全文获取类型
收费全文 | 966篇 |
免费 | 28篇 |
国内免费 | 4篇 |
专业分类
化学 | 729篇 |
晶体学 | 5篇 |
力学 | 18篇 |
数学 | 122篇 |
物理学 | 124篇 |
出版年
2023年 | 6篇 |
2022年 | 16篇 |
2021年 | 18篇 |
2020年 | 18篇 |
2019年 | 17篇 |
2018年 | 9篇 |
2017年 | 8篇 |
2016年 | 23篇 |
2015年 | 17篇 |
2014年 | 31篇 |
2013年 | 50篇 |
2012年 | 80篇 |
2011年 | 62篇 |
2010年 | 43篇 |
2009年 | 52篇 |
2008年 | 66篇 |
2007年 | 67篇 |
2006年 | 65篇 |
2005年 | 49篇 |
2004年 | 41篇 |
2003年 | 30篇 |
2002年 | 43篇 |
2001年 | 14篇 |
2000年 | 18篇 |
1999年 | 14篇 |
1998年 | 12篇 |
1997年 | 3篇 |
1996年 | 9篇 |
1995年 | 3篇 |
1994年 | 8篇 |
1993年 | 4篇 |
1992年 | 4篇 |
1991年 | 6篇 |
1990年 | 3篇 |
1989年 | 4篇 |
1988年 | 6篇 |
1987年 | 5篇 |
1985年 | 9篇 |
1984年 | 4篇 |
1983年 | 8篇 |
1982年 | 8篇 |
1981年 | 4篇 |
1976年 | 2篇 |
1956年 | 1篇 |
1955年 | 24篇 |
1954年 | 2篇 |
1933年 | 1篇 |
1932年 | 1篇 |
1913年 | 1篇 |
1872年 | 1篇 |
排序方式: 共有998条查询结果,搜索用时 15 毫秒
71.
An automatic system based on multisyringe flow injection analysis (MSFIA) and lab-on-valve (LOV) flow techniques for separation and pre-concentration of 226Ra from drinking and natural water samples has been developed. The analytical protocol combines two different procedures: the Ra adsorption on MnO2 and the BaSO4 co-precipitation, achieving more selectivity especially in water samples with low radium levels. 相似文献
72.
Consumption of non-steroidal anti-inflammatory drugs (NSAIDs) is increasing and with it the danger of environmental pollution
by pharmaceutical residues. Publications regarding NSAIDs in the environment not only show that they are toxic to many animal
species, but also highlight the need for robust analytical methods for monitoring the level of such contaminants in environmental
matrices. In our study we selected the four most widely used NSAIDs in Slovenia and Central Europe, ibuprofen, naproxen, ketoprofen
and diclofenac, and studied their extraction from sediment samples. We examined several extraction techniques (ultrasonic
extraction, Soxhlet extraction, pressurized liquid extraction, supercritical fluid extraction and microwave-assisted extraction)
using a spiked sediment sample and determined optimal extraction conditions. After extraction we applied a clean-up step,
derivatisation of the analytes and gas chromatography with mass spectrometric detection (GC-MSD) and selected the most appropriate
extraction procedure. The optimised analytical method chosen for analysis of sediment samples consisted of microwave-assisted
extraction, clean-up of the extract with SPE, derivatisation with MSTFA and determination with GC-MSD. The optimised procedure
was applied to the analysis of two environmental river samples taken from the vicinity of Novo mesto, the biggest town in
the south eastern part of Slovenia with 62,000 inhabitants, a hospital and a pharmaceutical factory in its vicinity. While
analysis of the sample taken upstream of the town showed no detectable amounts of NSAIDs, analysis of samples taken downstream
showed quantifiable levels of two of the studied NSAIDs (naproxen and ketoprofen). Besides these two NSAIDs, river water samples
sampled at the same time and location on the River Krka also showed the presence of diclofenac. Sampling on the River Krka
and other Slovene rivers will in the future be repeated at different sampling points in order to track down the main sources
of pollution. 相似文献
73.
Ana I. Olives Emiliano E. Romero-Ale M. Antonia Martín Benito del Castillo Pilar López-Alvarado J. Carlos Menéndez 《Journal of inclusion phenomena and macrocyclic chemistry》2007,57(1-4):553-559
Carbazole-9-carbonyl chloride (C9CC) and 9-carbazolylacetic acid (9CAA) were selected as model fluorescent reagents. The effect
of different chemically modified cyclodextrins (CDs) added to the aqueous solutions of these reagents was studied in water
and in buffered aqueous solutions at pH 4.5 and 8.8. The CDs employed were 2-hydroxypropyl-β-cyclodextrin (HP-βCD), 2,3-di-O-methyl-β-cyclodextrin (DM-βCD) and 2,3,6-tri-O-methyl-β-cyclodextrin (TM-βCD). The inclusion of these reagents inside the cavities of the CDs was verified and this process
can affect the derivatization reaction because CDs can modify the reactivity of the guest molecules. The basic conditions
necessary for the derivatization reaction between C9CC and amines lead to the formation of carbazole anion through hydrolysis
followed by decarboxylation. In the presence of CDs, the hydrolysis-decarboxylation of carbazole-9-carbonyl chloride is faster
than in buffered aqueous homogeneous solutions. The behaviour observed for these reagents in aqueous solutions of CDs was
compared to the one observed in basic ethanolic solutions. These changes are particularly noticeable in the case of 2,3-di-O-methyl-β-CD and 2-hydroxypropyl-β-CD. The characteristics of the fluorescent reagents are compared to carbazole and 9-methylcarbazole
as model compounds.
This paper was presented at XIIIth International Cyclodextrin Symposium. Torino, Italy, May 14–17, 2006. 相似文献
74.
Leszek Wachowski Antoni Grodzicki Piotr Piszczek Monika Richert Magdalena Hofman 《Reaction Kinetics and Catalysis Letters》2007,91(1):93-99
Hydrogenation of styrene has been applied as a test reaction to study the catalytic activity of TiO2 deposited by the CVD (chemical vapour deposition) method on the surface of a carbonaceous material enriched in nitrogen (CN). 相似文献
75.
Andrés G. León Ana I. Olives M. Antonia Martín Benito del Castillo 《Journal of inclusion phenomena and macrocyclic chemistry》2007,57(1-4):577-583
The use of cyclodextrins (CDs) in HPLC as mobile phase additives provides a flexible alternative for the separation of chemically
related compounds because these separations can be performed on conventional columns and are economically advantageous over
the use of chiral stationary phases. The present paper describes the influence of the presence of β-cyclodextrin (β-CD) and
2-hydroxypropyl-β-cyclodextrin (HPβ-CD) on the separation of the β-carboline alkaloids norharmane, harmane and harmine. The
nature of the stationary phase (reverse phases C1 and C18) affects the chromatographic separations and also the stability of the inclusion complexes that are developed. The changes
in the proportion of the organic solvents at constant concentration of CDs (3 mM for β-CD and 15 mM for HPβ-CD) modify the
retention factors (k′) for all alkaloids studied. The nature of the organic solvent in the mobile phase also changes the chromatographic parameters.
The logarithm of the capacity factor (k′) is linearly increased with the proportion of water in the hydro-organic mobile phase (ethanolic or methanolic) but the
slopes obtained vary depending on the CD added to the mobile phase. The role of competitive equilibria, i.e., chromatographic
distribution and inclusion complexes formation is discussed.
This paper was presented at XIIIth International Cyclodextrin Symposium. Torino, Italy, May, 14–17, 2006 相似文献
76.
Juan Manuel Benito David Rodríguez-Lucena José Luis Jiménez Blanco Carmen Ortiz Mellet José Manuel García Fernández 《Journal of inclusion phenomena and macrocyclic chemistry》2007,57(1-4):147-150
A new concept for the de novo synthesis of artificial glyconanocavities is presented. The use of alternating α,α′-trehalose building blocks and (thio)urea segments allows the efficient synthesis of a new family of cyclooligosaccharides,
namely cyclotrehalans (CTs), featuring a convex-shaped cavity with an apolar environment. CTs are designed to exhibit molecular inclusion abilities
similar to that of cyclodextrins (CDs). Contrary to CDs, CTs expose the monosaccharide β-face to the inner cavity, while the (thio)urea tethers provides some conformational adaptability. High-yielding syntheses
of a series of CTs and a preliminary evaluation of their inclusion properties are reported. 相似文献
77.
Isidoro Lpez Mehmed Z. Ertem Somnath Maji Jordi Benet‐Buchholz Anke Keidel Uwe Kuhlmann Peter Hildebrandt Christopher J. Cramer Victor S. Batista Antoni Llobet 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2014,126(1):209-213
The homogeneous catalysis of water oxidation by transition‐metal complexes has experienced spectacular development over the last five years. Practical energy‐conversion schemes, however, require robust catalysts with large turnover frequencies. Herein we introduce a new oxidatively rugged and powerful dinuclear water‐oxidation catalyst that is generated by self‐assembly from a mononuclear catalyst during the catalytic process. Our kinetic and DFT computational analysis shows that two interconnected catalytic cycles coexist while the mononuclear system is slowly and irreversibly converted into the more stable dinuclear system: an extremely robust water‐oxidation catalyst that does not decompose over extended periods of time. 相似文献
78.
Masllorens E Rodríguez M Romero I Roglans A Parella T Benet-Buchholz J Poyatos M Llobet A 《Journal of the American Chemical Society》2006,128(16):5306-5307
Three new Ru-aqua complexes containing a mixed carbene and pyridylic ligands with general formulas [Ru(CNC)(bpy)(H2O)](PF6)2 (1) (CNC is 2,6-bis(butylimidazol-2-ylidene)pyridine; bpy is 2,2'-bipyridine) and cis-/trans-[Ru(CNC)(nBu-CN)(H2O)](PF6)2 (cis-2 and trans-2) (nBu-CN is 2-(butylimidazol-2-ylidene)pyridine) have been prepared and structurally characterized both in the solid state (monocrystal X-ray diffraction analysis for 1 and for the related complex trans-[Ru(Br)(CNC)(nBu-CN)](PF6)) and in solution (for all of them) through NMR. The electrochemical properties of these three Ru-aqua complexes have been investigated by cyclic voltammetry, differential pulse voltammetry and Coulombimetric techniques. It is found that, for complex 1 at pH 7, the difference between the IV/III and the III/II redox couples (DeltaE1/2) is 50 mV, which is the smallest ever reported for this type of complex. On the other hand, for complexes cis-2 and trans-2, the oxidation state III is unstable with respect to disproportionation to II and IV. The reactivity of their Ru=O species has been tested toward cis-beta-methylstyrene oxidation, and it has been compared to [Ru(O)(trpy)(bpy)]2+. An inverse correlation between the degree of cis/trans-epoxide isomerization and DeltaE1/2 is found. In particular, for complexes cis-2 and trans-2, which have a DeltaE1/2 < 0, the epoxidation is highly stereoselective, yielding only cis-epoxide. 相似文献
79.
A membrane-assisted solvent extraction (MASE) method is presented for the extraction of several non-ionic organophosphorus chemicals from wastewaters samples followed by LC-MS/MS determination. The method was developed for a variety of chlorinated phosphates (trichloroethyl, tichloropropyl) and non-chlorinated phosphates (triphenyl, tributyl) used as flame retardants and for plasticizers such as triethylhexyl and tris-butoxyethyl phosphate. Parameters such as extracting solvent, sample volume and ionic strength, extraction temperature and time were optimized. The final method provides good quantification limits (1-25 ng L(-1)) and linearity (R2>0.9978). Method precision was also good at high concentrations (5% mean RSD at the 500 ng L(-1) level) but decreased at lower concentrations (20% mean RSD at the 20 ng L(-1) level). MASE yields lower matrix effects than SPE in a successive LC-MS/MS analysis of these compounds, avoiding the need for standard addition for quantification. When applied to wastewater samples comparable results were obtained using either MASE with internal standard calibration or SPE with standard addition. 相似文献
80.
Ballester P Costa A Castilla AM Deyà PM Frontera A Gomila RM Hunter CA 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(7):2196-2206
Three isomeric zinc bisporphyrins have been prepared by covalently linking together two aminoporphyrins with an isophthalic acid derivative. The porphyrins differ in the substitution pattern on the meso phenyl groups, that is, ortho, meta, or para. Titrations carried out by UV-visible and 1H NMR spectroscopy have been used to map out the stabilities and the stoichiometries of the complexes formed with 1,4-diazabicyclo[2.2.2]octane (DABCO) in chloroform. The ortho- and meta-substituted bisporphyrins form 1:1 intramolecular sandwich complexes. The para-substituted bisporphyrin cannot adopt the cofacial conformation required for this type of complex and forms a higher order 2:2 intermolecular assembly, which is stable over a wide range of DABCO concentrations. 相似文献