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81.
Makowska J Bagiñska K Makowski M Jagielska A Liwo A Kasprzykowski F Chmurzyñski L Scheraga HA 《The journal of physical chemistry. B》2006,110(9):4451-4458
We compared the ability of two theoretical methods of pH-dependent conformational calculations to reproduce experimental potentiometric titration curves of two models of peptides: Ac-K5-NHMe in 95% methanol (MeOH)/5% water mixture and Ac-XX(A)7OO-NH2 (XAO) (where X is diaminobutyric acid, A is alanine, and O is ornithine) in water, methanol (MeOH), and dimethyl sulfoxide (DMSO), respectively. The titration curve of the former was taken from the literature, and the curve of the latter was determined in this work. The first theoretical method involves a conformational search using the electrostatically driven Monte Carlo (EDMC) method with a low-cost energy function (ECEPP/3 plus the SRFOPT surface-solvation model, assumming that all titratable groups are uncharged) and subsequent reevaluation of the free energy at a given pH with the Poisson-Boltzmann equation, considering variable protonation states. In the second procedure, molecular dynamics (MD) simulations are run with the AMBER force field and the generalized Born model of electrostatic solvation, and the protonation states are sampled during constant-pH MD runs. In all three solvents, the first pKa of XAO is strongly downshifted compared to the value for the reference compounds (ethylamine and propylamine, respectively); the water and methanol curves have one, and the DMSO curve has two jumps characteristic of remarkable differences in the dissociation constants of acidic groups. The predicted titration curves of Ac-K5-NHMe are in good agreement with the experimental ones; better agreement is achieved with the MD-based method. The titration curves of XAO in methanol and DMSO, calculated using the MD-based approach, trace the shape of the experimental curves, reproducing the pH jump, while those calculated with the EDMC-based approach and the titration curve in water calculated using the MD-based approach have smooth shapes characteristic of the titration of weak multifunctional acids with small differences between the dissociation constants. Nevertheless, quantitative agreement between theoretically predicted and experimental titration curves is not achieved in all three solvents even with the MD-based approach, which is manifested by a smaller pH range of the calculated titration curves with respect to the experimental curves. The poorer agreement obtained for water than for the nonaqueous solvents suggests a significant role of specific solvation in water, which cannot be accounted for by the mean-field solvation models. 相似文献
82.
Franciszek Grabowski 《Physica A》2010,389(16):3081-1286
The paper is concerned with the new logistic equation of arbitrary order which describes the performance of complex executive systems X vs. number of tasks N, operating at limited resources K, at non-extensive, heterogeneous self-organization processes characterized by parameter f. In contrast to the classical logistic equation which exclusively relates to the special case of sub-extensive homogeneous self-organization processes at f=1, the proposed model concerns both homogeneous and heterogeneous processes in sub-extensive and super-extensive areas. The parameter of arbitrary order f, where −∞<f<+∞, depends on both the coefficient of external resource utilization u=N/K, where 0<u<1, and the internal microscopic character of realized processes related to the depth of feedback β. The coefficient β directly influences self-organization of processes by the change of microscopic parameters Vi, Si, i and Z, where Vi is the number of references (visit) to the ith component of the system during the service of each task, Si is the time of serving the task by the ith component, and Z is the think time of a given process. In the general case of complex system, parameters Vi, Si, i and Z can have values in the range from 0 to +∞. In this way the new equation includes all possible cases of a complex executive system’s operation. Furthermore, it allows us to define the optimal matching point between X and N with f as the parameter. It also helps to balance the load in complex systems and to equip artificial systems with self-optimization mechanisms similar to those observed in natural systems. 相似文献
83.
84.
A model for the adsorption of poly(γ-benzyl-L-glutamate) from solution onto alkali-halide substrates
The isothermal adsorption of high-and low-molecular-weight poly(γ-benzyl-L-glutamate) (PBLG) onto (100) alkali-halide surfaces has been studied with the intent of developing a model for the adsorption process. Data from both the solution and adsorbed phases were correlated in this study. The α-helical conformation was maintained in solution and on the substrate for all molecular weights investigated. Fourier-transform infrared spectroscopy proved to be a sensitive technique for conformational analysis of adsorbed PBLG films as thin as 200 Å. High-molecular-weight PBLG was adsorbed as an interwoven fibrous mat, homogeneously covering the entire substrate surface. This morphology was independent of the substrate. Adsorption of low-molecular-weight PBLG took on a lamellar morphology, often exhibiting epitaxial orientation in the substrate 〈100〉 directions. Quasielastic laser light scattering studies showed that low-molecular-weight PBLG had a much higher degree of aggregation in nonpolar solvents than did high-molecular-weight PBLG. The proposed adsorption model is essentially a modification of one suggested by Silberberg. It involves segmental adsorption of aggregates, as opposed to single macromolecules of PBLG. The unadsorbed segments of the aggregates form a concentrated interfacial region directly above the substrate. Further interaction and association of PBLG may occur in this region, thus reducing the epitaxial ordering influence of the substrate. Consequently, polymers which do not interact strongly in the concentrated interfacial region, such as polyethylene, are expected to be adsorbed epitaxially, as has been reported. 相似文献
85.
Maria Gdaniec Franciszek Sączewski Tomasz Dębowski 《Journal of chemical crystallography》1995,25(12):813-821
Crystals consisting of two distinct chemical entities, tautomers of each other, in exact 11 ratio, have been obtained and their structure determined by X-ray analysis. The crystals of C9H11N3·C9H11N3 are monoclinic,P21/c,a=15.674(3),b=17.085(3),c=13.758(3)Å, =90.78(2)°,Z=8. There are two hydroxylamine and two aminonitrone molecules in the asymmetric unit. Hydrogen bonds connect those molecules into chiral layers. Layers of opposite chirality alternate andthe crystal is centrosymmetric as a whole. Within those layers chains of tautomers joined by very strong O–H... O and strong N–H... N bonds can be recognized. Proton transfer along those chains with simultaneous rearrangement of -bonds within the molecules would result in interconversion of tautomers and would affect chirality of the layer. 相似文献
86.
2-hydroxylamino-4,5-dihydroimidazolium-O-sulfonate (1) has been prepared by reacting 2-chloro-4,5-dihydroimidazole with hydroxylamine-O-sulfonic acid. Deprotonated compound 1a containing both the nucleophilic endocyclic nitrogen atoms and electrophilic exocyclic nitrogen was used for the syntheses of 3-substituted 6,7-dihydro-5H-imidazo[2,1-c][1,2,4]oxadiazoles 2-9 and 6,7-dihydro-5H-imidazo[2,1-c][1,2,4]thiadiazole-3-thione (10) by tandem nucleophilic addition-electrophilic amination reaction. The method promises utility in the synthesis of a variety of other heterocycles. On the other hand, the convenient routes to 7,8-dihydroimidazo[1,2-c][1,3,5]thiadiazine-2,4(6H)-dithione (16) and 2,6,7,8-tetrahydroimidazo[1,2-a][1,3,5]triazine-4(3H)-thione derivative (17) are reported starting from compound 1. The structures of the compounds prepared were established by elemental analyses, IR, NMR, and MS spectra, and in some instances X-ray analyses. 相似文献
87.
Franciszek Warkusz 《Progress in Surface Science》1980,10(3):287-382
A general expression for the function of electron scattering in thin films of a grain structure is derived which takes account of both the external and internal size effects. Limiting and particular values of this function are given for polycrystalline and single-crystal films. Theoretical results are compared to the experimental results obtained for aluminium and tin. The relationship between size effects and resistivity, temperature coefficient of resistance, longitudinal and transverse strain coefficient of resistance and thermoelectric power is studied. The dependence of the orientation of monocrystalline copper and aluminium films on thermal stress, the direction of the energy density of elastic strains and the anistropy of elastoresistance coefficients in these films are examined. An expression for the gauge factor in single-crystal metal films of given orientations is derived. 相似文献
88.
Winkler W Buhl F Arenhövel-Pacuła A Hachuła U 《Analytical and bioanalytical chemistry》2003,376(6):934-937
A simple and very sensitive method for the spectrophotometric determination of manganese in pharmacological preparations and vegetable fertilizers is proposed. The method is based on the formation of a blue coloured complex of Mn (II) with 9-phenyl-2,3,7-trihydroxy-6-fluorone (PF) in the presence of cetylpyridinium chloride (CP) and Triton X-100. Optimum concentrations of PF, CP, Triton X-100 and pH ensuring maximum absorbance were defined. The complex Mn(II)-PF-CP-Triton X-100 shows maximum absorbance at 591 nm with the molar absorptivity value 1.77x10(5 )L mol(-1 )cm(-1). The detection limit of the method is 0.004 microg mL(-1). The Beer's law is obeyed for manganese concentrations in the range 0.02-0.2 microg mL(-1). The effect of foreign ions was elucidated. The statistical evaluation of the method was carried out for six determination using 5 microg Mn and the following results were obtained: standard deviation 0.021, confidence interval 5.05+/-0.05 microg Mn. The method has been applied for the determination of manganese in pharmacological preparations (Biovital, Kinder Biovital) and vegetable fertilizers (Hydrovit 100, Florovit). 相似文献
89.
Zbigniew Gregorowicz Franciszek Buhl und Barbara Piwowarska 《Fresenius' Journal of Analytical Chemistry》1962,188(1):2-6
Zusammenfassung Es wird ein volumetrisches Verfahren zur Bestimmung von Rhodanid mit Quecksilber(II)-nitratmaßlösung gegen Variaminblau als Redoxindicator angegeben. Auch Quecksilber(II)-ionen können bestimmt werden, wenn man mit einem Überschuß an Rhodanid versetzt und mit der Maßlösung zurücktitriert. Die Methode liefert genaue und wenig streuende Resultate.
I. Mitteilung: Gregorowicz, Z., u. F. Buhl: diese Z. 173, 115 (1960). 相似文献
Summary Thiocyanate can be determined by titration with mercuric nitrate standard solution using Variamine blue as redox indicator. The determination of mercuric ions is possible by adding an excess of thiocyanate and back titrating with the standard solution. Results are exact and deviations are small.
I. Mitteilung: Gregorowicz, Z., u. F. Buhl: diese Z. 173, 115 (1960). 相似文献
90.