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排序方式: 共有446条查询结果,搜索用时 15 毫秒
31.
Perin Gelson Silva Cristian M. Borges Elton L. Duarte José E. G. Goulart Helen A. Silva Rafaela B. Schumacher Ricardo F. 《Research on Chemical Intermediates》2016,42(6):5873-5885
Research on Chemical Intermediates - We describe a simple and rapid method for selective synthesis of 4-thiomethyl-1,3-dioxolan-2-ones promoted by KF/Al2O3 using PEG-400 as solvent under microwave... 相似文献
32.
The literature about direct solid sample analysis of the past 10–15 years using electrothermal atomic absorption spectrometry
has been reviewed. It was found that in the vast majority of publications aqueous standards were reported as having been used
for calibration after careful program optimization. This means the frequently expressed claim that certified reference materials
with a matrix composition and analyte content close to that of the sample have to be used for calibration in solid sample
analysis is not confirmed in the more recent literature. There are obviously limitations, and there are examples in the literature
where even calibration with certified reference materials did not lead to accurate results. In these cases the problem is
typically associated with spectral interferences that cannot be corrected properly by the systems available for conventional
line source atomic absorption spectrometry, including Zeeman-effect background correction. Using high-resolution continuum
source atomic absorption spectrometry, spectral interferences become visible owing to the display of the spectral environment
at both sides of the analytical line at high resolution, which makes program optimization straightforward. Any spectrally
continuous background absorption is eliminated automatically, and even rapidly changing background absorption does not cause
any artifacts, as measurement and correction of background absorption are truly simultaneous. Any kind of fine-structured
background can be eliminated by “subtracting” reference spectra using a least-squares algorithm. Aqueous standards are used
for calibration in all published applications of high-resolution continuum source atomic absorption spectrometry to direct
solid sample analysis.
This contribution is based on a presentation given at the Colloquium for Analytical Atomic Spectroscopy (CANAS ‘07) held March
18–21, 2007 in Constance, Germany. 相似文献
33.
Garrido JM Marques MP Silva AM Macedo TR Oliveira-Brett AM Borges F 《Analytical and bioanalytical chemistry》2007,388(8):1799-1808
The drugs of abuse cocaine (C), heroin (H), and morphine (M) have been studied to enable understanding of the occurrence of
cocaine–opioid interactions at a molecular level. Electrochemical, Raman, and NMR studies of the free drugs and their mixtures
were used to study drug–drug interactions. The results were analyzed using data obtained from quantum-mechanical calculations.
For the cocaine–morphine mixture (C–MH), formation of a binary complex was detected; this involved the 3-phenolic group and
the heterocyclic oxygen of morphine and the carbonyl oxygen and the methyl protons of cocaine’s methyl ester group. NMR studies
conducted simultaneously also revealed C–MH binding geometry consistent with theoretical predictions and with electrochemical
and vibrational spectroscopy results. These results provide evidence for the occurrence of a cocaine–morphine interaction,
both in the solid state and in solution, particularly for the hydrochloride form. A slight interaction, in solution, was also
detected by NMR for the cocaine–heroin mixture.
Figure "Schematic representation of the proposed model for cocaine:morphine salt interaction" 相似文献
34.
The phenomenon known as non-enzymatic glycation is described as the reaction of reducing sugars with basic amino groups of proteins and nucleic acids, as well as with simple amines, without enzyme mediation. Non-enzymatic model glycation reactions that make use of low-molecular-weight compounds make an important contribution in the elucidation of glicated processes in vitro and in vivo. Four alpha-dicarbonyl compounds, aldehydic (glyoxal, methylglyoxal and phenylglyoxal) and ketonic (diacetyl), were reacted with the modified amino acid N(alpha)-acetyl-L-lysine (AcLys) in an attempt to establish structure/activity relationships for the reactivity of alpha-dicarbonyls with the amine compound. Electrospray ionization mass spectrometry (ESI-MS) combined with tandem mass spectrometry (MS/MS) and collision-induced dissociation (CID) was used to identify and characterize reagents, intermediates and reaction products. The formation of dicarbonyl-derived lysine dimers was observed exclusively. Especially, attention is drawn to alkyl- (asymmetrical dicarbonyl systems) and carboxyl- (glyoxal system) substituted imidazolium ions, at ring position 2.The main differences observed in the reactions studied were related to the reactivity with the diimine intermediate. This intermediate can react either with a non-hydrated dicarbonyl molecule at the aldehydic carbonyl, or with a mono-hydrated one at the ketonic carbonyl, particularly for asymmetrical dicarbonyls. For 2-carboxyl-substituted imidazolium ion (glyoxal reaction), besides the usual keto-enol rearrangement from the diol group, an alternative reaction pathway (proton abstraction) appears to contribute also for the imidazolium ring-closure process. Moreover, the formation of imidazolium ring structures can depend on several factors, namely, the presence (or absence) of electron donor substituents at the formed diol, the degree of stability of the new electrophile generated and/or the equilibrium concentration of the non- and mono-hydrated dicarbonyl forms in solution, the last being particularly important for asymmetrical dicarbonyls.The results reported reveal the complexity of reactivity as well as the diversity of imidazolium molecular structures. 相似文献
35.
Pedro A. Fernandes Elsa S. Henriques Vineet Pande Maria. J. Ramos Ana R. R. Maia André A. S. Almeida Bruno F. B. Silva Carla M. S. Ribeiro César F. B. Ribeiro David S. M. Ribeiro Diana A. P. Fonseca Eva M. S. Cunha Filipe R. N. C. Maia Joana A. A. Pereira João P. G. Pacheco Joaquim A. A. D. Ferreira Liliana R. C. Matos Manuel A. B. P. Pinto Maria C. S. Borges Paulo J. C. R. Magalhães Pedro F. R. D. Teixeira Pedro N. B. C. Veloso Ricarte J. F. Ferreira Sandro S. Gomes Tiago F. Barros Tiago S. J. T. Selão Virgínia M. M. C. Fernandes 《Theoretical chemistry accounts》2005,113(4):197-204
We present a series of new inhibitors of the association between nuclear factor kappa B (NF-B) and the corresponding B site in DNA. They were designed using the lead compound 15-deoxy-12,14 -prostaglandin J2 (PGJ2), which is a natural product with demonstrated inhibitory efficiency for this system. First, the binding mode of PGJ2 to NF-B was unraveled by GOLD docking calculation. Subsequently, substitutions were made to PGJ2 to optimize its association with NF-B. Care was taken not to strongly increase the reactivity of the new compounds, and to keep the overall shape, size and hydrophilicity of the lead compound, which should render them a similar bioavailability. Molecular mechanics calculations were performed to decide on the suitability of the substitutions, and to evaluate the energies of association with NF-B. Density functional theory calculations were performed also to study the overall reactivity of the substituted drugs towards NF-B. Important general conclusions were obtained, concerning the improvement of these natural inhibitors; namely, a set of rational methodologies were deduced to improve the association between the PGJ2 derivatives and NF-B, and their efficiency demonstrated by generating a set of substituted complexes, some of them with a very much increased affinity for NF-B, opening new doors to enlarge the therapeutic capabilities of this class of drugs. 相似文献
36.
Rauter AP Martins A Borges C Ferreira J Jorge J Bronze MR Coelho AV Choi YH Verpoorte R 《Journal of chromatography. A》2005,1089(1-2):59-64
The anti-hyperglycemic flavonoid extract obtained from Genista tenera was first studied by liquid chromatography (LC)-diode array detection (DAD) which showed the presence of two major compounds. One of them was identified as genistein-7-O-glucoside. Luteolin-7-O-glucoside was detected as a minor constituent, while luteolin-7,3'-di-O-glucoside and rutin were found in trace amounts. LC-DAD-ESI-MS and NMR were used to confirm the structure of these compounds and allowed the elucidation of the structure of the unknown major compound, which is the flavonoid 5,7,4'-trihydroxyisoflavone-8-C-glucoside. 相似文献
37.
E.G. Góes P. Nicolucci I.C. Nali C.A. Pelá J.L. Bruço J.C. Borges D.T. Covas 《Radiation Physics and Chemistry》2010,79(6):673-677
Blood irradiation can be performed using a dedicated blood irradiator or a teletherapy unit. A thermal device providing appropriate storage conditions during blood components irradiation with a teletherapy unit has been recently proposed. However, the most appropriated volume of the thermal device was not indicated. The goal of this study was to indicate the most appropriated blood volume for irradiation using a teletherapy unit in order to minimize both the dose heterogeneity in the volume and the blood irradiation time using these equipments. Theoretical and experimental methods were used to study the dose distribution in the blood volume irradiated using a linear accelerator and a cobalt-60 therapy machine. The calculation of absorbed doses in the middle plane of cylindrical acrylic volumes was accomplished by a treatment planning system. Experimentally, we also used cylindrical acrylic phantoms and thermoluminescent dosimeters to confirm the calculated doses. The data obtained were represented by isodose curves. We observed that an irradiation volume should have a height of 28 cm and a diameter of 28 cm and a height of 35 cm and a diameter of 35 cm, when the irradiation is to be performed by a linear accelerator and a cobalt-60 teletherapy unit, respectively. Calculated values of relative doses varied from 93% to 100% in the smaller volume, and from 66% to 100% in the largest one. A difference of 5.0%, approximately, was observed between calculated and experimental data. The size of these volumes permits the irradiation of blood bags in only one bath without compromising the homogeneity of the absorbed dose over the irradiated volume. Thus, these irradiation volumes can be recommend to minimize the irradiation time when a teletherapy unit is used to irradiate blood. 相似文献
38.
39.
J. C. Anjos J. Sá Borges A. P. C. Malbouisson F. R. A. Simão 《Zeitschrift fur Physik C Particles and Fields》1984,23(3):243-245
Critical temperatures forSU(N)-singlet bags are determined and a deconfinement phase transition is exhibited. 相似文献
40.
Margarida S. Miranda José E. Rodríguez‐Borges Joaquim C. G. Esteves da Silva Xerardo García‐Mera 《Journal of Physical Organic Chemistry》2012,25(6):515-522
The cycloaddition between glyoxylate imines possessing two chiral auxiliaries, N‐(R)‐ or N‐(S)‐1‐phenylethyl and 8‐phenylmenthyl or 8‐phenylneomenthyl, and cyclopentadiene is described. Computational calculations using density functional theory with the Becke, three‐parameter, Lee–Yang–Parr functional and the 6‐31G(d) basis set were performed to better understand the highly diastereoselective mechanism and the exo‐selectivity observed experimentally for these ionic aza‐Diels–Alder reactions. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献