全文获取类型
收费全文 | 2348篇 |
免费 | 74篇 |
国内免费 | 10篇 |
专业分类
化学 | 1809篇 |
晶体学 | 31篇 |
力学 | 30篇 |
数学 | 141篇 |
物理学 | 421篇 |
出版年
2022年 | 15篇 |
2020年 | 24篇 |
2019年 | 53篇 |
2018年 | 18篇 |
2017年 | 21篇 |
2016年 | 40篇 |
2015年 | 26篇 |
2014年 | 32篇 |
2013年 | 110篇 |
2012年 | 126篇 |
2011年 | 129篇 |
2010年 | 63篇 |
2009年 | 76篇 |
2008年 | 122篇 |
2007年 | 122篇 |
2006年 | 134篇 |
2005年 | 110篇 |
2004年 | 72篇 |
2003年 | 80篇 |
2002年 | 88篇 |
2001年 | 52篇 |
2000年 | 39篇 |
1999年 | 30篇 |
1998年 | 28篇 |
1997年 | 22篇 |
1996年 | 23篇 |
1995年 | 15篇 |
1993年 | 26篇 |
1992年 | 28篇 |
1991年 | 28篇 |
1990年 | 37篇 |
1989年 | 26篇 |
1988年 | 28篇 |
1987年 | 24篇 |
1986年 | 33篇 |
1985年 | 33篇 |
1984年 | 41篇 |
1983年 | 30篇 |
1982年 | 35篇 |
1981年 | 32篇 |
1980年 | 35篇 |
1979年 | 29篇 |
1978年 | 38篇 |
1977年 | 31篇 |
1976年 | 21篇 |
1975年 | 19篇 |
1974年 | 23篇 |
1973年 | 22篇 |
1972年 | 14篇 |
1967年 | 14篇 |
排序方式: 共有2432条查询结果,搜索用时 15 毫秒
61.
62.
CuBr2-catalyzed three-component coupling of N-benzylallylamine, ethyl glyoxalate, and terminal alkynes afforded glycinate-tethered 1,6-enynes, which were further transformed into polycyclic pyrrole-2-carboxylates via iridium-catalyzed cycloisomerization/Diels-Alder cycloaddition/dehydrogenation sequence under conventional and microwave heating conditions. The corresponding β-amino acid analogues were obtained from the Ir-catalyzed reaction of enynes prepared from lithium amides of allylamines with methyl non-2-en-4-ynoate. The Cu-catalyzed Mannich-type condensation was further extended to the synthesis of the glycinate-tethered dienyne and cyclopropylenyne, which were subjected to Rh-catalyzed cycloisomerizations to furnish bicyclic amino acids. 相似文献
63.
A rhodium-catalyzed isomerization of alpha-arylpropargyl alcohols to indanones has been developed under mild conditions. Considering the ease of preparation of these propargyl alcohols (terminal alkynes + aromatic aldehydes), this method provides a new way of constructing indanones with high efficiency. By the mechanistic investigations using deuterium-labeled substrates, it has also been demonstrated that the reaction goes through an unexpected cascade, with a 1,4-hydrogen shift being the turnover-limiting step of the catalytic cycle. 相似文献
64.
Five Ci-Se...O...Se-Ci atoms in anthraquinone and 9-(methoxy)anthracene bearing phenylselanyl groups at 1,8-positions align linearly, the origin of which is shown to be a nonbonded 5c-6e interaction of the five atoms. 相似文献
65.
Reaction of α-nitro olefins with secondary amines in the presence of palladium(O) catalyst affords allylic amines. 相似文献
66.
67.
Takashi Abe Eiji Hayashi Hajime Baba Kazuo Kodaira Shunji Nagase 《Journal of fluorine chemistry》1980,15(5):353-380
Five kinds of 2-alkyl-substituted oxanes like 2-ethyloxane, 2-n-propyloxane, 2-iso-propyloxane, 2-n-butyloxane and 2-n-amyloxane were fluorinated electrochemically to give the corresponding perfluoro(2-alkyloxane)s. The perfluoro(2-alkyloxane)s were obtained in good yields from these starting materials together with isomeric perfluoro(2-alkyloxolane)s, perfluoro(2-alkyl-5-methyloxolane)s and perfluoro(dialkyl ether)s. The purification of the perfluoro(2-alkyloxane)s which contained small amounts of isomeric perfluoro(2-alkyloxolane)s was successfully achieved by recovering the former unreacted after treating these mixture with anhydrous aluminum chloride at 150 /sR 160 °C during /sR 48 hrs in order to convert the latter into the easy-separable perfluoro(2,5,5- trichloro-2-alkyloxolane)s. Small quantities of new perfluoro(5,5-dichloroalkanoyl chloride)s were also among the chlorination products. The spectroscopic data as well as the physical properties of these new fluorination products, and perfluoro(2,5,5-trichloro-2-alkyloxolane)s and perfluoro(5,5-dichloroalkanoyl chloride)s are presented. 相似文献
68.
Prof. Dr. Akira Watanabe Dr. Hisako Tamai Mrs. Kyoko Hayashi 《Colloid and polymer science》1977,255(8):773-781
Summary The adsorption of ionic dyes, the builder effect, and the interaction between ionic dyes and surface active agents were investigated by using the electrocapillarity at oil/water interfaces. The oil phase was the solution of tetra-butylammonium chloride, sodium cetylsulphate, cetylpyridinium chloride or stearylamine in methylisobutylketone, and the water phase contained various dyes in addition to the inorganic electrolyte. The interfacial tension decreased over the anodic (or cathodic) polarization range, when anionic (or cationic) dyes were added to the water phase, thus indicating the dye adsorption at the interface. The interfacial excess of dye ions was found to increase linearly with the cubic roots of dye concentration and of ionic strength of the water phase. The counterion binding took place at the oil/water interface between anionic (or cationic) dyes and positive (or negative) head groups of surface active agent ions adsorbed at the interface. It was found that the equilibrium constants of binding between anionic dyes and cationic surface active agents at the oil/water interface were of the same order of magnitude as, and a little larger than, those obtained at the water bulk phase.
With 12 figures and 2 tables 相似文献
Zusammenfassung An Öl-Wasser-Grenzflächen wurde die Adsorption ionischer Farbstoffe, die Füllerwirkung und die Wechselwirkung zwischen ionischen Farbstoffen und grenzflächenaktiven Verbindungen mit Hilfe der Elektrokapillarität untersucht. Die Ölphase bestand aus einer Lösung von Tetrabutylammoniumchlorid, Natriumcetylsulfat und Cetylpyridiniumchlorid oder Stearylamin in Methylisobutylketon. Die wässerige Phase enthielt verschiedene Farbstoffe und anorganische Elektrolyte. Die Grenzflächenspannung wurde durch anodische (bzw. kathodische) Polarisation erniedrigt, wenn anionischer (bzw. kationischer) Farbstoff zur Wasserphase gegeben wurde. Dies zeigt an, daß Farbstoff an der Grenzfläche adsorbiert wird. Die Grenzflächenkonzentration des Farbstoffions nimmt linear mit der Kubikwurzel der Farbstoffkonzentration und mit der Ionenstärke zu. Als Gegenionen für die anionischen (bzw. kationischen) Farbstoffe an der Grenzfläche wirken die positiven (bzw. negativen) Endgruppen der Tenside, die ebenfalls in der Grenzschicht adsorbiert sind. Die Gleichgewichtskonstanten für die Wechselwirkung zwischen den ionischen Farbstoffen und den grenzflächenaktiven Verbindungen sind an der Öl-Wasser-Grenzfläche von der gleichen Größenordnung bzw. geringfügig größer als in der wässerigen Volumenphase.
With 12 figures and 2 tables 相似文献
69.
H. Tsuchiya M. Sato H. Hayashi H. Kato H. Kureshiro T. Hayashi 《Chromatographia》1996,43(7-8):419-425
Summary Reversed-phase ion-pair HPLC with fluorimetric detectors connected in series was studied for the simultaneous determination of two tetrahydro--carbolines and two -carbolines. After additon of internal standards the samples were reacted with fluorescamine, and then subjected to serial extractions to remove their precursor (tryptamine) from the analytical system. This treatment not only suppressed the artefactual formation of tetrahydro--carbolines and -carbolines during analysis, but also effectively purified them. Under optimum conditions, using trifluoroacetic acid as counter ion, all analytes were separated within 14 min and without major interfering peaks. The quantitative ranges were 0.25–80.0 ng mL–1 for both tetrahydro--carbolines and 0.1–30.0 ng mL–1 for both -carbolines. Replicate spiking experiments showed that recovery from most of the samples tested was over 90% and the relative standard deviation ranged from 0.7 to 10.4% within and between assays. The proposed method was applicable to various materials such as soy sauce, vinegar, ketchup, tabasco, beer, wine, sake, whisky, brandy, cows' milk, coffee, cocoa, cheese, cigarette smoke and urine. 相似文献
70.
M. M. R. Talukder M. M. Zaman Y. Hayashi J. C. Wu T. Kawanishi 《Applied biochemistry and biotechnology》2007,141(1):77-83
The thermostability of Cromobacterium viscosum lip ase (EC 3.1.1.3) entrapped in AOT (sodium bis-[2-ethylhexyl] sulfosuccinate) reverse micelles was in creased by the addition
of short-chain polyethylene glycol (PEG 400). Two different approaches were considered: (1) the determination of half-life
time and (2) the mechanistic analysis of deactivation kinetics. The half-life of lipase entrapped in AOT/isooctane reverse
micelles with PEG 400 at 60°C was 28h, ninefold higher than that in reverse micelles without PEG 400. The lip ase entrapped
in both reverse micellar systems followed a series-type deactivation mechanism involving two first-order steps. The deactivation
constant for the first step at 60°C in PEG containing reverse micelles was 0.055 h11, 11-fold lower than that in reverse micelles without PEG, whereas it remained almost constant for the second step. The inactivation
energy of the lip ase entrapped in reverse micelles with and without PEG 400 was 88.12 and 21.97 kJ/mol, respectively. 相似文献