全文获取类型
收费全文 | 660篇 |
免费 | 110篇 |
国内免费 | 37篇 |
专业分类
化学 | 443篇 |
晶体学 | 3篇 |
力学 | 10篇 |
综合类 | 1篇 |
数学 | 25篇 |
物理学 | 325篇 |
出版年
2023年 | 8篇 |
2022年 | 4篇 |
2021年 | 13篇 |
2020年 | 24篇 |
2019年 | 18篇 |
2018年 | 18篇 |
2017年 | 13篇 |
2016年 | 23篇 |
2015年 | 33篇 |
2014年 | 27篇 |
2013年 | 61篇 |
2012年 | 32篇 |
2011年 | 55篇 |
2010年 | 57篇 |
2009年 | 43篇 |
2008年 | 50篇 |
2007年 | 52篇 |
2006年 | 34篇 |
2005年 | 35篇 |
2004年 | 39篇 |
2003年 | 35篇 |
2002年 | 29篇 |
2001年 | 17篇 |
2000年 | 15篇 |
1999年 | 16篇 |
1998年 | 10篇 |
1997年 | 9篇 |
1996年 | 4篇 |
1995年 | 5篇 |
1994年 | 6篇 |
1993年 | 4篇 |
1991年 | 1篇 |
1990年 | 2篇 |
1989年 | 1篇 |
1986年 | 1篇 |
1985年 | 2篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1976年 | 2篇 |
1975年 | 1篇 |
1972年 | 1篇 |
排序方式: 共有807条查询结果,搜索用时 937 毫秒
1.
This review demonstrates the multiple roles of surfactants in aqueous micellar catalysis. It covers the design and recent applications of proline-based amphiphile PS-750-M, including completely organic solvent-free amide couplings, C–H fluorination of arenes and heteroarenes achieved via radical pathway facilitated by the shielding effect of micelles. In addition, it critically sheds light on selective hydrogenolysis and cross-couplings of water-sensitive acid chlorides in water, catalyzed by phosphine ligand-free Pd (0) nanoparticles. The metal-micelle interaction responsible for catalytic activities as probed by various spectroscopic techniques is also discussed. 相似文献
2.
Nitin P. Lobo K.V. Ramanathan T. Narasimhaswamy 《Magnetic resonance in chemistry : MRC》2020,58(11):988-1009
In this review, methods to obtain the orientational order of topologically variant molecular mesogens using by one- and two-dimensional (2D) solid-state 13C nuclear magnetic resonance (NMR) spectroscopy are described. Besides 13C chemical shifts, the 13C─1H dipolar couplings measured from 2D-separated local field (SLF) technique are used for computing the order parameters of a variety of mesogens. The investigated molecules are composed of a variable number of rings in the core, that is, core ranging from simply one ring to five rings. Among the mesogens investigated, a special focus has been placed on mesogens with thiophene rings, which are gaining popularity as liquid crystalline organic semiconductors. The replacement of a phenyl ring by thiophene in the core has a dramatic influence on molecular topology, as observed from the measured order parameters. The review highlights the advantages of the 2D SLF method for understanding the local dynamics and for mapping the topology of mesogens through the measured order parameters. SLF NMR studies of as many as 24 molecular mesogens that vary in terms of the molecular structure as well as topology are covered in the review. Order parameters of the rings have been estimated from the 13C─1H dipolar couplings in the nematic, smectic A, smectic C, and tilted hexatic phases as well as in B1 and B2 mesophases of various mesogens. It is anticipated that, in the years to come, the 2D SLF method would provide advanced molecular information on structurally complex mesogens that are emerging in liquid crystal science through the incessant efforts of synthetic chemists. The mini review covers the orientational order of topologically variant molecular mesogens determined by 1D and 2D solid-state 13C NMR spectroscopy. Accordingly, rod-like, bent-core, and thiophene mesogens were subjected to 2D SLF measurements to get the order parameters from which the topology was established. The replacement of phenyl ring by thiophene and its influence on order parameters as well as on molecular topology is also discussed. 相似文献
3.
Dr. Si-Yong Qin Yan Jiang Dr. Han Sun Han Liu Prof. Ai-Qing Zhang Prof. Xinxiang Lei 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(39):17245-17251
Residual dipolar coupling (RDC), a robust anisotropic NMR parameter for structural elucidation of organic molecules, is only accessible in an anisotropic environment. Herein, we introduce a novel alignment medium based on the molecular self-assembly of oligopeptide amphiphile (OPA). This medium is compatible with different intermediate and polar solvent systems, such as CD3OD, [D6]DMSO, and D2O. The preparation of the OPA-based medium is simple and rapid, while only very weak background signals were observed from OPAs. Furthermore, we show that the purity of OPA has only a minor influence on the quality of the RDC data. These advantages allow RDC measurements of organic molecules with different polarities and solubilities with high efficiency and accuracy. 相似文献
4.
Phase modulation in dipolar-coupled A2 spin systems: effect of maximum state mixing in H NMR in vivo
Leif Schrder Christian Schmitz Peter Bachert 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2004,171(2):69
Coupling constants of nuclear spin systems can be determined from phase modulation of multiplet resonances. Strongly coupled systems such as citrate in prostatic tissue exhibit a more complex modulation than AX connectivities, because of substantial mixing of quantum states. An extreme limit is the coupling of n isochronous spins (An system). It is observable only for directly connected spins like the methylene protons of creatine and phosphocreatine which experience residual dipolar coupling in intact muscle tissue in vivo. We will demonstrate that phase modulation of this “pseudo-strong” system is quite simple compared to those of AB systems. Theory predicts that the spin-echo experiment yields conditions as in the case of weak interactions, in particular, the phase modulation depends linearly on the line splitting and the echo time. 相似文献
5.
Keyang Ding Angela M. Gronenborn 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2004,167(2):554-258
Sensitivity-enhanced 2D IPAP experiments using the accordion principle for measuring one-bond 13C'-13Calpha and 1Halpha-13Calpha dipolar couplings in proteins are presented. The resolution of the resulting spectra is identical to that of the decoupled HSQC spectra and the sensitivity of the corresponding 1D acquisitions are only slightly lower than those obtained with 3D HNCO and 3D HN(COCA)HA pulse sequences due to an additional delay 2Delta. For cases of limited resolution in the 2D 15N-1HN HSQC spectrum the current pulse sequences can easily be modified into 3D versions by introducing a poorly digitized third dimension, if so desired. The experiments described here are a valuable addition to the suites available for determination of residual dipolar couplings in biological systems. 相似文献
6.
7.
François Loiseau Jean-Mary Simone David Carcache Pavel Bobal Reinhard Neier 《Monatshefte für Chemie / Chemical Monthly》2007,138(2):121-129
Summary. Free radical couplings from furan, as cheap starting material, were studied in view of developing a rapid strategy en route to the synthesis of derivatives of nonactin. The chain containing the alcohol function was introduced in one or two steps
in 86% yield. For the introduction of the second chain with the ester function two different coupling methods were tested.
Starting from the advanced intermediates obtained nonactin derivatives can be prepared by catalytic hydrogenation of the furan
ring. 相似文献
8.
It is shown that in-situ
166mHo (I = 7) in a spherical single crystal of HoF3 can be used as sensitive internal thermometer to thermally detect NMR (NMR-TDNO) from the 100% abundant stable 165Ho (I = 7/2) nuclei. In addition, new 166mHo NMRON results are reported. Both the 166mHo NMRON and 165Ho NMR-TDNO spectra show three distinct quadrupolar split sub-resonances, in zero applied field. The data is used to make
estimates of the Ho magnetic moments and quadrupole parameters for the 166mHo and 166mHo sites. 相似文献
9.
Nessan J. Kerrigan 《Tetrahedron letters》2004,45(49):9087-9090
The samarium(II) iodide-mediated coupling of ketones with β-alkoxyacrylates gives β-hydroxy-γ-butyrolactones in moderate yield. The process has been applied to the asymmetric synthesis of an antifungal, γ-butyrolactone natural product. 相似文献
10.
Christian Kluger Wolfgang H. Binder 《Journal of polymer science. Part A, Polymer chemistry》2007,45(3):485-499
Block copolymers on basis of poly(oxanorbornenes) bearing functional moieties in their side‐chains are prepared via a combination of ROMP‐methods and 1,3‐dipolar‐“click”‐reactions. Starting from N‐substituted‐ω‐bromoalkyl‐oxanorbornenes and alkyl‐/perfluoroalkyl‐oxanorbornenes, block copolymers with molecular weights up to 25,000 g mol?1 were generated. Subsequent nucleophilic exchange‐reactions yielded the block‐copolymers functionalized with ω‐azidoalkyl‐moieties in one block. The 1,3‐azide/alkine‐“click” reactions with a variety of terminal alkynes in the presence of a catalyst system consisting of tetrakis(acetonitrile)hexafluorophosphate copper(I) and tris(1‐benzyl‐5‐methyl‐1H‐ [1,2,3]triazol‐4‐ylmethyl)‐amine furnished the substituted block copolymers in high yields, as proven by NMR‐spectroscopy. The resulting polymers were investigated via temperature‐dependent SAXS‐methods, revealing their microphase separated structure as well as their temperature‐dependent behavior. The presented method offers the generation of a large set of different block‐copolymers from only a small set of starting materials because of the high versatility of the “click” reaction, thus enabling a simple and complete functionalization after the initial polymerization reaction. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 485–499, 2007 相似文献