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61.
The concentration dependence of the205Tl chemical shifts of Tl+ and of (CH3)2Tl+ ions was determined in several solvents with NO
3
–
and ClO
4
–
counterions. In general, increased ion pairing caused a low-frequency shift of the205Tl resonance, with the exceptions of (CH3)2TlNO3 inn-butylamine and TlNO3 in N,N-dimethylformamide (DMF) and in hexamethylphosphorotriamide (HMPA). In HMPA,205Tl linewidths of both Tl+ and (CH3)2Tl+ increased dramatically with dilution below 0.1M. Analysis of the data allowed ion-pair formation constants and205Tl chemical shifts for the ion-paired cation and for the free (solvated) cation to be estimated for some of the solvents. 相似文献
62.
63.
64.
Blandamer M. J. Briggs B. Cullis P. M. Last P. Engberts J. B. F. N. Kacperska A. 《Journal of Thermal Analysis and Calorimetry》1999,55(1):29-35
The gel to liquid-crystal transition for vesicles in aqueous solution formed by dimethyldi-n-octadecylammonium bromide (DOAB)
occurs at 44.7°C. Moreover, the shapes of the scans recorded by a sensitive DSC microcalorimeter are very similar when the
vesicular solutions are prepared starting with solid DOAB and comparable amounts of either solid urea or solid alkylureas.
Therefore, the DOAB vesicles in aqueous solution accommodate this class of solutes without marked changes in the melting temperature
and the enthalpy of the transition. The contrast with effects of added surfactants and simple organic solutes such as THF
and ethanol is particularly significant.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
65.
66.
C. C. Briggs T. H. Lilley J. Rutherford S. Woodhead 《Journal of solution chemistry》1974,3(8):649-658
The activity coefficients of calcium chloride in aqueous solutions of the amino acids glycine, β-alanine, γ-amino butyric acid, ε-aminocaproic acid, and glycylglycine have been determined with cells utilizing a calcium ion-exchange electrode. Application of the cross-differential relationship allows a measure of the variation of the salting-in parameter across the series of amino acids. The variation of this parameter is discussed using a semiempirical approach developed earlier. 相似文献
67.
Mercury electrodes are oxidized in the presence of ethynyloestradiol, a synthetic steroid. The direct polarographic response obtained is based on the formation of an insoluble mercury compound. In analytical applications of this response, interference from laevonorgestrel, which is found in extracts from combined oral contraceptives, renders the direct determination of ethynyloestradiol impossible. However, reverse-phase liquid chromatography provides separation of the two steroids prior to electrochemical detection of both species at either static or dropping mercury electrodes. Linear response is obtained for 2.5–15.0 μg ml-1 ethynyloestradiol and 12.5–50 μg ml-1 laevonorgestrel with 20-μl injections. A microprocessor-based waveform generator is used to optimize the electro-chemical detection. The method is applied to pharmaceutical formulations; the data obtained agree satisfactorily with the nominal contents. Cathodic stripping voltammetry is briefly investigated for very low levels of ethynyloestradiol; the detection limit is around 5 × 10-9 M on standard solutions. 相似文献
68.
The adsorption of Na and the coadsorption of Na and O2 on Ag(110) have been studied by XPS and UPS. Adsorption of Na results in a rapid decrease in the work function. Δφ reaching a limiting value of ?2.0 eV at θNa = 0.5 and thereafter remaining constant. In the coverage range 0 < θNa <1 adsorption of O2 onto the Na dosed surface always results in an increase in the work function to an almost constant value of Δφ ~ ?1 eV. At the same time the XPS data show that the Na/O stoichiometry of the oxygen saturated surface remains essentially constant and independent of the initial Na dose. Calibration experiments using sodium formate as a standard compound indicate that this surface phase has the stoichiometry Na2O. For θNa > 1 there is a sharp change in behaviour; the work function of the oxygen saturated surface begins to decrease rapidly, and eventually falls below the value for the Na covered surface itself at θNa ~ 1.5. The XP spectrum now shows the appearance of a new oxygen peak which increases in intensity as θNa increases beyond unity. The UP spectra indicate that the binding energy of the surface orbital derived from Na (3s) is increased by ~6.7 eV as compared with the free atom value, and the emission at ~3 eV below EF, which is associated with surface oxygen is not greatly affected by the presence of Na. These results are discussed against the background of information already available from LEED, Auger, and thermal desorption studies, and we attempt to give a consistent interpretation of the properties of the system at coverages both below and above one monolayer. 相似文献
69.
Caroline R.S. Briggs David O’Hagan Judith A.K. Howard 《Journal of fluorine chemistry》2003,119(1):9-13
The conformation of 2-fluoro-N-(2-fluoroethyl)-propionamide 4 in the solid state indicates the influence of both a β-fluorine-amide gauche effect and an α-fluoroamide effect. The structure reveals the influence of two recently observed stereoelectronic effects associated with the CF bond, which has resulted in the successful prediction of the solid state conformation of amide 4. A gauche relationship (−69.9°) was observed for atoms N(1)C(4)C(5)F and a syn planar (2.0°) relationship was observed for N(1)C(3)C(2)F. The paper demonstrates the predictive power of using the CF bond as a tool in influencing the conformation of amides and peptides. 相似文献
70.
Experimental, molecular modeling, and model compound studies suggest that favorable fullerene[60] pi-stacking interactions in the ground state and in syn-transition states account for the high cis stereoselectivities observed in the reactions between C(60) and 6, 13-disubstituted pentacenes. 相似文献