全文获取类型
收费全文 | 2100篇 |
免费 | 61篇 |
国内免费 | 5篇 |
专业分类
化学 | 1617篇 |
晶体学 | 17篇 |
力学 | 24篇 |
综合类 | 1篇 |
数学 | 86篇 |
物理学 | 421篇 |
出版年
2022年 | 12篇 |
2021年 | 12篇 |
2020年 | 24篇 |
2019年 | 31篇 |
2018年 | 16篇 |
2017年 | 11篇 |
2016年 | 27篇 |
2015年 | 44篇 |
2014年 | 42篇 |
2013年 | 102篇 |
2012年 | 112篇 |
2011年 | 129篇 |
2010年 | 65篇 |
2009年 | 44篇 |
2008年 | 122篇 |
2007年 | 128篇 |
2006年 | 111篇 |
2005年 | 103篇 |
2004年 | 85篇 |
2003年 | 85篇 |
2002年 | 88篇 |
2001年 | 55篇 |
2000年 | 47篇 |
1999年 | 24篇 |
1998年 | 16篇 |
1997年 | 15篇 |
1996年 | 22篇 |
1995年 | 15篇 |
1994年 | 24篇 |
1993年 | 26篇 |
1992年 | 26篇 |
1991年 | 25篇 |
1990年 | 25篇 |
1989年 | 17篇 |
1988年 | 23篇 |
1987年 | 32篇 |
1986年 | 29篇 |
1985年 | 35篇 |
1984年 | 25篇 |
1983年 | 16篇 |
1982年 | 22篇 |
1981年 | 15篇 |
1980年 | 20篇 |
1979年 | 24篇 |
1978年 | 25篇 |
1977年 | 20篇 |
1976年 | 19篇 |
1975年 | 25篇 |
1974年 | 22篇 |
1973年 | 20篇 |
排序方式: 共有2166条查询结果,搜索用时 765 毫秒
41.
Fischer indolization of ethyl pyruvate 2-methoxyphenylhydrazone (1) with p-toluene-sulfonic acid in benzene in the presence of an enolizable dicarbonyl or an indolic compound gave either an indole product having an active methine group at C6 or a novel type of 3,6′-biindole compound. Structures of the products were established by NMR spectra and chemical evidence. 相似文献
42.
Osamu Sangen Singo Masuda Hidehiko Nakano Kazusige Yamana Kenichi Sasaki Naoki Ikeda 《Journal of polymer science. Part A, Polymer chemistry》1992,30(6):1171-1175
A new synthetic pathway of A–B–A tri-block copolyether which is composed of a hydrophilic poly(oxyethylene) unit as an A part and a hydrophobic poly(oxy-2-methyl-trimethylene) unit as a B part is proposed. Telechelic α-tosyl-ω-tosyloxypoly(oxy-2-methyl-trimethylene) derived from tosylation of poly(oxy-2-methyl-trimethylene glycol) (PMTG) was allowed to react with poly(ethylene glycol) (PEG) in the presence of sodium hydroxide. Tg of the resulting A–B–A tri-block copolyether (PEMG) (M?n = 1600) was ?72°C and its specific gravity [D415] was 1.055. 相似文献
43.
Some water-soluble organic iodine compounds (aqueous solution) can be analyzed for iodine by isotopically exchanging with labelled elementary iodine (organic solution). The method is applicable to exchange systems in which the rate of the exchange is rather small but measurable, before the exchange equilibrium is attained. The iodine content of iodoaromatic amino acids such as 5-iodouracil and 3,5-diiodotyrosine can be determined within an error of ±4%. 相似文献
44.
A continuous-flow hydride generator is modified and miniaturized for the determination of picogram amounts of arsenic by atomic absorption spectrometry. A 300-μl sample is dropped into a teflon cup and pumped into an alkaline sodium tetrahydroborate stream, which is acidified in a reaction coil. The evolved hydride is swept with argon through a phase separator into an electrically-heated quartz absorption cell and the absorbance is recorded. To eliminate differences in sensitivity between arsenic(III) and arsenic(V) without prereduction by potassium iodide, it is important that arsenic(V) be mixed with tetrahydroborate prior to mixing with hydrochloric acid. The method has a detection limit of 0.08 ng As ml?1 (24 pg) and the calibrations is linear up to 50 ng As ml?1. The relative standard deviation for 10 replicate measurements is 5.4% for 0.5 ng As ml?1. The addition of potassium iodide and hydroxylamine is confirmed to be effective in minimizing some interferences. The sampling rate is 90 h?1. Results for NBS biological and steel reference materials demonstrate applicability of the technique. 相似文献
45.
Efficient routes to hitherto unknown 1d-2,5-di-azido-di-deoxy-allo-inositol, 1d-2,5-di-amino-di-deoxy-allo-inositol, 1l-1-azido-1-deoxy-chiro-inositol and 1l-1-amino-1-deoxy-chiro-inositol were developed by using cheaply available myo-inositol as the starting material. Preliminary investigations on the enzyme inhibitory properties were done. The methodology reported is amenable to gram scale synthesis and thus can find application in natural product synthesis. 相似文献
46.
Paul H. Chen William F. Kuhn Fritz Will Robert M. Ikeda 《Journal of mass spectrometry : JMS》1970,3(2):199-209
The mass spectra of three bicyclic γ-lactones have been studied, and the fragmentation pathways have been proposed with the aid of accurate mass measurements and metastable transitions. An unusually low [M ? CH3] ion and the presence of an abundant [M ? C5H9] ion in the mass spectrum of dihydroactinidiolide were interpreted as a possible rearrangement involving a methyl migration. The eliminations of methyl radical, carbon monoxide and ketene are important processes in many cases. 相似文献
47.
Yoshihisa Kurasawa Tomoyoshi Hosaka Yuko Matsumoto Aiko Ishikura Kazue Ikeda Atsushi Takada Ho Sik Kim Yoshihisa Okamoto 《Journal of heterocyclic chemistry》1994,31(6):1697-1700
The reaction of 7-chlorotetrazolo[1,5-α]quinoxaline 5-oxide 6a with acetic anhydride gave 7-chloro-5-(7-chlorotetrazolo[1,5-α]quinoxalin-4-yl)-4,5-dihydro-4-oxotetrazolo[1,5-α]quinoxaline 7a , while the reaction of 7-chloro-1,2,4-triazolo[4,3-α]quinoxaline 5-oxide 6b with acetic anhydride afforded 7-chloro-5-(7-chloro-1,2,4-triazolo[4,3-α]quinoxalin-4-yl)-4,5-dihydro-4-oxo-1,2,4-triazolo[4,3-α]quinoxaline 7b and 7-chloro-4,5-dihydro-4-oxo-1,2,4-triazolo[4,3-α]quinoxaline 8b . The reaction of compound 6a or 6b with acetic anhydride/acetic acid provided 7-chloro-4,5-dihydro-4-oxo-tetrazolo[1,5-α]quinoxaline 8a or compound 8b , respectively. 相似文献
48.
Photoirradiation at >300 nm of aqueous suspensions of several natural crystal specimens and synthesized crystallites of mercury(II) sulfide (HgS) induced deaminocyclization of optically active or racemic lysine into pipecolinic acid (PCA) under deaerated conditions. This is the first example, to the best of our knowledge, of photoinduced chemical reactions of natural biological compounds over natural minerals. It was found that the natural HgS crystals had activity higher than those of synthesized ones but lower than those of other sulfides of transition metals, e.g., CdS and ZnS, belonging to the same II-IV chalcogenides. In almost all of the photoreactions, decompostion of HgS occurred to liberate hydrogen sulfide (H(2)S) and Hg(2+), and the latter seemed to have undergone in-situ reductive deposition on HgS as Hg(0) after a certain induction period (24-70 h) during the photoirradiation, as indicated by the darkened color of the suspensions. The formation of PCA, presumably through combination of oxidation of lysine and reduction of an intermediate, cyclic Schiff base, could also be seen after a certain induction time of the Hg(0) formation. This was supported by the fact that the addition of small amount of Hg(2+) (0.5 wt % of HgS) increased the PCA yield by almost 2-fold. We also tried to elucidate certain aspects of the plausible stereochemical reactions in relation to the chiral crystal structure of HgS. Although, in some experiments, slight enantiomeric excess of the product PCA was observed, the excess was below or equal to the experimental error and no other supporting analytical data could not be obtained; we cannot conclude the enantiomeric photoproduction of PCA by the natural chiral HgS specimen. 相似文献
49.
Ishikawa T Shimizu K Ishii H Ikeda S Saito S 《The Journal of organic chemistry》2001,66(11):3834-3847
To explore a novel concept for controlling diastereoselectivity, systematic studies on the sense and degree of diastereotopic groups and face selections in intramolecular [3 + 2] (nitrile oxide and nitrone) and [2 + 2 + 1] (Pauson-Khand) cycloadditions have been conducted. Optically pure methyl (S)-3,4-O-isopropylidene-3,4-dihydroxybutanoate (5) and methyl (S)-2,3-O-isopropylidene-2,3-dihydroxypropanoate (6) were converted to substrate aldehydes (1-4) that bear geminal allyl groups and four types of controllers with the intention of imparting a stereochemical bias to the allylic groups and their faces. The controllers involve 1,2-bis(tert-butyldimethylsiloxy), 1,3-bis(tert-butyldimethylsiloxy), 1,2-acetonide, and 1,3-acetonide groups, which are referred to as 1,2-(TBDMSO)(2), 1,3-(TBDMSO)(2), 1,3-dioxolane, and 1,3-dioxane, respectively. Twelve runs of cycloaddition reactions as combinations between the three types of reactions and the four types of substrates were performed to provide bicyclo[4.3.0] or -[3.3.0] adducts of synthetic importance in which isoxazolidine, isoxazoline, or cyclopentenone segments were fused. For every case, high levels of diastereoselectivity have been achieved: >99% (in eight cases), 82%, and 76% for the discrimination of diastereotopic groups and 68-->99% for the discrimination of diastereotopic faces. On the basis of the absolute structures of the cycloadducts, plausible stereochemical models are proposed. 相似文献
50.
Hiroyuki Ishibashi Motofumi Okada Atsuko Akiyama Kazuyuki Nomura Masazumi Ikeda 《Journal of heterocyclic chemistry》1986,23(4):1163-1166
2,3-Dihydro-5H-1,4-benzoxathiepins were prepared by intramolecular Friedel-Crafts reactions of ethyl α-[2-(aryloxy)ethylthio]-α-chloroacetates or by acid-catalyzed cyclizations of ethyl α-[2-(aryloxy)ethylsulfinyl]-acetate. 1,2,3,5-Tetrahydro-4,1-benzothiazepines were similarly prepared. 相似文献