全文获取类型
收费全文 | 241篇 |
免费 | 7篇 |
专业分类
化学 | 149篇 |
晶体学 | 5篇 |
力学 | 17篇 |
数学 | 41篇 |
物理学 | 36篇 |
出版年
2023年 | 2篇 |
2022年 | 4篇 |
2021年 | 2篇 |
2020年 | 3篇 |
2019年 | 5篇 |
2018年 | 6篇 |
2017年 | 7篇 |
2016年 | 12篇 |
2015年 | 10篇 |
2014年 | 3篇 |
2013年 | 14篇 |
2012年 | 12篇 |
2011年 | 11篇 |
2010年 | 13篇 |
2008年 | 11篇 |
2007年 | 12篇 |
2006年 | 9篇 |
2005年 | 12篇 |
2004年 | 15篇 |
2003年 | 6篇 |
2002年 | 3篇 |
2001年 | 2篇 |
2000年 | 5篇 |
1999年 | 2篇 |
1998年 | 3篇 |
1997年 | 2篇 |
1995年 | 4篇 |
1994年 | 4篇 |
1993年 | 5篇 |
1992年 | 3篇 |
1989年 | 3篇 |
1986年 | 4篇 |
1985年 | 1篇 |
1984年 | 4篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1981年 | 5篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1975年 | 1篇 |
1973年 | 1篇 |
1972年 | 5篇 |
1968年 | 1篇 |
1967年 | 1篇 |
1965年 | 1篇 |
1960年 | 2篇 |
1958年 | 2篇 |
1957年 | 1篇 |
1956年 | 1篇 |
1943年 | 1篇 |
排序方式: 共有248条查询结果,搜索用时 31 毫秒
231.
The rotational constants A,B and C are functions of the vibrational quantum numbers for the out-of-plane bending modes of trimethylene sulfide and trimethylene selenide. Infrared and microwave spectroscopy have been used to determine the dependence between these parameters. The height and shape of the barrier arising from combination of the ring bending modes is also determined. CNDO/2 calculations are used to predict the position of the hydrogen atoms of the methylene groups in trimethylene sulfide and trimethylene selenide. The four-membered rings are non-planar for several vibrational states of the ring bending modes. 相似文献
232.
In potentiometric studies of metal-ligand equilibria a given model is tested by adjustment of the corresponding stability constants with the purpose of obtaining as good a fit as possible to the experimental data. When judging the goodness-of-fit it is of great importance to be in control of any possible systematic error in concentrations or electrode parameters. The molybdate-oxalate system is used to demonstrate how one crucial parameter influences the evaluations of goodness-of-fit. The system is simple, having only one complex species, MoO(3)oxal(2-), with logarithmic stability constant 13.816 in the pH-range 4-7. 相似文献
233.
Mendiola JA Marín FR Hernández SF Arredondo BO Señoráns FJ Ibañez E Reglero G 《Journal of separation science》2005,28(9-10):1031-1038
Spirulina platensis microalga has been extracted on a pilot scale plant using supercritical fluid extraction (SFE) under various extraction conditions. The extraction yield and the antioxidant activity of the extracts were evaluated in order to select those extracts with both the highest antioxidant capacity and a good extraction yield. These extracts were characterized using LC coupled to diode array detection (DAD) and LC coupled to mass spectrometry (MS) with two different interfaces, atmospheric pressure chemical ionization (APCI) and electrospray (ESI) which allowed us to perform tandem MS by using an ion trap analyzer. The best extraction conditions were as follows: CO2 with 10% of modifier (ethanol) as extraction solvent, 55 degrees C (extraction temperature) and 220 bar (extraction pressure). Fractionation was achieved by cascade depressurization providing two extracts with different activity and chemical composition. Several compounds have been identified in the extracts, corresponding to different carotenoids previously identified in Spirulina platensis microalga along with chlorophyll a and some degradation products. Also, the structure of some phenolic compounds could be tentatively identified. The antioxidant activity of the extracts could be attributed to some of the above mentioned compounds. 相似文献
234.
Summary Various questions posed by P. Nyikos concerning ultrafilters on and chains in the partial order (, <*) are answered. The main tool is the oracle chain condition and variations of it.The first author is partially supported by the basic research fund of the Israeli Academy. The second author is partially supported by NSERC and was a guest of Rutgers University while the research on this paper was being done. The authors would also like to thank P. Nyikos for his valuable comments on early versions of this paper. This is number 465 on the first author's list of publications 相似文献
235.
Jānis Cīrulis 《Central European Journal of Mathematics》2008,6(1):189-190
We characterise those Hilbert algebras that are relatively pseudocomplemented posets.
相似文献
236.
Chemistry of Heterocyclic Compounds - Recent investigations of azidoazomethine-tetrazole (azide-tetrazole) tautomeric equilibrium reported from 2014 to 2019 are summarized. Pyridine, pyrimidine,... 相似文献
237.
238.
239.
Ramírez P Santoyo S García-Risco MR Señoráns FJ Ibáñez E Reglero G 《Journal of chromatography. A》2007,1143(1-2):234-242
A new, specially designed column has been developed for fractionation of supercritical fluid extract of rosemary by using a preparative supercritical fluid chromatography system (Prep-SFC). The column evaluated in this work was prepared using a new packing method consisting of a combination of slurry and supercritical CO2 with commercial silica particles coated with a stationary phase commonly used in gas chromatography, such as SE-54 (5% phenyl-, 95% methylsilicone). The new packing procedure provided columns with reasonable efficiencies, with high stability and useful at high-pressure range. A 25 cm x 10 mm i.d. column packed with silica particles coated with 3% of SE-54 was prepared, and its separation power was tested for isolating fractions with high antioxidant and/or antimicrobial activity from a supercritical rosemary extract. The SFC conditions were selected based on a previous work done with a commercial LC-Diol packed column (130 bar, 80 degrees C), and different percentages of modifier in the mobile phase were tested (5 and 10%). Two cyclones were employed to collect the fractions which were then characterized by HPLC-diode array detection (DAD), GC, and in vitro antioxidant and antimicrobial assays. The use of coated packed columns allowed the fractionation of a complex mixture of rosemary supercritical extract with a minimum amount of modifier in the mobile phase (5% ethanol). At the optimum conditions it was possible to obtain two very active fractions in terms of antioxidant and antimicrobial activity, with no residual rosemary aroma and with improved activities compared to the original supercritical extract. 相似文献
240.
Pachiappa N. Shanmugam Kalv I. Hemanth Kumar Paramasiva N. T. Perumal 《Journal of heterocyclic chemistry》2007,44(4):827-830
A diastereoselective synthesis of 4,5‐dihydrofurans by oxidative addition of 1,3‐dicarbonyl compounds with chalcones mediated by Mn(OAc)3 in moderate to good yields is reported. 相似文献