全文获取类型
收费全文 | 396篇 |
免费 | 22篇 |
国内免费 | 3篇 |
专业分类
化学 | 378篇 |
晶体学 | 1篇 |
力学 | 2篇 |
数学 | 11篇 |
物理学 | 29篇 |
出版年
2023年 | 6篇 |
2022年 | 10篇 |
2021年 | 21篇 |
2020年 | 22篇 |
2019年 | 15篇 |
2018年 | 6篇 |
2017年 | 10篇 |
2016年 | 29篇 |
2015年 | 14篇 |
2014年 | 12篇 |
2013年 | 34篇 |
2012年 | 42篇 |
2011年 | 47篇 |
2010年 | 26篇 |
2009年 | 16篇 |
2008年 | 30篇 |
2007年 | 21篇 |
2006年 | 17篇 |
2005年 | 15篇 |
2004年 | 13篇 |
2003年 | 6篇 |
2002年 | 4篇 |
1997年 | 1篇 |
1990年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1923年 | 1篇 |
排序方式: 共有421条查询结果,搜索用时 31 毫秒
411.
Cahlíková L Macáková K Zavadil S Jiros P Opletal L Urbanová K Jahodár L 《Natural product communications》2011,6(5):603-606
The underivatized alkaloid mixture extracted from the bulbs of Chlidanthus fragrans Herb. was investigated by capillary GC/MS for the first time. Fifteen known Amaryllidaceae alkaloids of five structure types were identified. The main alkaloids were tazzetine (9, tazettine-type), chlidanthine (2, galanthamine-type), belladine (8, belladine-type) and lycorine (12, lycorine-type). The alkaloid extract from the bulbs showed promising human blood acetylcholinesterase (IC50 = 20.1 +/- 2.9 microg/mL) and human plasma butyrylcholinesterase (IC50 = 136.8 +/- 6.9 microg/mL) inhibitory activity. 相似文献
412.
Janečková L Kalíková K Vozka J Armstrong DW Bosáková Z Tesařová E 《Journal of separation science》2011,34(19):2639-2644
Cyclofructans (CFs), a new class of chiral selectors, have been recently introduced for application in liquid chromatography and capillary electrophoresis. So far, derivatized CFs have performed interesting separation possibilities for a variety of compounds. The current work is focused on characterization of three different CF-based chiral stationary phases (CF-based CSPs), i.e. isopropyl carbamate cyclofructan 6 (IP-CF6), R-naphthylethyl carbamate cyclofructan 6 (RN-CF6) and dimethylphenyl carbamate cyclofructan 7 (DMP-CF7). The linear free energy relationship (LFER) model was used to reveal the dominant interactions participating in the complex retention mechanism. A set of 44 different test solutes, with known solvation parameters, was used to determine the regression coefficients of the LFER equation under two mobile-phase compositions in normal separation mode. The LFER results showed that hydrogen bond acidity, hydrophobicity and dipolarity/polarizibility mostly affect the retention and separation process on the CF-based columns in the studied separation systems. 相似文献
413.
Lux F Mignot A Mowat P Louis C Dufort S Bernhard C Denat F Boschetti F Brunet C Antoine R Dugourd P Laurent S Vander Elst L Muller R Sancey L Josserand V Coll JL Stupar V Barbier E Rémy C Broisat A Ghezzi C Le Duc G Roux S Perriat P Tillement O 《Angewandte Chemie (International ed. in English)》2011,50(51):12299-12303
414.
Lucie Nováková Axel Vaast Catherine Stassen Ken Broeckhoven Mauro De Pra Remco Swart Gert Desmet Sebastiaan Eeltink 《Journal of separation science》2013,36(7):1192-1199
We report on the optimization of nano‐LC gradient separations of proteomic samples that vary in complexity. The gradient performance limits were visualized by kinetic plots depicting the gradient time needed to achieve a certain peak capacity, while using the maximum system pressure of 80 MPa. The selection of the optimal particle size/column length combination and corresponding gradient steepness was based on scouting the performance of 75 μm id capillary columns packed with 2, 3, and 5 μm fully porous silica C18 particles. At optimal gradient conditions, peak capacities up to 500 can be obtained within a 120 min gradient using 2 μm particle‐packed capillary columns. Separations of proteomic samples including a cytochrome c tryptic digest, a bovine serum albumin tryptic digest, a six protein mix digest, and an Escherichia coli digest are demonstrated while operating at the kinetic‐performance limit, i.e. using 2‐μm packed columns, adjusting the column length and scaling the gradient steepness according to sample complexity. Finally, good run‐to‐run retention time stability (RSD values below 0.18%) was demonstrated applying ultra‐high pressure conditions. 相似文献
415.
Qiu H Loukotková L Sun P Tesařová E Bosáková Z Armstrong DW 《Journal of chromatography. A》2011,1218(2):270-279
New stationary phases for hydrophilic interaction liquid chromatography (HILIC) were synthesized by covalently attaching native cyclofructan 6 (CF6) to silica gel. The chromatographic characteristics of the new stationary phases were evaluated and compared to three different types of commercial HILIC columns. The CF6 columns produced considerably different retention and selectivity patterns for various classes of polar analytes, including nucleic acid compounds, xanthines, β-blockers, salicylic acid and its derivatives, and maltooligosaccharides. Univariate optimization approaches were examined including organic modifier (acetonitrile) contents and buffer pH and salt concentration. The thermodynamic characteristic of the CF6 stationary phase was investigated by considering the column temperature effect on retention and utilizing van't Hoff plots. CF6 based stationary phases appear to have exceptionally broad applicability for HILIC mode separations. 相似文献
416.
Lucie ?echováPetr Jansa Michal ŠálaMartin Dra?ínský Antonín HolýZlatko Janeba 《Tetrahedron》2011,67(5):866-871
The microwave-assisted decomposition of DMF was thoroughly studied and the reaction conditions (temperature, solvent effect, and effect of additives, such as acids, bases, and salts) were optimized for its use in amination reactions. The applicability of this expedient methodology in purine chemistry and with various formamides is demonstrated. 相似文献
417.
Lucie AliouaneBaptiste Rigaud Emmanuel PfundLudovic Jean Pierre-Yves RenardThierry Lequeux 《Tetrahedron letters》2011,52(28):3681-3685
The one step synthesis of difluoromethylphosphonamidates from dialkylphosphonates is reported. The addition of lithiated amides onto dialkyl difluoromethylphosphonates afforded the corresponding phosphonamidates, as potential prodrug precursors. The remarkable high stability of these phosphonamidates in acidic medium was studied by 31P NMR. 相似文献
418.
Anger E Rudolph M Norel L Zrig S Shen C Vanthuyne N Toupet L Williams JA Roussel C Autschbach J Crassous J Réau R 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(50):14178-14198
Acetylacetonato-platina[6]- and -platina[7]helicenes have been prepared from 2-pyridyl-substituted benzophenanthrene ligands by following a two-step cycloplatination reaction. The photophysical properties (UV-visible absorption and emission behavior) and chiroptical properties (circular dichroism and molar rotation) of the resolved enantiomers have been measured. These metallahelicenes constitute a novel family of easily accessible helicene derivatives that exhibit large and tuneable chiroptical properties that can be rationalized theoretically and compared to the parent [6]- and [7]carbohelicenes. Furthermore, they are red phosphors at room temperature and their large chiroptical properties can be modulated by oxidation of the metal center to Pt(IV). Hetero- and homochiral diastereomeric bis(metallahelicene)s that possess a rare Pt(III)-Pt(III) scaffold bridged by benzoato ligands have also been prepared. It is shown that the heterochiral (P,M)-bis(Pt(III)-[6]helicene) 9a(1) can isomerize into the homochiral (P,P)- and (M,M)-bis(Pt(III)-[6]helicene) 9a(2). Spectral assignments and an analysis of the optical rotation of these systems were made with the help of time-dependent density functional theory. The calculations highlight the contributions of the metal centers to the chiroptical properties. For 9a(1) and 9a(2), σ-π conjugation between the helicenes and the Pt-Pt moiety may contribute strongly to the optical rotation and electronic circular dichroism. 相似文献
419.
Kalachova K Pulkrabova J Drabova L Cajka T Kocourek V Hajslova J 《Analytica chimica acta》2011,707(1-2):84-91
In this study, a new rapid and flexible method for the simultaneous determination of 18 key representatives of polychlorinated biphenyls (PCBs), 7 polybrominated diphenyl ethers (PBDEs), and 32 polycyclic aromatic hydrocarbons (PAHs) in fish and shrimps by gas chromatography coupled to mass spectrometry (GC-MS) was developed and validated. A substantial simplification of sample processing prior to quantification step was achieved: after addition of water to homogenized sample, transfer of hydrophobic analytes into ethyl acetate was supported by added inorganic salts. Bulk fat, contained in crude organic extract obtained by partition, was subsequently removed on a silica minicolumn. This approach enabled to process six samples in less than 1h; moreover, the volume of an extraction solvent and consumption of other chemicals can be significantly reduced compared to, e.g., traditional Soxhlet extraction followed by gel permeation chromatography. The recoveries of target analytes were in the range of 73-120% even at the lowest spiking level (1 μg kg(-1)), repeatabilities (relative standard deviations, RSDs) ranged from 1 to 20%. Under optimized GC-MS conditions (time-of-flight mass analyzer, TOF), the limits of quantification (LOQs) were as follows: PCBs 0.1-0.5 μg kg(-1), PBDEs 0.5 μg kg(-1), and PAHs 0.05-0.25 μg kg(-1). Ambient mass spectrometry employing a direct analysis in real time (DART) ion source was shown as an effective tool for fat control in extract, which is needed during the method development and examination of unknown samples prior to the analysis. Further extension of a method scope by other similar analytes is easily possible. 相似文献
420.
Lucie Blondeau Suzy Surble Eddy Foy Hicham Khodja Magali Gauthier 《Journal of Energy Chemistry》2021,(4):124-128
A composite In-Pb:carbon was successfully synthetized by a two-step mechanochemical synthesis in order to obtain an adequate particles size and structure to investigate the electrochemical reactivity of the In-Pb solid solution towards Mg.A potential synergetic coupling of electroactive elements In and Pb was examined using electrochemical and ex situ X-ray diffraction analyses.The potential profile of the solid solution indicates the formation of Mg2Pb and Mg In.However,the diffraction study suggests a peculiar electrochemically-driven amorphization of Mg In during the magnesiation,in strong contrast to Mg In crystallization in In-based and In Bi-based electrodes reported in the literature.Combining In and Pb favors the amorphization of Mg In and a high first magnesiation capacity of about 550 m Ah g-1,but is thereafter detrimental to the material’s reversibility.These results emphasize the possible influence of electrochemically-driven amorphization and crystallization processes on electrochemical performance of battery materials. 相似文献