全文获取类型
收费全文 | 7971篇 |
免费 | 412篇 |
国内免费 | 196篇 |
专业分类
化学 | 4889篇 |
晶体学 | 83篇 |
力学 | 323篇 |
综合类 | 25篇 |
数学 | 1209篇 |
物理学 | 2050篇 |
出版年
2023年 | 79篇 |
2022年 | 204篇 |
2021年 | 224篇 |
2020年 | 223篇 |
2019年 | 230篇 |
2018年 | 211篇 |
2017年 | 188篇 |
2016年 | 280篇 |
2015年 | 268篇 |
2014年 | 290篇 |
2013年 | 550篇 |
2012年 | 507篇 |
2011年 | 557篇 |
2010年 | 345篇 |
2009年 | 312篇 |
2008年 | 391篇 |
2007年 | 332篇 |
2006年 | 335篇 |
2005年 | 288篇 |
2004年 | 218篇 |
2003年 | 222篇 |
2002年 | 188篇 |
2001年 | 163篇 |
2000年 | 142篇 |
1999年 | 112篇 |
1998年 | 71篇 |
1997年 | 65篇 |
1996年 | 75篇 |
1995年 | 56篇 |
1994年 | 96篇 |
1993年 | 67篇 |
1992年 | 79篇 |
1991年 | 66篇 |
1990年 | 64篇 |
1989年 | 49篇 |
1988年 | 51篇 |
1987年 | 43篇 |
1986年 | 52篇 |
1985年 | 67篇 |
1984年 | 67篇 |
1983年 | 38篇 |
1982年 | 67篇 |
1981年 | 58篇 |
1980年 | 66篇 |
1979年 | 55篇 |
1978年 | 61篇 |
1977年 | 52篇 |
1976年 | 49篇 |
1975年 | 38篇 |
1973年 | 39篇 |
排序方式: 共有8579条查询结果,搜索用时 15 毫秒
11.
The examination of 19F chemical shifts for ca. 650 F-alkylated compounds of general formula CF3(CF2)nCF2X led to the following conclusions: the CF2 groups α to X are very sensitive to the nature of X, and are spread over a range of 85 ppm. The effect of the length of the F-alkyl chain decreases rapidly, so that δCF2(α) can already be considered as characteristic of X for n = 1 or 2 for most practical purposes. Solvent effects (in 9 different solvents having ε = 1.8 to 52.1) were found to be rather small except for the F-alkyl iodides. A chart which indicates the domain in which the CF2(α) resonance signal is to be expected is given for 42 different series of F-alkylated compounds; it is expected to provide the synthetic chemist with a useful tool for the identification and characterization of such compounds. 相似文献
12.
13.
De Wasch K Le Bizec B De Brabander H André F Impens S 《Rapid communications in mass spectrometry : RCM》2001,15(16):1442-1447
In previous work (Le Bizec et al., Rapid Commun. Mass Spectrom. 2000; 14: 1058), it was demonstrated that a boar meal intake could lead to possible false accusations of abuse of 17beta-nortestosterone in antidoping control. The aim of the present study was to identify and quantify endogenous 19-norsteroids in boar edible tissue at concentrations that can alter the steroid urinary profile in humans, and lead to excretion of 19-norandrosterone (19-NA) and 19-noretiocholanolone (19-NE). The samples were analysed in two laboratories. The methodologies used for extraction and detection (GC/MS(EI) and LC/MS/MS(APCI+)) are compared and discussed. 19-Norandrostenedione (NAED), 17beta- and 17alpha-nortestosterone (bNT, aNT), and 17beta- and 17alpha-testosterone (bT, aT) were quantified. The largest concentrations of NAED and bNT were observed in testicles (83 and 172 microg/kg), liver (17 and 63 microg/kg) and kidney (45 and 38 microg/kg). A correlation between the bNT and NAED content of a typical meal prepared with boar parts and the excreted concentrations of 19-NA and 19-NE in human urine was demonstrated. 相似文献
14.
E. G. Le Roux M. Peisach C. A. Pineda M. A. B. Pougnet 《Journal of Radioanalytical and Nuclear Chemistry》1988,120(1):97-104
The split-root technique was used to study the effect of varying the growth media on the elemental content of nutrient elements in the roots of grape vines. The varieties 2-1 (R99 x Jacquez) and Sauvignon blanc (Vitis vinifera) were grown in Hoagland water culture with and without added aluminium. The elemental concentrations of Mg, Al, Si, P. S, Cl, K and Ca in the dried roots were determined by PIXE. Roots grown in Al-rich media were deficient in Mg and Ca, but enriched in Al. There was a correlation between Al and Si but the uptake differed in the two varieties. 相似文献
15.
Differential scanning calorimetry has been used to study the heats of fusion and melting transitions of n-octadecyl derivatives of polyvinyl alcohol. These derivatives were obtained by grafting paraffinic chains on polyvinyl alcohol, at various proportions. The melting transitions are independent of the degree of substitution; the heats of fusion are proportional to the fraction of crystallizable units. Using Flory's equilibrium crystallization theory, it is possible to interpret these results assuming, as demonstrated previously, that the chemical grafting is sequential with two unreacted hydroxyl groups between two adjacent paraffinic side-chains. 相似文献
16.
F. Zammattio J. D. Brion L. Belachmi G. Le Baut 《Journal of heterocyclic chemistry》1991,28(8):2013-2019
(2-Styrylchromon-8-yl)acetic acids, structural analogs of (flavon-8-yl)acetic acid (FAA) have been synthesized with satisfactory yields according to two different methods. The 1H and 13C nmr data fovor the S-trans stereoisomers. 相似文献
17.
The dependence of electron density, superdelocalizability and localization energy on the value of the parameter for the inductive effect of the methyl group in 1,4-dimethyl naphthalene is exactly the same when calculated by the Omega or by the Pople-Pariser-Parr techniques.
This work was done with financial assistance from the National Research Council of Canada. L.-K. H. wishes to acknowledge the award of a Colombo Plan scholarship. 相似文献
Zusammenfassung Die Abhängigkeit der Elektronendichte, der Superdelokalisierbarkeit und der Lokalisierungsenergie vom Wert des Parameters für den induktiven Effekt der Methylgruppe in 1,4-Dimethylnaphthalin ist genau dieselbe, wenn man diese Größen mit der Omegatechnik oder der PPP-Methode berechnet.
Résumé La densité électronique, la super délocalisabilité et l'énergie de localisation du 1,4 diméthyl naphtalène dépendent de la même manière de la valeur du paramètre de l'effet inductif du groupe méthyle selon que les calculs sont effectués par la méthode oméga ou par la méthode de Pople-Pariser-Parr.
This work was done with financial assistance from the National Research Council of Canada. L.-K. H. wishes to acknowledge the award of a Colombo Plan scholarship. 相似文献
18.
Iodine and bromine in the presence of sodium benzenesulphinate react with olefins in acetone solution to give halosulphones resulting from an apparent steric direction of attack at the intermediate halonium ion. A straightforward preparation of benzenesulphonyl iodide from benzenesulphonyl chloride and sodium iodide is also described. 相似文献
19.
G. Le Bas C. de Rango N. Rysanek G. Tsoucaris 《Journal of inclusion phenomena and macrocyclic chemistry》1984,2(3-4):861-867
Studies of chiroptical properties need separation or at least enrichment of enantiomers. The separation is difficult with chiral conformers of molecules with very low barriers of internal rotation. However, in association with cyclodextrin, these labile molecules can exhibit a strong Cotton effect in solution: one chiral conformer is favoured by complexation. The cyclodextrins have the advantage to yield inclusion complex in solution as well as crystalline clathrates. Therefore, the absolute configuration of the guest can be obtained by determining the structure of the cyclodextrin. In this work, the first CD spectrum of 4-helicene is recorded and crystal structures of several clathrates of labile molecules are studied. 相似文献
20.
The title compound is a crucial intermediate in the biomimetic conversion of vincadifformine 1 into vincamine 7. Its configuration at C-16 is established by a combination of chemical and spectroscopic evidence. Iodine oxidation converts 14 into the bridged lactam 18, thus proving a β configuration for the hydroxy group at C-16. The same reaction applied to vindoline 19 gives 21 identical with one of the compounds obtained by microbiological transformation of 19. The 13C NMR spectra of derivatives 3 and 8 (obtained by oxidation of vincadifformine) show that oxidation proceeds with introduction of the substituent at C-16, with a β-configuration. The alcohol 3 however, posesses a different conformation due to strong hydrogen bonding with N-4. 相似文献