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161.
Alfonso I Bolte M Bru M Burguete MI Luis SV 《Chemistry (Weinheim an der Bergstrasse, Germany)》2008,14(29):8879-8891
The efficient synthesis of large-ring pseudopeptidic macrocycles through a multicomponent [2+2] reductive amination reaction is described. The reaction was entirely governed by the structural information contained in the corresponding open-chain pseudopeptidic bis(amidoamine) precursors, which have a rigid (R,R)-cyclohexane-1,2-diamine moiety. A remarkable match/mismatch relationship between the configurations of the chiral centers of the cyclic diamine and those of the peptidic frame was observed. The macrocyclic tetraimine intermediates have been studied in detail by NMR spectroscopy, circular dichroism (CD), and molecular modeling, and the results support the appropriate preorganization induced by the match combination of the chiral centers. We have also synthesized the corresponding open-chain bis(imine) model compounds. The structural studies (NMR spectroscopy, CD, modeling) of these systems showed an intrinsically lower reactivity of the mismatch combination, even when the product of the reaction was acyclic. In addition, a synergistic effect between the two chiral substructures for the correct folding of the molecules was observed. Finally, X-ray analysis of the HCl salt of one of the macrocycles showed an interesting pattern; the macrocyclic rings stack in columnar aggregates leaving large interstitial channels filled with water-solvated chloride anions. 相似文献
162.
The efficient synthesis of new open-chain enantiopure polyamines bearing (R,R)- and/or (S,S)-trans-cyclohexane-1,2-diamine moieties is described. The key step for the synthetic procedure is the selective monoalkylation of the cyclohexanebis(sulfonamide) core, which allows the subsequent functionalization of this moiety. Compounds bearing different combinations of absolute configurations, length of the aliphatic spacers and terminal groups have been prepared. As a demonstration of the potential utility of the obtained compounds, the preliminary DNA binding abilities of some of them have been studied by UV-measurements of melting temperatures (Tm). The effects of the absolute configuration of the corresponding chiral centers and the length of the spacer separating the cyclohexanediamine moieties on the strength of the interaction with DNA are also discussed. 相似文献
163.
Pasquale Longo Alfonso Grassi Fabia Grisi Stefano Milione 《Macromolecular rapid communications》1998,19(5):229-233
1-Vinylcyclohexene was polymerized in the presence of several homogeneous catalytic systems consisting of methylaluminoxane and group 4 metallocenes such as CpTiCl3, [isopropyl(cyclopentadienyl)(1-fluorenyl)]ZrCl2,rac-[ethylenebis(1-indenyl)] ZrCl2, (CH3)2Si(Cp)2ZrCl2, CpZrCl3. The structure of the polymers depends on the catalyst. In fact, with CpTiCl3, [isopropyl(cyclopentadienyl)(1-fluorenyl)]ZrCl2 and rac-[ethylenebis-(1-indenyl)]ZrCl2 the polymers are chemo-, regio- and stereoregular with 1,4 cis, 1,4 trans and 1,2 isotactic structure, respectively. 相似文献
164.
Norman S. Allen Alfonso Chirinos-Padron John H. Appleyard Terence J. Henman 《Polymer Degradation and Stability》1983,5(2):105-112
The effect of calcium and nickel stearates on the photo-stabilising action and photo-decomposition of one calcium and two nickel metal chelates in polypropylene has been examined using infra-red and ultraviolet derivative spectroscopy. Whereas the presence of calcium stearate antagonises the photo-stabilising action of one nickel chelate, Irgastab 2002, it strongly synergises with the other, Cyasorb UV 1084. The presence of nickel stearate synergises with the calcium chelate, Irganox 1425, in unprocessed polymer whereas, in processed polymer, it exhibits antagonism. Rates of photo-decomposition of the metal chelates are also affected by metal stearates. The data suggests that, with these nickel chelates, there might be some degree of metal exchange with certain metal stearates. 相似文献
165.
Carreño MC Gonzalez-López M Latorre A Urbano A 《The Journal of organic chemistry》2006,71(13):4956-4964
Two alternative routes are described for the synthesis of 8-aryl-2-tetralones (1). Route A starts from alpha-tetralone 3 and involves 3 or 4 steps, with the selective Na-EtOH reduction of 1-aryl-7-methoxynaphthalenes 2 being the key step. The exclusive reduction of the A ring of naphthalenes 2 occurs when the aryl group at C-1 has no substituent at the ortho positions, affording tetrahydronaphthalenes 11. Reduction of the B ring of 2 becomes the major process when the aryl fragment has two substituents at the ortho positions, affording 8-aryl-2-tetralones 1 as the major component. Route B involves 5 steps starting from 2-tetralone 5, with the key step being the Suzuki coupling with triflate 4. This approach allows the synthesis of 8-aryl-2-tetralones 1 with no substituent at the ortho positions of the aryl fragment and with naphthalene and anthracene rings at C-8. 相似文献
166.
Anna Maria Candela Alfonso Romero Miguel Sánchez 《Archive for Rational Mechanics and Analysis》2013,208(1):255-274
We analyze the extendability of the solutions to a certain second order differential equation on a Riemannian manifold (M, g), which is defined by a general class of forces (both prescribed on M or depending on the velocity). The results include the general time-dependent anholonomic case, and further refinements for autonomous systems or forces derived from a potential are obtained. These extend classical results for Lagrangian and Hamiltonian systems. Several examples show the optimality of the assumptions as well as the utility of the results, including an application to relativistic pp-waves. 相似文献
167.
Alfonso?Maiellaro Francesco?Romeo Roberta?CitroEmail author 《The European physical journal. Special topics》2018,227(12):1397-1404
We investigate the topological properties of a Kitaev ladder, i.e., a system made of two Kitaev chains coupled together by transversal hopping and pairing term, t1 and Δ1, respectively. Using the Chern number invariant, we present the topological phase diagram of the system. It is shown that beyond a non-topological phase, the system exhibits a topological phase either with four or two Majorana (zero energy) modes. In particular, we find that for some critical values of the transversal hopping t1, and at a given transversal paring Δ1, the topological phase survives also when the Kitaev criterion for the single chain (Δ > 0, |μ| < 2t) is violated. Using a tight-binding analysis for a finite-size system we numerically check the bulk-edge correspondence. 相似文献
168.
Grzegorz Graff Alfonso Ruiz-Herrera 《Journal of Dynamics and Differential Equations》2014,26(1):93-107
In this paper we discuss a topological treatment for the planar system 0.1 $$\begin{aligned} z'=f(t,z)+g(t,z) \end{aligned}$$ where $f:\mathbb {R}\times \mathbb {R}^{2}\longrightarrow \mathbb {R}^{2}$ and $g:\mathbb {R}\times \mathbb {R}^{2}\longrightarrow \mathbb {R}^{2}$ are $T$ -periodic in time and $g(t,z)$ is bounded. Namely, we study the effect of $g(t,z)$ in two different frameworks: isochronous centers and time periodic systems having subharmonics. The main tool employed in the proofs consists of a topological strategy to locate fixed points in the class of orientation preserving embedding under the condition of some recurrence properties. Generally speaking, our topological result can be considered as an extension of the main result in Brown (Pac J Math 143:37–41, 1990) (concerning two cycles) to any recurrent point. 相似文献
169.
Barreca D Carraro G Devi A Fois E Gasparotto A Seraglia R Maccato C Sada C Tabacchi G Tondello E Venzo A Winter M 《Dalton transactions (Cambridge, England : 2003)》2012,41(1):149-155
Iron oxide is a key multi-functional material in many different fields of modern technology. The β-Fe(2)O(3) cubic phase, one of the least studied Fe-O systems, was obtained by Chemical Vapor Deposition (CVD) using for the first time a Fe(II) β-diketonate diamine complex, Fe(hfa)(2)·TMEDA, as the molecular source (hfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate; TMEDA = N,N,N',N'-tetramethylethylenediamine). The strong visible light absorption of β-Fe(2)O(3) deposits highlights their possible functional application in photocatalytic hydrogen production under solar light. A comprehensive investigation on the Fe(ii) complex, performed by a joint experimental-theoretical approach, explains the molecular origin of its excellent thermal behaviour and reveals why this species is a successful precursor for the CVD of iron oxide nanostructures. 相似文献
170.
Kauffman DR Alfonso D Matranga C Qian H Jin R 《Journal of the American Chemical Society》2012,134(24):10237-10243
Atomically precise, inherently charged Au(25) clusters are an exciting prospect for promoting catalytically challenging reactions, and we have studied the interaction between CO(2) and Au(25). Experimental results indicate a reversible Au(25)-CO(2) interaction that produced spectroscopic and electrochemical changes similar to those seen with cluster oxidation. Density functional theory (DFT) modeling indicates these changes stem from a CO(2)-induced redistribution of charge within the cluster. Identification of this spontaneous coupling led to the application of Au(25) as a catalyst for the electrochemical reduction of CO(2) in aqueous media. Au(25) promoted the CO(2) → CO reaction within 90 mV of the formal potential (thermodynamic limit), representing an approximate 200-300 mV improvement over larger Au nanoparticles and bulk Au. Peak CO(2) conversion occurred at -1 V (vs RHE) with approximately 100% efficiency and a rate 7-700 times higher than that for larger Au catalysts and 10-100 times higher than those for current state-of-the-art processes. 相似文献