全文获取类型
收费全文 | 4183篇 |
免费 | 110篇 |
国内免费 | 13篇 |
专业分类
化学 | 3181篇 |
晶体学 | 11篇 |
力学 | 71篇 |
数学 | 635篇 |
物理学 | 408篇 |
出版年
2021年 | 37篇 |
2020年 | 36篇 |
2019年 | 38篇 |
2016年 | 68篇 |
2015年 | 80篇 |
2014年 | 80篇 |
2013年 | 160篇 |
2012年 | 127篇 |
2011年 | 132篇 |
2010年 | 99篇 |
2009年 | 92篇 |
2008年 | 120篇 |
2007年 | 131篇 |
2006年 | 152篇 |
2005年 | 147篇 |
2004年 | 128篇 |
2003年 | 127篇 |
2002年 | 130篇 |
2001年 | 82篇 |
2000年 | 63篇 |
1999年 | 67篇 |
1998年 | 69篇 |
1997年 | 54篇 |
1996年 | 74篇 |
1995年 | 57篇 |
1994年 | 63篇 |
1993年 | 66篇 |
1992年 | 78篇 |
1991年 | 78篇 |
1990年 | 55篇 |
1989年 | 83篇 |
1988年 | 70篇 |
1987年 | 83篇 |
1986年 | 70篇 |
1985年 | 98篇 |
1984年 | 102篇 |
1983年 | 59篇 |
1982年 | 94篇 |
1981年 | 78篇 |
1980年 | 76篇 |
1979年 | 91篇 |
1978年 | 69篇 |
1977年 | 82篇 |
1976年 | 64篇 |
1975年 | 57篇 |
1974年 | 73篇 |
1973年 | 49篇 |
1972年 | 25篇 |
1971年 | 42篇 |
1970年 | 24篇 |
排序方式: 共有4306条查询结果,搜索用时 31 毫秒
61.
62.
Yutaka Morita Manfred Hesse Hans Schmid Avijit Banerji Julie Banerji Asima Chatterjee Willi E. Oberhnsli 《Helvetica chimica acta》1977,60(4):1419-1434
Alstonia scholaris: The structure of the indole alkaloid nareline Besides the known akuammidine, picralinal, picrinine and pseudoakuammigine a new indole alkaloid called nareline (M=352) was isolated from Alstonia scholaris R. BR. , which belongs to the plant family of Apocynaceae. Its structure 2 was deduced by single crystal X-ray diffraction. 2 represents the absolute configuration. The spectroscopic data of 2 and its derivatives (Scheme 1) as well as their chemical behavior support this structure. In biogenetic sense nareline is related to the bases akuammiline ( 4 ) and picraline ( 5 ) (Scheme 2). In contrast to those the C-atom 5 is exocyclic and represents an aldehyde group which forms together with the oxygen atom of the N (4)-hydroxylamine group a cyclic half acetale. - By oxidation (CrO3/CH3COOH) of 2 the oxindol derivative 19 (oxonareline) is formed which contains a cyclic acetal as a partial structure element (Scheme 4). 相似文献
63.
Summary Meldrum's acid (1) reacts with isothiocyanates and LiH as base, yielding after alkylation the ketene-S,N-acetales3,4 and5. Protonation of the lithium-thiolate2 gives the thioamides7 with tautomers8, which can be methylated with diazomethane to9. From1 and7 it is possible to obtain the amides12.2 reacts with -bromo-carboxylic esters and bromo-acetone, yielding thiazolidones15,16 and thiazolines18. 相似文献
64.
Schormann M Garratt S Hughes DL Green JC Bochmann M 《Journal of the American Chemical Society》2002,124(38):11266-11267
The reaction of the tin-substituted propene Me3Sn(R)CHCH=CHR (R = SiMe3) with MCl4 in dichloromethane in the presence of Me3SnCl gives the first examples of isolable sec-alkyl carbocation salts, [HC{CH(R)SnMe3}2]+M2Cl9- (M = Zr, Hf). The compounds are thermally stable and, unlike previously isolated trialkyl carbocations, do not require superacidic media or weakly coordinating anions for stability. The crystal structure and DFT calculations suggest polarization of the Sn substituents and hyperconjugation as the reason for the unexpected stability. The stabilizing effect of tin is significantly stronger than that of Si. The carbocations are effective initiators for the polymerization of isobutene, isoprene, and alpha-methylstyrene. 相似文献
65.
Dietrich Estel Manfred Mohnke Frans Biermans Harald Rotzsche 《Journal of separation science》1995,18(7):403-412
This paper deals with the separation of alkanes, naphthenes, and aromatic compounds in naphtha and reformate, on a newly developed apolar high resolution GC column. The selectivity of this apolar phase has been compared with those of squalane, DB-1, and SE-30. A total of 95 hydrocarbons were reliably identified, mostly by GC-MS. Repeated measurements of Kováts retention indices are presented as evidence for the reproducible manufacture of fused silica columns coated with this phase. 相似文献
66.
A new synthetic route for the preparation of the betain-like compound (CH3N)6P4F8 from (CH3NPF3)2,N-methyl-hexamethyldisilazane andN,N-dimethyl-urea has been found. The steps of this multi-stage reaction could be rationalized to a far extent.
10. Mitteilung:Kubjacek M., Utvary K., Mh. Chem.112, 305 (1981). 相似文献
67.
Christian Grugel Manfred Lehnig Wilhelm P. Neumann Josef Sauer 《Tetrahedron letters》1980,21(3):273-276
Tristannanes used as photochemical precursors for the synthesis of organic compounds react via triplet states followed by homolytic CSn and SnSn bond scission. 相似文献
68.
α-Diazo phosphonates with a conjugated 1,3-diene unit are synthesised by the Bamford-Stevens reaction ( →→). They undergo [4+2]-cycloadditions with the dienophile to form the tetrahydrotriazolopyridazines , which possess an unchanged diazo group. In contrast, dimethyl acetylenedicarboxylate () reacts exclusively with the diazo dipole of to yield the 3H-pyrazoles , which rearrange to by sigmatropic PO-shifts and hydrolyse to form . The diazo compound isomerises to the pyrazole when heated in benzene. 相似文献
69.
Loss of vinyl alcohol from 1-amino-3-aryloxy-2-propanols under electron impact Under electron impact compounds of type 1 (see Scheme 1) split off 44 mass units from the molecular ion. This unusual reaction was studied using derivatives and deuterium labelled compounds. It could be demonstrated that for this fragmentation reaction 16 is the important structural feature from which H2(C3)?C(2)HOH (44 mass units) is lost. The preferred reaction mechanism involves a transition state in which four members of the side chain are involved (Scheme 2, mechanism 2). 相似文献
70.
Contribution to the mass spectrometry of substituted α,ω-alkane diamines The main mass spectral fragmentation pattern of compounds of types 1 to 4 is discussed. After loss of C6H5 · CH2 · from the molecular ion the acid correspondin to the N,N-disubstituted residue is splitted off. The mechanism of this fragmentation reaction depends on the member of CH2-groups between the two nitrogen atoms (Schemes 1 and 3) and on the substitution pattern of both nitrogens (Scheme 2). 相似文献