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81.
Recent experiments in low-intensity fluorescence spectrometry, flash photolysis and continuous photolysis at 300°K in an aqueous medium have provided data for uracil and thymine from which all the pertinent phenomenological characteristics (Φfl0′, Φisc0′, Φ′chem) of the 0′ level can be evaluated. It is necessary to postulate considerable internal conversion 1S → 0S with τic ≈ 1–2 × 10−12 sec, this process correlating with the dielectric relaxation time of water. 相似文献
82.
A routine fluorimetric procedure is described for the determination of trace amounts of uranium in zircon. It employs the direct extraction of uranyl nitrate with ethyl acetate using phosphate as a retainer for zirconium. Submicrogram amounts or uranium are separated in the presence of 100,000 times the amount of zirconium. The modified procedure has been worked out using synthetic mixtures of known composition and zircon. Results of analyses have an accuracy of 97–98% of the contained uranium and a standard deviation of less than 2.5%. 相似文献
83.
Ligand Bite Angle‐Dependent Palladium‐Catalyzed Cyclization of Propargylic Carbonates to 2‐Alkynyl Azacycles or Cyclic Dienamides 下载免费PDF全文
David S. B. Daniels Alison S. Jones Dr. Amber L. Thompson Dr. Robert S. Paton Dr. Edward A. Anderson 《Angewandte Chemie (International ed. in English)》2014,53(7):1915-1920
The regioselectivity of the palladium‐catalyzed cyclization of propargylic carbonates with sulfonamide nucleophiles is critically dependent on the bite angle of the bidentate phosphine ligand. Ligands with small bite angles favor attack on the central carbon atom of an allenylpalladium intermediate to afford cyclic dienamide products, whereas the use of those with large bite angles leads to alkynyl azacycles, with high stereoselectivity. A computational analysis of the reaction pathway is also presented. 相似文献
84.
The Maron theory of polymer solutions was used to reexamine the data of Brown et al. and Allen et al. [1,2] on the vapor pressures of polypropylene solutions in diisopropyl and diethyl ketones. The results obtained show that the interaction parameters for these systems depend on concentration and temperature, and that these parameters are able to reproduce the observed data so long as the polymers are amorphous. However, in the presence of polymer order, deviations from theory are observed which lead to 31 and 61% crystallinity for the two polypropylene samples used in these studies. 相似文献
85.
Abstract A photobiological study was camed out on the bacterium Escherichia coli in order to determine whether stimulation of growth occurred after irradiation of an inoculum with coherent red light. No enhancement or inhibition of growth was observed for cultures of the bacterium following irradiation of inocula with a Helium-neon laser (continuous wave, λ= 632.8 nm) at irradiances of 7.7 × 1015 and 1.8 × 1016 photons cm−2 s−1 using fluences of 4.5 × 10−-1 and 4.5 J cm−2 at each irradiance. Bacterial growth in irradiated and control cultures was monitored during a growth period of ca 2 h using a viable count technique after inocula in the early exponential phase had been diluted with fresh growth medium. These results do not provide support for the work of Kam et al . (1983, Nuov. Cim . 2D, 1138–1144), and Tiphlova and Karu (1988, Photochem. Photobiol . 48 , 467–471), which appear to show substantial enhancement of E. coli growth under these conditions. 相似文献
86.
Abstract— The corrected normalized emission spectrum, quantum yield, and emission anisotropy are reported for partially-stacked CpA excited at 266 nm. Utilizing parameters determined separately for cytidine and AMP, the emission spectrum of CpA is quantitatively resolved into components characteristic of the two monomers plus a red shifted emission characteristic of complex formation. The results fit accurately to a simple stacking model in which monomer-like fluorescence originates only from the unstacked fraction and "complex" fluorescence from the stacked fraction. It is shown that the stacked fraction absorbs in a manner qualitatively distinct from the unstacked fraction and the "complex" emission is really the excited state of a ground state complex. This emission, which peaks at 440 nm and contains at least two species can be resolved into three Gaussian components. It is suggested that the emitting species may originate in distinct stacked conformations. 相似文献
87.
Riggins JN Pratt DA Voehler M Daniels JS Marnett LJ 《Journal of the American Chemical Society》2004,126(34):10571-10581
3-(2'-Deoxy-beta-D-erythro-pentofuranosyl)pyrimido[1,2-alpha]purin-10(3H)-one (M1dG) is the major product of the reaction of deoxyguanosine with malondialdehyde (MDA). M1dG blocks replication by DNA polymerases in vitro and is mutagenic in vivo. M1dG reacts with hydroxide to form the N2-(3-oxo-1-propenyl)deoxyguanosine anion (N2OPdG-). This reaction is pH-dependent and reverses under neutral and acidic conditions to form M1dG. Here we describe the kinetics and mechanism of the ring-closure reaction in both the nucleoside and oligonucleotides. Kinetic analysis of absorbance and fluorescence changes demonstrates that ring-closure is biphasic, leading to the rapid formation of an intermediate that slowly converts to M1dG in a general-acid-catalyzed reaction. The dependence of the rate of the rapid phase on pH reveals the pKa for protonated N2OPdG is 6.9. One-dimensional 1H NMR and DQF-COSY experiments identified two distinct intermediates, N2OPdG-H and 8-hydroxy-6,7-propenodeoxyguanosine (HO-Prene-dG), that are formed upon acidification of N2OPdG-. Characterization of ring-closure in single-stranded and in melted duplex oligonucleotides shows M1dG formation is also acid-catalyzed in single-stranded oligonucleotides and that the denaturation of an oligonucleotide duplex enhances ring-closure. This work details the complexity of ring-closure in the nucleoside and oligonucleotides and provides new insight into the role of duplex DNA in catalyzing ring-opening and ring-closing of M1dG and N2OPdG. 相似文献
88.
Of the known trinuclear dipyridylamido complexes of the first-row transition metals, M(3)(dpa)(4)Cl(2) (dpa is the anion of di(2-pyridyl)amine, M = Cr, Co, Ni, Cu), the one-electron-oxidation products of only Cr(3)(dpa)(4)Cl(2) and Co(3)(dpa)(4)Cl(2) have been isolated previously. Here we report one-electron-oxidation products of Ni(3)(dpa)(4)Cl(2) (1) and Cu(3)(dpa)(4)Cl(2) (3): Ni(3)(dpa)(4)(PF(6))(3) (2) and [Cu(3)(dpa)(4)Cl(2)]SbCl(6) (4). While there are no Ni-Ni bonds in 1, the Ni-Ni distances in 2 are 0.15 A shorter than those in 1, very suggestive of metal-metal bonding interactions. In contrast, the oxidation of 3 to 4 is accompanied by a lengthening of the Cu-Cu distances, as expected for an increase in electrostatic charge between positively charged nonbonded metal ions, which is further evidence against Cu-Cu bonding in either 3 or 4. A qualitative model of the electronic structures of all [M(3)(dpa)(4)Cl(2)](n+) (n = 0, 1) compounds is presented and discussed. 相似文献
89.
Peter Daniels Frank Lichtenberg Sander van Smaalen 《Acta Crystallographica. Section C, Structural Chemistry》2003,59(2):i15-i17
Crystals of pentalanthanum pentatitanium heptadecaoxide (La5Ti5O17 with 0.3% oxygen excess, or LaTiO3.41) have been synthesized by floating‐zone melting, and the structure has been solved using single‐crystal X‐ray diffraction intensities. The monoclinic (P21/c) structure consists of perovskite‐like slabs of vertex‐sharing TiO6 octahedra, which are separated by additional oxygen layers. The slabs are five octahedra wide. Due to the adjustment of the TiO6 octahedra to meet the coordination requirements of the La3+ cations, a superstructure develops along the a axis. 相似文献
90.
H.A.M. de Gronckel H. Kohlstedt C. Daniels 《Applied Physics A: Materials Science & Processing》2000,70(4):435-441
Aluminum films with thicknesses ranging from 1 nm to 12 nm have been sputtered on 20 nm thick Co layers. The properties of
the Co/Al bilayers were studied by X-ray photoemission spectroscopy (XPS) and spin-echo nuclear magnetic resonance (NMR).
Both methods show independently that a 1 nm Al film covers the Co surface completely. XPS and NMR also showed that layers
thicker than 1 nm Al are not oxidized completely in ambient air. Similarities to and deviations from niobium with Al overlayers
(Nb/Al) are described. Prerequisites for the fabrication of tunneling magnetoresistance devices based on Co or NiFe ferromagnets
and an aluminum oxide barrier are discussed.
Received: 7 July 1999 / Accepted: 11 November 1999 / Published online: 8 March 2000 相似文献