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91.
Wing Ying Chow Brendan P. Norman Norman B. Roberts Lakshminarayan R. Ranganath Christian Teutloff Robert Bittl Melinda J. Duer James A. Gallagher Hartmut Oschkinat 《Angewandte Chemie (International ed. in English)》2020,59(29):11937-11942
Alkaptonuria (AKU) is a rare disease characterized by high levels of homogentisic acid (HGA); patients suffer from tissue ochronosis: dark brown pigmentation, especially of joint cartilage, leading to severe early osteoarthropathy. No molecular mechanism links elevated HGA to ochronosis; the pigment's chemical identity is still not known, nor how it induces joint cartilage degradation. Here we give key insight on HGA‐derived pigment composition and collagen disruption in AKU cartilage. Synthetic pigment and pigmented human cartilage tissue both showed hydroquinone‐resembling NMR signals. EPR spectroscopy showed that the synthetic pigment contains radicals. Moreover, we observed intrastrand disruption of collagen triple helix in pigmented AKU human cartilage, and in cartilage from patients with osteoarthritis. We propose that collagen degradation can occur via transient glycyl radicals, the formation of which is enhanced in AKU due to the redox environment generated by pigmentation. 相似文献
92.
Susanne Bhr Sabine Brinkmann‐Chen Marc Garcia‐Borrs John M. Roberts Dimitris E. Katsoulis K. N. Houk Frances H. Arnold 《Angewandte Chemie (International ed. in English)》2020,59(36):15507-15511
Compared to the biological world's rich chemistry for functionalizing carbon, enzymatic transformations of the heavier homologue silicon are rare. We report that a wild‐type cytochrome P450 monooxygenase (P450BM3 from Bacillus megaterium, CYP102A1) has promiscuous activity for oxidation of hydrosilanes to give silanols. Directed evolution was applied to enhance this non‐native activity and create a highly efficient catalyst for selective silane oxidation under mild conditions with oxygen as the terminal oxidant. The evolved enzyme leaves C?H bonds present in the silane substrates untouched, and this biotransformation does not lead to disiloxane formation, a common problem in silanol syntheses. Computational studies reveal that catalysis proceeds through hydrogen atom abstraction followed by radical rebound, as observed in the native C?H hydroxylation mechanism of the P450 enzyme. This enzymatic silane oxidation extends nature's impressive catalytic repertoire. 相似文献
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The recent publication of Reuschling and co-workers2 on the synthesis of a carbapenem utilizing, as the key step in the sequence, the stereospecific C-C coupling of 3-substituted-acetoxy-2-azetidinones with cuprates, prompts us to report our related work in this area. 相似文献
96.
Bruno Casella Gareth Roberts Osnat Stramer 《Methodology and Computing in Applied Probability》2011,13(4):835-854
A broad class of implicit or partially implicit time discretizations for the Langevin diffusion are considered and used as
proposals for the Metropolis–Hastings algorithm. Ergodic properties of our proposed schemes are studied. We show that introducing
implicitness in the discretization leads to a process that often inherits the convergence rate of the continuous time process.
These contrast with the behavior of the naive or Euler–Maruyama discretization, which can behave badly even in simple cases.
We also show that our proposed chains, when used as proposals for the Metropolis–Hastings algorithm, preserve geometric ergodicity
of their implicit Langevin schemes and thus behave better than the local linearization of the Langevin diffusion. We illustrate
the behavior of our proposed schemes with examples. Our results are described in detail in one dimension only, although extensions
to higher dimensions are also described and illustrated. 相似文献
97.
D. Robert Lloyd Peter J. Roberts Ian H. Hillier Ian C. Shenton 《Molecular physics》2013,111(5):1549-1556
He(I), He(II) and variable temperature Ne(I) photoelectron spectra of sulphur trioxide are reported. All six of the expected ionic states have been detected and assigned; the assignment differs from a previous one. Ab initio calculations in a medium basis set have been carried out and spectral intensities have been calculated; agreement between the calculations and the spectra is good. The calculations show a substantial S(3d) character in the S-O bond, and the spectra are consistent with this. 相似文献
98.
Marco Rahm David Schurig Daniel A. Roberts Steven A. Cummer David R. Smith John B. Pendry 《Photonics and Nanostructures》2008,6(1):87-VII
The technique of applying form-invariant, spatial coordinate transformations of Maxwell’s equations can facilitate the design of structures with unique electromagnetic or optical functionality. Here, we illustrate the transformation-optical approach in the designs of a square electromagnetic cloak and an omni-directional electromagnetic field concentrator. The transformation equations are described and the functionality of the devices is numerically confirmed by two-dimensional finite element simulations. The two devices presented demonstrate that the transformation optic approach leads to the specification of complex, anisotropic and inhomogeneous materials with well directed and distinct electromagnetic behavior. 相似文献
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James B. Gerken Claire Badger Christopher Bisbee Sasha Gardner Yan Qi Víctor Durà Vilà John D. Roberts 《Journal of Physical Organic Chemistry》2008,21(3):193-197
Conformational preferences of glutaric, 3‐hydroxyglutaric and 3‐methylglutaric acid, and their mono‐ and dianions have been investigated with the aid of NMR spectroscopy. In contrast to succinic acid, glutaric acid displays essentially statistical conformational equilibria in polar and non‐polar solutions of high and low hydrogen‐bonding ability with no clear evidence for intramolecular hydrogen‐bonding interactions. The acid ionization constant ratios, K 1/K2, in D2O and DMSO of glutaric, 3‐hydroxyglutaric, and 3‐methylglutaric acids also indicate that intramolecular interactions are much less important than, or indeed insignificant, for shorter‐chain acids. FTIR studies on 3‐methylglutaric acid indicate some preference for either association with solvent or dimerization, depending on the solvent, rather than intramolecular hydrogen bonding. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献