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Total enantioselective synthesis of the natural (-)-codonopsinine was accomplished in seven steps with an overall yield of approximately 16% starting from the five-membered endocyclic enecarbamate 4. The total synthesis features a highly efficient and stereoselective Heck arylation of endocyclic enecarbamate 4 with p-methoxybenzenediazonium tetrafluoroborate and a stereoselective epoxidation/epoxide opening sequence as key steps. 相似文献
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Beatrice Severino 《Tetrahedron》2009,65(1):206-1637
In this paper, we describe the synthesis of conformationally constrained dipeptide mimetic derivatives. Microwave flash heating was used in several synthetic steps providing the opportunity to perform the reactions in dramatically shortened time as well as to increase the obtained yields. The efficiency of the methodology makes it useful in order to prepare other dipeptides containing the 4-amino-tetrahydro-2-benzazepin-3-one motif. 相似文献
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Jin Akiyama Hiroshi Era Severino V. Gervacio Mamoru Watanabe 《Journal of Graph Theory》1989,13(5):571-573
Let the finite, simple, undirected graph G = (V(G), E(G)) be vertex-colored. Denote the distinct colors by 1,2,…,c. Let Vi be the set of all vertices colored j and let <Vi be the subgraph of G induced by Vi. The k-path chromatic number of G, denoted by χ(G; Pk), is the least number c of distinct colors with which V(G) can be colored such that each connected component of Vi is a path of order at most k, 1 ? i ? c. We obtain upper bounds for χ(G; Pk) and χ(G; P∞) for regular, planar, and outerplanar graphs. 相似文献
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We consider transport of a solute obeying linear kinetic sorption under unsteady flow conditions. The study relies on the
vertical unsaturated flow model developed by Indelman et al. [J. Contam. Hydrol. 32 (1998), 77–97] to account for a cycle of infiltration and redistribution. One of the main features of this type of transport,
as compared with the case of a continuous water infiltration, is the finite depth of solute penetration. In the infiltration
stage an analytical solution that generalizes the previous results of Lassey [Water Resour. Res. 24 (1988), 343–350] and Severino and Indelman [J. Contam. Hydrol. 70 (2004), 89–115] is derived. This solution accounts for quite general initial solute distributions in both the mobile and
immobile concentration. When the redistribution is also considered, two timescales become relevant, namely: (i) the desorption
rate k−1, and (ii) the water application time tap. In particular, we have assumed that the quantity ε =(k tap)−1 can be regarded as a small parameter so that a perturbation analytical solution is obtained. At field-scale the concentration
is calculated by means of the column model of Dagan and Bresler [Soil Sci. Soc. Am. J. 43 (1979), 461–467], i.e. as ensemble average over an infinite series of randomly distributed and uncorrelated soil columns.
It is shown that the heterogeneity of hydraulic properties produces an additional spreading of the plume. An unusual phenomenon
of plume contraction is observed at long times of solute propagation during the drying period. The mean solute penetration
depth is studied with special emphasis on the impact of the variability of the saturated conductivity upon attaining the maximum
solute penetration depth. 相似文献
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Brandão GP de Campos RC Luna AS de Castro EV de Jesus HC 《Analytical and bioanalytical chemistry》2006,385(8):1562-1569
A procedure for the determination of As in diesel, gasoline and naphtha at μg L−1 levels by GFAAS is proposed. Sample stabilization was achieved by the formation of three component solutions prepared by
mixing appropriate volumes of the samples propan-1-ol and nitric acid aqueous solution. This mixture resulted in a one-phase
medium, which was indefinitely stable. No changes in the analyte signals were observed over several days in spiked samples,
proving long-term stabilization ability. The use of conventional (Pd) and permanent (Ir) modification was investigated and
the former was preferred. Central composite design multivariate optimization defined the optimum microemulsion composition
as well as the temperature program. In this way, calibration using aqueous analytical solutions was possible, since the same
sensitivity was observed in the investigated microemulsion media and in 0.2% v/v HNO3. Coefficients of correlation larger than 0.999 and an As characteristic mass of 22 pg were observed. Recoveries (n=4) obtained from spiked samples were 98±4, 99±3 and 103±5%, and the limits of detection in the original samples were 1.8,
1.2 and 1.5 μg L−1 for diesel, gasoline and naphtha, respectively. Validation was performed by the analysis of a set of commercial samples by
independent comparative procedures. No significant difference (Student’s t-test, p<0.05) was observed between comparative and proposed procedure results. The total determination cycle lasted 4 min for diesel
and 3 min for gasoline and naphtha, equivalent to a sample throughput of 7 h−1 for diesel and 10 h−1 for gasoline and naphtha. 相似文献