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81.
This paper reports the study of backbone cleavages in the collision-induced negative-ion mass spectra of the [M - H](-) anions of some synthetic modifications of the bioactive amphibian peptide citropin 1 (GLFDVIKKVASVIGGL-NH(2)). The peptides chosen for study contain no amino acid residues which could effect facile side-chain cleavage, i.e. Ser (-CH(2)O, side-chain cleavage) and Asp (-H(2)O) are replaced by Ala or Lys. We expected that such peptides should exhibit standard and pronounced peaks due to alpha cleavage ions (and to a lesser extent beta cleavage ions) in their collision-induced negative-ion spectra. This expectation was realised, but the spectra also contained peaks formed by a new series of cleavage anions. These are produced following cyclisation of the C-terminal CONH(-) moiety at carbonyl functions of amide groups along the peptide backbone; effectively transferring the NH of the C-terminal CONH(-) group to other amino acid residues. We have called the product anions of these processes beta' ions, in order to distinguish them from standard beta ions. Some beta' ions also fragment directly to some other beta' ions of smaller mass. The reaction coordinates of alpha,beta and beta' backbone processes have been calculated at the HF/6-31G*//AM1 level theory for simple model systems. The initial cyclisation step of the beta' sequence is barrierless and exothermic. Subsequent steps have a maximum barrier of +40 kcal mol(-1), with the overall reaction being endothermic by some 30 kcal mol(-1) at the level of theory used. These calculations take no account of the complexity of the conformationally flexible peptide system, and it is surprising that each of the two reacting centres can 'find' each other in such a large system. 相似文献
82.
A unique matrix system consisting mostly of 4-nitrophenol has shown to be very effective for matrix-assisted laser desorption/ionization time-of-flight mass spectrometric analysis of large DNA oligomers when a cooled sample stage was used to prevent the sublimation of this matrix under vacuum. Using this 4-nitrophenol matrix with UV laser desorption, detection of picomole quantities of DNA oligomers containing up to approximately 800 nucleotides was routinely achieved. The effectiveness of this matrix was further demonstrated by the observation of a double-stranded DNA oligomer larger than 1000 base pairs, seen as a denatured single-stranded species, with a molecular ion mass exceeding 300 000 Da. The potential applications of 4-nitrophenol as a matrix for DNA sizing are discussed. 相似文献
83.
Polyoxometalates (POMs) electrostatically bind to silica nanoparticles coated with cationic aluminum oxide "(Si/AlO2)n+" to form a new type of material (the anionic POMs replace Cl- counterions associated with the cationic surface sites). Association of a new approximately D3h POM of formula [(FeIII(OH2)2)3(A-alpha-PW9O34)2]9- (1) with the cationic nanoparticles (to form "K81/(Si/AlO2)") was studied in detail. Elemental analysis, particle sizes from both laser light scattering and TEM before and after association of 1, the size of 1 from X-ray crystallography, and other methods provide mutually consistent data that indicate about 58 K8[(FeIII(OH2)2)3(A-alpha-PW9O34)2]- monoanions associate with the average nanoparticle (diameter of the K81/(Si/AlO2) product = approximately 17 nm). While heterogeneity of the cationic sites and roughness of the (Si/AlO2)n+ surfaces make the associated POMs structurally nonuniform, the equivalent of approximately 1 monolayer of 1 is present in K81/(Si/AlO2). Remarkably, while 1, the precursor (Si/AlO2)n+, and the components of 1, each alone, are inactive as catalysts for O2/air-based oxidation of sulfides or aldehydes in solution, K81/(Si/AlO2) is an active catalyst for both reactions (facile reaction with air at low temperature). 相似文献
84.
Lonnon DG Craig DC Colbran SB Bernhardt PV 《Dalton transactions (Cambridge, England : 2003)》2004,(5):778-787
Copper(II) bromide and chloride complexes of the new heptadentate ligand 2,6-bis(bis(2-pyridylmethyl)amino)methylpyridine (L) have been prepared. For the bromide complexes, chains of novel, approximately C2-symmetric, chiral [Cu2(L)Br2]2+ 'wedge-shaped' tectons are found. The links between the dicopper tectons and the overall chirality and packing of the chains are dictated by the bromide ion content, not the counter anion. In contrast, the chloride complexes exhibit linked asymmetric [Cu2(L)Cl3]+ tectons with distinct N3CuCl2 and N4CuCl2 centres in the solid. The overall structures of the dicopper bromide and chloride units persist in solution irrespective of the halide. The redox chemistry of the various species is also described. 相似文献
85.
The structures of the compounds initially reported to be 7·Bi2O3·ZnO and 96·Bi2O3·4Fe2O3, have been determined by X ray methods. Three dimensional, absorption corrected diffractometer data were used and atomic parameters were refined by least-squares procedures. The structures are isomorphous, cubic witha = 10.194(3)and10.179(3)A?, respectively, and space group I23. Each Bi3+ ion is surrounded by five oxygen atoms that form an incomplete octahedral arrangement with BiO distances ranging from 2.07–2.60A?. The6s2inert electron pair completes the octahedron. The Bi3+ ions are vibrating anisotropically. Tetrahedral sites in the structures contain 61 and 46 electrons, respectively. These values are consistent with a statistical distribution of Zn2+ and Bi5+ ions or Fe3+ and Bi5+ ions on these sites. Molar ratios are derived that agree with the observed distributions of electron density and give rise to perfectly stoichiometric systems, devoid of cationic or anionic vacancies. The compositions studied correspond to Bi3+24Bi5+Fe3+O40 and Bi3+36Bi5+2ZnO60 and they are optical enantiomorphs.It is proposed that a reduction in the percentage composition of Bi2O5 leads to metastable phases, in which all atomic positions remain fully occupied but some tetrahedral sites contain Bi3+ ions. The end product of the series is γ-Bi2O3 in which 50% of these sites contain Bi3+ and the remainder Bi5+ ions. We believe that γ-Bi2O3 is Bi3+25Bi5+O40. 相似文献
86.
A new type of coordination network polymer involving the redox-active polyanion, PV(2)Mo(10)O(40)(5)(-), and bridging -Cu(II)(OH(2))(4)- units, [[(Cu(II)(OH(2))(4))(3)(OH)]PV(2)Mo(10)O(40)](n)() (1), has been characterized by X-ray crystallography and several other methods. It is the first efficient heterogeneous (insoluble) catalyst for selective and rapid sulfoxidation using only the ambient environment (air at room temperature). Catalytic activity is enhanced by soluble nitrate in nontoxic perfluoropolyether (PFPE) media. 相似文献
87.
beta-Galactosidase was incubated for 60 min with the fluorogenic substrate resorufin-beta-D-galactopyranoside, which is converted by the action of the enzyme into resorufin and galactose. A 160 pL aliquot of reaction mixture was analyzed by capillary electrophoresis utilizing laser-induced fluorescence detection. Based on the detection of the resorufin formed, the limit of detection of beta-galactosidase was 1.5 x 10(-15) M or 900 molecules of enzyme in a 1 microL sample. 相似文献
88.
Eric J Amis Craig J Carriere F.Henry M Nestler John L Schrag John D Ferry 《Analytica chimica acta》1986
A new system of data acquisition and analysis has been developed for the modified Birnboim-Schrag multiple lumped resonator apparatus which is used to measure the viscoelastic properties of long-chain macromolecules in dilute solution. The modifications to the original apparatus include improved temperature control and vibration isolation. The original theory has been reworked so that each resonance mode can be characterized by a greater number of data points. The entire data acquistion/processing system is automated and placed under computer control; a correlation for mode coupling is derived. The modifications result in improved precision in the measured viscoelastic moduli and increased instrumental reliability. 相似文献
89.
90.
Harth E Van Horn B Lee VY Germack DS Gonzales CP Miller RD Hawker CJ 《Journal of the American Chemical Society》2002,124(29):8653-8660
A novel approach is presented for the controlled intramolecular collapse of linear polymer chains to give well-defined single-molecule nanoparticles whose structure is directly related to the original linear polymer. By employing a combination of living free radical polymerization and benzocyclobutene (BCB) chemistry, nanoparticles can be routinely prepared in multigram quantities with the size being accurately controlled by either the initial degree of polymerization of the linear chain or the level of incorporation of the BCB coupling groups. The latter also allows the cross-link density of the final nanoparticles to be manipulated. In analogy with dendritic macromolecules, a significant reduction of up to 75% in the hydrodynamic volume is observed on going from the starting random coil linear chains to the corresponding nanoparticles. The facile nature of the living free radical process also permits wide variation in monomer selection and functional group incorporation and allows novel macromolecular architectures to be prepared. Furthermore, the use of block copolymers functionalized with benzocyclobutene groups in only one of the blocks gives, after intramolecular collapse, a hybrid architecture in which a single linear polymer chain is attached to the globular nanoparticle. 相似文献