全文获取类型
收费全文 | 589篇 |
免费 | 29篇 |
专业分类
化学 | 420篇 |
晶体学 | 10篇 |
力学 | 10篇 |
数学 | 60篇 |
物理学 | 118篇 |
出版年
2023年 | 13篇 |
2022年 | 15篇 |
2021年 | 31篇 |
2020年 | 18篇 |
2019年 | 13篇 |
2018年 | 11篇 |
2017年 | 11篇 |
2016年 | 25篇 |
2015年 | 18篇 |
2014年 | 14篇 |
2013年 | 36篇 |
2012年 | 60篇 |
2011年 | 53篇 |
2010年 | 20篇 |
2009年 | 23篇 |
2008年 | 27篇 |
2007年 | 36篇 |
2006年 | 34篇 |
2005年 | 30篇 |
2004年 | 20篇 |
2003年 | 16篇 |
2002年 | 25篇 |
2001年 | 12篇 |
2000年 | 6篇 |
1999年 | 6篇 |
1998年 | 4篇 |
1997年 | 5篇 |
1996年 | 11篇 |
1995年 | 2篇 |
1994年 | 1篇 |
1993年 | 2篇 |
1992年 | 2篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1982年 | 1篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1962年 | 1篇 |
排序方式: 共有618条查询结果,搜索用时 31 毫秒
101.
(1,10-Phenanthroline)bis(piperidinecarbodithioato-S,S′)cadmium(II), [Cd(pipdtc)2(1,10-phen)] (1) and (2,2′-bipyridine)bis(piperidinecarbodithioato-S,S′)cadmium(II), [Cd(pipdtc)2(bipy)] (2) adducts were prepared and the crystal structures are reported. Cd–S and Cd–N distances and the angles subtended at cadmium are almost the same in both complexes but the Cd–S distances in the adducts are longer than those in Cd(pipdtc)2 (3) complexes due to the presence of an additional neutral ligand. Thioureide C–N distance in 1 and 2 are supported by νC–N bands observed at 1471 and 1470 cm−1, respectively. S2p binding energies for the adducts show a significant reduction in value compared to the parent dithiocarbamate indicating the weakening of the Cd–S bond on adduct formation. The observed reduction in binding energy is due to the increased electron density on the metal in the adducts. The cyclic voltammetric study on the complexes also show an increase of electron density on cadmium in the adducts compared to Cd(pipdtc)2. 相似文献
102.
Patricia Lara Margarita Paneque Prof. Dr. Manuel L. Poveda Prof. Dr. Laura L. Santos Dr. José E. V. Valpuesta Verónica Salazar Dr. Ernesto Carmona Prof. Dr. Salvador Moncho Gregori Ujaque Dr. Agustí Lledós Prof. Dr. Celia Maya Dr. Kurt Mereiter Prof. Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(36):9046-9057
Experimental and theoretical studies on equilibria between iridium hydride alkylidene structures, [(TpMe2)Ir(H){?C(CH2R)ArO }] (TpMe2=hydrotris(3,5‐dimethylpyrazolyl)borate; R=H, Me; Ar=substituted C6H4 group), and their corresponding hydride olefin isomers, [(TpMe2)Ir(H){R(H)C? C(H)OAr}], have been carried out. Compounds of these types are obtained either by reaction of the unsaturated fragment [(TpMe2)Ir(C6H5)2] with o‐C6H4(OH)CH2R, or with the substituted anisoles 2,6‐Me2C6H3OMe, 2,4,6‐Me3C6H2OMe, and 4‐Br‐2,6‐Me2C6H2OMe. The reactions with the substituted anisoles require not only multiple C? H bond activation but also cleavage of the Me? OAr bond and the reversible formation of a C? C bond (as revealed by 13C labeling studies). Equilibria between the two tautomeric structures of these complexes were achieved by prolonged heating at temperatures between 100 and 140 °C, with interconversion of isomeric complexes requiring inversion of the metal configuration, as well as the expected migratory insertion and hydrogen‐elimination reactions. This proposal is supported by a detailed computational exploration of the mechanism at the quantum mechanics (QM) level in the real system. For all compounds investigated, the equilibria favor the alkylidene structure over the olefinic isomer by a factor of between approximately 1 and 25. Calculations demonstrate that the main reason for this preference is the strong Ir–carbene interactions in the carbene isomers, rather than steric destabilization of the olefinic tautomers. 相似文献
103.
Francisco J. Lara Ana M. García-Campaña Christian Neusüss Fermín Alés-Barrero 《Journal of chromatography. A》2009,1216(15):3372-3379
An in-line solid-phase extraction-capillary electrophoresis method with UV–vis detection was developed for the monitoring of residues of five sulfonamides (sulfadoxin, sulfadimethoxine, sulfamerazine, sulfachloropyridazine and sulfamethoxazole) in tap, bottled mineral and river waters. For this purpose an analyte concentrator was constructed, based on the introduction of a small portion of a solid-phase extraction sorbent into the electrophoretic capillary to carry out an in-line concentration step, improving sensitivity. A detailed study was carried out to optimize parameters affecting the in-line solid-phase extraction process, such as the design of the concentrator device, type of sorbent and conditions of elution and injection. The proposed method is simple for the environmental monitoring of these antibiotic residues in waters, allowing the direct injection of the samples without any off-line pretreatment and achieving limits of detection between 0.3 and 0.6 μg/L. Recoveries ranging 52.2–109.2% and relative standard deviations below 13.4% were obtained. 相似文献
104.
Mohamad Sleiman Randy L. Maddalena Lara A. Gundel Hugo Destaillats 《Journal of chromatography. A》2009,1216(45):7899-7905
Tobacco-specific nitrosamines (TSNAs) are some of the most potent carcinogens in tobacco and cigarette smoke. Accurate quantification of these chemicals is needed to help assess public health risks. We developed and validated a specific and sensitive method to measure four TSNAs adsorbed to model surfaces and secondhand smoke (SHS) particles using gas chromatography–ion-trap tandem mass spectrometry. In an 18-m3 room-sized chamber, a smoking machine generated realistic concentrations of SHS that were actively sampled on Teflon-coated fiber glass (TCFG) filters, and passively sampled on cellulose substrates. A simple solid–liquid extraction protocol using methanol as solvent was successfully applied to both substrates with recoveries ranging from 85 to 115%. For each TSNA, tandem MS parameters were optimized and the major fragmentation pathways were elucidated. The method showed excellent performance, with a linear dynamic range from 2 to 1000 ng mL−1, low detection limits (S/N > 3) of 30–300 pg mL−1 and precision with experimental errors below 10% for all compounds. Moreover, no interfering peaks were observed, indicating a high selectivity of MS/MS without the need for a sample clean-up step. This method provides a suitable analytical tool to detect and quantify traces of TSNA in indoor environments polluted with SHS. 相似文献
105.
Lara Bianchi Giovanni Petrillo Egon Rizzato Elda Severi Marco Stenta Cinzia Tavani 《Tetrahedron》2009,65(1):336-757
(1E,3Z)-1-Aryl-4-methanesulfonyl-2-nitro-1,3-butadienes (8), derived from the initial ring-opening of 3-nitrothiophene (5), have been found to undergo a facile base-induced cyclization leading to thiopyran S,S-dioxides (9), thus furnishing a further example of effective ring-enlargement from 5- to 6-membered sulfur heterocycles. Compounds 9 are obtained as single racemic mixtures in satisfactory yields; they still contain a nitrovinylic moiety, which can be exploited for further modifications targeted to new derivatives endowed with either synthetic or pharmacological potentialities e.g., in the field of L-type Ca2+-channel blockers. 相似文献
106.
Applications of capillary electrophoresis to the determination of antibiotics in food and environmental samples 总被引:1,自引:0,他引:1
Ana M. García-Campaña Laura Gámiz-Gracia Francisco J. Lara Monsalud del Olmo Iruela Carmen Cruces-Blanco 《Analytical and bioanalytical chemistry》2009,395(4):967-986
In this paper we review applications of capillary electrophoresis (CE) to the determination of antibiotic residues in food
derived from animals and in environmental samples. Although many CE methods have been used to determine antibiotics in the
pharmaceutical field (drug quality control or therapeutic monitoring in biological samples), food and environmental applications
have been increasing in recent years. Due to the maximum residue limits established by the EU, in Directive 2377/90/EEC, for
foodstuffs of animal origin and considering the low levels that can be found in environmental or waste waters or soils, different
strategies to increase sensitivity have been developed, including off-line preconcentration, on-line stacking modes to use
higher sample volumes, or in-line solid-phase extraction. Also, several detection techniques, such as fluorescence, laser-induced
fluorescence, electrochemical detection, or mass spectrometry have been used; the last of these also enables unequivocal identification
of the residues, required by Commission Decision 2002/657/EC. All these aspects will be discussed in this paper, in relation
to the main groups of antibiotics used in veterinary and human medicine, for which applications in food and environmental
samples have been developed by using CE as an efficient alternative to liquid chromatography. 相似文献
107.
Weber (2009) suggested that counterexamples can be generated by a syntactic proof production, apparently contradicting our earlier assertion (Alcock & Inglis, 2008). Here we point out that this ostensible difference is the result of Weber working with theoretical definitions that differ slightly from ours. We defend our approach by arguing that Weber’s relies upon an as yet unspecific metric for gauging the amount of work conducted in each representation system, and that it does not recognize an important asymmetry between the status of representation systems in the context of undergraduate mathematics. 相似文献
108.
We present a parallel implementation of the integral equation formalism of the polarizable continuum model for Hartree-Fock and density functional theory calculations of energies and linear, quadratic, and cubic response functions. The contributions to the free energy of the solute due to the polarizable continuum have been implemented using a master-slave approach with load balancing to ensure good scalability also on parallel machines with a slow interconnect. We demonstrate the good scaling behavior of the code through calculations of Hartree-Fock energies and linear, quadratic, and cubic response function for a modest-sized sample molecule. We also explore the behavior of the parallelization of the integral equation formulation of the polarizable continuum model code when used in conjunction with a recent scheme for the storage of two-electron integrals in the memory of the different slaves in order to achieve superlinear scaling in the parallel calculations. 相似文献
109.
A new route to 15-hydroxydehydroabietic acid derivatives from abietic acid (11), via abieta-8,13(15)-dien-18-oic acid (12), is reported. Utilizing this, the first synthesis of bioactive terpenes 3, 6, 9 and 10, as well as natural diol 4 and lactones 7-8 was achieved. 相似文献
110.
Lara B. Anderson James Gray Dan Grayson Yang-Hui He André Lukas 《Communications in Mathematical Physics》2010,297(1):95-127
We present a practical, algebraic method for efficiently calculating the Yukawa couplings of a large class of heterotic compactifications
on Calabi-Yau three-folds with non-standard embeddings. Our methodology covers all of, though is not restricted to, the recently
classified positive monads over favourable complete intersection Calabi-Yau three-folds. Since the algorithm is based on manipulating
polynomials it can be easily implemented on a computer. This makes the automated investigation of Yukawa couplings for large
classes of smooth heterotic compactifications a viable possibility. 相似文献