全文获取类型
收费全文 | 264675篇 |
免费 | 3136篇 |
国内免费 | 714篇 |
专业分类
化学 | 140533篇 |
晶体学 | 3750篇 |
力学 | 11744篇 |
综合类 | 18篇 |
数学 | 32399篇 |
物理学 | 80081篇 |
出版年
2021年 | 2745篇 |
2020年 | 2876篇 |
2019年 | 3362篇 |
2018年 | 4558篇 |
2017年 | 4522篇 |
2016年 | 6582篇 |
2015年 | 3795篇 |
2014年 | 5918篇 |
2013年 | 11778篇 |
2012年 | 9980篇 |
2011年 | 11539篇 |
2010年 | 8698篇 |
2009年 | 8670篇 |
2008年 | 10803篇 |
2007年 | 10571篇 |
2006年 | 9513篇 |
2005年 | 8479篇 |
2004年 | 7767篇 |
2003年 | 6906篇 |
2002年 | 6673篇 |
2001年 | 6868篇 |
2000年 | 5199篇 |
1999年 | 3933篇 |
1998年 | 3351篇 |
1997年 | 3410篇 |
1996年 | 3333篇 |
1995年 | 2891篇 |
1994年 | 3041篇 |
1993年 | 2956篇 |
1992年 | 3293篇 |
1991年 | 3181篇 |
1990年 | 3107篇 |
1989年 | 3108篇 |
1988年 | 2973篇 |
1987年 | 3026篇 |
1986年 | 2845篇 |
1985年 | 3686篇 |
1984年 | 3729篇 |
1983年 | 3104篇 |
1982年 | 3115篇 |
1981年 | 3044篇 |
1980年 | 2856篇 |
1979年 | 3186篇 |
1978年 | 3129篇 |
1977年 | 3328篇 |
1976年 | 3281篇 |
1975年 | 3058篇 |
1974年 | 2978篇 |
1973年 | 3056篇 |
1972年 | 2271篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
941.
A. N. Kost L. A. Sviridova G. A. Golubeva Yu. N. Portnov 《Chemistry of Heterocyclic Compounds》1970,6(3):344-348
The opening of the pyrazolidine ring at the nitrogen-nitrogen bond in 1-arylpyrazolidine under the influence of acidic agents which leads to the formation of N-(3-aminoalkyl)indoles is described. The method enables one to obtain indoles with branched aminoalkyl radicals.For part XIII see [1]. 相似文献
942.
Reaction of 1, 3-dimethylbenzimidazolone and its 5-methyl and 5-chloro derivatives with -valerolactone in presence of anhydrous aluminum chloride gives the corresponding 6-(1-methyl-3-carboxypropyl)-benzimidazolones. 相似文献
943.
A. N. Veselkov L. N. Dymant S. S. Dreval' 《Theoretical and Experimental Chemistry》1987,23(3):346-350
The chemical shifts of the protons in the proflavine and acridine orange molecules in aqueous solution were measured by 1H NMR spectroscopy. The equilibrium constants for the association of the molecules and the chemical shifts of the protons in the monomers and associates were obtained from the concentration dependence of the proton chemical shifts. The most probable structures for the dimers of the dyes were calculated on the basis of the obtained chemical shifts, and a comparative analysis was made.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 23, No. 3, pp. 373–376, May–June, 1987. 相似文献
944.
The pathways in the microbiological transformation of pyridine and its derivatives (alkylpyridines, hydroxypyridines, pyridinecarboxylic acids, alkaloids, and condensed systems with a pyridine ring) under the influence of various microorganisms and several enzyme systems are examined. It is noted that oxidation, reduction, hydrolysis, and destruction of the aromatic ring are the most characteristic processes.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1299–1313, October, 1978. 相似文献
945.
Syntheses of New N-Vinylpyrroles The reactions of pyrrolyl potassium ( 1 ) with (ethoxymethylene)malonic acid derivatives 2a–e yielded the carbanions 3a–e , which could be hydrolyzed to 4a–e , but with the exception of 4b they were not isolated, because a transformation to the N-vinylpyrroles 5a , c–e by elimination of ethanol took place; 1 reacted with 2b at 80°C to give 4b and 6 . Hydrolysis of 4b with KOH yielded 4g , which eliminated 1 mol of ethanol to form 5f , decarboxylation of which led to N-vinylpyrrole 7 . By cyclization of 5e under various conditions the pyrrolizines 8a , b are obtained, the hydrolysis of which did not give ketone 9 but only amino alcohol 10 . Some other cyclizations of 11a–c and 13 yielded the 3H-pyrrolizine derivatives 12a–c and 14 , respectively. 相似文献
946.
Zh. M. Putieva L. G. Mzhel'skaya T. T. Gorovits E. S. Kondratenko N. K. Abubakirov 《Chemistry of Natural Compounds》1976,11(6):756-761
Summary Alternative structures for the acyloside chain of acanthophylloside B and C have been established.Institute of the Chemistry of Plant Substances, Academy of Sciences of the Uzbek SSR. Translated from Khimiya Prirodnykh Soedinenii, No. 6, pp. 728–734, November–December, 1975. 相似文献
947.
[reaction: see text] The enediamine tautomer of a variety of substituted amidine free bases reacts with nitric oxide (NO) to produce compounds containing a carbon-bound diazeniumdiolate [R1R2R3C-N(O)=NO-] functional group (previously called "nitrosohydroxylamines"). The new reaction has been shown to be quite general, although the nature of the products does vary. Amidines containing more than one replaceable hydrogen produce polydiazeniumdiolates as intermolecular salts, while those in which only one diazeniumdiolation can occur provide zwitterionic salts. These diazeniumdiolated amidines are shown to be useful NO donor compounds which undergo very slow spontaneous dissociation on dissolution in pH 7.4 phosphate buffer to produce mixtures of NO and nitrous oxide containing mostly NO. The most advantageous manifestation of the new discovery is the preparation of the monodiazeniumdiolated amidine zwitterions. Reaction of the medically relevant alpha-adrenergic agonists tetrahydrozoline and idazoxan produced monodiazeniumdiolated amidine zwitterions from which NO release was observed for up to 28 days and showed little sign of ending. The reaction should be applicable to a variety of pharmaceutical agents, including NO synthase inhibitors, antitumor agents, and antibacterials. 相似文献
948.
Chmutova G. A. Zverev V. V. Pudovik M. A. Khailova N. A. Pudovik A. N. 《Russian Journal of General Chemistry》2003,73(11):1696-1701
The stability of phosphorylated ureas and thioureas containing various substituents at the P and N atoms and the stability of their cyclization products, the corresponding diazaphospholidines and/or oxaza(thiaza)phospholines, was evaluated by PM3, MNDO, and DFT calculations. The thermal effects of cyclizations involving phosphorylated ureas (or thioureas) and of separate steps of their synthesis were calculated. According to the calculations, diazaphospholidine structures are preferably formed from substituted ureas and thiazaphospholine structures, from their thio analogs; formation of five-membered heterorings with the P-N bond is thermodynamically preferable as compared to their six-membered analogs. 相似文献
949.
p-Toluenesulfinic acid is an effective catalyst for condensation of acetoacetic ester with urea and aliphatic or aromatic aldehydes, leading to formation of 4-substituted 6-methyl-5-ethoxycarbonyl-1,2,3,4-tetrahydropyrimidin-2-ones.M. V. Lomonosov State Academy of Fine Chemical Technology, Moscow 117571, Russia. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 979–982, July, 1998. 相似文献
950.
L. N. Raevskaya N. I. Il’chenko Yu. I. Pyatnitskii N. V. Pavlenko 《Theoretical and Experimental Chemistry》1997,33(2):78-81
The catalytic transformation of methane into higher hydrocarbons was studied under unsteady conditions on oxides of various
metals (Pb, Fe, Nb, Ti, Cd, Mo, Mg, Zn, Zr, Bi, Mn), and also on Sr-, La-, Y-, and Mn-containing oxides of perovskite structure.
It is shown that the effectiveness of the catalysts not only depends on their chemical composition, but also is determined
to a considerable degree by the specific surface of the oxides.
L. V. Pisarzhevskii Institute of Physical Chemistry, Academy of Sciences of Ukraine, 252039 Kiev, Prosp. Nauki, 31, Ukraine.
Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 33, No. 2, pp. 91–95, March–April, 1997. 相似文献