全文获取类型
收费全文 | 2413篇 |
免费 | 78篇 |
国内免费 | 6篇 |
专业分类
化学 | 1871篇 |
晶体学 | 25篇 |
力学 | 46篇 |
数学 | 108篇 |
物理学 | 447篇 |
出版年
2022年 | 16篇 |
2021年 | 19篇 |
2020年 | 31篇 |
2019年 | 34篇 |
2018年 | 29篇 |
2017年 | 15篇 |
2016年 | 46篇 |
2015年 | 39篇 |
2014年 | 45篇 |
2013年 | 90篇 |
2012年 | 105篇 |
2011年 | 111篇 |
2010年 | 74篇 |
2009年 | 83篇 |
2008年 | 137篇 |
2007年 | 96篇 |
2006年 | 135篇 |
2005年 | 136篇 |
2004年 | 108篇 |
2003年 | 94篇 |
2002年 | 74篇 |
2001年 | 51篇 |
2000年 | 60篇 |
1999年 | 38篇 |
1998年 | 26篇 |
1997年 | 37篇 |
1996年 | 35篇 |
1995年 | 21篇 |
1994年 | 39篇 |
1993年 | 26篇 |
1992年 | 42篇 |
1991年 | 22篇 |
1990年 | 29篇 |
1989年 | 28篇 |
1988年 | 28篇 |
1987年 | 41篇 |
1986年 | 28篇 |
1985年 | 35篇 |
1984年 | 34篇 |
1983年 | 27篇 |
1982年 | 37篇 |
1981年 | 37篇 |
1980年 | 38篇 |
1979年 | 35篇 |
1978年 | 24篇 |
1977年 | 33篇 |
1976年 | 26篇 |
1975年 | 19篇 |
1974年 | 15篇 |
1973年 | 15篇 |
排序方式: 共有2497条查询结果,搜索用时 187 毫秒
71.
72.
73.
74.
Miura T Goto K Waragai H Matsumoto H Hirose Y Ohmae M Ishida HK Satoh A Inazu T 《The Journal of organic chemistry》2004,69(16):5348-5353
The Bfp-OH, a novel fluorous protecting reagent, was able to be easily prepared. The Bfp group was readily introduced to a carbohydrate, removed in high yield, and recyclable after cleavage. The use of the Bfp group made it possible to synthesize a pentasaccharide by minimal column chromatography purification. Each synthetic intermediate was able to be easily purified only by simple fluorous-organic solvent extraction and monitored by TLC, NMR, and MS. 相似文献
75.
Kondo F Saito H Hayashi R Onda H Kobayashi S Matsumoto M Suzuki M Ito Y Oka H Nakanishi T Shimizu A 《The Analyst》2003,128(11):1360-1364
We developed a rapid and reliable identification method for Shiga toxins in Shiga toxin-producing Escherichia coli (STEC) using immunoprecipitation and high-performance liquid chromatography-electrospray ionization mass spectrometry (HPLC-ESI-MS). Polyclonal antisera specific for Shiga toxin 1 (Stx1) and Shiga toxin 2 (Stx2) were raised in rabbits so as to be used for the immunoprecipitation. The immunoprecipitaion was carried out by mixing sample solutions with 50 microl each of the antisera to Stx1 and Stx2 followed by allowing the mixed solutions to stand for 30 min. The quantity required to obtain the immunoprecipitate was more than 0.5 microg of Shiga toxins. HPLC-ESI-MS analysis of the resulting immunoprecipitates provided accurate molecular weight information on Shiga toxins, leading to direct evidence for the presence of these toxins. It requires at most two days to perform our procedure from toxin extraction to measurement of HPLC-ESI-MS whereas the previous method using isolation procedures required about two weeks to complete. The usefulness of the present method has been demonstrated by identifying Stx1, Stx2 and a variant of Stx2 (Stx2e) in the immunoprecipitates prepared from STEC strains. 相似文献
76.
Chemical modification of cinoxacin was studied with the aim of improving its antibacterial activity and spectrum. Alkylation of ethyl 6,7,8-trifluoro- and 6,7-difluoro-4-hydroxycinnoline-3-carboxylates (1 and 7) with alkyl iodide or dialkyl sulfate gave ethyl 1-alkyl-6,7,8-trifluoro- and 6,7-difluoro-1,4-dihydro-4-oxocinnoline-3-carboxylates (2 and 8), together with the isomeric anhydro-bases 3 and 9 of 2-alkyl-3-ethoxycarbonyl-6,7,8-trifluoro- and 6,7-difluoro-4-hydroxycinnolinium hydroxides, respectively. Acid-catalyzed hydrolysis of the 1-alkyl derivatives 2 and 8 gave the corresponding carboxylic acids 4 and 10. The same treatment of 3 and 9, accompanied with decarboxylation of the inner salts 5 and 11, afforded the anhydro-bases 6 and 12 of 2-alkyl-4-hydroxycinnolinium hydroxides, respectively. Displacement reactions of 4 and 10 with nucleophiles such as amine, alkoxide and thiolate gave 7-substituted 1-alkyl-6,8-difluoro- and 6-fluoro-1,4-dihydro-4-oxocinnoline-3-carboxylic acids (13 and 17-35). Antibacterial activities of these compounds were evaluated and compared with those of cinoxacin and norfloxacin. Some compounds showed a broader spectrum and more potent activity than cinoxacin, but were considerably inferior in activity to norfloxacin. 相似文献
77.
Y Nishikawa T Shindo K Ishii H Nakamura T Kon H Uno J Matsumoto 《Chemical & pharmaceutical bulletin》1989,37(5):1256-1259
A new series of oxopyridinecarboxamide derivatives 3a--g and 5a were synthesized and evaluated for their antiallergic activity. 1,4-Dihydro-7-methyl-4-oxo-1,8-naphthyridine-3-carboxamides 3a and 5a exhibited potent antiallergic activity (inhibitory rates of 80.7 and 88.3%, respectively, at 20 mg/kg, p.o.) in the rat passive cutaneous anaphylaxis (PCA) test and also exhibited much more potent in vitro inhibitory activity than caffeic acid against the enzyme 5-lipoxygenase (5-LO). Their in vitro antihistamine activity, however, was weaker than that of ketotifen. Compounds 3a and 5a are viewed as promising candidates for antiallergic agents. 相似文献
78.
Aldehydes are easily converted to 1-bromoolefins or terminal acetylenes by the use of Wittig reaction of bromomethylenetriphenylphosphorane which is prepared from bromomethyltriphenylphosphonium bromide with potassium tert-butoxide. 相似文献
79.
Yoshihisa Kurasawa Tomoyoshi Hosaka Kazue Ikeda Yuko Matsumoto Aiko Ishikura Atsushi Takada Ho Sik Kim Yoshihisa Okamoto 《Journal of heterocyclic chemistry》1994,31(2):527-533
The 3-(arylhydrazono)methyl-2-oxo-1,2-dihydroquinoxalines 9–11 were synthesized by the reaction of the quinoxalines 6–8 with various p-substituted benzenediazonium salts. Compounds 9–11 showed the tautomeric equilibria between the hydrazone imine A and diazenyl enamine B forms in dimethyl sulfoxide media. The substituent effect on the tautomer ratios of A to B was studied by the nmr spectroscopy to clarify that the presence of the ester group R2 on the hydrazone carbon and electron-donating p-substituent R1 on the side chain benzene ring exhibited a tendency to increase the ratios of the tautomer A . 相似文献
80.
Takaya Matsumoto 《Catalysis Surveys from Asia》2007,11(1-2):31-48
In this article, two kinds of our transition metal-catalyzed olefin arylations are summarized and discussed. The first one
is Ir-catalyzed novel anti-Markovnikov hydroarylation of olefins with benzene. Using this reaction catalyzed by [Ir(μ-acac-O,O′,C3)(acac-O,O′)(acac-C3)]2 (acac = acetylacetonato), 1, straight-chain alkylarenes, which were not obtainable by the conventional Friedel-Crafts aromatic alkylation with olefins,
were able to be successfully synthesized directly from arenes and olefins with the higher selectivity than that of branched
alkylarenes. This is the first efficient catalyst which shows the desirable high regioselectivity. The reaction of benzene
with propylene gave n-propylbenzene and cumene in 61% and 39% selectivities, respectively, and the reaction of benzene and styrene afforded 1,2-diphenylethane
in 98% selectivity. The reaction of alkylarene and olefin showed meta and para orientations. A wide range of olefins and arenes
can be employed for the synthesis of alkylarenes. The mechanism of the reaction involves C–H bond activation of benzene by
Ir center to form Ir–phenyl species. The second reaction is Rh-catalyzed oxidative arylation of ethylene with benzene to directly
produce styrene, namely one-step synthesis of styrene. The reaction of benzene with ethylene catalyzed by Rh(ppy)2(OAc) (ppyH = 2-phenylpyridine, OAc = acetate), 3 with Cu oxidizing agent gave styrene and vinyl acetate in 77% and 23% selectivities, respectively, in contrast to those by
Pd(OAc)2, 47% of styrene and 53% of vinyl acetate. The mechanism of the reaction involves Rh-mediated C–H bond activation of benzene,
which appears to be a rate-determining step. Furthermore, Rh complexes in a Rh(I) oxidation state at the beginning of the
reaction work as catalysts for the reaction by addition of acacH and O2 without any oxidizing agent, like Cu salt. 相似文献