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221.
Piergiorgio Pecorari Marcella Rinaldi M. Paola Costi Luciano Antolini 《Journal of heterocyclic chemistry》1992,29(6):1449-1455
Derivatives of two new molecular structures, namely, 7,8-dihydro-6H,10H-[1,2,5]thiadiazolo[3′,4′:4,5]pyrimido[2,1-b][1,3]thiazin-10-one and 6,7-dihydro-9H-thiazolo[3,2-a][1,2,5]thiadiazolo[3,4-d][pyrimidin-9-one, and derivatives of N-substituted sulfamic acid, namely, (8-amino-3,4-dihydro-2H,6H-pyrimido[2,1-b][1,3]thiazin-6-on-7-yl)sulfamic acid and (7-amino-2,3-dihydro-5H-thiazolo[3,2-a]pyrimidin-5-on-6-yl)sulfamic acid, were separated out as by-products in the reduction reaction of 8-amino-3,4-dihydro-7-nitroso-2H,6H-pyrimido[2,1- b][1,3]thiazin-6-one and 7-amino-2,3-dihydro-6-nitroso-5H-thiazolo[3,2-a]pyrimidin-5-one derivatives, respectively, with sodium hydrosulfite. A mechanism of reaction, which hypothesizes the action of sodium hydrosulfite in an asymmetic form, is proposed. The results of single-crystal X-ray investigation on 7,8-dihydro-6H,10H-[1,2,5]thiadiazolo[3′,4′:4,5]pyrimido[2,1-b][1,3]thiazin-10-one (R = 0.032 for 863 reflections) and (8-amino-3,4-dihydro-2H,6H-pyrimido[2,1-b]- [1,3]thiazin-6-on-7-yl)sulfamic acid, sodium salt (R = 0.028 for 3507 reflections) are reported. 相似文献
222.
Esposito V Randazzo A Piccialli G Petraccone L Giancola C Mayol L 《Organic & biomolecular chemistry》2004,2(3):313-318
NMR, molecular dynamics and mechanics calculations, and CD spectroscopy were used to characterise three tetramolecular quadruplex complexes: [d(TG(Br)GGT)](4), [d(TGG(Br)GT)](4) and [d(TGGG(Br)T)](4), where G(Br) indicates an 8-bromoguanine residue. All three quadruplexes are characterised by a 4-fold symmetry with all strands parallel to each other and, differently to what has been observed for other parallel quadruplex structures, with a tetrad (formed by 8-Br-dGs) in a syn conformation. The whole of the data demonstrates that the replacement in turn of different dG residues with 8-Br-dG in the sequence 5[prime or minute]-TGGGT-3[prime or minute] affects the resulting structures in different ways, leading to different CD profiles and thermal stabilities. Particularly, [d(TG(Br)GGT)](4) and [d(TGG(Br)GT)](4) are more stable than the unmodified sequence, whereas [d(TGGG(Br)T)](4) is much less stable than the natural counterpart. The conformational features found in the three quadruplexes might, in principle, amplify the range of applicability of synthetic oligonucleotides as aptamers or catalysts, by providing novel structural motifs with different molecular recognition capabilities from those of native DNA sequences. 相似文献
223.
Luciano Farias Almeida Edvan Cirino Silva Mario Cesar Ugulino Araujo 《Analytica chimica acta》2003,486(1):143-148
A novel strategy for implementing the automatic standard addition method (SAM) is described. By using a flow-batch system that presents the intrinsic favourable characteristics of the flow and batch techniques, the proposed strategy performs fast standard additions with sufficient flexibility and versatility and employs only one standard solution per analyte. To calculate the analyte concentration, a mathematical model based on a classical SAM and flow variables of the system was developed. The proposed flow-batch SAM was applied to copper determination by flame atomic absorption spectrometry (AAS) in sugar cane-made alcoholic beverages, known as “Cachaça”, available in Brazil. A SAM has been recommended for these analyses because “Cachaças” presents a significantly different composition causing matrix effects and copper determination by calibration using matrix-matching standards can yield inaccurate results. The results show good agreement between the obtained values with the proposed flow-batch SAM and a manual SAM. The mean relative errors and overall standard deviations were always <1.0% (n=6) and 0.2 mg l−1, respectively, for 1.0-7.0 mg l−1 Cu. By using five standard addition levels, the sample throughput was 70 h−1 and the consumption of sample and standard solution were 1.5 and 0.5 ml per analysis, respectively. 相似文献
224.
Brian T. Heaton Luciano Longhetti D. Michael P. Mingos Clive E. Briant Peter C. Minshall Brian R.C. Theobald Luigi Garlaschelli Ugo Sartorelli 《Journal of organometallic chemistry》1981,213(1):333-350
The crystal structures of Rh4(CO)10(PPh3)2 and Rh4(CO)9P(OPh)33 are reported. 31P-1H NMR studies on Rh4(CO)12-x {P(OPh)3}x(X 1, 2 and 3) show that each derivative exists as only one isomer in solution whereas the analogous triphenylphosphine derivatives can exist as different isomers. A quantitative redistribution of triphenylphosphites occurs on mixing Rh4-(CO)12-xLx with Rh4(CO)12-yLy (L P(OPh)3; x 0, 1, 2, y x + 2; x 0, y x + 4) to give Rh4(CO)12-zLz[z (x + y)]; a related rapid intermolecular randomisation of carbonyls occurs on mixing Rh4(12CO)12 with Rh4(13CO)12. 相似文献
225.
Luciano Galeone Carmela Mastroserio 《Numerical Methods for Partial Differential Equations》1996,12(5):615-626
We consider nonlinear semi-discrete problems that derive by reaction diffusion systems of partial differential equations, when finite difference methods or Faedo Galerkin methods are used for spatial discretization. The aim of this article is to give sufficient conditions for the contractivity of the θ-method, in a norm generated by a positive diagonal matrix G. We show that the numerical contractivity property is obtained if some matrices, constructed by means of the Jacobian matrix of nonlinear term, are M-matrices. © 1996 John Wiley & Sons, Inc. 相似文献
226.
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228.
Luciano Luciani Federico Milani Roberto Zannetti Vito Di Noto 《Macromolecular Symposia》1993,66(1):55-70
Among many precursors and catalysts for alpha-olefins polymerization, one seems to be particularly interesting, because it has not been completely clarified yet. We refer especially to precursors obtained via reaction between Mg-alkyls and SiCl4. The products of this reaction are not well known; in fact, under some operating conditions, a special form of MgCl2 is obtained, showing x-ray diffraction peaks in the angular region lower than 15° (2 theta), which corresponds to the 5.9 A interplanar spacing, characteristic of alpha-MgCl2. Under other conditions, MgCl2 is obtained in the well known and strongly disordered delta structure. By employing these precursors, some catalytic systems for alpha-olefins polymerization have been prepared. In this paper, the peculiar aspects of these precursors and catalysts are described, particularly focusing on the correlation between structure and performances in ethylene and propylene polymerization. 相似文献
229.
Luciano Caseli Thatyane Morimoto Nobre Denise Abatti Kasper Silva Watson Loh Maria Elisabete Darbello Zaniquelli 《Colloids and surfaces. B, Biointerfaces》2001,22(4):2667-321
The surface activity of the poly–[block (ethylene oxide)]–poly [block (propylene oxide)]–poly [block (ethylene oxide)] copolymers (EO)x–(PO)y–(EO)x adsorbed together with dihexadecyl phosphoric acid (DHP), a synthetic phospholipid, is analyzed from their surface pressure and surface potential isotherms. The block copolymers of (EO)x–(PO)y–(EO)x with variable molecular weight (1100–14 000) were dissolved in the subphase for DHP monolayers. The concentration of the copolymers within the aqueous subphase were selected to render an initial surface tension of 60 mN/m. The simultaneous adsorption of the copolymer and DHP is attested by the observation of a liquid expanded state at large areas, absent for pure DHP monolayers. Above some critical surface pressure all copolymers cited above are expelled from the interface. The surface potential isotherms, which give information on the component of the molecular dipole moment normal to the plane of the monolayer, are interpreted in terms of changes in the copolymer conformation as well as in terms of the copolymer desorption from the air–liquid interface. For an equal hydrophobic/hydrophilic ratio, the size of the chains or molecular weight is decisive in the mechanism of the copolymer expulsion from the air–liquid interface. 相似文献
230.
Eddie A. Maier Bernard Griepink Philippe Quevauviller Luciano De Angelis Herbert Muntau 《Mikrochimica acta》1990,102(1-3):87-99
Analyses of nutritive elements are routinely performed in grass or hay for animal feed in order to improve the state of health and growth of domestic animals. To control the quality of such determinations, the Community Bureau of Reference (BCR) so far has produced two certified reference materials: Hay Powder (CRM 129) and Rye Grass (CRM 281). After a careful preparation procedure of the materials, a homogeneity study and a long-term stability study, the contents of the elements Ca, K, Mg, P, S, Zn, I, N and Kjeldahl-N were certified in CRM 129, whereas CRM 281 was certified for As, B, Cd, Cu, Hg, Mn, Mo, Ni, Pb, Sb, Se and Zn. This paper presents the certification exercise and especially concentrates on the elements I, B and Mo as examples of analytical work. Indicative values for Co, Cr, Fe, Cl and Na were also obtained. 相似文献