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121.
Summary The influence of Cr2O7
2–, MnO4
–, Ce4+, H2O2 and S2O8
2– as oxidising agents in lead determination as volatile covalent hydride using NaBH4 is reported in this paper.The reaction conditions for every oxidising agents (pH, quantity and concentration of the oxidising agent, quantity and concentration of the NaBH4 and reaction time) are optimized, determining sensitivity and detection limit by measuring the peak height.The recovery of lead from solution has been measured by flameless atomic absorption spectroscopy.A linear relationship the logarithm of absorbance values and the redox potential of the system is obtained with a correlation coefficient of 0.999. The results and their interpretation are given in this paper.
Paper presented to Euroanalysis V. Cracow, August 1984. 相似文献
Der Einflu oxidierender Reagentien auf die Bleibestimmung mittels Flammenatomabsorptionsspektrometrie nach Hydridbildung
Zusammenfassung Es wird über den Einfluß der Oxidantien Cr2O7, MnO4 –, Ce4+, H2O2 und S2O8 2 – auf die Bleibestimmung als kovalentes Hydrid berichtet. Für jede oxidierende Substanz werden die Reaktionsbedingungen optimiert (pH, Menge und Konzentration des Oxidans, Menge und Konzentration von NaBH4 und Reaktionszeit), um Empfindlichkeit und Nachweisgrenze durch Peakhöhenauswertung zu bestimmen.Die Wiederfindungsrate wurde durch elektrothermale Atomabsorptionsspektrometrie von Blei in der Lösung bestimmt.Der Zusammenhang zwischen dem Logarithmus der Extinktion und dem Redoxpotential des Oxidans ergibt eine lineare Beziehung mit einem Korrelationskoeffizienten von 0.999. Die Ergebnisse und deren Interpretation werden in dieser Arbeit beschrieben.
Paper presented to Euroanalysis V. Cracow, August 1984. 相似文献
122.
The rotational dynamics of HCl in liquid Ar has been studied by means of molecular-dynamics simulations. We calculate the lifetimes of weakly bound HCl-Ar dimers induced by the anisotropic pair interaction. It is shown that, although lifetimes are small with respect to the reorientational decorrelation, the time interval between the breaking down and formation of the next dimer is negligibly small. Thus, with respect to the rotational dynamics of the probe, the effect is similar to that and eventually would cause a time-stable complex. This provokes a peculiar hindered rotation of the diatomic in the liquid which is macroscopically embodied in the infrared spectrum of the solution as a Q-branch nonexistent otherwise. 相似文献
123.
Carabias-Martínez R Rodríguez-Gonzalo E Domínguez-Alvarez J García Pinto C Hernández-Méndez J 《Journal of chromatography. A》2003,1005(1-2):23-34
A preconcentration study based on the cloud point phenomenon was carried out for a set of triazine herbicides, three of them chloro-substituted and three of them methylthio-substituted. Concentration factors and recoveries were calculated as function of the percentage of the non-ionic surfactant Triton X-114 employed. From these values, obtained from a cloud point extraction (CPE) procedure, the distribution coefficient between the Triton X-114 micelles and water, Kc, prior to CPE was calculated for each triazine and related to the corresponding octanol-water partition coefficient, Kow. In order to confirm the results obtained with the triazine herbicides, two sets of data from chemically different organic pollutants--organophosporous and chlorophenols--obtained from the literature were assessed, concluding that they display a similar behaviour to that of the triazine herbicides. This can be used to predict the CPE behaviour of other organic pollutants from their octanol-water partition coefficients. The Kc values were compared with the analyte concentration ratio in the surfactant-rich phase and aqueous phase (Ksa) with a view to obtaining a link between the analyte behaviour prior to and after cloud point extraction procedures. 相似文献
124.
Samuel González-Mancebo Javier Lacadena Yolanda García-Alonso Jesús Hernández-Benito Emilio Calle Julio Casado 《Monatshefte für Chemie / Chemical Monthly》2002,133(2):157-166
Summary. Nitrosation reactions of phenol, o-cresol, 2,6-dimethylphenol, o-tert-butylphenol, 2-hydroxyacetophenone, and 2-allylphenol in water and water/acetonitrile were studied. Kinetic monitoring of
the reactions was accomplished by spectrophotometric analysis of the nitrosated products at 345 nm. The dominant reaction
was C-nitrosation via a mechanism consisting of an attack on the nitrosatable substrate by NO+/NO2H2
+ followed by a slow proton transfer. The values of the rate constants of phenolic C-nitrosation were increased by electron
donating substituents, and a good Hammett correlation was observed with ρ = −6.1. The results also revealed the strong effect of pH and the permitivity of the reaction medium on the rate constant, whose maximum values were observed for pH ≈ 3, decreasing strongly for higher pH values. The study in water/acetonitrile with up to 25% acetonitrile showed that it is possible to inhibit the reaction strongly
by increasing the percentage of the organic component. The conclusions drawn show that (i) it is possible to predict the rate of nitrosation of phenolics as a function of the meta-substituents on the phenol ring and (ii) the nitrosation of phenolics can be strongly inhibited by increasing the pH of the reaction medium as well as by lowering its dielectric constant.
Received July 13, 2001. Accepted (revised) September 18, 2001 相似文献
125.
Carabias-Martínez R Rodríguez-Gonzalo E Domínguez-Alvárez J Hernández-Méndez J 《Electrophoresis》2002,23(3):494-501
The separation and determination of a mixture of chloro- and methylthiotriazines in water samples by both micellar electrokinetic capillary chromatography (MEKC) and nonaqueous capillary zone electrophoresis (NA-CZE) were compared. The characteristics of both methods proved to be very similar in terms of separation efficiency and analysis times, but application of these methods for the analysis of triazines in natural waters, with a prior preconcentration step, revealed significant differences. A preconcentration step by solid-phase extraction (SPE) with Oasis HLB cartridges was accomplished for the determination of triazines at sub-ppb levels in drinking and river waters; when NA-CZE was used after this SPE step, electropherograms with fewer interferences and more stable baselines were obtained than when separation was carried out using MEKC. Another aspect related to the application to real samples was the lack of precision encountered upon evaluating the electrophoretic signals generated when using SPE coupled with NA-CZE. Here, we demonstrate the importance of choosing an appropriate internal standard for analyte quantification. It is recommended that a triazine belonging to the same family as that of the triazine to be determined should be used as internal standard. 相似文献
126.
Curutchet C Bofill JM Hernández B Orozco M Luque FJ 《Journal of computational chemistry》2003,24(10):1263-1275
This study examines the contribution of electrostatic and polarization to the interaction energy in a variety of molecular complexes. The results obtained from the Kitaura-Morokuma (KM) energy decomposition analysis at the HF/6-31G(d) level indicate that, for intermolecular distances around the equilibrium geometries, the polarization energy can be determined as the addition of the polarization energies of interacting blocks, as the mixed polarization term is typically negligible. Comparison of KM and QM/MM results shows that the electrostatic energy determined in the KM method is underestimated (in absolute value) by QM/MM methods. The reason of such underestimation can be attributed to the simplified representation of treating the interaction between overlapping charge distribution by the interaction of a QM molecule with a set of point charges. Nevertheless, the polarization energies calculated by KM and QM/MM methods are in close agreement. Finally, a consistent, automated strategy to derive charge distributions that include implicitly polarization effects in pairwise, additive force fields is presented. The strategy relies in the simultaneous fitting of electrostatic and polarization energies computed by placing a suitable perturbing particle at selected points around the molecule. The suitability of these charges to describe molecular interactions is discussed. 相似文献
127.
Dai QH Tommos C Fuentes EJ Blomberg MR Dutton PL Wand AJ 《Journal of the American Chemical Society》2002,124(37):10952-10953
The use of side chains as catalytic cofactors for protein mediated redox chemistry raises significant mechanistic issues as to how these amino acids are activated toward radical chemistry in a controlled manner. De novo protein design has been used to examine the structural basis for the creation and maintenance of a tryptophanyl radical in a three-helix bundle protein maquette. Here we report the detailed structural analysis of the protein by multidimensional NMR methods. An interesting feature of the structure is an apparent pi-cation interaction involving the sole tryptophan and a lysine side chain. Hybrid density functional calculations support the notion that this interaction raises the reduction potential of the W degrees /WH redox pair and helps explain the redox characteristics of the protein. This model protein system therefore provides a powerful model for exploring the structural basis for controlled radical chemistry in protein. 相似文献
128.
R. M. Masegosa M. A. Llorente M. G. Prolongo I. Hernández-Fuentes A. Horta 《Colloid and polymer science》1984,262(11):862-866
Measurements of preferential sorption,(
3), determined by refractometry, and swelling equilibrium,
3
–1, of PMMA networks have been carried out in the cosolvent mixture MeCN+BuOH at 25 and 49 C. With an intermediate mixture composition,
3
–1 passes through a maximum at both temperatures. At 25 C MeCN is preferentially adsorbed by the network over most of the composition range, but a small inversion is detected. At 49 C MeCN is preferentially adsorbed over all the composition range.The behavior of the system crosslinked PMMA/MeCN + BuOH is compared with the results obtained for solutions of linear PMMA in mixtures formed by the same two solvents, MeCN and BuOH. 相似文献
129.
Elena E. Ferapontova John Castillo Dmitri Hushpulian Vladimir Tishkov Tatiana Chubar Irina Gazaryan Lo Gorton 《Electrochemistry communications》2005,7(12):1291-1297
Direct electron transfer (DET) reactions of recombinant tobacco peroxidase (rTOP), namely direct electroreduction of Compound I/Compound II and heme Fe3+/2+ conversion, were studied on gold electrodes. rTOP of wild type, non-glycosylated, was produced using an Escherichia coli expression system. At pH 5.0, the redox potential for direct electrochemical transformation of the Fe3+/2+ of the peroxidase heme was −143 mV vs. AgAgCl, and 0.26 ± 0.07 pmol of the adsorbed rTOP were in DET contact with the gold electrode. The total amount of the adsorbed rTOP estimated from QCM data was 53 ± 5 pmol/cm2 or 1.67 pmol when referred to the surface area of the electrodes used for electrochemical measurements. Of 1.67 pmol of adsorbed rTOP, only 0.76 pmol were catalytically active. DET between Au and the enzyme was also studied in the reaction of the bioelectrocatalytic reduction of H2O2 by cyclic voltammetry and amperometric detection of H2O2 at +50 mV with rTOP-modified Au electrodes placed in a wall-jet flow-through electrochemical cell. Maximal bioelectrocatalytic current response of the rTOP-modified gold electrodes to H2O2 was observed at pH 5.0 and stemmed from its bioelectrocatalytic reduction based on DET between Au and the active site of rTOP. Kinetic analysis of the DET reactions gave 52% of the adsorbed rTOP molecules active in DET reactions (0.4 pmol of adsorbed catalytically active rTOP, correspondingly), which correlated well with the non-catalytic-voltammetry data. DET was characterised by a heterogeneous ET rate constant of 13.2 s−1, if one takes into account the QCM data, and 19.6 s−1, if the amount of rTOP estimated from the data on DET transformation of Fe3+/2+ couple of rTOP is considered. The sensitivity for H2O2 obtained for the rTOP-modified Au electrodes was 0.7 ± 0.1 A M−1 cm−2. These are the first ever-reported data on DET reactions of anionic plant peroxidases on bare gold electrodes. 相似文献
130.
Ernesto R. González 《Journal of Electroanalytical Chemistry》1978,90(3):431-438
The adsorption of azide ions on mercury from aqueous solutions of NaN3+NaF at constant ionic strength has been studied by measurements of the double layer capacity. Taking into account changes in the structure of the diffuse layer, it was possible to show that the properties of the inner layer are similar to those observed previously in solutions of pure NaN3, and that the same adsorption isotherm applies to both systems. 相似文献