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151.
The use of the intramolecular vinylogous aldol reaction for the preparation of an advanced intermediate for the synthesis of peloruside A is described. The reaction was applied to compound 19 and proceeds in high yield and good levels of diastereoselectivity. Application of the Achmatowicz reaction to this intermediate provided the corresponding pyranone, a late stage intermediate well positioned for conversion to the natural product. 相似文献
152.
The inclusion behavior of an intramolecular charge transfer (ICT) fluorescent probe namely; 2-[3-(4-dimethylamino-phenyl)-allylidene]-tetralone (DMAPT) in organized assemblies of aqueous micellar, α- and β-cyclodextrins (CDs) and bovine serum albumin (BSA) pockets have been studied using steady-state absorption and fluorescence spectroscopy. The fluorescence characteristics (energy and intensity) of DMAPT are highly sensitive to the properties of the medium. The ICT maximum is strongly blue-shifted with a great enhancement of the fluorescence intensity upon addition of different surfactants, confirming the solubilization of DMAPT in the hydrophobic micellar assembly. In addition, the fluorescence of DMAPT is more sensitive to the nature and concentration of the added CDs. In α- or β-CD solutions, the fluorescence intensity increases strongly (by 6 and 23 orders of magnitude, respectively). Upon encapsulation in the CD cavity, the molecular flexibility decreases due to the geometrical restrictions of the CD nanocavity which decreases the non radiative transition via the free rotation around the single and/or double bonds of the butadiene bridge. This was supported by finding that the fluorescence quantum yield of DMAPT increases with increasing the viscosity of the medium. The binding constants of DMAPT with micelles, α- and β-CD solutions have been calculated and were found to be highly dependent on the nature of the used surfactants or CDs. The thermodynamic parameters have been also determined and the difference in magnitude between the formed α- and β-CD-DMAPT inclusion complexes is discussed on the basis of the cavity size. Finally, the binding constant of DMAPT with bovine serum albumin was calculated, indicating the relative stability of the DMAPT-BSA complex. The energy transfer distance between BSA as a donor and DMAPT as an acceptor was obtained following the fluorescence quenching of BSA by DMAPT, via resonance mechanism as a quencher. 相似文献
153.
Houali Allal Azahra Mustafa El Bardouni Tarek Ferro García Maria Angeles Piňero García Francisco Chham Essaid 《Journal of Radioanalytical and Nuclear Chemistry》2019,322(2):271-280
For the first time, atmospheric aerosols were collected in Tetouan city (Morocco), from June 2015 to February 2017, through which 7Be activity was measured. We studied data variation by time series analyses and the impact of local meteorological factors on it. 7Be concentrations, in mBq/m3, ranged from 1.65 to 7.79 with mean value of 4.69. 7Be behaviour is mainly influenced by air temperature, precipitations, relative humidity and atmospheric pressure. A prediction model was suggested and validated. This study helps understanding climatic variables impact on the environmental processes as aerosols transport and deposit, and to predict 7Be behaviour in Mediterranean regions like Tetouan.
相似文献154.
J. Felipe Perez Segovia Tarek Beji Bart Merci 《Flow, Turbulence and Combustion》2018,101(4):1059-1072
In the present paper an evaporation model is implemented and assessed in a Computational Fluid Dynamics (CFD) code named ISIS. First, the influence of the cell size and time step on the temperature field is studied via simulations with a prescribed fuel Mass Loss Rate (MLR). Then, the evaporation model is assessed using predictive simulations. The experimental scenario is a 30 cm-diameter heptane pool fire. The average fuel Mass Loss Rate Per Unit Area (MLRPUA) is predicted within 5.5% deviation from the experimental value. In addition, an analysis of the temperature and heat fluxes at the surface of the liquid, the mass transfer coefficient and the temperature inside the liquid is performed. 相似文献
155.
Belal TS 《Journal of fluorescence》2008,18(5):771-780
A simple, rapid, selective and sensitive spectrofluorimetric method was described for the analysis of three nitrofuran drugs,
namely, nifuroxazide (NX), nitrofurantoin (NT) and nitrofurazone (NZ). The method involved the alkaline hydrolysis of the
studied drugs by warming with 0.1 M sodium hydroxide solution then dilution with distilled water for NX or 2-propanol for
NT and NZ. The formed fluorophores were measured at 465 nm (λ
Ex 265 nm), 458 nm (λ
Ex 245 nm) and 445 nm (λ
Ex 245 nm) for NX, NT and NZ, respectively. The reaction pathway was discussed and the structures of the fluorescent products
were proposed. The different experimental parameters were studied and optimized. Regression analysis showed good correlation
between fluorescence intensity and concentration over the ranges 0.08–1.00, 0.02–0.24 and 0.004–0.050 μg ml−1 for NX, NT and NZ, respectively. The limits of detection of the method were 8.0, 1.9 and 0.3 ng ml−1 for NX, NT and NZ, respectively. The proposed method was validated in terms of accuracy, precision and specificity, and it
was successfully applied for the assay of the three nitrofurans in their different dosage forms. No interference was observed
from common pharmaceutical adjuvants. The results were favorably compared with those obtained by reference spectrophotometric
methods. 相似文献
156.
The effect of ultrasound on flow through a capillary using the pendant drop method was investigated. Water was injected into a 0.1 mm Hastelloy C-276 capillary tube submersed into several mineral oils with different viscosity, and kerosene. The average drop rate per minute was measured at several ultrasonic intensities. We observed that there exists a peak drop rate at a characteristic intensity, which strongly depends on oil viscosity and the interfacial tension between water and the oil. The semi-quantitative results reveal that the remarkable change in the interfacial forces between oil and water could be the explanation to the enhancement of oil recovery when the ultrasonic waves are applied. 相似文献
157.
Combinations of N-halosuccinimide and tetrachlorosilane in acetonitrile were found to be efficient systems for the selective α-monohalogenation of carbonyl compounds as well as for benzylic halogenation under mild conditions. 相似文献
158.
Thermoplastic elastomers require domains that act as physical crosslinks, and these can be either glassy regions or crystallites. For crystallites in a polymer such as polypropylene (PP), the challenge is to develop polymerizations that form the long stereoregular chain sequences required to produce crystallites large enough to act as stable temporary crosslinks but not so many of them that the material becomes a significantly crystalline thermoplastic rather than an elastomer. For PP, the requirement is for long isotactic sequences within predominantly elastomeric atactic chains, and catalysts required to achieve such an unusual stereoblock structure are now available. This provided encouragement for the simulations described herein that illustrate some relationships between polymerization mechanisms and distributions of isotactic sequences (particularly those long enough to crystallize). A Windle‐type Monte Carlo algorithm was then applied to arrays of the representative PP chains in searches for matches of crystallizable sequences. This approach provided estimates of degrees of crystallinity, melting points, interfacial free energies, standard free energies of fusion, and Young's moduli at small extensions. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 840–853, 2002 相似文献
159.
El-Gogary TM El-Gendy EM 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2003,59(11):2635-2644
Two psoralen derivatives (probes) were prepared. Their geometries were optimized at the Hartree-Fock (HF) and Density Functional (B3LYP) levels employing 6-31G** and cc-pVDZ basis sets. Their interaction with DNA was investigated using spectrophotometric and computational techniques. Both of them have shown strong binding to calf thymus DNA. The red-shift and hypochromism that detected in the spectrum were taken as an evidence for the strong interaction between these probes and DNA. The spectrophotometric DNA titration data were treated by two different methodologies to calculate the intercalation affinity. Half-reciprocal plots gave binding constants of 5.5065 x 10(4) and 6.4727 x 10(4) for 8-butoxypsoralen (8-BOP) and 8-hexoxypsoralen (8-HOP), respectively. Schatchard plots gave a comparable intercalation binding constants and also the surface binding constants along with the number of intercalated probe molecules per base pair. The interaction between these probes and DNA were studied theoretically. The energy of interaction was computed using molecular mechanics method. Strength of interaction of these probes with different types of DNA was computed and compared. Calculated energies of interaction were compared with the observed intercalation affinities. HOMO and LUMO energies were computed and used to account for the strength of interaction. 相似文献
160.
Wehbe J Kassem T Rolland V Rolland M Tabcheh M Roumestant ML Martinez J 《Organic & biomolecular chemistry》2003,1(11):1938-1942
A short asymmetric synthesis of optically pure beta-substituted beta-hydroxy aspartates is described. The key step is an aldol reaction between a glycine enolate derived from an oxazinone intermediate used as chiral auxiliary and various alpha-keto esters. Excellent diastereomeric excesses are obtained. 相似文献