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541.
Garcia P Harrak Y Diab L Cordier P Ollivier C Gandon V Malacria M Fensterbank L Aubert C 《Organic letters》2011,13(11):2952-2955
A variety of allenynamides can undergo cycloisomerization reactions in the presence of silver triflate thus leading to the formation of N-containing heterocycles incorporating cross-conjugated trienes. Access to new dienic 4-piperidinone and azepane motifs was achieved. An extension to one-pot tandem sequences involving silver-catalyzed cycloisomerization/Diels-Alder reaction was also examined. 相似文献
542.
An edge-colored directed graph is observable if an agent that moves along its edges from node to node is able to determine his position in the graph after a sufficiently long observation of the edge colors, and without accessing any information about the traversed nodes. When the agent is able to determine his position only from time to time, the graph is said to be partly observable. Observability in graphs is desirable in situations where autonomous agents are moving on a network and they want to localize themselves with limited information. In this paper, we completely characterize observable and partly observable graphs and show how these concepts relate to other concepts in the literature. Based on these characterizations, we provide polynomial time algorithms to decide observability, to decide partial observability, and to compute the minimal number of observations necessary for finding the position of an agent. In particular we prove that in the worst case this minimal number of observations increases quadratically with the number of nodes in the graph. We then consider the more difficult question of assigning colors to a graph so as to make it observable and we prove that two different versions of this problem are NP-complete. 相似文献
543.
Liu Q Beier C Evans J Lee T He S Smalyukh II 《Langmuir : the ACS journal of surfaces and colloids》2011,27(12):7446-7452
We report alignment of anisotropic amphiphilic dye molecules within oblate and prolate anisotropic micelles and lamellae, the basic building blocks of surfactant-based lyotropic liquid crystals. Absorption and fluorescence transition dipole moments of these dye molecules orient either parallel or orthogonal to the liquid crystal director. This alignment enables three-dimensional visualization of director structures and defects in different lyotropic mesophases by means of fluorescence confocal polarizing microscopy and two-photon excitation fluorescence polarizing microscopy. The studied structures include nematic tactoids, Schlieren texture with disclinations in the calamitic nematic phase, oily streaks in the lamellar phase, developable domains in the columnar hexagonal phase, and various types of line defects in the discotic cholesteric phase. Orientational three-dimensional imaging of structures in the lyotropic cholesterics reveals large Burgers vector dislocations in cholesteric layering with singular disclinations in the dislocation cores that are not common for their thermotropic counterparts. 相似文献
544.
545.
Silvestri F Jordan M Howes K Kivala M Rivera-Fuentes P Boudon C Gisselbrecht JP Schweizer WB Seiler P Chiu M Diederich F 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(22):6088-6097
The substrate scope of the [2+2] cycloaddition-cycloreversion (CA-CR) reaction between electron-deficient (2,2-dicyanovinyl)benzene (DCVB) or (1,2,2-tricyanovinyl)benzene (TCVB) derivatives and N,N-dimethylanilino (DMA)-substituted acetylenes was investigated. The structural features of the cyanobutadiene products of these transformations were examined and the rates of selected CA-CR reactions were measured. Rate constants for reactions utilizing pentafluorinated TCVB and DCVB were found to be one to two orders of magnitude larger than those for the unsubstituted analogues. Multiple, consecutive CA-CR reactions were performed with substrates incorporating two reactive 2,2-cyanovinyl or 4-ethynylanilino sites. 1,4-Bis(2,2-dicyanovinyl)-2,3,5,6-tetrafluorobenzene and 1,4-bis[(4'-dihexylamino)phenylethynyl]benzene were selected as suitably reactive monomers for the synthesis of regular [AB] oligomers wherein the push-pull chromophores were formed in the chain-growth step. Oligomers of two types were isolated: macrocyclic [AB](n) and open-chain B[AB](n) oligomers, with n≤4. 相似文献
546.
Silva FL Fischer DC Tavares JF Silva MS de Athayde-Filho PF Barbosa-Filho JM 《Molecules (Basel, Switzerland)》2011,16(2):1070-1102
Bidens pilosa L. is a cosmopolitan annual herb, known for its traditional use in treating various diseases and thus much studied for the biological activity of its extracts, fractions and isolated compounds. Polyacetylenes and flavonoids, typical metabolite classes in the Bidens genus, predominate in the phytochemistry of B. pilosa. These classes of compounds have great taxonomic significance. In the Asteraceae family, the acetylene moiety is widely distributed in the Heliantheae tribe and some representatives, such as 1-phenylhepta-1,3,5-triyne, are noted for their biological activity and strong long-wave UV radiation absorbance. The flavonoids, specifically aurones and chalcones, have been reported as good sub-tribal level markers. Natural products from several other classes have also been isolated from different parts of B. pilosa. This review summarizes the available information on the 198 natural products isolated to date from B. pilosa. 相似文献
547.
Gulaim A. Seisenbaeva Suresh GohilVadim G. Kessler Michel Andrieux Corinne Legros Patrick Ribot M. Brunet E. Scheid 《Applied Surface Science》2011,257(6):2281-2290
Strontium titanate SrTiO3 thin films have attracted interest as a possible gate dielectric material. Preparation of its high quality coatings is hindered by difference in volatility of the homometallic precursors - strontium beta-diketonates and titanium alkoxides. The only earlier known single-source precursor, a sec-alkoxide derivative Sr2Ti2(thd)4(OiPr)8, has limited volatility. Bimetallic primary alkyl chain complexes, Sr4Ti2O(thd)4(OR)10(ROH)2, R = Et, nPr, are stable and volatile, but possess a wrong composition. Highly volatile precursor Sr2Ti2(thd)4(OiBu)8 has been prepared using an iso-alkoxide, combining proper ligand size with the sterical requirements, and characterized by multivariate evaporation analysis. Its evaporation is associated with complete decomposition into homometallic species, which, however, are evaporated in a single step. This permits to successfully use this novel precursor for SrTiO3 thin film deposition by DLI-MOCVD technique in a sufficiently broad established temperature range. Using optimized experimental conditions, 100 nm thick strontium titanate films with high permittivity have been successfully obtained on (1 0 0) Si. 相似文献
548.
Dr. Martin Štefko Dr. Manolis D. Tzirakis Dr. Benjamin Breiten Dr. Marc‐Olivier Ebert Oliver Dumele Dr. W. Bernd Schweizer Dr. Jean‐Paul Gisselbrecht Prof. Dr. Corinne Boudon Marten T. Beels Prof. Dr. Ivan Biaggio Prof. Dr. François Diederich 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(38):12693-12704
A series of donor–acceptor‐substituted alkynes, 2 a – f , was synthesized in which the length of the π‐conjugated polyyne spacer between the N,N‐diisopropylanilino donor and the 1,1,4,4‐tetracyanobuta‐1,3‐diene (TCBD) acceptor was systematically changed. The effect of this structural change on the optoelectronic properties of the molecules and, ultimately, their third‐order optical nonlinearity was comprehensively investigated. The branched N,N‐diisopropyl groups on the anilino donor moieties combined with the nonplanar geometry of 2 a – f imparted exceptionally high solubility to these chromophores. This important property allowed for performing INADEQUATE NMR measurements without 13C labeling, which, in turn, resulted in a complete assignment of the carbon skeleton in chromophores 2 a – f and the determination of the 13C–13C coupling constants. This body of data provided unprecedented insight into characteristic 13C chemical shift patterns in push–pull‐substituted polyynes. Electrochemical and UV/Vis spectroscopic studies showed that the HOMO–LUMO energy gap decreases with increasing length of the polyyne spacer, while this effect levels off for spacers with more than four acetylene units. The third‐order optical nonlinearity of this series of molecules was determined by measuring the rotational averages of the third‐order polarizabilities (γrot) by degenerate four‐wave mixing (DFWM). These latter studies revealed high third‐order optical nonlinearities for the new chromophores; most importantly, they provided fundamental insight into the effect of the conjugated spacer length in D–A polyynes, that can be exploited in the future design of suitable charge‐transfer chromophores for applications in optoelectronic devices. 相似文献
549.
An improved synthesis of a water-soluble deep-cavity cavitand (octa-acid, 1) is presented. Previously (Gibb, C.L.D.; Gibb, B.C. J. Am. Chem. Soc. 2004, 126, 11408–11409), we documented access to host 1 in eight (non-linear) steps starting from resorcinol; a synthesis that required four steps involving chromatographic purification. Here, we reveal a modified synthesis of host 1. Consisting of seven (non-linear) steps, this new synthesis involves only one chromatographic step, and avoids a minor impurity observed in the original approach. This improved synthesis is therefore useful for the laboratories that are investigating the properties of these types of host. 相似文献
550.
Dr. Kara Howes Dr. Abdelhalim Belbakra Dr. Andrea Listorti Dr. W. Bernd Schweizer Dr. Philippe Reutenauer Dr. José‐Lorenzo Alonso‐Gómez Dr. Claudio Chiorboli Lorenz M. Urner Dr. Jean‐Paul Gisselbrecht Prof. Dr. Corinne Boudon Dr. Nicola Armaroli Prof. Dr. François Diederich 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(1):202-216
The synthesis, electrochemical, and photophysical properties of five multicomponent systems featuring a ZnII porphyrin (ZnP) linked to one or two anilino donor‐substituted pentacyano‐ (PCBD) or tetracyanobuta‐1,3‐dienes (TCBD), with and without an interchromophoric bridging spacer (S), are reported: ZnP‐S‐PCBD ( 1 ), ZnP‐S‐TCBD ( 2 ), ZnP‐TCBD ( 3 ), ZnP‐(S‐PCBD)2 ( 4 ), and ZnP‐(S‐TCBD)2 ( 5 ). By means of steady‐state and time‐resolved absorption and luminescence spectroscopy (RT and 77 K), photoinduced intramolecular energy and electron transfer processes are evidenced, upon excitation of the porphyrin unit. In systems equipped with the strongest acceptor PCBD and the spacer ( 1 , 4 ), no evidence of electron transfer is found in toluene, suggesting ZnP→PCBD energy transfer, followed by ultrafast (<10 ps) intrinsic deactivation of the PCBD moiety. In the analogous systems with the weaker acceptor TCBD ( 2 , 5 ), photoinduced electron transfer occurs in benzonitrile, generating a charge‐separated (CS) state lasting 2.3 μs. Such a long lifetime, in light of the high Gibbs free energy for charge recombination (ΔGCR=?1.39 eV), suggests a back‐electron transfer process occurring in the so‐called Marcus inverted region. Notably, in system 3 lacking the interchromophoric spacer, photoinduced charge separation followed by charge recombination occur within 20 ps. This is a consequence of the close vicinity of the donor–acceptor partners and of a virtually activationless electron transfer process. These results indicate that the strongly electron‐accepting cyanobuta‐1,3‐dienes might become promising alternatives to quinone‐, perylenediimide‐, and fullerene‐derived acceptors in multicomponent modules featuring photoinduced electron transfer. 相似文献