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171.
Guzzo MR Uemi M Donate PM Nikolaou S Machado AE Okano LT 《The journal of physical chemistry. A》2006,110(36):10545-10551
In this work, the interaction between fisetin (3,3',4',7-tetrahydroxyflavone) (Fis) and cyclodextrins (CDs) (alpha and beta) was studied through UV-vis absorption, steady-state fluorescence, induced circular dichroism, and (1)H NMR experiments with dependence on temperature and pH. Some experimental data were compared with quantum-mechanics studies based on the SAM1 (AMPAC) semiempirical model, as well as with the B3LYP and MPW1PW91 functional models from density functional theory using the 6-311G and 3-21G basis sets. The spectroscopic measurements show that Fis does not form stable complexes with alpha-CD. On the other hand, at pH 4.0 and 6.5, the complex Fis-beta-CD is formed in a Fis:beta-CD 1:1 stoichiometry and an equilibrium constant (K) of 900 +/- 100 M(-1). In basic medium (pH 11.5), K decreases to 240 +/- 90 M(-1) because Fis deprotonation leads to its better solubilization in water. Molecular modeling points out that Fis is not totally inserted into the inner cavity of beta-CD. The formation of the inclusion complex renders an environment that enhances intramolecular excited state proton transfer. The inclusion complex is formed preferentially via entry of the Fis phenyl group into beta-CD. 相似文献
172.
Determination of phenol, resorcinol and hydroquinone in air samples by synchronous fluorescence using partial least-squares (PLS) 总被引:4,自引:0,他引:4
Pistonesi MF Di Nezio MS Centurión ME Palomeque ME Lista AG Fernández Band BS 《Talanta》2006,69(5):1265-1268
The determination of phenolic compounds is of great importance owing to their high toxicity. Some of them are present in tobacco smoke and it is important for their monitoring in air of closed room. A simple, rapid and sensitive method was developed for simultaneous determination of hydroquinone, resorcinol and phenol in this kind of samples. Synchronous fluorescence technique was used and the data were processed by using the partial least-squares (PLS) chemometric algorithm. The concentrations for experimental calibration matrix were varied between 0.02 and 0.2 mg L−1 for hydroquinone, between 0.05 and 0.6 mg L−1 for resorcinol and between 0.05 and 0.4 mg L−1 for phenol in accordance with the national legislation. The cross-validation method was used to select the number of factors. To check the accuracy of the proposed method a recovery study on real samples was carried out. 相似文献
173.
Lee VY Fukawa T Nakamoto M Sekiguchi A Tumanskii BL Karni M Apeloig Y 《Journal of the American Chemical Society》2006,128(35):11643-11651
((t)Bu(2)MeSi)(2)Sn=Sn(SiMe(t)Bu(2))(2) 1, prepared by the reaction of (t)Bu(2)MeSiNa with SnCl(2)-diox in THF and isolated as dark-green crystals, represents the first example of acyclic distannene with a Sn=Sn double bond that is stable both in the crystalline form and in solution. This was proved by the crystal and NMR spectral data of 1. Distannene 1 has these peculiar structural features: a shortest among all acyclic distannenes Sn=Sn double bond of 2.6683(10) A, a nearly planar geometry around both Sn atoms, and a highly twisted Sn=Sn double bond. The reactions of 1 toward carbon tetrachloride and phenylacetylene also correspond to the reactivity anticipated for the Sn=Sn double bond. The one-electron reduction of 1 with potassium produced the distannene anion radical, the heavy analogue of alkene ion radicals, for which the particular crystal structure and low-temperature EPR behavior are also discussed. 相似文献
174.
We present a theoretical study of the dispersion relation of surface-plasmon resonances of mesoscopic metal-dielectric-metal microspheres. By analyzing the solutions to Maxwell's equations, we obtain a simple geometric condition for which the system exhibits a band of surface-plasmon modes whose resonant frequencies are weakly dependent on the multipole number. Using a modified Mie calculation, we find that a large number of modes belonging to this flat-dispersion band can be excited simultaneously by a plane wave, thus enhancing the absorption cross section. We demonstrate that the enhanced absorption peak of the sphere is geometrically tunable over the entire visible range. 相似文献
175.
Merlin Carl Tim Fischbach Peter Koepke Russell Miller Miriam Nasfi Gregor Weckbecker 《Archive for Mathematical Logic》2010,49(2):249-273
Infinite time register machines (ITRMs) are register machines which act on natural numbers and which are allowed to run for arbitrarily many ordinal steps. Successor steps are determined by standard register machine commands. At limit times register contents are defined by appropriate limit operations. In this paper, we examine the ITRMs introduced by the third and fourth author (Koepke and Miller in Logic and Theory of Algorithms LNCS, pp. 306–315, 2008), where a register content at a limit time is set to the lim inf of previous register contents if that limit is finite; otherwise the register is reset to 0. The theory of these machines has several similarities to the infinite time Turing machines (ITTMs) of Hamkins and Lewis. The machines can decide all ${\Pi^1_1}Infinite time register machines (ITRMs) are register machines which act on natural numbers and which are allowed to run for arbitrarily many ordinal steps.
Successor steps are determined by standard register machine commands. At limit times register contents are defined by appropriate
limit operations. In this paper, we examine the ITRMs introduced by the third and fourth author (Koepke and Miller in Logic
and Theory of Algorithms LNCS, pp. 306–315, 2008), where a register content at a limit time is set to the lim inf of previous
register contents if that limit is finite; otherwise the register is reset to 0. The theory of these machines has several
similarities to the infinite time Turing machines (ITTMs) of Hamkins and Lewis. The machines can decide all P11{\Pi^1_1} sets, yet are strictly weaker than ITTMs. As in the ITTM situation, we introduce a notion of ITRM-clockable ordinals corresponding to the running times of computations. These form a transitive initial segment of the ordinals. Furthermore
we prove a Lost Melody theorem: there is a real r such that there is a program P that halts on the empty input for all oracle contents and outputs 1 iff the oracle number is r, but no program can decide for every natural number n whether or not n ? r{n \in r} with the empty oracle. In an earlier paper, the third author considered another type of machines where registers were not
reset at infinite lim inf’s and he called them infinite time register machines. Because the resetting machines correspond
much better to ITTMs we hold that in future the resetting register machines should be called ITRMs. 相似文献
176.
Giulio Rastelli Alberto Del Rio Gianluca Degliesposti Miriam Sgobba 《Journal of computational chemistry》2010,31(4):797-810
In the drug discovery process, accurate methods of computing the affinity of small molecules with a biological target are strongly needed. This is particularly true for molecular docking and virtual screening methods, which use approximated scoring functions and struggle in estimating binding energies in correlation with experimental values. Among the various methods, MM‐PBSA and MM‐GBSA are emerging as useful and effective approaches. Although these methods are typically applied to large collections of equilibrated structures of protein‐ligand complexes sampled during molecular dynamics in water, the possibility to reliably estimate ligand affinity using a single energy‐minimized structure and implicit solvation models has not been explored in sufficient detail. Herein, we thoroughly investigate this hypothesis by comparing different methods for the generation of protein‐ligand complexes and diverse methods for free energy prediction for their ability to correlate with experimental values. The methods were tested on a series of structurally diverse inhibitors of Plasmodium falciparum DHFR with known binding mode and measured affinities. The results showed that correlations between MM‐PBSA or MM‐GBSA binding free energies with experimental affinities were in most cases excellent. Importantly, we found that correlations obtained with the use of a single protein‐ligand minimized structure and with implicit solvation models were similar to those obtained after averaging over multiple MD snapshots with explicit water molecules, with consequent save of computing time without loss of accuracy. When applied to a virtual screening experiment, such an approach proved to discriminate between true binders and decoy molecules and yielded significantly better enrichment curves. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010 相似文献
177.
The application of abrasive stripping voltammetry for the determination of the corrosion potential of metal powders is described.
Micro amounts of a metal are fixed on a paraffin-impregnated graphite electrode immersed in suitable electrolyte solutions
and polarization curves are recorded. The value of the corrosion potential is determined by Tafel analysis. The precision
of the method is ±0.5% (mean rel. dev.) of the final potential value.
Received: 24 March 1998 / Accepted: 27 August 1998 相似文献
178.
Maykel González Torres José Rogelio Rodríguez Susana Vargas Miriam Estevez González Ma. del Pilar Carreón-Castro Gabriel López Calzada 《International Journal of Polymer Analysis and Characterization》2015,20(5):469-479
Biodegradable graft copolymers were prepared by gamma radiation-induced graft polymerization of two vinyl monomers, vinyl acetate and vinyl alcohol, onto poly[(R)-3-hydroxybutyric acid]. Success of the grafting reaction was verified by Fourier-transform infrared and nuclear magnetic resonance spectroscopy. Thermal remolding was used to create membranes from the copolymers. We determined tribological and mechanical properties of the membranes obtained. The lowest elongation at break in tensile testing is seen for P(3HB) and the highest for P(3HB-g-VA). Up to 5 N or so, the highest scratch resistance is exhibited by P(3HB-g-VA). Piezoelectric behavior is seen for P(3HB-g-VA) while P(3HB-g-VAc) and plain P(3HB) showed no electric response. Explanation of the piezoelectric behavior in terms of molecular structures is provided. 相似文献
179.
180.