全文获取类型
收费全文 | 600篇 |
免费 | 52篇 |
国内免费 | 2篇 |
专业分类
化学 | 494篇 |
晶体学 | 2篇 |
力学 | 11篇 |
数学 | 70篇 |
物理学 | 77篇 |
出版年
2023年 | 17篇 |
2022年 | 9篇 |
2021年 | 14篇 |
2020年 | 23篇 |
2019年 | 22篇 |
2018年 | 13篇 |
2017年 | 8篇 |
2016年 | 35篇 |
2015年 | 27篇 |
2014年 | 23篇 |
2013年 | 43篇 |
2012年 | 44篇 |
2011年 | 56篇 |
2010年 | 33篇 |
2009年 | 16篇 |
2008年 | 44篇 |
2007年 | 49篇 |
2006年 | 32篇 |
2005年 | 49篇 |
2004年 | 25篇 |
2003年 | 21篇 |
2002年 | 14篇 |
2001年 | 10篇 |
2000年 | 4篇 |
1999年 | 2篇 |
1998年 | 6篇 |
1997年 | 2篇 |
1996年 | 5篇 |
1995年 | 2篇 |
1994年 | 2篇 |
1993年 | 3篇 |
1991年 | 1篇 |
排序方式: 共有654条查询结果,搜索用时 15 毫秒
191.
Peña D Minnaard AJ Boogers JA de Vries AH de Vries JG Feringa BL 《Organic & biomolecular chemistry》2003,1(7):1087-1089
The combination of monodentate ligands in the rhodium-catalysed enantioselective hydrogenation enables a new approach when searching for the optimal activity and enantioselectivity in catalysis. 相似文献
192.
Jiang XB Minnaard AJ Hessen B Feringa BL Duchateau AL Andrien JG Boogers JA de Vries JG 《Organic letters》2003,5(9):1503-1506
Secondary phosphine oxides were prepared from R(1)PCl(2) and R(2)MgBr, followed by hydrolysis. They were obtained in an enantiopure form by preparative chiral HPLC. These new monodentate ligands were tested in the iridium-catalyzed hydrogenation of imines at 25 bar. Enantioselectivities up to 76% were obtained at L/Ir = 2. Addition of pyridine (Pyr/Ir = 1:2) raised the ee to 83%. Using pyridine as an additive allowed reduction of the L/Ir ratio to 1 without reduction of ee. [reaction: see text] 相似文献
193.
Jeroen Kuipers 《Mathematical Methods of Operations Research》1998,47(3):499-510
We consider bin packing games introduced by Faigle and Kern (1993) and we restrict ourselves to the subclass of games for which all bins have unit capacity and all items are larger than 1/3. We adopt the taxation model of Faigle and Kern and we prove that for a tax-rate of = sk7/1 the -core is always non empty. The bound is sharp, since for every < sk7/1 there exist instances of the bin packing game within our sublass with an empty -core. 相似文献
194.
Bugaev LA van Bokhoven JA Sokolenko AP Latokha YV Avakyan LA 《The journal of physical chemistry. B》2005,109(21):10771-10778
The local aluminum structure in zeolite mordenite was studied at temperatures up to 1000 K in a vacuum by Al K-edge X-ray absorption near-edge structure (XANES) spectroscopy. The interatomic aluminum-oxygen distances and the number of coordinating oxygen atoms were determined by Fourier transform analyses of experimental Al K-edge XANES spectra and the fits of the nearest oxygen atoms contributions, using a limited number of variables. The values of fixed parameters for Fourier transform and fit are established from the spectrum of Na-mordenite, considered the reference compound for the studied zeolites H-mordenites, which was also used to test the accuracy and the stability of the determined structural parameters. To reveal the aluminum coordination in H-mordenite at various temperatures, the Fourier transform peak of the coordinating oxygen polyhedron was fitted first with a single-shell model, and the obtained structural information was refined by the fits, on the basis of the most plausible models for the aluminum coordination environment. The choice of such models for each temperature was performed according to the qualitative predictions on the aluminum local atomic structure provided by the preedge data analysis and 27Al magic angle spinning (MAS) NMR experiments. By this method, the presence of sixfold aluminum atoms, aside from the fourfold ones, in H-mordenite at room temperature was revealed quantitatively, and the concentrations of these mixed coordinations were determined; the structural distortion of the oxygen tetrahedron around aluminum in dehydrated H-mordenite at T = 575 K was found to be strong, and the corresponding Al-O distances for this distortion were obtained; for H-mordenite at 985 K, the presence of threefold coordinated aluminum atoms, aside from the fourfold ones, was revealed, and an estimate of the amount of threefold aluminum was given. 相似文献
195.
Duursma A Lefort L Boogers JA de Vries AH de Vries JG Minnaard AJ Feringa BL 《Organic & biomolecular chemistry》2004,2(12):1682-1684
A library of 96 unique monodentate phosphoramidite ligands has been synthesized in solution and used in the asymmetric conjugate addition of potassium vinyltrifluoroborate to enones resulting in up to 88% ee. 相似文献
196.
Preparation and properties of starch-based colloidal microgels 总被引:4,自引:0,他引:4
Novel starch microgels were prepared by emulsion cross-linking and characterized with respect to shape, volume, and mass density. Starch microgels appear to be negatively charged ( approximately -50 mV), with a particle size varying as a function of the type of cross-linker (ca. 0.25-10 microm). Environmental scanning electron microscopy observations show a dependence of the particle swelling on the cross-linking density. Viscosimetry reveals that starch microgels behave as charged polymers, where the reduced viscosity increases with dilution (anomalous viscosity behavior) for sufficiently low kappaalpha (ca. kappaalpha <3), the ratio of the particle radius (a), and the Debye length (kappa(-1)). Analogous results are obtained for reference-charged rigid silica spheres, which approach the hard sphere limit for increasing ionic strengths. The shape of the microgels appears to play a minor role in the anomalous viscosity behavior, which is more likely dominated by electrostatic effects. 相似文献
197.
Tromp M van Bokhoven JA van Strijdonck GP van Leeuwen PW Koningsberger DC Ramaker DE 《Journal of the American Chemical Society》2005,127(2):777-789
Pd K-edge X-ray absorption near-edge spectroscopy (XANES) is used to probe the unoccupied molecular orbitals in bidentate diphosphine Pd complexes. Complexes containing a series of bidentate diphosphine ligands (PP) are examined to study the effect of the ligand bite angle on the charge redistribution in these complexes. Different coordinating moieties (XX) have been used to induce a range of Pd oxidation states. A full interpretation of the Pd K-edge XANES data is presented. The negative second derivative of these XANES data provides direct information on the energy and electronic distribution of the different unoccupied molecular orbitals probed. The charge redistributions within the complexes, as reflected in the effective Pd oxidation state, are indicated by both the intensity of the first edge feature, the "Pd d peak", and the energy of the second edge feature, the "Pd p peak", which can be easily observed in the negative second derivative of the XANES data. Additionally, the changing covalent interaction between the Pd and coordinated moieties via the Pd p orbitals is reflected directly in the energy splitting of the "Pd p" peak. Thus, investigation of these (PP)Pd(XX) complexes, some used as catalysts in organic synthesis, with XANES spectroscopy provides new essential information on their electronic properties. Further, the XANES analysis techniques described in this paper can be applied to investigate the unoccupied molecular orbitals and charge redistributions within a wide range of samples. 相似文献
198.
Raveendra I. Mathad Bernhard Jaun Oliver Flögel James Gardiner Markus Löweneck Jeroen D. C. Codée Peter H. Seeberger Dieter Seebach Michael K. Edmonds Florian H. M. Graichen Andrew D. Abell 《Helvetica chimica acta》2007,90(12):2251-2273
To further study the preference of the antiperiplanar (ap) conformation in α‐fluoro‐amide groups, two β‐peptides, 1 and 2 , containing a (2‐F)‐β3hAla and a (2‐F)‐β2hPhe residue, have been synthesized. Their NMR‐solution structures in CD3OH were determined and compared with those of non‐F‐substituted analogs, 3 and 4a . While we have found in a previous investigation (Helv. Chim. Acta 2005 , 88, 266) that a stereospecifically introduced F‐substituent in the central position of a β‐heptapeptide is capable of ‘breaking’ the 314‐helical structure by enforcing the F? C? C?O ap‐conformation, we could now demonstrate that the same procedure leads to a structure with the unfavorable ca. 90° F? C? C?O dihedral angle, enforced by the 314‐helical folding in a β‐tridecapeptide (cf. 1 ; Fig. 4). This is interpreted as a consequence of cooperative folding in the longer β‐peptide. A F‐substituent placed in the turn section of a β‐peptidic hairpin turn was shown to be in an ap‐arrangement with respect to the neighboring C?O bond (cf. 2 ; Fig. 7). Analysis of the non‐F‐substituted β‐tetrapeptides (with helix‐preventing configurations of the two central β2/β3‐amino acid residues) provides unusually tight hairpin structural clusters (cf. 3 and 4a ; Figs. 8 and 9). The skeleton of the β‐tetrapeptide H‐(R)β3hVal‐(R)β2hVal‐(R)β3hAla‐(S)β3hPhe‐OH ( 4a ) is proposed as a novel, very simple backbone structure for mimicking α‐peptidic hairpin turns. 相似文献
199.
Jeroen van Westrenen A. Dean Sherry Won Dae Kim Anne E. Vivian Duane C. Hrncir 《Journal of chemical crystallography》1993,23(11):885-890
C5H10N2O3S,1, monoclinicC2/c,a=17.545(5),b=6.822(8),c=13.928(2),=108.36(7),Z=8, MoK, =0.7107 Å,=3.56cm–1,R=0.061,R
w
=0.099. C8H16N3O6S2Na·2H2O,2, orthorhombicPna21,a=17.964(8),b=10.157(1),c=8.263(4),Z=4, MoK, =0.7107 Å,=4.07 cm–1,R=0.040,R
w
=0.043. C10H20N4O6S2·2H2O,3, monoclinicP21/n,a=9.142(9),b=13.576(7),c=14.028(8),=94.096(8),Z=4, MoK, =0.7107 Å,=3.37 cm–1,R=0.063,R
w
=0.074. Compounds1, 2, and3 were prepared from the pH-dependent sulfomethylation of the di- and polyazamacrocycles. 相似文献
200.
In this paper we introduce the ℬ-prenucleolus for a transferable utility game (N,v), where ℬ⊆2
N
. The ℬ-prenucleolus is a straightforward generalization of the ordinary prenucleolus, where only the coalitions in ℬ determine
the outcome. We impose a combinatorial structure on the collection ℬ which enables us to compute the ℬ-prenucleolus in ?(n
3|ℬ|) time. The algorithm can be used for computing the nucleolus of several classes of games, among which is the class of
minimum cost spanning tree games.
Received: September 4, 1995 / Accepted: May 5, 1997?Published online June 8, 2000 相似文献