全文获取类型
收费全文 | 2411篇 |
免费 | 64篇 |
国内免费 | 2篇 |
专业分类
化学 | 1380篇 |
晶体学 | 28篇 |
力学 | 66篇 |
数学 | 210篇 |
物理学 | 793篇 |
出版年
2021年 | 17篇 |
2020年 | 20篇 |
2019年 | 21篇 |
2016年 | 65篇 |
2015年 | 42篇 |
2014年 | 38篇 |
2013年 | 92篇 |
2012年 | 69篇 |
2011年 | 85篇 |
2010年 | 71篇 |
2009年 | 58篇 |
2008年 | 95篇 |
2007年 | 99篇 |
2006年 | 92篇 |
2005年 | 59篇 |
2004年 | 63篇 |
2003年 | 51篇 |
2002年 | 58篇 |
2001年 | 66篇 |
2000年 | 57篇 |
1999年 | 55篇 |
1998年 | 28篇 |
1997年 | 31篇 |
1996年 | 42篇 |
1995年 | 36篇 |
1994年 | 32篇 |
1993年 | 30篇 |
1992年 | 55篇 |
1991年 | 29篇 |
1990年 | 48篇 |
1989年 | 30篇 |
1988年 | 39篇 |
1987年 | 23篇 |
1986年 | 35篇 |
1985年 | 45篇 |
1984年 | 34篇 |
1983年 | 31篇 |
1982年 | 30篇 |
1981年 | 29篇 |
1980年 | 44篇 |
1979年 | 36篇 |
1978年 | 27篇 |
1977年 | 40篇 |
1976年 | 25篇 |
1975年 | 29篇 |
1974年 | 23篇 |
1973年 | 22篇 |
1971年 | 23篇 |
1967年 | 26篇 |
1966年 | 21篇 |
排序方式: 共有2477条查询结果,搜索用时 15 毫秒
11.
Acyl- and Alkylidenephosphines. XXIII. Synthesis and Structure of [Bis(trimethylsilylsulfano)methylidene]phosphines Analogous to the phenyl derivative 1a [2] tert-butyl- 1b , mesityl- 1c and methylbis-(trimethylsilyl)phosphine 1 d react with carbon disulfide to give the corresponding [bis(trimethylsilylsulfano)methylidene]phosphines 4 . Only in case of the mesitylphosphine 1 c the intermediate compounds 2 and 3 could be detected by n.m.r. spectroscopic methods; thermally unstable [bis(trimethylsilylsulfano)methylidene]methylphosphine 4 d dimerizes rapidly [1]. [Bis(trimethylsilylsulfano)methylidene]phenylphosphine 4 a crystallizes in the monoclinic centrosymmetric space group P21/c with following dimensions of the unit cell determined at ?95 ± 3°C: a = 1386.4(8); b = 1036.0(7); c = 1281.7(8) pm; ß = 101.23(4)°; Z = 4. An X-ray structure determination (R = 0.032) proves the constitution of this compound as already derived from its nmr spectra. Characteristic bond lengths and angles are: P?C 170; P? C(phenyl) 183; C? S 176; S? Si 219 pm; C? P?C 107; P?C? S 124 and 120; S? C? S 116 and C? S? Si 111°. 相似文献
12.
Acyl- and Alkylidenephosphines. XXII. Synthesis and Structure of 1, 3-Dimethyl-2,2,4,4-tetrakis(trimethylsilylsulfano)-1,3-diphosphetane At ?30°C methylbis(trimethylsilyl)phosphine reacts with carbon disulfide to give a red adduct first which rearranges to [bis(trimethylsilylsulfano)methylidene]methylphosphine 1a . In contrast to the thermally stable phenyl derivative 1b [2], this compound with its insufficiently shielded P?C group dimerizes fast with increasing temperature. 1,3-Dimethyl-2,2,4,4-tetrakis(trimethylsilylsulfano)-1,3-diphosphetane 2a formed by this reaction, crystallizes in the triclinic space group P1 with following dimensions of the unit cell, determined at a temperature of measurement of ?80 ± 3°C: a = 1024.7(3); b = 1360.2(5); c = 1326.3(6)pm; α = 117.85(4); ß = 111.05(3); γ = 72.09(3)°; Z = 2. Due to ring folding at the P1? P2 axis of 149.1°, the molecule shows pseudosymmetry Cs. Characteristic averaged bond lengths and angles obtained at an Rw-value of 0.030, are: P? C(endocyclic) 188 and 191; P? CH3 184; C? S 183; S? Si 216 pm; C? P? CH3 105; P? C? S 113; S? C? S 114; C? S? Si 108; P? C? P 90 and C? P? C 86°. 相似文献
13.
James Y. Becker Larry L. Miller Frank R. Stermitz 《Journal of Electroanalytical Chemistry》1976,68(2):181-191
The electrochemical oxidation of the alkaloid laudanosine (Ia) to O-methylflavinantine (II) has been studied in acetonitrile solvent. Using cyclic voltammetry, rotating disc voltammetry and preparative electrolyses on several alkaloids, simple aliphatic amines and aromatic compounds, some aspects of the mechanism of this coupling reaction are elucidated. The first anodic wave for laudanosine at platinum has Ep=0.55 V vs. Ag/Ag+. The electrode rapidly becomes partially passivated at potentials above 0.5 V. This is due to a film which “dissolves” below 0.5 V, at a rate independent of the potential. It is shown that the reaction (Ia)→(II) proceeds at 0.5 V by initial oxidation of the amine moiety. If acids such as sodium bicarbonate are added to the anolyte the amine is protonated causing the first wave to disappear. Oxidation at 1.1 V under these acidic conditions produces the same product, but more rapidly and in significantly higher yield because electrode filming and side reactions resulting from the amine oxidation are abrogated. 相似文献
14.
15.
16.
17.
18.
19.
Graham R. Underwood Brajeswar Paul M. Alice Becker 《Journal of heterocyclic chemistry》1976,13(6):1229-1232
Pyridoxol, protected by acetylation of the hydroxyl groups, has been converted to its N-oxide which upon reaction with perfluoroacetie anhydride yields a 2-nor-2-hydroxymethylpyridoxol derivative as an intermediate. This compound undergoes acyl migration from the 3-position. Protection of the pyridoxol hydroxyls by benzylalion followed by the same treatment yields the unrearranged α2-hydroxy derivative. This compound has been converted to a series of α2-substituted pyridoxols (X = -Cl, -Br, -OCOCH3, -OCH3, -OC2H5). 相似文献
20.
A unique matrix system consisting mostly of 4-nitrophenol has shown to be very effective for matrix-assisted laser desorption/ionization time-of-flight mass spectrometric analysis of large DNA oligomers when a cooled sample stage was used to prevent the sublimation of this matrix under vacuum. Using this 4-nitrophenol matrix with UV laser desorption, detection of picomole quantities of DNA oligomers containing up to approximately 800 nucleotides was routinely achieved. The effectiveness of this matrix was further demonstrated by the observation of a double-stranded DNA oligomer larger than 1000 base pairs, seen as a denatured single-stranded species, with a molecular ion mass exceeding 300 000 Da. The potential applications of 4-nitrophenol as a matrix for DNA sizing are discussed. 相似文献