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221.
Bartolomê Vilanova Josefa Donoso Francisco Muoz Francisco García-Blanco 《Helvetica chimica acta》1996,79(7):1793-1802
The degradation of cefaclor ( 1 ), an oral cephalosporin antibiotic, was studied at 37° in a neutral aqueous medium by HPLC and 1H-NMR. Under these conditions, 1 underwent intramolecular aminolysis by the 7-side-chain NH2 group on the β-lactam moiety to give a piperazine-2,5-dione. The most prominent peak in the HPLC profile of a degradation solution from 1 was isolated by prep. HPLC. Mechanistically, the formation of this degradation product cis- 11 from 1 involves the contraction from a six-membered cephem ring to a five-membered ring, which presumably takes place via a common episulfonium ion intermediate 9 (see Scheme). Loss of the Cl-atom from 3-chloro-3-cephem is a general reaction subsequent to β-lactam ring opening. 相似文献
222.
A. Mndez-Vilas M.G. Donoso J.L. Gonzlez-Carrasco M.L. Gonzlez-Martín 《Colloids and surfaces. B, Biointerfaces》2006,52(2):157
Surface topography of polished and blasted samples of a Ti6Al4V biomaterial has been studied using an atomic force microscope. Surface RMS roughness and surface area have been measured at different scales, from 1 to 50 μm, while at distances below 10 μm the surface RMS roughness in both kinds of samples is not very different, this difference becomes significant at larger scanning sizes. This means that the surface roughness scale that could have a main role in cell adhesion varies depending on the size, shape and flexibility of participating cells. This consideration suggests that in cell–material interaction studies, surface roughness should not be considered as an absolute and independent property of the material, but should be measured at scales in the order of the cell sizes, at least if a microscopic interpretation of the influence of roughness on the adhesion is intended. The microscopic information is contrasted with that coming from a macroscopic approach obtained by contact angle measurements for polar and non-polar liquids whose surface tension is comprised in a broad range. Despite the very large differences of contact angles among liquids for each surface condition, a similar increase for the blasted surface with respect to the polished has been found. Interpretation of these results are in accordance with the microscopic analysis done through the use of a functional roughness parameter, namely the valley fluid retention index, evaluated from the AFM images, which has been shown not to correlate with the RMS roughness, one of the most commonly used roughness parameter. 相似文献
223.
Juan Frau Josefa Donoso Francisco Muoz Francisco Gracia-Blanco 《Helvetica chimica acta》1996,79(2):353-362
We used semi-empirical and ab initio calculations to investigate the nucleophilic attack of the OH? ion on the β-lactam carbonyl group. Both allowed us to detect reaction intermediates pertaining to proton-transfer reactions rather than the studied reaction. We also used the PM3 semi-empirical method to investigate the influence of the solvent on the process. The AMSOL method predicts the occurrence of a potential barrier of 20.7 kcal/mol due to the desolvation of the OH? ion in approaching the β-lactam carbonyl group. Using the supermolecular approach and a H2O solvation sphere of 20 molecules around the solute, the potential barrier is lowered to 17.5 kcal/mol, which is very close to the experimental value (16.7 kcal/mol). 相似文献
224.
Water clusters (H2On and (D2On (n相似文献
225.
Calorimetric energy measurements associated with the different peaks involved during the linear heating of Cu-20 at.% Mn were
performed employing cold-rolled and quenched materials. Unlike the situation in the quenched alloy in which disperse-short-range
order (DSRO) is developed, in the deformed alloy such a process is inhibited by the segregation of solute atoms to partial
dislocations. This conclusion was drawn by determining the dislocation density in the deformed alloy on the basis of expressions
governing the energy release which accompanies the pinning process. After the computed value for the dislocation density had
been employed to evaluate the energy evolved during recrystallization, it is verified that this energy does not differ much
from the experimentally measured value. The above interpretation of the behaviour exhibited by this alloy is also supported
by kinetics and solute balance analyses. After tensile tests performed in the deformed alloy subjected to a low thermal heat
treatment at 423 K for 30 min, an increase in 0.2 offset YS (yield stress) of 25 and 21% in UTS (ultimate tensile strength) was achieved, if compared with those values corresponding to the deformed alloy without heat
treatment. However, the as quenched alloy subsequently heat treated at 560 K for 30 min exhibits an increase of 10% in 0.2%
offset YS, and a very small increment in UTS with respect to the solely quenched alloy condition.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
226.
Miguel A. Vázquez Segura Josefa Donoso Francisco Muñoz INÉS Fernandez de Piérola Francisco Garcia Blanco Gerardo R. Echevarria 《Photochemistry and photobiology》1993,57(S1):923-928
Abstract— Absorption and fluorescence spectra of 5'-deoxypyridoxal (DPL) in various pure solvents and mixtures were recorded both at room temperature and over the range10–65°C. The areas under the absorption bands were analyzed to obtain the mole fraction (fN , fz ) of two tautomers (the zwitterionic, Z, and neutral, N, forms) in the ground state. The following spectral parameters were determined from the fluorescence spectra: Stokes shift (Δ v ), fluorescence quantum yield of the neutral form (QN ), fluorescence ratio of the neutral to the zwitterionic form (øN /øZ ) and the rate constant of tautomerization ( k 1 ) from Z to N in the excited state. Some of these parameters (fN , Δ v , QN , k 1 ) were found to depend on the proton donor character of the solvent, whereas others (øN /øZ ) depended on its dipole moment. Thus, the absorption and fluorescence spectra of DPL allow one to obtain information on the polarity and the concentration of –OH groups on its environment. 相似文献
227.
Ren D. Peralta Ramiro Infante Gladis Cortez Luis Villarreal Jaime Wisniak 《Thermochimica Acta》2002,390(1-2):47-53
Densities of the binary systems of cyclohexane with ethyl acrylate (EA), butyl acrylate (BA), methyl methacrylate (MMA), and styrene have been measured as a function of the composition, at 298.15 K and atmospheric pressure, using an Anton Paar DMA 5000 oscillating U-tube densitometer. The calculated excess volumes were correlated with the Redlich–Kister equation and with a series of Legendre polynomials. The excess volumes are positive for the systems reported here. 相似文献
228.
Bartolom Vilanova Francisco Muoz Josefa Donoso Francisco García Blanco 《Helvetica chimica acta》1993,76(8):2789-2802
Alkaline hydrolysis (pH 10.5) of the three 7-(oxyiminoacyl)cephalosporins 1a–c (cefuroxime, ceftazidime, and ceftriaxone) was studied at 37° using HPLC and 1H-NMR techniques. The 7-epicephalosporin 2 , the 3-methylidene compound 3 , and the 6-epimer 4 of the 3-methylidene compound 3 were identified for each cephalosporin as the major degradation products under the conditions used; ceftazidime ( 1b ) yielded also the Δ2-isomer 5b (Scheme 1). A kinetic scheme was developed to account for the production of these compounds, and the different kinetic constants involved in the process were calculated. The experimental results show that the presence of a pyridinio group at position C–C(3) favours the appearance of the Δ2-isomer, which was detected mainly in cephalosporins bearing an ester function at C(4). The presence of an oxyimino group at C? CONH? C(7) facilitates epimerization at C(7) (→ 2 ), whereas that of an electron-withdrawing group at C? C(3) results in a increased formation constant for the 3-methylidene compound 3 . The 3-methylidene compounds 3a–c produced by the three cephalosporins on cleavage of the β-lactam ring all underwent epimerization at C(6) to yield the corresponding 6-epimer 4 . 相似文献
229.
We used semiempirical and ab initio calculations to investigate the nucleophilic attack of the hydroxyl ion on the β-lactam carbonyl group. Both allowed us to detect reaction intermediates pertaining to proton-transfer reactions. We also used ab initio calculations and the PM3 semiempirical method to investigate the influence of the solvent on the process. The AMSOL method predicts the occurrence of a potential energy barrier of 20.7 kcal mol−1 due to the desolvation of the hydroxyl ion in approaching the β-lactam carbonyl group. Using the supermolecular approach and a water solvation sphere of 20 molecules around the solute, the potential energy barrier is lowered to 17.5 kcal mol−1. Ab initio calculations using the SCRF method predict a potential energy barrier of 13.6 kcal mol−1. These three values, especially the last two, are very close to the experimental value of 16.7 kcal mol−1. 相似文献
230.
Miguel Coll Juan Frau Bartolomé Vilanova Josefa Donoso Francisco Muñoz 《Journal of computer-aided molecular design》2001,15(9):819-833
Various electrostatic and structural parameters for a series of classical and non-classical -lactams were determined and compared in order to ascertain whether some specific -lactams possess antibacterial or -lactamase inhibitory properties. The electrostatic parameters obtained, based on the Distributed Multipole Analysis (DMA) of high-quality wavefunctions for the studied structures, suggest that some non-classical -lactams effectively inhibit the action of -lactamases. As shown in this work, such electrostatic parameters provide much more reliable information about the antibacterial and inhibitory properties of -lactams than do structural parameters. 相似文献