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141.
Covalent and reversible cluster molecules were synthesized by an A3B2 type gelation. Crosslinking of three-arm hydroxyl-terminated star polymers with 2,4-toluenediisocyanate gave branched polymers, while the reversible analogue was made by crosslinking of tertiary amine-terminated star polymers with bis(4-hydroxy-3,5-dinitrophenyl) adipate. Gelation process was followed by static and dynamic light scattering. The extent of reacted groups was measured with UV spectroscopy. Growth of the covalent clusters could be described in terms of percolation scaling laws. The experimental gel point (POH, cr = 0.70) was shifted significantly from the theoretical predicted gel point (POH, cr = 0.50), indicating extensive ring formation during the gelation. The reversible endlinking reaction gave no macroscopic gelation, though increase of the cluster dimensions was observed. Ring formation proved to be an important side reaction in both cases; however, the ring formation ability seems to change in a different manner during the course of a gelation. 相似文献
142.
143.
Cristóbal Valenzuela Calahorro Fernando José García Barros Angeles Díaz Díez Alvaro Bernalte García Eduardo Sabio Rey 《Monatshefte für Chemie / Chemical Monthly》1992,123(1-2):9-16
Summary Five Cu(II), Pd(II), Cd(II), Pt(IV), and Au(III) complexes of 6-chloropurine have been obtained. The complexes were characterized by elemental analysis, IR,1H-NMR and13C-NMR spectroscopy. On the basis of these data the structure of the complexes and the coordination of the ligand have been proposed. Thus, the physical and chemical methods supported evidence that in acidic medium, with exception of the Cu(II) complex, 6-chloropurine acts in the monoprotonated form neutralizing the charge of [PdCl4]2–, [CdCl4]2–, [AuCl4]– and [PtCl6]2– anions. The thermal behaviour of the complexes has also been studied.
Metallkomplexe von 6-Chlorpurin
Zusammenfassung Es wurden fünf Komplexe von 6-Chlorpurin mit Cu(II), Pd(II), Cd(II), Pt(IV) und Au(III) erhalten. Die Komplexe wurden mittels Elementaranalysen, IR,1H-NMR und13C-NMR charakterisiert. Auf der Basis dieser Daten wurde eine Komplexstruktur und eine bestimmte Koordination der Liganden vorgeschlagen. Physikalische und chemische Methoden beweisen, daß im sauren Bereich [mit der Ausnahme von Cu(II)] das 6-Chlorpurin in der monoprotonierten Form koordiniert, wobei die Ladung von [PdCl4]2–, [CdCl4]2–, [AuCl4]– und [PtCl6]2– jeweils neutralisiert wird. Das thermische Verhalten der Komplexe wurde ebenfalls untersucht.相似文献
144.
145.
An integrated mechanical model for fiber-laden membranes is presented and representative predictions of relevance to cellulose
ordering and orientation in the plant cell wall are presented. The model describes nematic liquid crystalline self-assembly
of rigid fibers on an arbitrarily curved fluid membrane. The mechanics of the fluid membrane is described by the Helfrich
bending-torsion model, the fiber self-assembly is described by the 2D Landau-de Gennes quadrupolar Q-tensor order parameter model, and the fiber-membrane interactions (inspired by an extension of the 2D Maier-Saupe model to
curved surfaces) include competing curvo-philic (curvature-seeking) and curvo-phobic (curvature-avoiding) effects. Analysis
of the free energy reveals three fiber orientation regimes: (a) along the major curvature, (b) along the minor curvature,
(c) away from the principal curvatures, according to the competing curvo-philic and curvo-phobic interactions. The derived
shape equation (normal stress balance) now includes curvature-nematic ordering contributions, with both bending and torsion
renormalizations. Integration of the shape and nematic order equations gives a complete model whose solution describes the
coupled membrane shape/fiber order state. Applications to cylindrical membranes, relevant to the plant cell wall, shows how
growth decreases the fiber order parameter and moves the fibers’ director from the axial direction towards the azimuthal orientation,
eventually leading to a state of stress predicted by pure membranes. The ubiquitous 54.7° cellulose fibril orientation with
respect to the long axis in a cylindrical plant cell wall is shown to be predicted by the preset model when the ratio of curvo-phobic
and curvo-philic interactions is in the range of the cylinder radius. 相似文献
146.
Vibha Mishra Rabindranath Mukherjee Jorge Linares Epiphane Codjovi Fran?ois Varret Max Lawson-Daku 《Hyperfine Interactions》2009,188(1-3):71-78
The spin-transition (1A1?5T2) behaviour of a new mononuclear iron(II) compound [FeII(L)3][PF6]2[L = 2-[3-(2′-pyridyl)pyrazole-1-ylmethyl]pyridine] has been investigated by 57Fe Mössbauer spectroscopy. Analysis of the Mössbauer spectra revealed low value of the quadrupole splitting of the high-spin state which reflects iron(II) to be in nearly cubic lattice site. Mössbauer spectra under light show the light-induced excited spin state trapping effect and the observed quadrupole splitting of the metastable high-spin state is found little sensitive to the high-spin fraction value. DFT calculations are in progress to document the almost cubic nature of the ligand-field acting on the iron atom. 相似文献
147.
Jan-Falk Kannengießer Max Briesenick Dr. Dennis Meier Dr. Volker Huch Dr. Bernd Morgenstern Prof. Dr. Guido Kickelbick 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(66):16461-16476
Organosilanols typically show a high condensation tendency and only exist as stable isolable molecules under very specific steric and electronic conditions at the silicon atom. In the present work, various novel representatives of this class of compounds were synthesized by hydrolysis of alkoxy- or chlorosilanes. Phenyl, 1-naphthyl, and 9-phenanthrenyl substituents at the silicon atom were applied to systematically study the influence of the aromatic substituents on the structure and reactivity of the compounds. Chemical shifts in 29Si NMR spectroscopy in solution, correlated well with the expected electronic situation induced by the substitution pattern on the Si atom. 1H NMR studies allowed the detection of strong intermolecular hydrogen bonds. Single-crystal X-ray structures of the alkoxides and the chlorosilanes are dominated by π-π interactions of the aromatic systems, which are substituted by strong hydrogen bonding interactions representing various structural motifs in the respective silanol structures. 相似文献
148.
Stéphanie Gracia Cédric Marion Jullien Rey Florence Popowycz Stéphane Pellet-Rostaing Marc Lemaire 《Tetrahedron letters》2012,53(25):3165-3168
Horner-Wadsworth Emmons olefination followed by asymmetric hydrogenation allowed the first synthetic access to the chiral thiotryptophan with good enantiomeric excess. Oxazolidinone formation followed by a Pictet–Spengler condensation provided the benzothiophenic analog of Azatoxin. 相似文献
149.
Max M. Majireck 《Tetrahedron letters》2010,51(27):3555-8861
A wide array of readily prepared pivalates of ketoximes can be converted to the corresponding ketones in good yields by treatment with iron powder in THF containing catalytic amounts of both trimethylsilyl chloride and glacial acetic acid at room temperature for 30 min, followed by a brief aqueous workup. 相似文献
150.
Max Engelstein 《Discrete and Computational Geometry》2010,44(3):645-653
We prove that the least-perimeter partition of the sphere into four regions of equal area is a tetrahedral partition. 相似文献