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11.
12.
Simon G. Bott Duddu Raja Gopal Annavajhula Durga Prasad Alan P. Marchand 《Journal of chemical crystallography》1993,23(12):1013-1017
The structure of a compound derived from a symmetrical cage diketone in which one of the ketone C=O groups has been converted into a ketal is reported. An investigation into reasons for the selective functionalization resulted in the isolation of acis, cisoid, cis linear triquinane derivative in which both ketones are converted to ketals that share a transannular oxygen atom. Crystal data:2, monoclinic,P21/c,a=7.6298(9),b=7.673(3),c=22.125(3)Å,=97.59(1)°,V=1283.9(5)Å3, andR=0.048 (1001 reflections);4, triclinic,P¯1,a=7.3864(9),b=9.6362(9),c=10.3889(9)Å,=75.749(6),=76.300(8), =83.100(8)°,V=694.8(4)Å3, andR=0.088 (1284 reflections). 相似文献
13.
Synthesis and Structure of Copper(I)Chalcogenolate-2,2′-Bipyridine Complexes [CuS(2,4,6-iPr3C6H2)]4(Bipy)2 and [CuSe(2,4,6-iPr3C6H2)]2(Bipy)2 The oligomeric homoleptical copper(I) chalcogenolate compounds [CuS(2,4,6-iPr3C6H2)]x=4,8 and [CuSe(2,4,6-iPr3C6H2)]6 react with 2,2′-bipyridine (Bipy) to yield the tetranuclear complex [CuS(2,4,6-iPr3C6H2)]4(Bipy)2 ( 4 ) and the dinuclear complex [CuSe(2,4,6-iPr3C6H2)]2(Bipy)2 ( 5 ). The structures of 4 and 5 were determined by X-ray analysis. In the eight-membered Cu4S4 core of 4 with chair conformation the copper atoms are linked by μ2-bridging selenolate ligands. Only two copper atoms are coordinated by 2,2′bipyridine. The corresponding copper(I) selenolate complex ( 5 ) forms a folded four-membered Cu2Se2 ring with μ2-bridging selenolate ligands. The Cu? Cu distance of 2.52 Å is relatively short. In contrast to the reaction performed with 2,2′-bipyridine, addition of phenantroline to 1 respectively 2 yields a dinuclear complex [CuS(2,4,6-iPr3C6H2)]2(Phen)2 ( 10 ). NMR spectroskopic and cryoscopic measurement of 4 show that this complex dissociates into smaller fragments in solution which undergo rapid exchange reactions. However, corresponding investigations performed on 5 indicate that the solid state structure is maintained in solution. The electrochemical behaviour of 4, 5 and 10 was studied in CH2Cl2 and in any case no reversible redox processes could be observed. 相似文献
14.
Alan P Marchand Bishwajit GangulyCarolina I Malagón Huiguo LaiWilliam H Watson 《Tetrahedron》2003,59(10):1763-1771
Diels-Alder cyclodimerization of 1,2,3,4,5-pentachlorocyclopentadiene (1) affords 2a as the exclusive reaction product. Diels-Alder cycloaddition of 1 to norbornadiene also proceeds stereoselectively to produce a single [4+2] cycloadduct, 4c. The structures of 2a and 4c were established unequivocally via application of single crystal X-ray crystallographic techniques. The origins of the observed diastereofacial selectivity in each of these cycloaddition processes have been investigated by application of semiempirical (AM1 Hamiltonian) and ab initio (Hartree-Fock 3-21G*) calculations. The computational results thereby obtained, which are based upon consideration of the kinetically favored transition state for each of the two cycloaddition reactions studied, are consistent with experiment. These semiempirical and ab initio methods also have been used to investigate the mechanisms of the Diels-Alder reactions that have been used to prepare aldrin and isodrin (7 and 8, respectively). The results thereby obtained suggest that isodrin formation via Diels-Alder cycloaddition of cyclopentadiene to 1,2,3,4,7,7-hexachloronorbornadiene proceeds with kinetic control of product stereochemistry. 相似文献
15.
Antifouling coatings are the most reliable way to prevent biofouling of immersed surfaces. As concerning the high toxicity of organotin paints, the tendency is the development of coatings, which do not present environmental risks. In this work, we prepare binders from modification of acrylic copolymers containing free carboxylic acid groups. Biocides chosen are tertiary ammonium salts; alkyl chain substituents with different length are considered. The chemical modifications of resins are carried out via a single step reaction. Modification extents are monitored through proton nuclear magnetic resonance and thermogravimetric analysis and the modified resins are characterised by Fourier transform infrared spectroscopy. The glass transition temperature of the acrylate systems is assessed by dynamical mechanical analysis (pin point method) and compared with data obtained by differential scanning calorimetry. The erosion and antifouling properties of the binders are followed during an exposure to marine environment by a visual observation. 相似文献
16.
M. M. Mason J. S. Morris V. L. Spate C. K. Baskett T. A. Nichols T. L. Horsman L. Le Marchand L. N. Kolonel S. Yukimoto 《Journal of Radioanalytical and Nuclear Chemistry》1998,232(1-2):29-34
Haem peroxidases are globular proteins with an iron-porphyrin complex as prosthetic group. They catalyze the oxidation of
substrate by peroxides, frequently via free-radical intermediates. The catalytic cycle involves changes in the redox states
of the prosthetic group, that can be monitored by changes in the intense absorption spectra. During the past decades, considerable
scientific effort has been put into the elucidation of the mechanisms of reactions catalyzed by these enzymes. Radiation-chemical
techniques have made an important contribution by providing information on the redox states of the enzymes and their interconversions,
as well as on the properties of the free-radical intermediates involved. 相似文献
17.
R. I. Damper Y. Marchand J.-D. S. Marsters A. I. Bazin 《Journal of Sol-Gel Science and Technology》1997,8(2):147-160
A common requirement in speech technology is to align two different symbolic representations of the same linguistic ‘message’.
For instance, we often need to align letters of words listed in a dictionary with the corresponding phonemes specifying their
pronunciation. As dictionaries become ever bigger, manual alignment becomes less and less tenable yet automatic alignment
is a hard problem for a language like English. In this paper, we describe the use of a form of the expectation-maximization
(EM) algorithm to learn alignments of English text and phonemes, starting from a variety of initializations. We use the British
English Example Pronunciation (BEEP) dictionary of almost 200,000 words in this work. The quality of alignment is difficult
to determine quantitatively since no ‘gold standard’ correct alignment exists. We evaluate the success of our algorithm indirectly
from the performance of a pronunciation by analogy system using the aligned dictionary data as a knowledge base for inferring
pronunciations. We find excellent performance—the best so far reported in the literature. There is very little dependence
on the start point for alignment, indicating that the EM search space is strongly convex. Since the aligned BEEP dictionary
is a potentially valuable resource, it is made freely available for research use. 相似文献
18.
M. Guillot F. Tchéou A. Marchand P. Feldmann 《Zeitschrift für Physik B Condensed Matter》1984,56(1):29-39
Using neutron diffraction data, the thermal evolution of the magnetic structure of Ho3Fe5O12 has been studied. Below the compensation temperature (T
comp=137 K), the umbrella structure parameters which fit the spontaneous ferrite magnetizations have been determined. AboveT
comp, the compound behaves as a Néel type ferrimagnet. Particular attentions have been given to the magnetic form factors of both iron sublattice on one hand and on the other hand to a rhombohedral distorsion fromI a3d toR
below 30 K and in first approximation toR
c forT>30 K. 相似文献
19.
Alan P. Marchand Anna Hazlewood Zilin Huang Satish Kumar Vadlakonda John-David R. Rocha Trevor D. Power Kata Mlinarić-Majerski Lada Klaić Goran Kragol Jeffrey C. Bryan 《Structural chemistry》2003,14(3):279-288
Bis-cage-annulated 18-crown-6 and 20-crown-6 macrocyclic ethers (i.e., 1 and 2, respectively) have been synthesized, and their alkali metal picrate extraction profiles have been determined. Host system 1 proved to be a significantly more avid alkali metal cation complexant than 2 and somewhat more avid than 18-crown-6. Both 1 and 18-crown-6 display modest selectivity toward K+ and Rb+. A stable host–guest complex was prepared by slow evaporation of a CH2Cl2–hexane solution of an equimolar mixture of 2 and potassium picrate. The X-ray crystal structure of this complex reveals that picrate anion functions as a bidentate ligand therein. The gas-phase interaction energy between the 2 K+ complex and picrate anion was calculated to be ca. –64.9 kcal mol–1, thereby indicating that participation of picrate anion as an additional bidentate ligand results in significant stabilization of complex 10. 相似文献
20.
Graeme Cooke Hugues A de Cremiers Florence M.A DuclairoirJulie Leonardi Georgina RosairVincent M Rotello 《Tetrahedron》2003,59(18):3341-3347
We describe the electrochemically controlled hydrogen bonding interactions between the isobutyl flavin/2,6-diferrocenylamidopyridine (2·5) and 9,10-phenanthrenequinone/1-ferrocenyl-3-hexylurea (4·6) dyads. Cyclic and square wave voltammetry studies have shown that the binding efficiencies between these moieties can be electrochemically actuated in non-polar (CH2Cl2 for 2·5) or polar (DMF for 4·6) organic solvents between three distinct states. 相似文献