全文获取类型
收费全文 | 490篇 |
免费 | 14篇 |
国内免费 | 1篇 |
专业分类
化学 | 394篇 |
晶体学 | 3篇 |
力学 | 11篇 |
数学 | 46篇 |
物理学 | 51篇 |
出版年
2023年 | 4篇 |
2022年 | 19篇 |
2021年 | 25篇 |
2020年 | 10篇 |
2019年 | 12篇 |
2018年 | 11篇 |
2017年 | 9篇 |
2016年 | 14篇 |
2015年 | 22篇 |
2014年 | 11篇 |
2013年 | 44篇 |
2012年 | 30篇 |
2011年 | 41篇 |
2010年 | 25篇 |
2009年 | 19篇 |
2008年 | 28篇 |
2007年 | 24篇 |
2006年 | 25篇 |
2005年 | 21篇 |
2004年 | 20篇 |
2003年 | 18篇 |
2002年 | 11篇 |
2001年 | 4篇 |
2000年 | 5篇 |
1999年 | 2篇 |
1998年 | 2篇 |
1997年 | 2篇 |
1996年 | 1篇 |
1995年 | 5篇 |
1994年 | 1篇 |
1993年 | 2篇 |
1992年 | 1篇 |
1991年 | 5篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1986年 | 4篇 |
1985年 | 5篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 3篇 |
1976年 | 3篇 |
1974年 | 3篇 |
1973年 | 1篇 |
排序方式: 共有505条查询结果,搜索用时 15 毫秒
211.
Cecilio Marquez Rosa M. Rabanal Serafin Valverde Liliana Eguren Aurea Perales José Fayos 《Tetrahedron letters》1981,22(29):2823-2826
Lolin, a new diterpene of the -clerodane type has been isolated from Teucrium capitatum. The X-ray structure and spectroscopic data of lolin is given. 相似文献
212.
Liliana Lamartina Renato Noto Caterina Arnone 《Journal of heterocyclic chemistry》1988,25(4):1083-1086
A uv-visible, 1H and 13C nmr study has been carried out to show that 4-, 1, and 5-formylthiophene-2-carboxylic acids, 2, give acetals in methanol. The ms data have confirmed the acetal formation, which has been studied kinetically by the uv technique. The substituent chemical shifts induced on the carboxy carbon atom and on the endocyclic carbon atoms in the acetals formed from 1 and 2 and in the corresponding anions have been used to recalculate σI and σR values by means of a dual substituent parameter treatment of data. 相似文献
213.
214.
Isabel A. Perillo María B. García Juan . Bisceglia Liliana R. Orelli 《Journal of heterocyclic chemistry》2002,39(4):655-661
A series of N‐arylhexahydropyrimidines la‐1 were synthesized by condensation of N‐aryl‐N'‐alkyl‐ (or aryl)‐1,3‐propanediamines 2a‐h with aldehydes. Reactions with formaldehyde proceeded in hydroalcoholic solution, while condensation with aromatic aldehydes required in general the use of activated molecular sieves. 1H NMR spectra of compounds la‐1 were analyzed and the results correlated with their conforma‐tional features. Derivatives devoid of a 2‐substituent la‐g show fast ring reversal and N‐inversion. The presence of a 2‐aryl group shifts the ring reversal equilibrium towards conformations where the 2‐aryl substituent is equatorial. Differential assignment of axial and equatorial hydrogens in these compounds was made on the basis of coupling constants and chemical shift values. In compound 1k spectral data suggest the axial orientation of the N‐methyl group. Such findings were confirmed in the corresponding NOESY spectrum. 相似文献
215.
Enrique Agull María Susana Rodríguez Viviana Ramos Liliana Albertengo 《Macromolecular bioscience》2003,3(10):521-530
The conversion of processed discarded material into valuable by‐products and alternative specialty materials has been identified as a timely challenge for food research and development associated with numerous applications of chitinous products. Chitin, chitosan, calcareous chitin, and chitosan, N‐acetylated chitosan, N‐methylene phosphonic chitosan (NMPC), and N‐lauryl‐N‐methylene phosphonic chitosan (LMPC) are being studied as a result of their broad range of food applications. These biopolymers offer a wide range of unique applications including formation of biodegradable films, immobilization of enzymes, preservation of foods from microbial deterioration, as additives (clarification and deacidification of fruits and beverages, emulsifier agents, thickening and stabilizing agents, color stabilization), and dietary supplements. This review summarizes some of the most important developments in this field. 相似文献
216.
The solubility diagram of the system NaAl(SO4)2–CsAl(SO4)2–H2O was investigated at 25°C. This is a system in which -and -alums participate. The fields of cristallization are outlined. There is one wide field of cesium aluminium alum and one, very narrow, of sodium aluminium alum. The eutonic point of the system lies at the composition of the liquid phase corresponding to 27.90 wt.% NaAl(SO4)2 and 0.008 wt.% CsAl(SO4)2. It was found that mixed crystals are not formed in the system. The solid phases were investigated by X-ray diffraction andDTA. 相似文献
217.
Quintanar L Gebhard M Wang TP Kosman DJ Solomon EI 《Journal of the American Chemical Society》2004,126(21):6579-6589
The multicopper oxidases are a family of enzymes that couple the reduction of O(2) to H(2)O with the oxidation of a range of substrates. Saccharomyces cerevisiae Fet3p and human ceruloplasmin (hCp) are members of this family that exhibit ferroxidase activity. Their high specificity for Fe(II) has been attributed to the existence of a binding site for iron. In this study, mutations at the E185 and Y354 residues, which are putative ligands for iron in Fet3p, have been generated and characterized. The effects of these mutations on the electronic structure of the T1 Cu site have been assessed, and the reactivities of this site toward 1,4-hydroquinone (a weak binding substrate) and Fe(II) have been evaluated and interpreted in terms of the semiclassical Marcus theory for electron transfer. The electronic and geometric structure of the Fe(II) substrate bound to Fet3p and hCp has been studied for the first time, using variable-temperature variable field magnetic circular dichroism (VTVH MCD) spectroscopy. The iron binding sites in Fet3p and hCp appear to be very similar in nature, and their contributions to the ferroxidase activity of these proteins have been analyzed. It is found that these iron binding sites play a major role in tuning the reduction potential of iron to provide a large driving force for the ferroxidase reaction, while still supporting the delivery of the Fe(III) product to the acceptor protein. Finally, the analysis of possible electron-transfer (ET) pathways from the protein-bound Fe(II) to the T1 Cu site indicates that the E185 residue not only plays a role in iron binding, but also provides the dominant ET pathway to the T1 Cu site. 相似文献
218.
The structure of 2-diethylamino-6-methyl-5-n-propyl-4(3H)-pyrimidinone has been studied by X-ray crystallography and quantum-chemical calculations. X-ray analysis established that 2-diethylamino-6-methyl-5-n-propyl-4(3H)-pyrimidinone exists exclusively as the lactam tautomer protonated at the N3 ring nitrogen in the solid state. Crystals of 2-diethylamino-6-methyl-5-n-propyl-4(3H)-pyrimidinone are monoclinic (space group P21/n); the unit-cell dimensions are: a = 11.0460(8) Å, b = 5.0064(4) Å, c = 22.8358(17) Å, = = 90°, = 90.521(1)°. In the crystal, molecules of 2-diethylamino-6-methyl-5-n-propyl-4(3H)-pyrimidinone are assembled in planar centrosymmetric dimers by strong resonance-assisted N—H···O intermolecular hydrogen bonds from the NH group of one molecule to the C=O of the adjacent molecule (N—H···O distance 2.804 Å). Bond distances and angles are generally similar to those reported for the corresponding tautomer of isocytosine and derivatives. Quantum-chemical calculations on 2-diethylamino-6-methyl-5-n-propyl-4(3H)-pyrimidinone are also reported in order to estimate the relative energies of the possible tautomeric forms; ab initio and DFT results predict the coexistence of the N3 and AH tautomers in the gas phase. There is excellent correspondence between the crystal and the HF/6-311G** or B3LYP/6-31G* calculated structures of the N3 lactam form; the largest deviations between the experimental and computed structures are mostly the effects of strong intermolecular H bonds in the crystal. 相似文献
219.
This work describes an analytical method for the screening of pesticide residues in water. The developed method identifies and quantitates a variety of pesticides in water samples such as organochlorine, organophosphate, chloronitrile, phthalimide, dicarboximide, and triazine. The analytical technique employed for the extraction and cleanup step involves solid-phase extraction with C-18 cartridges. The identification and determination of concentration is carried out with gas chromatography. Two columns of different polarity and two specific detectors are used. An electron capture detector is used for organochrolorine, and a thermoinic detector is used for organonitrogen and organophosphate. The good resolution achieved with the temperature program and the combination of columns is shown in the chromatograms obtained. The limits of detection, percent recovery, and their respective standard deviations are determined for each product. According to the results obtained, this method permits monitoring of the pesticides mentioned, many of which are moderately to highly toxic to aquatic organisms. 相似文献
220.
An apparatus containing cross molecular and pulsed electron beams has been used to obtain distributions in kinetic energy and angle of fast (? 0.5 eV) positive ions produced through dissociative ionization of N2 and O2 by impact of 50 to 2000 eV electrons. Four main O+ ion groups are observed with peak energies of 0.8, 2.0, 3.0, and 5.0 eV. Two main N+ groups peaking at 2.0 and 3.0 eV are seen. Angular distributions of both N+ and O+ ions are essentially isotropic for electron-beam-ion detection angles from 30° to 110°. 相似文献