全文获取类型
收费全文 | 651篇 |
免费 | 16篇 |
国内免费 | 2篇 |
专业分类
化学 | 410篇 |
晶体学 | 1篇 |
力学 | 17篇 |
数学 | 98篇 |
物理学 | 143篇 |
出版年
2024年 | 2篇 |
2023年 | 4篇 |
2022年 | 8篇 |
2021年 | 11篇 |
2020年 | 9篇 |
2019年 | 7篇 |
2018年 | 7篇 |
2017年 | 11篇 |
2016年 | 16篇 |
2015年 | 20篇 |
2014年 | 24篇 |
2013年 | 36篇 |
2012年 | 32篇 |
2011年 | 40篇 |
2010年 | 19篇 |
2009年 | 27篇 |
2008年 | 48篇 |
2007年 | 36篇 |
2006年 | 42篇 |
2005年 | 48篇 |
2004年 | 34篇 |
2003年 | 26篇 |
2002年 | 36篇 |
2001年 | 6篇 |
2000年 | 3篇 |
1999年 | 11篇 |
1998年 | 10篇 |
1997年 | 7篇 |
1996年 | 6篇 |
1995年 | 9篇 |
1994年 | 6篇 |
1993年 | 4篇 |
1992年 | 2篇 |
1991年 | 4篇 |
1990年 | 2篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1986年 | 6篇 |
1985年 | 4篇 |
1984年 | 4篇 |
1983年 | 3篇 |
1981年 | 5篇 |
1980年 | 7篇 |
1979年 | 4篇 |
1978年 | 4篇 |
1973年 | 2篇 |
1946年 | 1篇 |
1944年 | 2篇 |
1917年 | 1篇 |
1897年 | 1篇 |
排序方式: 共有669条查询结果,搜索用时 0 毫秒
121.
MSPD procedure for determining buprofezin,tetradifon, vinclozolin,and bifenthrin residues in propolis by gas chromatography–mass spectrometry 总被引:1,自引:0,他引:1
dos Santos TF Aquino A Dórea HS Navickiene S 《Analytical and bioanalytical chemistry》2008,390(5):1425-1430
A simple and effective extraction method based on matrix solid-phase dispersion (MSPD) was developed to determine bifenthrin,
buprofezin, tetradifon, and vinclozolin in propolis using gas chromatography–mass spectrometry in selected ion monitoring
mode (GC–MS, SIM). Different method conditions were evaluated, for example type of solid phase (C18, alumina, silica, and Florisil), the amount of solid phase and eluent (n-hexane, dichloromethane, dichloromethane–n-hexane (8:2 and 1:1, v/v) and dichloromethane–ethyl acetate (9:1, 8:2 and 7:3, v/v)). The best results were obtained using 0.5 g propolis, 1.0 g silica as dispersant sorbent, 1.0 g Florisil as clean-up sorbent,
and dichloromethane–ethyl acetate (9:1, v/v) as eluting solvent. The method was validated by analysis of propolis samples fortified at different concentration levels
(0.25 to 1.0 mg kg−1). Average recoveries (four replicates) ranged from 67% to 175% with relative standard deviation between 5.6% and 12.1%. Detection
and quantification limits ranged from 0.05 to 0.10 mg kg−1 and 0.15 to 0.25 mg kg−1 propolis, respectively. 相似文献
122.
123.
Understanding solvation in hydrofluoroalkane (HFA) propellants is of great importance for the development of novel pressurized metered-dose inhaler (pMDI) formulations. HFA-based pMDIs are not only the most widely used inhalation therapy devices for delivering small drug molecules to the respiratory tract, but they also hold promise as vehicles for the delivery of therapeutic biomolecules to and through the lungs. In this work we use binding energy calculations to determine the degree of interaction between HFA propellants and candidate HFA-philes, including a methyl-based tail (isohexane, ISO), and fragments of poly(ethylene oxide) (EO), poly(propylene oxide) (PO), and poly(lactide) (LA). The distinct nature of solvation forces of the two HFA propellants approved by the FDA for use in pMDIs, 1,1,1,2-tetrafluoroethane (HFA134a) and 1,1,1,2,3,3,3-heptafluoropropane (HFA227), is also studied. Binding energy (Ebst) calculations demonstrated that an increase in tail polarity through the addition of oxygen atoms in the fragment backbone provides for sites capable of interacting with the HFA propellant molecules, thus enhancing the stabilization energy of the complexes. The interaction energy between HFA227 and LA (EbHFA227-LA = -24.7 kJ.mol(-1)) is significantly more favorable than that between HFA227 and its hydrocarbon analog (EbHFA227-ISO = -10.0 kJ.mol(-1)). However, it was shown that not only the fragment polarity is of relevance in stabilizing the complexes. The accessibility of the oxygen atoms in the fragments of interest is also relevant. Cluster studies indicate that although both oxygen atoms in the LA fragment are available to form H-bonds with the propellant molecules, the ether oxygen in PO is accessible to only one propellant molecule, thus decreasing significantly the stabilization energy of the cluster. The results shown here serve as a guide for the design of novel HFA-philes for HFA-based pMDIs. 相似文献
124.
Chuev GN Fedorov MV Chiodo S Russo N Sicilia E 《Journal of computational chemistry》2008,29(14):2406-2415
We have tested the reference interaction site model (RISM) for the case of the hypernetted chain (HNC) and the partially linearized hypernetted chain (PLHNC) closures improved by a repulsive bridge correction (RBC) for ionic hydrated species. We have analyzed the efficiency of the RISM/HNC+RBC and RISM/PLHNC+RBC techniques for decomposition of the electrostatic and the nonpolar hydration energies on the energetic and the enthalpic parts for polyatomic ions when the repulsive bridge correction is treated as a thermodynamic perturbation, and investigate the repulsive bridge effect on the electrostatic potential induced by solvent on solute atoms. For a number of univalent and bivalent atomic ions, molecular cations, and anions, the method provides hydration energies deviating only by several percents from the experimental data. In most cases, the enthalpic contributions to the free energies are also close to the experimental results. The above models are able to satisfactory predict the hydration energies as well as the electrostatic potential around the ionic species. For univalent atomic ions, they also provide qualitative estimates of the Samoilov activation energies. 相似文献
125.
Driscoll JA Allard MM Wu L Heeg MJ da Rocha SR Verani CN 《Chemistry (Weinheim an der Bergstrasse, Germany)》2008,14(31):9665-9674
Herein, we describe the synthesis and characterization of a novel series of single-tail amphiphiles LPyCn (Py=pyridine, Cn=C18, C16, C14, C10) and their copper(II)-containing complexes, which are of relevance for patterned films. The N-(pyridine-2-ylmethyl)alkyl-1-amine ligands and their complexes [CuIICl2(LPyC18)] (1), [CuIICl2(LPyC16)] (2), [CuIICl2(LPyC14)] (3), [CuIIBr2(LPyC18)] (4), [CuIIBr2(LPyC16)] (5), and [CuIIBr2(LPyC10)] (6) were synthesized, isolated, and characterized by means of mass spectrometry, IR and NMR spectroscopies, and elemental analysis. Complexes 1, 2, 3, and 6 had their molecular structure solved by X-ray diffraction methods, which showed that the local geometry around the metal center is distorted square planar. With the aim of using these species as precursors for redox-responsive films, an assessment of their electrochemical properties involved cyclic voltammetry in different solvents, with different supporting electrolytes and scan rates. Density functional theory calculations of relevant species in bulk and at interfaces were used to evaluate their electronic structure and dipole moments. The morphology and order of the resulting films at the air/water interface were studied by isothermal compression and Brewster angle microscopy. Biphasic patterned Langmuir films were observed for all complexes except 3 and 6, and dependence on the chain length and the nature of the halogen coligand determine the characteristics of the isotherms and their intricate topology. Complexes 3 and 6, which have shorter chain lengths, failed to exhibit organization. These results exemplify the first comprehensive study of the behavior of single-tail metallosurfactants, which are likely to lead to high-end technological applications based on their patterned films. 相似文献
126.
Wiesner M Revell JD Tonazzi S Wennemers H 《Journal of the American Chemical Society》2008,130(17):5610-5611
The peptide H-D-Pro-Pro-Glu-NH2 is a highly effective catalyst for conjugate addition reactions between aldehydes and nitroethylene. Only 1 mol % of H-d-Pro-Pro-Glu-NH2 and a 1.5-fold excess of aldehyde with respect to nitroethylene suffice to obtain gamma-nitroaldehydes and, after reduction, monosubstituted gamma-nitroalcohols in excellent yields and optical purities. The products can be readily converted into gamma2-amino acids, thereby opening an effective direct entry into this important class of compounds. 相似文献
127.
Claudio Garola Sandro Sozzo Junde Wu 《International Journal of Theoretical Physics》2016,55(5):2500-2528
The ESR model has been recently proposed in several papers to offer a possible solution to the problems raising from the nonobjectivity of physical properties in quantum mechanics (QM) (mainly the objectification problem of the quantum theory of measurement). This solution is obtained by embodying the mathematical formalism of QM into a broader mathematical framework and reinterpreting quantum probabilities as conditional on detection rather than absolute. We provide a new and more general formulation of the ESR model and discuss time evolution according to it, pointing out in particular that both linear and nonlinear evolution may occur, depending on the physical environment. 相似文献
128.
Sandro Coriasco Karoline Johansson Joachim Toft 《Journal of Fourier Analysis and Applications》2016,22(2):285-333
We illustrate the composition properties for an extended family of \({\text {SG}}\) Fourier integral operators. We prove continuity results on modulation spaces, and study mapping properties of global wave-front sets for such operators. These extend classical results to more general situations. For example, there are no requirements on homogeneity for the phase functions. Finally, we apply our results to the study of the propagation of singularities, in the context of modulation spaces, for the solutions to the Cauchy problems for the corresponding linear hyperbolic operators. 相似文献
129.
Understanding solvation in hydrofluoroalkanes: ab initio calculations and chemical force microscopy 总被引:1,自引:0,他引:1
Understanding solvation in hydrofluoroalkane (HFA) propellants is of great importance for the development of novel pressurized metered-dose inhaler (pMDI) formulations. HFA-based pMDIs are not only the most widely used inhalation therapy devices for treating lung diseases, but they also hold promise as vehicles for the systemic delivery of biomolecules to and through the lungs. In this work we propose a combined microscopic experimental and computational approach to quantitatively relate the chemistry of moieties to their HFA-philicity. Binding energy calculations are used to determine the degree of interaction between a propellant HFA and candidate fragments. We define a new quantity, the enhancement factor E, which also takes into account fragment-fragment interactions. This quantity is expected to correlate well with the solubility and the ability of the moieties of interest to impart stability to colloidal dispersions in HFAs. We use a methyl-based (CH) segment and its fluorinated analog (CF) to test our approach. CH is an important baseline case since it represents the tails of surfactants in FDA-approved pMDIs. CF was chosen due to the improved solubility and ability of this chemistry to stabilize aqueous dispersions in HFAs. Adhesion force from Chemical Force Microscopy (CFM) is used as an experimental analog to the binding energy calculations. The force of interaction between a chemically modified AFM tip and substrate is measured in a model HFA, which is a liquid at ambient conditions. Silanes with the same chemistry as the fragments used in the ab initio calculations allow for direct comparison between the two techniques. The CFM results provide an absolute scale for HFA-philicity. Single molecule (pair) forces calculated from the CFM experiments are shown to be in very good agreement to the E determined from the ab initio calculations. The ab initio calculations and CFM are corroborated by previous experimental studies where propellants HFAs are seen to better solvate the CF functionality. 相似文献
130.
Berná J Brouwer AM Fazio SM Haraszkiewicz N Leigh DA Lennon CM 《Chemical communications (Cambridge, England)》2007,(19):1910-1912
Hydrogen bonding in a [2]rotaxane is shown to stabilise the phenolate anion of a coumaric amide chromophore by almost 3 pKa units; however, the effect on the UV spectral shift in the anion is small and, significantly given the photochemistry of PYP, despite the hydrogen bonding olefin photoisomerisation in the anionic rotaxane remains heavily suppressed. 相似文献