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991.
The article aims at giving a critical overview of the passage from bare to modified electrode systems, proposed for amperometric sensing. An introduction is made with reference to the development of modified electrodes since the beginning, i.e. from the 1970s. A subsequent time border is arbitrarily identified around the middle of 1990s. Since those years, many novelties, coming in part from chemistry and in big part from physics and engineering, induced deep changes in the approach to the electroanalytical research; they are tentatively outlined. The term ??research?? is intentionally used to underline that wide room is still available to applications, waiting for efforts by electrochemists. It is also emphasised that the nature of the modifying systems has been only occasionally tailor-made, i.e. designed and developed on purpose. A strict enough connection between the electrochemists and the chemists expert in synthesis has been and still is often scarce. This notwithstanding, the number of novel potentially interesting materials suitable to develop electroanalytical systems highly effective in applications is so high that the next future of electroanalysis seems definitely promising. A brief critical analysis of electrode systems based on similar materials is made.   相似文献   
992.
An instrument for fluorescence spectrometry and FTICR-MS has been developed to study ions produced by a commercial ESI source.
  相似文献   
993.
We define the Virasoro algebra action on imaginary Verma modules for affine ${\mathfrak{sl}(2)}$ and construct an analogue of the Knizhnik?CZamolodchikov equation in the operator form. Both these results are based on a realization of imaginary Verma modules in terms of sums of partial differential operators.  相似文献   
994.
We consider one dimensional systems of particles interacting with each other through long range interactions that are translation invariant. We seek quasi-periodic equilibrium states. Standard arguments show that if there are continuous families of quasi-periodic ground states, the system can have large scale motion, if the family of ground states is discontinuous, the system is pinned down. The transition between the two cases is called breakdown of analyticity and has been widely studied. We use recently developed fast and efficient algorithms to compute all the continuous families of ground states even close to the boundary of analyticity. We show that the boundary of analyticity can be computed by monitoring some appropriate norm of the computed solutions. We implemented these algorithms on several models. We found that there are regions where the boundary is smooth and the breakdown satisfies scaling relations. In other regions, the scalings seem to be interrupted and restart again. We present a renormalization group explanation of these phenomena. This suggest that the renormalization group may have some complicated global behavior.  相似文献   
995.
The disappearance of the hydrophobic effect in the gas phase due to the absence of an aqueous surrounding raises a long-standing question: can noncovalent complexes that are exclusively bound by hydrophobic interactions in solution be preserved in the gas phase? Some reports of successful detection by mass spectrometry of complexes largely stabilized by hydrophobic effect are questionable by the presence of electrostatic forces that hold them together in the gas phase. Here, we report on the MS-based analysis of model supramolecular complexes with a purely hydrophobic association in solution, β-cyclodextrin, and synthetic adamantyl-containing ligands with several binding sites. The stability of these complexes in the gas phase is investigated by quantum chemical methods (DFT-M06). Compared with the free interaction partners, the inclusion complex between β-cyclodextrin and adamantyl-containing ligand is shown to be stabilized in the gas phase by ΔG = 9.6 kcal mol–1. The host–guest association is mainly enthalpy-driven due to strong dispersion interactions caused by a large nonpolar interface and a high steric complementarity of the binding partners. Interference from other types of noncovalent binding forces is virtually absent. The complexes are successfully detected via electrospray ionization mass spectrometry, although a high dissociation yield is also observed. We attribute this pronounced dissociation of the complexes to the collisional activation of ions in the atmospheric interface of mass spectrometer. The comparison of several electrospray-based ionization methods reveals that cold spray ionization provides the softest ion generation conditions for these complexes.  相似文献   
996.
The synthesis of azamacrocycles in which the ring nitrogens are regioselectively functionalized is described. An organozinc palladium(0)-catalyzed coupling with an appropriately functionalized bromopyridine generated a key intermediate, which was transformed in two steps to a desired precursor and subjected to an intramolecular N-alkylation to effect a macrocyclization affording selectively protected azamacrocycles 1-3.  相似文献   
997.
This work proposes the use of digital image-based method for determination of total acidity in red wines by means of acid-base titration without using an external indicator or any pre-treatment of the sample. Digital images present the colour of the emergent radiation which is complementary to the radiation absorbed by anthocyanines present in wines. Anthocyanines change colour depending on the pH of the medium, and from the variation of colour in the images obtained during titration, the end point can be localized with accuracy and precision. RGB-based values were employed to build titration curves, and end points were localized by second derivative curves. The official method recommends potentiometric titration with a NaOH standard solution, and sample dilution until the pH reaches 8.2-8.4. In order to illustrate the feasibility of the proposed method, titrations of ten red wines were carried out. Results were compared with the reference method, and no statistically significant difference was observed between the results by applying the paired t-test at the 95% confidence level. The proposed method yielded more precise results than the official method. This is due to the trivariate nature of the measurements (RGB), associated with digital images.  相似文献   
998.
Spin forbidden 1,2-cycloadditions of triplet methylene to alkenes have been theoretically studied as an example of the two-state reactivity paradigm in organic chemistry. The cycloadditions of triplet methylene to ethylene and the (E)- and (Z)-2-butene isomers show spin inversion after the transition state and therefore with no effect on the reaction rate. A local analysis shows that while triplet methylene addition to alkenes leading to the formation of a biradical intermediate is driven by spin polarization, the ring closure step to yield cyclopropane is a pericyclic process. We have found that at the regions in the potential energy surface where the spin crossover is likely to occur, the spin potential in the direction of increasing spin multiplicity, mu(+)(s), tends to equalize the one in the direction of decreasing spin multiplicity, mu(-)(s). This equalization facilitates the spin transfer process driven by changes in the spin density of the system.  相似文献   
999.
Summary The reaction of trans-[Tcv(OH)(O)(DMPE)2]2+ (DMPE = 1,2-bis(dimethylphosphino)ethane) with a series of arenethiols in base produces the novel complexes cis-[TcIII-(SC6H4X-p)2 (DMPE)2]+ for X = H, Cl, Me, OMe, t-Bu. One of the complexes has been characterized by X-ray crystallography: cis-[Tc(SPh)2(DMPE)2]PF6, chemical formula Tc1S2P5F6C24H42, crystallizes in the orthorhombic space group P21nb with Z = 4 and lattice parameters a = 9.311(1) Å, b=11.190(2) Å, c = 31.936(4) Å, Vol = 3327.3(8) Å3. The final weighted R-value was 0.033. Averaged structural parameters are Tc—S = 2.29(2) Å, Tc—P (trans to P) = 2.42(1) Å, Tc—P (trans to S) = 2.49(3) Å, Tc—S—C = 118.5(5)°. The complexes have been characterized by FAB mass spectrometry and u.v.-vis. spectroscopy. The visible region charge transfer bands are diagnostic for cis geometry in the [Tc(SR)2(DMPE)2]+ complexes. Electrochemical and spectroelectrochemical measurements show reversible TcIII/II redox couples in the range -0.19V to -0.38V versus Ag/AgCl (3 M NaCl). Irreversible couples are exhibited at ca. -1.1 V to -1.2 V for TcII/I and +0.7V to +0.9V for TcIV/III. Variation in redox potential is discussed in terms of sulphur nucleophilicity.On leave from the Department of Chemistry, The University of Tsukuba, Tsukuba, Ibaraki 305, Japan.On leave from Dipartimento di Chimica dell'Universita' di Sassari, Via Vienna, 2-Sassari, Italy.  相似文献   
1000.
Steric interactions between aryl and heterocyclic moieties in 2-substituted-2,3-dihydro-3-o-tolyl(chlorophenyl)-4(1H)-quinazolinones 1a-j produce sufficient restriction to rotation about the aryl C? N bond that the presence of torsional isomers may be detected at room temperature. Diastereomeric population and free energy of activation for rotation have been calculated by 1H nmr spectra. Probably due to a preferred axial position of R2 substituent no dramatic variation both in A /B ratio and in ΔG≠ value has been observed for 1a-f . The comparison between 1a and 1j ΔG values allows to formulate a hypothesis on the structure of the transition state.  相似文献   
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