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排序方式: 共有121条查询结果,搜索用时 156 毫秒
101.
Seitaro Koshino Akira Takikawa Keiichi Ishida Prof. Dr. Tohru Taniguchi Prof. Dr. Kenji Monde Prof. Dr. Eunsang Kwon Dr. Shigenobu Umemiya Prof. Dr. Yujiro Hayashi 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(20):4524-4530
The pot-economical, highly enantioselective synthesis of axially chiral biaryls was developed by using one-pot organocatalyst-mediated domino and aromatization reactions as key steps. The axial information of the precursor, which also has central chirality, was completely inverted in the final biaryls. The inversion of the axial information occurred in the conversion of the central chirality to the axial chirality of an oxidative aromatization step. 相似文献
102.
Dr. Seitaro Koshino Prof. Dr. Tohru Taniguchi Prof. Dr. Kenji Monde Prof. Dr. Eunsang Kwon Prof. Dr. Yujiro Hayashi 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(63):15786-15794
Enantiodivergent one-pot synthesis of biaryls was developed using a catalytic amount of a single chiral source. A domino organocatalyst-mediated enantioselective Michael reaction and aldol condensation provided centrally chiral dihydronaphthalenes with excellent enantioselectivity, from which an enantiodivergent chirality conversion from central-to-axial chirality was achieved. Both enantiomers of biaryls were obtained with excellent enantioselectivity. All transformations can be conducted in a single reaction vessel. A plausible reaction mechanism for the enantiodivergence is proposed. 相似文献
103.
Yoshitaka Yamaguchi Noriyuki Suzuki Arno Fries Takaya Mise Hiroyuki Koshino Yuji Ikegami Hiromichi Ohmori Akira Matsumoto 《Journal of polymer science. Part A, Polymer chemistry》1999,37(3):283-292
Stereospecific polymerization of 1‐hexene under high pressures (up to 1,000 MPa = ca. 10,000 atm) using metallocene/methylaluminoxane (MAO) catalysts was investigated. Several C2‐symmetric ansa‐metallocenes, their meso‐isomers, and two Cs‐symmetric ansa‐metallocenes were employed as catalyst precursors. In the course of this study, novel C2‐symmetric germylene‐bridged ansa‐metallocenes, (rac‐[Me2Ge(η5‐C5H‐2,3,5‐Me3)2MCl2] (M = Zr, rac‐4a; M = Hf, rac‐4b), have been prepared. High pressures induced enhancement of the catalytic activity and the molecular weight of the polymers in most of the catalysts. The maximum of both the catalytic activity and the molecular weight of the polymers was mostly observed at 100–500 MPa in each catalyst, although the enhanced ratio was smaller than that observed for nonbridged metallocenes. Isospecificity of the C2‐symmetric ansa‐metallocene catalysts was essentially maintained even under high pressure. Highly isotactic polyhexene ([mmmm] = 91.6%) with very high molecular weight (Mw = 2,360,000) was achieved by rac‐4b under 250 MPa. High pressures slightly decreased syndiotacticity when the Cs‐symmetric ansa‐metallocene, isopropylidene(1‐η5‐cyclopentadienyl)(9‐η5‐fluorenyl)zirconium dichloride 5, was employed. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 283–292, 1999 相似文献
104.
Cobalt Phenanthroline–Indole Macrocycles as Highly Active Electrocatalysts for Oxygen Reduction
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Martin Quernheim Dr. Haiwei Liang Dr. Qi Su Prof. Dr. Martin Baumgarten Dr. Nobuyoshi Koshino Dr. Hideyuki Higashimura Prof. Dr. Klaus Müllen 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(44):14178-14183
The replacement of scarce and expensive platinum species poses a challenge in fuel‐cell development. The design and synthesis of a novel type of CoII–N4 macrocyclic complex, [CoN4], based on the phenanthroline–indole macrocyclic ligand (PIM) is reported. This unique ligand allows the formation of mono‐ and dinuclear complexes with defined active sites that facilitate the direct four‐electron reduction of oxygen. Electrochemical measurements revealed that the [CoN4]/C (20 wt %) catalysts have a high activity and long‐term stability for the oxygen‐reduction reaction (ORR) under alkaline conditions, similar to the Pt/C catalyst. These structurally well‐defined complexes represent a nonprecious alternative to platinum species for future fuel‐cell applications. 相似文献
105.
Frontispiece: Cobalt Phenanthroline–Indole Macrocycles as Highly Active Electrocatalysts for Oxygen Reduction
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106.
Observation of a Metastable P‐Heterocyclic Radical by Muonium Addition to a 1,3‐Diphosphacyclobutane‐2,4‐diyl
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Prof. Dr. Shigekazu Ito Dr. Yasuhiro Ueta Kota Koshino Prof. Dr. Kenji M. Kojima Dr. Iain McKenzie Prof. Dr. Koichi Mikami 《Angewandte Chemie (International ed. in English)》2018,57(28):8608-8613
A 1,3‐diphosphacyclobutane‐2,4‐diyl contains a unique unsaturated cyclic unit, and the presence of radical‐type centers have been expected as a source of functionality. This study demonstrates that the P‐heterocyclic singlet biradical captures muonium (Mu=[μ+e?]), the light isotope of a hydrogen radical, to generate an observable P‐heterocyclic paramagnetic species. Investigation of a powder sample of 2,4‐bis(2,4,6‐tri‐t‐butylphenyl)‐1‐t‐butyl‐3‐benzyl‐1,3‐diphosphacyclobutane‐2,4‐diyl using muon (avoided) level‐crossing resonance (μLCR) spectroscopy revealed that muonium adds to the cyclic P2C2 unit. The muon hyperfine coupling constant (Aμ) indicated that the phosphorus atom bearing the t‐butyl group trapped muonium to provide a metastable P‐heterocyclic radical involving the ylidic MuP(<)=C moiety. The observed regioselective muonium addition correlates the canonical formula of 1,3‐diphosphacyclobutane‐2,4‐diyl. 相似文献
107.
Modified two-dimensional (2D) triple-resonance H(C)P and H(P)C experiments based on INEPT/HMQC and double-INEPT schemes are applied to the study of organophosphorus compounds at natural abundances. The implementation of effective (1)H--(13)C gradient selection, additional purging pulsed field gradients, spinlock pulses, and improved phase cycling is demonstrated to allow weak correlation signals based on long-range couplings to be readily observed. Through the combination of two heteronuclear long-range coupling constants, (n)J(CH) and (n)J(PC) in H(C)P experiments or (n)J(PH) and (n)J(PC) in H(P)C experiments, protons can be correlated to a second heteronucleus through 4-7 chemical bonds. These experiments thus overcome the inherit limitations of classical (1)H-X HMBC experiments, which require a nonzero value of the heteronuclear coupling constant (n)J(XH). Ultra-broadband inversion composite pulses are successfully employed in the H(P)C INEPT/HMQC and H(P)C double-INEPT pulse sequences to increase the utility of the experiments and the quality of obtained spectra. This work extends and completes a set of 2D phase-sensitive triple-resonance experiments applicable at natural abundances, and also offers insight into the methodology of triple-resonance experiments and the application of pulsed field gradients. A one-dimensional triple-resonance experiment employing carbon detection is suggested for accurate determination of small (n)J(PC). 相似文献
108.
The reaction of ate-complexes formed from 1,2-dimethoxy-ethenyllithium and trialkylboranes, with BF3:Et2O followed by hydrolysis with hydrochloric acid gives the corresponding aldehydes in good yields. 相似文献
109.
Takahashi S Hongo Y Ogawa N Koshino H Nakata T 《The Journal of organic chemistry》2006,71(16):6305-6308
This paper describes a second-generation synthesis of an antitumor tetrahydropyran (THP) acetogenin, pyragonicin. The key step involved an olefin cross-metathesis between the THP segment and the terminal gamma-lactone residue. The coupling reaction in the presence of Grubbs' second-generation catalyst resulted in an unseparable mixture of a desired coupling product and its one-carbon eliminated product while the use of Grubbs' first-generation catalyst afforded the former exclusively. A novel MOM-migrating reaction found in a cyclization reaction is also discussed. 相似文献
110.
The quantum Hall effect is studied numerically in modulated two-dimensional electron systems in the presence of disorder. Based on the scaling property of the Hall conductivity as well as the localization length, the critical energies where the states are extended are identified. We find that the critical energies, which are distributed to each of the subbands, combine into one when the disorder becomes strong, in the way depending on the symmetry of the disorder and/or the periodic potential. 相似文献