全文获取类型
收费全文 | 12094篇 |
免费 | 557篇 |
国内免费 | 58篇 |
专业分类
化学 | 8791篇 |
晶体学 | 39篇 |
力学 | 296篇 |
数学 | 1884篇 |
物理学 | 1699篇 |
出版年
2023年 | 93篇 |
2022年 | 142篇 |
2021年 | 202篇 |
2020年 | 292篇 |
2019年 | 308篇 |
2018年 | 145篇 |
2017年 | 168篇 |
2016年 | 440篇 |
2015年 | 477篇 |
2014年 | 493篇 |
2013年 | 648篇 |
2012年 | 921篇 |
2011年 | 925篇 |
2010年 | 574篇 |
2009年 | 494篇 |
2008年 | 734篇 |
2007年 | 709篇 |
2006年 | 648篇 |
2005年 | 631篇 |
2004年 | 495篇 |
2003年 | 405篇 |
2002年 | 394篇 |
2001年 | 184篇 |
2000年 | 153篇 |
1999年 | 147篇 |
1998年 | 151篇 |
1997年 | 149篇 |
1996年 | 123篇 |
1995年 | 94篇 |
1994年 | 89篇 |
1993年 | 101篇 |
1992年 | 73篇 |
1991年 | 70篇 |
1990年 | 57篇 |
1989年 | 72篇 |
1988年 | 56篇 |
1987年 | 40篇 |
1986年 | 45篇 |
1985年 | 61篇 |
1984年 | 50篇 |
1983年 | 50篇 |
1982年 | 51篇 |
1981年 | 63篇 |
1980年 | 49篇 |
1979年 | 50篇 |
1978年 | 45篇 |
1977年 | 44篇 |
1976年 | 50篇 |
1975年 | 43篇 |
1973年 | 36篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
61.
The Schiff base of all-trans-retinal was investigated in organic solution by 1H- and 13C-NMR. at high field. Complete assignment of the 1H-NMR. peaks of N-butyl-(all-trans-retinylidene)amine ( 2 ) and the N-butyl-(all-trans-retinylidene)-ammonium ion ( 3 ) was achieved by INDOR (internuclear double resonance). The vicinal proton coupling constants of the polyene chain show that the π-bond orders remain unchanged in N-butyl-(all-trans-retinylidene)amine relative to all-trans-retinal ( 1 ), but change towards larger π-delocalization in the N-butyl-(all-trans-retinylidene)ammonium ion. At ?-61° only one isomer of N-butyl-(all-trans-retinylidene)ammonium was observed. This was shown to be trans at the imine linkage and independent of the solvent. The trifluoroacetic acid counter-ion can approach the positive charge of the N-atom in the weakly polar solvent dichloromethane but not in the leveling solvent methanol. In dichloromethane the nature of the 1:1 complex is a H-bonded (O?…H-N+) ion-pair whose rate of breaking and forming is rapid at RT. Strong stabilization of the ion-pair resulted from homo-conjugation with a second molecule of trifluoroacetic acid. Excess of acid efficiently diminished the isomerization rate at the C,N-bond. 相似文献
62.
The NMR spectra of a series of five 3-pyridinols in dioxane and dimethyl sulfoxide have been correlated with electron densities calculated by the Extended Hückel MO method. The data were found to fit a straight line of the form y=ax+b. The effects of hydrogen-bonding between pyridinol molecules on the correlation have been considered, and they have been estimated quantitatively for 6-methyl-3-pyridinol via Extended Hückel Calculations on a hydrogen-bonded model.
Zusammenfassung Die Kernresonanzspektren von fünf 3-Pyridinolen in Dioxan und Dimethylsulfoxyd wurden mit den Elektronendichten aus dem erweiterten Hückelverfahren korreliert, und zwar ergibt sich eine lineare Beziehung. Der Einfluß von intermolekularen Wasserstoffbrücken ist für einen Fall (die 6-Methyl-Verbindung) durch EHT-Rechnungen festgestellt und in den übrigen Fällen geschätzt worden.
Résumé Les spectres NMR d'une série de cinq 3-pyridinols dans le dioxane et le diméthyle-sulfoxyde ont été corrélés avec les densités électroniques calculées par la méthode de Hückel Etendue. Les données se placent correctement sur une droite y=ax+b. Les effets de la liaison hydrogène entre les molécules de pyridinol ont été envisagés et estimés quantitativement pour le 6-méthyl-3-pyridinol par des calculs EHT sur un modèle à liaison hydrogène.相似文献
63.
Christian E. Madu 《Tetrahedron》2007,63(23):5019-5029
A variety of aryl enynes have been constructed from o-iodophenol derivatives containing ortho-tert-butyl groups via O-alkylation and a Sonogashira cross-coupling reaction. These substrates undergo efficient thermal and oxidative intramolecular Pauson-Khand reactions leading to the formation of sterically congested cyclopentenones, as well as the formation of medium-sized rings, although in the latter case with unusual regioselectivity. Incorporation of a TMS moiety on the alkyne group in a higher homolog led to cyclization via the normal mode, albeit in relatively low yield. 相似文献
64.
The electrochemical characteristics of Haloperidol and related compounds, representative neuroleptics of the butyrophenone family, have been investigated as a function of pH and concentration by direct-current, alternating-current and differential-pulse polarography and cyclic voltammetry at a hanging mercury drop electrode. A single cathodic wave representing an irreversible two-electron reduction is obtained, and its half-wave potential differs from that characteristic of aromatic ketone reduction. Adsorption processes disturb the wave behaviour and an adsorption prewave is observed at high concentrations. Quantitative measurements were successful in the concentration range 1 x 10(-4)-1 x 10(-6)M (0.4 mg/l.), the lower concentration representing the detection limit by differential-pulse polarography. 相似文献
65.
Sirockin F Sich C Improta S Schaefer M Saudek V Froloff N Karplus M Dejaegere A 《Journal of the American Chemical Society》2002,124(37):11073-11084
There has recently been considerable interest in using NMR spectroscopy to identify ligand binding sites of macromolecules. In particular, a modular approach has been put forward by Fesik et al. (Shuker, S. B.; Hajduk, P. J.; Meadows, R. P.; Fesik, S. W. Science 1996, 274, 1531-1534) in which small ligands that bind to a particular target are identified in a first round of screening and subsequently linked together to form ligands of higher affinity. Similar strategies have also been proposed for in silico drug design, where the binding sites of small chemical groups are identified, and complete ligands are subsequently assembled from different groups that have favorable interactions with the macromolecular target. In this paper, we compare experimental and computational results on a selected target (FKBP12). The binding sites of three small ligands ((2S)1-acetylprolinemethylester, 1-formylpiperidine, 1-piperidinecarboxamide) in FKBP12 were identified independently by NMR and by computational methods. The subsequent comparison of the experimental and computational data showed that the computational method identified and ranked favorably ligand positions that satisfy the experimental NOE constraints. 相似文献
66.
67.
Enantioselective C-P cross-coupling of diarylphosphines and ortho-substituted aryl iodides has been achieved with >90%ee, using an in situ catalyst prepared from Et,Et-FerroTANE, Pd(2)(dba)(3) x CHCl(3) and LiBr. 相似文献
68.
The nitrosation of some γ,δ-unsaturated β-diketo compounds affords the 3-substituted 4-oxo-5,6-dihydro-1,2,4H-oxazines. These compounds are converted to the isomeric 3-oxo-1-pyrroline 1-oxides by a facile thermal rearrangement. 相似文献
69.
70.
Christian Weiske 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》1974,86(13):488-488