectroscopic evidence. They are structural analogs of the polyoxins. As a nucleobase, neopolyoxin C possesses uracil, while neopolyoxins A and B contain the imidazoline moiety. A ring contraction reaction of pyrimidine nucleoside into imidazoline nucleoside played a key role in the structure determination of the nucleoside moiety. A similar transformation was suggested for the biosynthesis of neopolyoxins A and B. 相似文献