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排序方式: 共有395条查询结果,搜索用时 15 毫秒
111.
Andrés Guerrero Rebeca Herrero Esther Quintanilla Dr. Juan Z. Dávalos Dr. José‐Luis M. Abboud Prof. Dr. Pedro B. Coto Dr. Dieter Lenoir Prof. Dr. 《Chemphyschem》2010,11(3):713-721
We show that the radical cations of adamantane (C10H16.+, 1 H.+) and perdeuteroadamantane (C10D16.+, 1 D.+) are stable species in the gas phase. The radical cation of adamantylideneadamantane (C20H28.+, 2 H.+) is also stable (as in solution). By using the natural 13C abundances of the ions, we determine the rate constants for the reversible isergonic single‐electron transfer (SET) processes involving the dyads 1 H.+/ 1 H, 1 D.+/ 1 D and 2 H.+/ 2 H. Rate constants for the reaction 1 H.++ 1 D? 1 H+ 1 D.+ are also determined and Marcus’ cross‐term equation is shown to hold in this case. The rate constants for the isergonic processes are extremely high, practically collision‐controlled. Ab initio computations of the electronic coupling (HDA) and the reorganization energy (λ) allow rationalization of the mechanism of the process and give insights into the possible role of intermediate complexes in the reaction mechanism. 相似文献
112.
Joaquín García Gloria Subías Vera Cuartero Javier Herrero‐Martin 《Journal of synchrotron radiation》2010,17(3):386-392
Here the correlation between the chemical shift in X‐ray absorption spectroscopy, the geometrical structure and the formal valence state of the Mn atom in mixed‐valence manganites are discussed. It is shown that this empirical correlation can be reliably used to determine the formal valence of Mn, using either X‐ray absorption spectroscopy or resonant X‐ray scattering techniques. The difficulties in obtaining a reliable comparison between experimental XANES spectra and theoretical simulations on an absolute energy scale are revealed. It is concluded that the contributions from the electronic occupation and the local structure to the XANES spectra cannot be separated either experimentally or theoretically. In this way the geometrical and electronic structure of the Mn atom in mixed‐valence manganites cannot be described as a bimodal distribution of the formal integer Mn3+ and Mn4+ valence states corresponding to the undoped references. 相似文献
113.
The “pinking” phenomenon which is known to occur in white PVC profiles has been investigated in detail in terms of test evaluations and analysis methods for determining the mechanism and nature of the species responsible. An artificial weathering test method using a Microscal unit with submersion of samples in water was established where colour measurements and FTIR analysis proved the similarity of functional groups present to those in naturally pinked samples. Analysis of the artificially pinked samples using hydroperoxide concentration analysis, fluorescence spectroscopy, microscopic FTIR and scanning electron microscopy showed that artificially pinked samples presented a more extended degree of degradation than samples that remained white during the exposure in the Microscal Unit. Short chain conjugated polyenes were also identified as the most likely functional groups responsible for the development of the pink colour and interactions between the titania pigment and the lead stabilisers would seem to have an influence in the pinking development. In addition, it was found that there was an increase in production of hydroperoxy radicals at the onset of colour development. Addition of trace lead dioxide accelerated the production of pinking. Different titanium dioxide grades used in the PVC profiles showed differences in pinking in the artificial test, also an increase in the titanium dioxide concentration decreased the formation of the pinking colouration in artificial conditions The use of different analysis methods (Microwave Dielectric Spectroscopy and ESR) showed differences in carrier radical production and carrier radical trapping between the pigments used suggesting that a small difference in the coatings could play an important part in the pinking phenomenon. Finally, a mechanism was proposed involving a redox reaction between the titanium dioxide and the lead stabiliser that could facilitate the production of short polyene chains in the PVC matrix producing the pink colouration. 相似文献
114.
Juan José Baeza‐Baeza Virginia Bernabé‐Zafón José Manuel Herrero‐Martínez Ernesto F. Simó‐Alfonso 《Electrophoresis》2010,31(6):1003-1010
A theoretical study on the retention behaviour and chromatographic performance of neutral solutes using a lauryl methacrylate‐based monolithic column under voltage gradient mode in CEC was carried out. Through a flexible mathematical function based on a modified Gaussian model, the peak shape of compounds was firstly fitted under constant and gradient voltage. Using the peak shape parameters and retention time, the estimation of global chromatographic performance, efficiency and peak capacity under several voltage conditions was performed. The influence of voltage gradient on the separation efficiency is discussed and simple equations are presented to calculate retention and peak widths under voltage gradient conditions. A comparison in terms of chromatographic performance of a test mixture of neutral solutes under constant and gradient voltage modes was also carried out. The experiments carried out under gradient voltage showed better efficiencies (172 000 plates/m) and lower peak widths than those obtained under constant voltage (52 000 plates/m). 相似文献
115.
In this contribution, the performance of three different extraction procedures towards the extraction of antioxidants from rosemary (Rosmarinus officinalis) is presented. Namely, pressurized liquid extraction (PLE), using water and ethanol as solvents, supercritical fluid extraction (SFE), using neat CO2 and supercritical CO2 modified with ethanol, as well as a novel extraction process called Water Extraction and Particle formation On-line (WEPO) are directly compared. Different extraction conditions including temperatures, times and pressures have been studied. The produced extracts have been characterized in terms of extraction yield, antioxidant activity (using the DPPH radical scavenging method) and total phenols (using the Folin method). Besides, all the extracts have been chemically characterized using a new quantitative UPLC-MS/MS method. This method allowed the determination of the main antioxidants present in rosemary, including, among others, rosmarinic acid, carnosic acid and carnosol, attaining detection limits as low as 2 ng/mL. The results obtained in this study show that PLE using ethanol at high temperatures (200 °C) was able to produce extracts with high antioxidant activity (EC50 8.8 μg/mL) and high yield (ca. 40%) while efficiently extracting antioxidants of diverse polarity, among them, carnosic and rosmarinic acids, regarded as the most important antioxidants present in rosemary. Nevertheless, in this work, the ability of the three studied environmentally friendly extraction techniques to obtain bioactives from natural sources is demonstrated. 相似文献
116.
A practical, brief and selective synthesis of limonoid CDE fragments from a readily available starting diketone is described. The key step is a cationic electrocyclization promoted by strong acids. In general the methodology has been demonstrated for compounds with sensitive furane and thiophene substituents to obtain diverse substituted indenones. Several of the compounds obtained show significant antifeedant activity against Spodoptera littoralis. 相似文献
117.
The determination of metoclopramide hydrochloride is spectrophotometrically determined by the Bratton-Marshall method in a flow injection assembly. The required nitrite is prepared on-line in the flow assembly by reducing a nitrate solution with the aid of a copperised cadmium solid-phase reactor. The calibration graph is linear over the range 0.5-85 mg l(-1), with a relative standard deviation (RSD) of 0.89%, and sample throughput of 51 samples h(-1). The method is easy and simple, and it is applied to determination of metoclopramide in some pharmaceutical formulations. The method eliminates the need for frequent preparation of unstable nitrite solutions. 相似文献
118.
A generalization of the Swift-Hohenberg (SH) equation is used to study several stationary patterns that appear in hydrodynamical instabilities. The corresponding amplitude equations allow one to find the stability of planforms with different symmetries. These results are compared with numerical simulations of a generalized SH equation (GSHE). The transition between different symmetries, the hysteretic effects, and the characteristics of the defects observed in experiments are well reproduced in these simulations. The existence of steady fronts between domains with different symmetries is also analyzed. Steady domain boundaries between hexagons and rolls, and between hexagons and squares are possible solutions in the amplitude equation framework and are obtained in numerical simulations for a full range of coefficients in the GSHE. 相似文献
119.
Multicriteria optimization, widely used in engineering, does not much used in the optimization of analytical signals. The aim of this paper is to show the usefulness of the desirability function to optimize instrumental responses obtained in instrumental analysis. The simultaneous optimization of a signal and of its variability is a generic question of interest to any chemical analyst. It is clear that the improvement of the two responses forms the basis of the validation of any analytical method, and affects all the figures of merit: accuracy (trueness and precision), capability of detection, robustness, sensitivity, etc. Furthermore, in the specific case of electroanalysis, an improvement in the signal may implicitly mean an increase of the signal in the blank, such that the “net signal” may not improve. This experimental approach (surface response methodology plus desirability) to multicriteria optimization has been applied to three cases of growing complexity. Thus, in the determination of Cu(II) by differential pulse anodic stripping voltammetry the simultaneous maximization of the peak current and minimization of its standard deviation is looked for. Whereas, in the determinations of Ni(II) and indomethacin by differential pulse adsorptive stripping voltammetry, the simultaneous maximization of the peak current and minimization of the blank signal is desired. In all the cases, the experimental conditions where the optima are found for each individual response are just opposite, so it is required to look for a certain compromise, that is achieved using the desirability function. 相似文献
120.
Chemometric characterisation of Basque and French ciders according to their polyphenolic profiles 总被引:3,自引:0,他引:3
Alonso-Salces RM Guyot S Herrero C Berrueta LA Drilleau JF Gallo B Vicente F 《Analytical and bioanalytical chemistry》2004,379(3):464-475
Polyphenolic compositions of Basque and French ciders were determined by HPLC-DAD following thiolysis, in order to characterise and differentiate these beverages and then develop a classification system capable of confirming the authenticities of both kinds of cider. A data set consisting of 165 cider samples and 27 measured features was evaluated using multivariate chemometric techniques, such as cluster analysis and principal component analysis, in order to perform a preliminary study of data structure. Supervised pattern recognition techniques such as linear discriminant analysis (LDA), K-nearest neighbours (KNN), soft independent modelling of class analogy (SIMCA), and multilayer feed-forward artificial neural networks (MLF-ANN) attained classification rules for the two categories using the chemical data, which produced satisfactory results. Authentication systems obtained by combining two of these techniques were proposed. We found that SIMCA and LDA or KNN models achieved 100% hit-rates, since LDA and KNN permit the detection of every Basque cider and SIMCA provides a model for Basque cider that excludes all French ciders. Polyphenolic profiles of the ciders provided enough information to be able to develop classification rules for identifying ciders according to their geographical origin (Basque or French regions). Chemical and organoleptic differences between these two types of cider are probably due to the original and distinctive cidermaking technologies used for their elaboration. Using polyphenic profiles, about 80% of French ciders could be distinguished according to their region of origin (Brittany or Normandy). Although their polyphenolic profiles did not provide enough information to achieve an authentication system for Breton and Norman ciders.Abbreviations AVI
Avicularin - CQA
Caffeoylquinic acid - CAF
Caffeic acid - CAT
(+)-catechin - CT-1, -2, -3
Unknown flavan-3-ols - DPn
Average degree of polymerization of procyanidins - EC
(–)-epicatechin - HCA-7
Ferulic acid - HCA-1, -2 ,-3, 4, -5, -6
Unknown hydroxycinnamic acids - HYP
Hyperin - IQC
Isoquercitrin - PC
Total procyanidins - PCM
p-Coumaric acid - PCQ
p-Coumaroylquinic acid - PL
Phloretin - PLG
Phloridzin - PLXG
phloretin-2-O-xyloglucoside - PPO
Polyphenoloxidase - QCE
Quercetin - QCI
Quercitrin - RUT
Rutin - CA
Cluster analysis - KNN
K-nearest neighbours - LDA
Linear discriminant analysis - MLF-ANN
Multilayer feed-forward-artificial neural network - PCA
Principal component analysis - PC1
First principal component - PC2
Second principal component - PC3
Third principal component - RMSE
Root medium square error - SD
Standard deviation - SIMCA
Soft independent modelling of class analogy - DAD
Diode array detector - HPLC
High Performance Liquid Chromatography - ND
Not detected 相似文献