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191.
We report a concise synthesis of A-factor, the prototypical γ-butyrolactone signalling compound of Streptomyces bacteria. In analogy to enzymatic reactions in A-factor biosynthesis, our synthesis features a tandem esterification-Knoevenagel condensation yielding a 2-acyl butenolide and a surprising, chemoselective conjugate reduction of this α,β-unsaturated carbonyl compound using sodium cyanoborohydride. 相似文献
192.
Two-dimensional π-systems are of current interest in the design of functional organic molecules, exhibiting unique behavior for applications in organic electronics, single-molecule devices, and sensing. Here we describe the synthesis and characterization of "push-pull macrocycles": electron-rich and electron-poor moieties linked by a pair of (matched) conjugated bridges. We have developed a two-component macrocyclization strategy that allows these structures to be synthesized with efficiencies comparable to acyclic donor-bridge-acceptor systems. Compounds with both cross-conjugated (m-phenylene) and linearly conjugated (2,5-thiophene) bridges have been prepared. As expected, the compounds undergo excitation to locally excited states followed by fluorescence from charge-transfer states. The m-phenylene-based systems exhibit slower charge-recombination rates presumably due to reduced electronic coupling through the cross-conjugated bridges. Interestingly, pairing the linearly conjugated 2,5-thiophene bridges also slows charge recombination. DFT calculations of frontier molecular orbitals show that the direct HOMO-LUMO transition is polarized orthogonal to the axis of charge transfer for these symmetrical macrocyclic architectures, reducing the electronic coupling. We believe the push-pull macrocycle design may be useful in engineering functional frontier molecular orbital symmetries. 相似文献
193.
Thode AM Kim KH Blackwell SB Greene CR Nations CS McDonald TL Macrander AM 《The Journal of the Acoustical Society of America》2012,131(5):3726-3747
An automated procedure has been developed for detecting and localizing frequency-modulated bowhead whale sounds in the presence of seismic airgun surveys. The procedure was applied to four years of data, collected from over 30 directional autonomous recording packages deployed over a 280 km span of continental shelf in the Alaskan Beaufort Sea. The procedure has six sequential stages that begin by extracting 25-element feature vectors from spectrograms of potential call candidates. Two cascaded neural networks then classify some feature vectors as bowhead calls, and the procedure then matches calls between recorders to triangulate locations. To train the networks, manual analysts flagged 219 471 bowhead call examples from 2008 and 2009. Manual analyses were also used to identify 1.17 million transient signals that were not whale calls. The network output thresholds were adjusted to reject 20% of whale calls in the training data. Validation runs using 2007 and 2010 data found that the procedure missed 30%-40% of manually detected calls. Furthermore, 20%-40% of the sounds flagged as calls are not present in the manual analyses; however, these extra detections incorporate legitimate whale calls overlooked by human analysts. Both manual and automated methods produce similar spatial and temporal call distributions. 相似文献
194.
Dan Archdeacon C. Paul Bonnington Nathaniel Dean Nora Hartsfield Katherine Scott 《Journal of Graph Theory》2001,38(1):42-64
A graph is outer‐cylindrical if it embeds in the sphere so that there are two distinct faces whose boundaries together contain all the vertices. The class of outer‐cylindrical graphs is closed under minors. We give the complete set of 38 minor‐minimal non‐outer‐cylindrical graphs, or equivalently, an excluded minor characterization of outer‐cylindrical graphs. We also give the obstruction sets under the related topological ordering and Y Δ‐ordering. © 2001 John Wiley & Sons, Inc. J Graph Theory 38: 42–64, 2001 相似文献
195.
Nathan Corbin Deng-Tao Yang Nikifar Lazouski Katherine Steinberg Karthish Manthiram 《Chemical science》2021,12(38):12847
Correction for ‘Suppressing carboxylate nucleophilicity with inorganic salts enables selective electrocarboxylation without sacrificial anodes’ by Nathan Corbin et al., Chem. Sci., 2021, DOI: 10.1039/D1SC02413B.We regret that there was a minor error in the structure of the benzyl chloride in Scheme 2, Fig. 2 and the ESI. The structure of the benzyl chloride should be 4-methyl benzyl chloride but was instead given as 3-methyl benzyl. The correct figure and scheme are shown below, and the ESI has been updated.Open in a separate windowFig. 2(A) Comparison of acid yields for non-sacrificial-anode and sacrificial-anode carboxylation of various substrates. (B) Ratio of carboxylic acid to nucleophilic side products (ester + carbonate + alcohol) for various systems and substrates. Effect of adding MgBr2 to the sacrificial-anode system on the (C) acid yield and (D) ratio of acid to SN2 side products for benzyl bromide. Acid yields are tabulated in Table S6.† ND: acid not detected (acid-to-SN2 ratio <0.1).Open in a separate windowScheme 2Substrate scope for the sacrificial-anode-free electrochemical carboxylation of organic halides. aStandard reaction conditions: 100 mM electrolyte, 100 mM substrate, 100 mM MgBr2, silver cathode, platinum anode, 20 sccm CO2, 2.2 mL DMF, −20 mA cm−2 for 3.5 h. TBA-Br was used for chlorinated substrates because bromide oxidizes more readily than chloride, and only a small amount of chloride was replaced by bromide (<1% for the alkyl chloride, ∼4% for the benzylic chloride). Yields are referenced to the initial amount of substrate and were calculated from 1H NMR spectroscopy using either 1,3,5-trimethoxybenzene or ethylene carbonate as internal standards. b−15 mA cm−2 instead of −20 mA cm−2. c150 mM MgBr2 instead of 100 mM MgBr2.The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers. 相似文献
196.
Eric W. Reinheimer Katherine A. Kantardjieff Xiang Ouyang Steven R. Herron Tiffany Lu Joe A. Casalnuovo 《Journal of chemical crystallography》2007,37(8):507-515
The crystal structures of four new diphosphinated chromium Fischer amino carbenes with the compositions fac-[(P-P)(CO)3Cr=C(NR2′)(R)] (R = Me, NR2′ = pyrrolidino, P-P = dppe, 1; R = Me, NR2′ = pyrrolidino, P-P = dppp, 2; R = Me, R′ = Me, P-P = dppe, 3; R = Me, R′ = Me, P-P = dppp, 4) have been determined at 243 K. Compound 1 crystallizes in the monoclinic system, space group P21 /n with a = 12.1597(11) Å, b = 20.1556(17) Å, c = 14.0557(12) Å, β = 114.163(3)°, V = 3143.0(5) Å3, and Z = 4. Compound 2 crystallizes in the triclinic system, space group P ? 1 with a = 7.4424(3) Å, b = 10.8830(5) Å, c = 20.6040(9) Å, α = 100.9880(10)°, β = 91.7650(10)°, γ = 97.6610(10)°, V = 1620.90(12) Å3, and Z = 2. Compound 3 crystallizes in the monoclinic system as a mono-solvate of d5-pyridine, space group P21 /c with a = 11.485(2) Å, b = 22.825(5) Å, c = 14.092(3) Å, β = 108.53(3)°, V = 3502.7(12) Å3, and Z = 4. Compound 4 crystallizes in the orthorhombic system, space group P2 1 2 1 2 1 with a = 8.359(3) Å, b = 15.364(6) Å, c = 23.784(9) Å, V = 3055(2) Å3, and Z = 4. Steric repulsions with the diphosphine ligand favor a conformation with the amino moiety directed away from the diphosphine backbone in 1–4. 相似文献
197.
Nathan Corbin Deng-Tao Yang Nikifar Lazouski Katherine Steinberg Karthish Manthiram 《Chemical science》2021,12(37):12365
Although electrocarboxylation reactions use CO2 as a renewable synthon and can incorporate renewable electricity as a driving force, the overall sustainability and practicality of this process is limited by the use of sacrificial anodes such as magnesium and aluminum. Replacing these anodes for the carboxylation of organic halides is not trivial because the cations produced from their oxidation inhibit a variety of undesired nucleophilic reactions that form esters, carbonates, and alcohols. Herein, a strategy to maintain selectivity without a sacrificial anode is developed by adding a salt with an inorganic cation that blocks nucleophilic reactions. Using anhydrous MgBr2 as a low-cost, soluble source of Mg2+ cations, carboxylation of a variety of aliphatic, benzylic, and aromatic halides was achieved with moderate to good (34–78%) yields without a sacrificial anode. Moreover, the yields from the sacrificial-anode-free process were often comparable or better than those from a traditional sacrificial-anode process. Examining a wide variety of substrates shows a correlation between known nucleophilic susceptibilities of carbon–halide bonds and selectivity loss in the absence of a Mg2+ source. The carboxylate anion product was also discovered to mitigate cathodic passivation by insoluble carbonates produced as byproducts from concomitant CO2 reduction to CO, although this protection can eventually become insufficient when sacrificial anodes are used. These results are a key step toward sustainable and practical carboxylation by providing an electrolyte design guideline to obviate the need for sacrificial anodes.Selective electrocarboxylation of nucleophilically susceptible organic halides without sacrificial anodes is enabled by inorganic salt additives, which suppress the nucleophilicity of anions in the electrolyte. 相似文献
198.
M. J. Welch Jennifer C. Colbert Lisa M. Gill Curtis S. Phinney Katherine E. Sharpless Lorna T. Sniegoski Laura J. Wood 《Analytical and bioanalytical chemistry》2001,370(1):42-47
In response to reference material needs expressed by the food industry and government regulators, the National Institute
of Standards and Technology (NIST) has developed a new Standard Reference Material (SRM) consisting of a canned meat product
with certified and reference values for a large number of constituents. SRM 1546 Meat Homogenate consists of a mixture of
finely ground pork and chicken prepared and canned by a commercial process. NIST determined the concentration levels of cholesterol,
sodium, calcium, iron, and seven fatty acids in this SRM using well defined methods and procedures. These analytes as well
as 34 other constituents or properties were determined in an interlaboratory comparison exercise involving 21 laboratories,
most of which are associated with the National Food Processors Association (NFPA) Food Industry Analytical Chemists Subcommittee
(FIACS). From statistical analysis of the data, NIST assigned certified concentrations for the eleven analytes measured at
NIST and reference concentrations for the proximates, six additional fatty acids, seven minerals, and seven water-soluble
vitamins. Information values without uncertainties are provided for the concentrations of six additional constituents for
which the uncertainties could not adequately be assessed. SRM 1546 will provide laboratories with a means to evaluate the
accuracy of the methods they use to assign nutrient levels to processed meats and similar products.
Received: 11 October 2000 / Revised: 15 December 2000 / Accepted: 21 December 2000 相似文献
199.
Frank Marken Yu-Chen Tsai Andrew J. Saterlay Barry A. Coles Daniel Tibbetts Katherine Holt Christiaan H. Goeting John S. Foord Richard G. Compton 《Journal of Solid State Electrochemistry》2001,5(5):313-318
Microwave activation of electrochemical processes has recently been introduced as a new technique for the enhancement and
control of processes at electrode|solution (electrolyte) interfaces. This methodology is extended to processes at glassy carbon
and boron-doped diamond electrodes. Deposition of both Pb metal and PbO2 from an aqueous solution of Pb2+ (0.1 M HNO3) are affected by microwave radiation. The formation of PbO2 on anodically pre-treated boron-doped diamond is demonstrated to change from kinetically sluggish and poorly defined at room
temperature to nearly diffusion controlled and well defined in the presence of microwave activation. Calibration of the temperature
at the electrode|solution (electrolyte) interface with the Fe3+/2+ (0.1 M HNO3) redox system allows the experimentally observed effects to be identified as predominantly thermal in nature and therefore
consistent with a localized heating effect at the electrode|solution interface. The microwave-activated deposition of PbO2 on boron-doped diamond remains facile in the presence of excess oxidizable organic compounds such as ethylene glycol. An
increase of the current for the electrocatalytic oxidation of ethylene glycol at PbO2/boron-doped diamond electrodes in the presence of microwave radiation is observed. Preliminary results suggest that the electrodissolution
of solid microparticles of PbO2 abrasively attached to the surface of a glassy carbon electrode is also enhanced in the presence of microwave radiation.
Electronic Publication 相似文献
200.
Let H be a semibounded perturbation of the Laplacian H
0 in L
2(
d
). For an admissible function sufficient conditions are given for the completeness of the scattering system (H), (H
0). If is the exponential function and if e– H
is an integral operator we denote the kernel of the difference D
= e– H
– e– H
0 by D
(x, y), > 0. The singularly continuous spectrum of H is empty ifd dx
d dy |D(x,y)| (1 + |y|2)< for some > 1. This result is applied to potential perturbations and to perturbations by imposing Dirichlet boundary conditions. 相似文献