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971.
In this article we study the first eigenvalue of the Laplacian on a compact manifold using stable bundles and balanced bases. Our main result is the following: Let M be a compact Kähler manifold of complex dimension n and E a holomorphic vector bundle of rank r over M. If E is globally generated and its Gieseker point Te is stable, then for any Kähler metric g on M\(\lambda _1 (M,g) \leqslant \frac{{4\pi h^0 (E)}}{{r(h^0 (E) - r)}} \cdot \frac{{\left\langle {C_1 (E) \cup [\omega ]^{n - 1} ,[M]} \right\rangle }}{{(n - 1)!vol(M,[\omega ])}}\) where ω = ωg is the Kähler form associated to g.By this method we obtain, for example, a sharp upper bound for λ1 of Kähler metrics on complex Grassmannians. 相似文献
972.
Mangiapia G Ricciardi R Auriemma F Rosa CD Celso FL Triolo R Heenan RK Radulescu A Tedeschi AM D'Errico G Paduano L 《The journal of physical chemistry. B》2007,111(9):2166-2173
The structure of poly(vinyl alcohol) (PVA) hydrogels formed as a result of freeze/thaw treatments of aqueous solutions of the polymer (11 wt % PVA) in the freshly prepared state is analyzed through the combined use of small (SANS) and ultrasmall (USANS) angle neutron scattering techniques. The structure of these hydrogels may be described in terms of polymer rich regions, with dimensions of the order of 1-2 microm, dispersed in a water rich phase, forming two bicontinuous phases. The PVA chains in the polymer rich phase form a network where the cross-linking points are mainly crystalline aggregates of PVA having average dimensions of approximately 45 A. The structural organization of freeze/thaw PVA hydrogel membranes does not change either after rehydration of dried gels or in the presence of a tensile force. Finally, addition of surfactant micelles inside the gel provides a formulation with both hydrophobic and hydrophilic regions, which demonstrates the potential of the system for drug delivery. Both SANS and EPR measurements show that sodium decylsulfate (C10OS) micelles do not significantly interact with the PVA gel. Variation of the gel structure by the number of freeze/thaw cycles should modulate the rate of release of an active constituent, for example, in a dermal patch. 相似文献
973.
Maekawa H Toniolo C Broxterman QB Ge NH 《The journal of physical chemistry. B》2007,111(12):3222-3235
Two-dimensional infrared (2D IR) spectra of Calpha-alkylated model octapeptides Z-(Aib)8-OtBu, Z-(Aib)5-L-Leu-(Aib)2-OMe, and Z-[L-(alphaMeVal)]8-OtBu have been measured in the amide I region to acquire 2D spectral signatures characteristic of 3(10)- and alpha-helical conformations. Phase-adjusted 2D absorptive spectra recorded with parallel polarizations are dominated by intense diagonal peaks, whereas 2D rephasing spectra obtained at the double-crossed polarization configuration reveal cross-peak patterns that are essential for structure determination. In CDCl3, all three peptides are of the 3(10)-helix conformation and exhibit a doublet cross-peak pattern. In 1,1,1,3,3,3-hexafluoroisopropanol, Z-[L-(alphaMeVal)]8-OtBu undergoes slow acidolysis and 3(10)-to-alpha-helix transition. In the course of this conformational change, its 2D rephasing spectrum evolves from an elongated doublet, characteristic of a distorted 3(10)-helix, to a multiple-peak pattern, after becoming an alpha-helix. The linear IR and 2D absorptive spectra are much less informative in discerning the structural changes. The experimental spectra are compared to simulations based on a vibrational exciton Hamiltonian model. The through-bond and through-space vibrational couplings are modeled by ab initio coupling maps and transition dipole interactions. The local amide I frequency is evaluated by a new approach that takes into account the effects of hydrogen-bond geometry and sites. The static diagonal and off-diagonal disorders are introduced into the Hamiltonian through statistical models to account for conformational fluctuations and inhomogeneous broadening. The sensitivity of cross-peak patterns to different helical conformations and the chain length dependence of the spectral features for short 3(10)- and alpha-helices are discussed. 相似文献
974.
Reactive oxygen species (ROS) have important functions in cell signaling and, when present at overly high levels, may cause oxidation of important biological molecules. Kinetic models to study diffusion of ROS inside of mitochondria often assume dynamics similar to that in solution. However, it is well-known that separation of proteins in the cytosol or inside of mitochondria, where ROS are most predominant, can be smaller than 1 nm. Diffusion of small molecules can be better regarded as a percolation process. In this article, we report results of diffusivity and residence of water and hydrogen peroxide in the proximity of proteins. In carrying out this study, we found some issues with the conventional way of computing residence times by means of survival time correlation functions. The main problem is that particles remaining on the surface of a protein for long times and for which one has very poor statistics contribute significantly to the short time behavior of the survival time correlation function. We mathematically describe this problem and propose methodology to overcome it. 相似文献
975.
Espinoza Catalán L Carrasco Altamirano H Cuellar Fritis M Gallardo Araya C Catalán Marín K 《Molecules (Basel, Switzerland)》2007,12(3):318-327
A selective route for the degradation of the unsaturated side chain of ent-labdanes has been devised, giving two useful synthons: 2beta-acetoxy-14,15,17-trinor-ent-labdane-8,13- dione (5) and 2beta-acetoxy-14,15-dinor-ent-labd-8(17)-en-13-one (7), the use of which for the preparation of terpenylquinone derivatives shall be reported elsewhere. 相似文献
976.
Mercolini L Grillo M Bartoletti C Boncompagni G Raggi MA 《Analytical and bioanalytical chemistry》2007,388(1):235-243
A high-performance liquid chromatographic method has been developed for the simultaneous determination of classical neuroleptics (chlorpromazine, haloperidol, loxapine and clotiapine), atypical antipsychotics (clozapine, quetiapine and risperidone) and their active metabolites (N-desmethylclozapine, clozapine N-oxide and 9-hydroxyrisperidone) in human plasma. Separation was obtained by using a C8 reversed-phase column and a mobile phase composed of 70% aqueous phosphate buffer containing triethylamine at pH 3.0 and 30% acetonitrile. The UV detector was set at 238 nm and amitriptyline was used as the internal standard. A careful pre-treatment procedure of plasma samples was developed, using solid-phase extraction with cyanopropyl cartridges, which gives high extraction yields (>or=93%). The limits of quantitation (LOQ) were always lower than 2.6 ng mL-1 and the limits of detection (LOD) were always lower than 0.9 ng mL-1 for all analytes. The method was applied with success to plasma samples from schizophrenic patients undergoing polypharmacy with two or more different antipsychotics. Precision data and accuracy results were satisfactory and no interference from other central nervous system (CNS) drugs was found. Hence the method is suitable for the therapeutic drug monitoring (TDM) of the analytes in psychotic patients' plasma. 相似文献
977.
978.
Highly Stereoselective [4+2] and [3+2] Spiroannulations of 2‐(2‐Oxoindolin‐3‐ylidene)acetic Esters Catalyzed by Bifunctional Thioureas
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Prof. Magda Monari Dr. Elisa Montroni Andrea Nitti Prof. Marco Lombardo Prof. Claudio Trombini Dr. Arianna Quintavalla 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(31):11038-11049
A new Michael–Michael cascade reaction between 2‐(2‐oxoindolin‐3‐ylidene)acetic esters 1 and nitroenoates 2 , catalyzed by bifunctional thioureas, is investigated. The combination of the two Michael reactions results in a novel and facile [4+2] or [3+2] spiroannulation process, which is characterized by the following features: 1) two carbon–carbon bonds and four stereocenters, including a quaternary spiro carbon, are formed under mild conditions; 2) an unprecedented and stereochemically defined substitution pattern on the spirocarbocyclic unit is obtained; 3) the double‐bond configuration of the donor–acceptor nitroenoate 2 determines the absolute configuration of the spiro center, whereas the remaining stereocenters are formed under control of the catalyst. The effect on the final stereochemical outcome of structural variations of each starting material, catalyst, and experimental conditions is analyzed in detail. In particular, the use of specifically designed chiral nitroenoates enables diverse polyfunctional spirocyclohexane derivatives containing six consecutive stereogenic centers to be constructed. To our knowledge, this is the first asymmetric organocatalytic strategy enabling both five‐ and six‐membered β‐nitro spirocarbocyclic oxindoles. 相似文献
979.
Prof. Claudio Toniolo Dr. Marco Crisma Prof. Alessandro Moretto Dr. Cristina Peggion Prof. Fernando Formaggio Prof. Carlos Alemán Prof. Carlos Cativiela Prof. Chandrasekharan Ramakrishnan Prof. Padmanabhan Balaram 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(40):13866-13877
Among the various types of α‐peptide folding motifs, δ‐turn, which requires a central cis‐amide disposition, has been one of the least extensively investigated. In particular, this main‐chain reversal topology has been studied in‐depth neither in linear/cyclic peptides nor in proteins. This Minireview article assembles and critically analyzes relevant data from a literature survey on the δ‐turn conformation in those compounds. Unpublished results from recent conformational energy calculations and a preliminary solution‐state analysis on a small model peptide, currently ongoing in our laboratories, are also briefly outlined. 相似文献
980.
Prof. Steven V. Ley Daniel E. Fitzpatrick Dr. Rebecca M. Myers Dr. Claudio Battilocchio Dr. Richard. J. Ingham 《Angewandte Chemie (International ed. in English)》2015,54(35):10122-10136
In this Review we describe how the advent of machines is impacting on organic synthesis programs, with particular emphasis on the practical issues associated with the design of chemical reactors. In the rapidly changing, multivariant environment of the research laboratory, equipment needs to be modular to accommodate high and low temperatures and pressures, enzymes, multiphase systems, slurries, gases, and organometallic compounds. Additional technologies have been developed to facilitate more specialized reaction techniques such as electrochemical and photochemical methods. All of these areas create both opportunities and challenges during adoption as enabling technologies. 相似文献