全文获取类型
收费全文 | 156篇 |
免费 | 2篇 |
国内免费 | 7篇 |
专业分类
化学 | 89篇 |
晶体学 | 2篇 |
数学 | 46篇 |
物理学 | 28篇 |
出版年
2022年 | 1篇 |
2021年 | 2篇 |
2020年 | 2篇 |
2019年 | 2篇 |
2018年 | 1篇 |
2017年 | 1篇 |
2016年 | 4篇 |
2015年 | 4篇 |
2014年 | 4篇 |
2013年 | 4篇 |
2012年 | 11篇 |
2011年 | 8篇 |
2010年 | 6篇 |
2009年 | 7篇 |
2008年 | 9篇 |
2007年 | 11篇 |
2006年 | 8篇 |
2005年 | 7篇 |
2004年 | 5篇 |
2003年 | 10篇 |
2002年 | 4篇 |
2000年 | 2篇 |
1998年 | 3篇 |
1997年 | 6篇 |
1996年 | 1篇 |
1995年 | 1篇 |
1994年 | 4篇 |
1993年 | 5篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1990年 | 2篇 |
1989年 | 1篇 |
1986年 | 3篇 |
1985年 | 1篇 |
1984年 | 2篇 |
1983年 | 3篇 |
1982年 | 1篇 |
1980年 | 3篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1976年 | 3篇 |
1975年 | 1篇 |
1971年 | 1篇 |
1964年 | 1篇 |
1959年 | 1篇 |
1934年 | 2篇 |
1915年 | 2篇 |
排序方式: 共有165条查询结果,搜索用时 0 毫秒
81.
The synthesis of the alkamides 2Z,4E-undeca-2,4-dien-8,10-diynoic acid isobutyl amide (1) and 2Z,4E-undeca-2,4-dien-8,10-diynoic acid isobutyl amide (5) was accomplished by organometallic coupling followed by introduction of the doubly unsaturated amide moiety. The distribution of these two amides in accessions of the nine species of Echinacea was determined. 相似文献
82.
Song HE Kirmaier C Schwartz JK Hindin E Yu L Bocian DF Lindsey JS Holten D 《The journal of physical chemistry. B》2006,110(39):19131-19139
Static and time-resolved optical measurements are reported for three cyclic hexameric porphyrin arrays and their self-assembled complexes with guest chromophores. The hexameric hosts contain zinc porphyrins and 0, 1, or 2 free base (Fb) porphyrins (denoted Zn(6), Zn(5)Fb, or Zn(4)Fb(2), respectively). The guest is a core-modified (O replacing one of the four N atoms) dipyridyl-substituted Fb porphyrin (DPFbO) that coordinates to zinc porphyrins of a host via pyridyl-zinc dative bonding. Each architecture is designed to have a gradient of excited-state energies for excitation funneling among the weakly coupled constituents of the host to the guest. Energy transfer to the lowest-energy chromophore(s) (coordinated zinc porphyrins or Fb porphyrins) within a hexameric host occurs primarily via a through-bond (TB) mechanism, is rapid ( approximately 40 ps), and is essentially quantitative (>or=98%). Energy transfer from a pyridyl-coordinated zinc porphyrin of the host to the guest in the Zn(6)*DPFbO complex has a yield of approximately 75%, a rate constant of approximately (0.7 ns)(-1), and significant F?rster through-space (TS) character. In the case of Zn(5)Fb*DPFbO, which has an additional TS route via the Fb porphyrin with a rate constant of approximately (20 ns)(-1), the yield of energy transfer to the guest is somewhat lower ( approximately 50%) than that for Zn(6)*DPFbO. Complex Zn(4)Fb(2)*DPFbO has an identical TS pathway via the Fb porphyrin plus an additional TS pathway involving the second Fb porphyrin (closer to the guest) with a rate constant of approximately (0.5 ns)(-1). This complex exhibits an energy-transfer yield to the guest that is significantly enhanced over that for Zn(5)Fb*DPFbO and comparable to that for Zn(6)*DPFbO. Collectively, the results for the various arrays suggest designs for similar host-guest complexes that are expected to exhibit much more efficient light harvesting and excitation trapping at the central guest chromophore. 相似文献
83.
Garrido G Koort E Ràfols C Bosch E Rodima T Leito I Rosés M 《The Journal of organic chemistry》2006,71(24):9062-9067
The acidity constants (pKa) of 11 bases (amines, anilines, pyridines, pyrrolidines, and iminophosphoranes) have been determined in tetrahydrofuran by potentiometry, complemented by conductometric measurements. The pK(a) values of the studied bases cover a wide absolute pH range of acidity in tetrahydrofuran, from 7.4 to 21.7. From the pK(a) values obtained, a scale of absolute acidity in tetrahydrofuran has been established, which has allowed calculation of the absolute pKa values of 77 bases from literature relative pK(a) data. 相似文献
84.
Eve Oja 《Journal of Mathematical Analysis and Applications》2006,323(1):666-679
Based on a new reformulation of the bounded approximation property, we develop a unified approach to the lifting of bounded approximation properties from a Banach space X to its dual X*. This encompasses cases when X has the unique extension property or X is extendably locally reflexive. In particular, it is shown that the unique extension property of X permits to lift the metric A-approximation property from X to X*, for any operator ideal A, and that there exists a Banach space X such that X,X**,… are extendably locally reflexive, but X*,X***,… are not. 相似文献
85.
We study a method of adding–removing knots that has been proposed in the literature for solving the smoothing problem with obstacles. The method uses the coefficients of natural splines in the expansion by radial basis functions. We present examples of cycling and counterexamples to possible use of some ideas. We also give some sufficient conditions for finiteness of the method. 相似文献
86.
We consider the affine equivariant sign covariance matrix (SCM) introduced by Visuri et al. (J. Statist. Plann. Inference 91 (2000) 557). The population SCM is shown to be proportional to the inverse of the regular covariance matrix. The eigenvectors and standardized eigenvalues of the covariance matrix can thus be derived from the SCM. We also construct an estimate of the covariance and correlation matrix based on the SCM. The influence functions and limiting distributions of the SCM and its eigenvectors and eigenvalues are found. Limiting efficiencies are given in multivariate normal and t-distribution cases. The estimates are highly efficient in the multivariate normal case and perform better than estimates based on the sample covariance matrix for heavy-tailed distributions. Simulations confirmed these findings for finite-sample efficiencies. 相似文献
87.
We study how to reduce the smoothing problems with obstacles to the solution of smoothing problems with weights. We prove that, for problems with obstacles in Hilbert spaces, and in the classical case, especially, in several variable problems, the associated Lagrangian has a saddle point. This implies the existence of equivalent problems with weights. 相似文献
88.
Eve Saarikoski Sami Lipponen Marja Rissanen Jukka Seppälä 《Cellulose (London, England)》2012,19(3):661-669
Alkaline solutions of cellulose and polyethylene-co-acrylic acid (PE-co-AA) were mixed in order to prepare homogeneous cellulose/PE-co-AA
blends. Different mixing methods were tested and the evolution of solutions was characterized by rheological methods and optical
microscopy. In addition to the solution stage studies, the precipitated dry blends were characterized by Differential Scanning
Calorimetry, Scanning Electron Microscopy, Fourier Transform Infrared and Dynamic Mechanical Analysis. Mixing the solutions
led to the formation of a suspension, consisting of PE-co-AA particles dispersed evenly throughout the cellulose solution.
Based on the conducted studies, it became clear that a uniform quality and the best mixing were attained with a method in
which the suspension was frozen and melted between the mixing. In dry blends the polymers were found to be mixed on 1 μm scale,
and the crystallization temperature of the PE-co-AA phase was increased ~6 °C due to the nucleation ability of the cellulose
phase. 相似文献
89.
John Bacsa Desmond Eve Kim R. Dunbar 《Acta Crystallographica. Section C, Structural Chemistry》2005,61(1):m58-m60
Single crystals of the title compound, [Co(C2O4)(H2O)2]n, have been prepared by hydrothermal methods and characterized by X‐ray diffraction analysis. The crystal structure consists of infinite one‐dimensional chains of diaquacobalt(II) units bridged by oxalate groups. These chains lie on twofold symmetry axes parallel to the b axis, and the [Co(C2O4)]n system is nearly planar within experimental error. The cobalt(II) coordination polyhedra are irregular octahedra, with oxalate O atoms at the equatorial positions and water molecules at the axial positions. The chains are linked by hydrogen bonds via the water molecules. 相似文献
90.
P. Kanarjov V. Reedo I. Oja Acik L. Matisen A. Vorobjov V. Kiisk M. Krunks I. Sildos 《Physics of the Solid State》2008,50(9):1727-1730
Luminescent films of TiO2: Sm were prepared by the sol-gel method using the spray pyrolysis technique. Various techniques (including IR absorption,
Raman, AFM, XPS, photoluminescence) were used to characterize the samples. After a thermal treatment up to 750°C, intense
Sm3+ luminescence with a well-resolved fine structure was observed under optical excitation within the fundamental absorption
band of the TiO2 host. After further thermal treatments up to 950°C, the luminescence was quenched, although no anataseto-rutile phase transformation
was observed. This behavior is attributed to nanocrystallinity and segregation of Sm ions in the surface layer.
The text was submitted by the authors in English. 相似文献