全文获取类型
收费全文 | 844篇 |
免费 | 50篇 |
国内免费 | 3篇 |
专业分类
化学 | 617篇 |
晶体学 | 7篇 |
力学 | 4篇 |
数学 | 152篇 |
物理学 | 117篇 |
出版年
2023年 | 8篇 |
2022年 | 10篇 |
2021年 | 19篇 |
2020年 | 17篇 |
2019年 | 27篇 |
2018年 | 12篇 |
2017年 | 9篇 |
2016年 | 34篇 |
2015年 | 20篇 |
2014年 | 30篇 |
2013年 | 37篇 |
2012年 | 36篇 |
2011年 | 41篇 |
2010年 | 27篇 |
2009年 | 31篇 |
2008年 | 56篇 |
2007年 | 37篇 |
2006年 | 45篇 |
2005年 | 52篇 |
2004年 | 30篇 |
2003年 | 28篇 |
2002年 | 24篇 |
2001年 | 11篇 |
2000年 | 9篇 |
1999年 | 7篇 |
1998年 | 9篇 |
1997年 | 5篇 |
1996年 | 12篇 |
1995年 | 7篇 |
1994年 | 10篇 |
1993年 | 9篇 |
1992年 | 4篇 |
1991年 | 11篇 |
1990年 | 7篇 |
1989年 | 5篇 |
1988年 | 10篇 |
1986年 | 10篇 |
1985年 | 11篇 |
1984年 | 13篇 |
1983年 | 11篇 |
1982年 | 7篇 |
1981年 | 9篇 |
1980年 | 9篇 |
1979年 | 13篇 |
1978年 | 14篇 |
1977年 | 6篇 |
1976年 | 9篇 |
1975年 | 6篇 |
1973年 | 5篇 |
1971年 | 5篇 |
排序方式: 共有897条查询结果,搜索用时 15 毫秒
141.
Dr. Paul J. Evans Dr. Jiangkun Ouyang Dr. Ludovic Favereau Dr. Jeanne Crassous Dr. Israel Fernández Dr. Josefina Perles Prof. Dr. Nazario Martín 《Angewandte Chemie (International ed. in English)》2018,57(23):6774-6779
A rigid, inherently chiral bilayer nanographene has been synthesized as both the racemate and enantioenriched M isomer (with 93 % ee) in three steps from established helicenes. This folded nanographene is composed of two hexa‐peri‐hexabenzocoronene layers fused to a [10]helicene, with an interlayer distance of 3.6 Å as determined by X‐ray crystallography. The rigidity of the helicene linker forces the layers to adopt a nearly aligned AA‐stacked conformation, rarely observed in few‐layer graphene. By combining the advantages of nanographenes and helicenes, we have constructed a bilayer system of 30 fused benzene rings that is also chiral, rigid, and remains soluble in common organic solvents. We present this as a molecular model system of bilayer graphene, with properties of interest in a variety of potential applications. 相似文献
142.
Alexandre V. Borovik Israel Gelfand Neil White 《Journal of Algebraic Combinatorics》1998,8(3):235-252
A symplectic matroid is a collection B of k-element subsets of J = {1, 2, ..., n, 1*, 2*, ...; n*}, each of which contains not both of i and i* for every i n, and which has the additional property that for any linear ordering of J such that i j implies j* i* and i j* implies j i* for all i, j n, B has a member which dominates element-wise every other member of B. Symplectic matroids are a special case of Coxeter matroids, namely the case where the Coxeter group is the hyperoctahedral group, the group of symmetries of the n-cube. In this paper we develop the basic properties of symplectic matroids in a largely self-contained and elementary fashion. Many of these results are analogous to results for ordinary matroids (which are Coxeter matroids for the symmetric group), yet most are not generalizable to arbitrary Coxeter matroids. For example, representable symplectic matroids arise from totally isotropic subspaces of a symplectic space very similarly to the way in which representable ordinary matroids arise from a subspace of a vector space. We also examine Lagrangian matroids, which are the special case of symplectic matroids where k = n, and which are equivalent to Bouchet's symmetric matroids or 2-matroids. 相似文献
143.
Rajesh Koner Israel Goldberg 《Acta Crystallographica. Section C, Structural Chemistry》2008,64(7):m264-m266
The title compound, catena‐poly[[[heptaaqualanthanum(III)]‐μ‐1,3‐dioxo‐2‐oxa‐1H,3H‐phenalene‐6,7‐dicarboxylato‐κ2O6:O7] hemi(4,8‐dicarboxynaphthalene‐1,5‐dicarboxylate) dihydrate], {[La(C14H4O7)(H2O)7](C14H6O8)0.5·2H2O}n, is a dihydrate of a coordination polymer between the dianion of naphthalene‐1,4,5,8‐tetracarboxylic 1,8‐anhydride and the heptahydrated lanthanum(III) ion, charge balanced by an additional 4,8‐dicarboxynaphthalene‐1,5‐dicarboxylate dianion that is located on an inversion centre and is not coordinated to the metal ion. The linear polymeric arrays adopt a comb‐like structure, and these pack in pairs with one chain interpenetrating another, like two parts of a zip, to optimize stacking interactions between their ligand fragments. All the components of this compound are further interlinked by an extensive pattern of O—H...O hydrogen bonds throughout the crystal structure. The main scientific significance of the results reported here is that they demonstrate for the first time the feasibility of coordination polymerization of the above organic ligand with lanthanide ions. The resulting polymer has a unique architecture. Finally, the reported structure is a rare example where the tetraacid and the diacid anhydride ligand species co‐exist in the same crystal. 相似文献
144.
145.
146.
Dr. Lucas Wickemeyer Prof. Dr. Israel Fernández Beate Neumann Dr. Hans-Georg Stammler Prof. Dr. Norbert W. Mitzel 《Angewandte Chemie (International ed. in English)》2023,62(11):e202216943
The reaction of the oxygen-bridged frustrated Lewis pairs (FLPs) tBu2P−O−Si(C2F5)3 ( 1 ) and tBu2P−O−AlBis2 ( 2 ) with azobenzene, promoted by UV irradiation, led to a selective complexation of the cis-isomer. The addition product of 2 is stable, while the adduct of 1 isomerizes in solution in an ortho-benzidine-like [3,3]-rearrangement by cleavage of the N−N bond, saturation of the nitrogen atoms with hydrogen atoms and formation of a new bond between two phenyl ortho-carbon atoms. Similar rearrangements take place with different para-substituted azobenzenes (R=Me, OMe, Cl) and di(2-naphthyl)diazene, while ortho-methylated azo compounds do not form adducts with 1 . All adducts were characterized by multinuclear NMR spectroscopy and elemental analyses and the mechanism of the rearrangement was explored by quantum-chemical calculations. 相似文献
147.
Jian Liu Zhen Zhao ChunMing Xu AiJun Duan GuiYuan Jiang JinSen Gao WenYong Lin Israel E. Wachs 《中国科学B辑(英文版)》2008,51(6):551-561
UV-Raman spectroscopy was used to study the molecular structures of TiO2 or ZrO2-supported vanadium oxide catalysts. The real time reaction status of soot combustion over these catalysts was detected by
in-situ UV-Raman spectroscopy. The results indicate that TiO2 undergoes a crystalline phase transformation from anatase to rutile phase with the increasing of reaction temperature. However,
no obvious phase transformation process is observed for ZrO2 support. The structures of supported vanadium oxides also depend on the V loading. The vanadium oxide species supported on
TiO2 or ZrO2 attain monolayer saturation when V loading is equal to 4 (4 is the number of V atoms per 100 support metal ions). Interestingly,
this loading ratio (V4/TiO2 and V4/ZrO2) gave the best catalytic activities for soot combustion reaction on both supports (TiO2 and ZrO2). The formation of surface oxygen complexes (SOC) is verified by in-situ UV Raman spectroscopy and the SOC mainly exist as carboxyl groups during soot combustion. The presence of NO in the reaction
gas stream can promote the production of SOC.
Supported by the National Natural Science Foundation of China (Grant Nos. 20473053, 20773163 and 20525621), the Beijing Natural
Science Foundation (Grant No. 2062020), and the 863 Program of China (Grant No. 2006AA06Z346) 相似文献
148.
Patricia Izquierdo-García Dr. Jesús M. Fernández-García Dr. Josefina Perles Dr. Israel Fernández Prof. Dr. Nazario Martín 《Angewandte Chemie (International ed. in English)》2023,62(7):e202218927
Scholl oxidation has become an essential reaction in the bottom-up synthesis of molecular nanographenes. Herein, we describe a Scholl reaction controlled by the electronic effects on the starting substrate ( 1 a , b ). Anthracene-based polyphenylenes lead to spironanographenes under Scholl conditions. In contrast, an electron-deficient anthracene substrate affords a helically arranged molecular nanographene formed by two orthogonal dibenzo[fg,ij]phenanthro-[9,10,1,2,3-pqrst]pentaphene (DBPP) moieties linked through an octafluoroanthracene core. Density Functional Theory (DFT) calculations predict that electronic effects control either the first formation of spirocycles and subsequent Scholl reaction to form spironanographene 2 , or the expected dehydrogenation reaction leading solely to the helical nanographene 3 . The crystal structures of four of the new spiro compounds (syn 2 , syn 9 , anti 9 and syn 10 ) were solved by single crystal X-ray diffraction. The photophysical properties of the new molecular nanographene 3 reveal a remarkable dual fluorescent emission. 相似文献
149.
Israel Cabasso Youxin Yuan Xinwei Wang 《Journal of polymer science. Part A, Polymer chemistry》2008,46(23):7677-7689
An electropolymerization of haloalkylhalosilanes (Cl? R? SiCH3Cl2) that possess two types of electroactive sites, that is, the C? Cl and Si? Cl bond is described. The one‐pot synthesis method is shown to yield branched polycarbosilanes having a regular carbon block‐spaced silicon backbone structure. A series of branched polycarbosilanes, [? R? SiCH3? ]n with R being ? CH2? , ? C2H4? , ? C3H6? , and ? CH2? C6H4? C2H4? , have been successfully electropolymerized with Mn up to 42,600 Dalton. Experimental and simulation cyclic voltammetry of these monomers and the computational examination of their LUMOs are applied to study the electropolymerization mechanism. The results suggest that polymerization proceeds by iterating steps involving (1) electroreduction of a C? Cl bond to a carbanion, which is catalyzed by silylanion radical [Cl SiCl(CH3)RCl] and/or Ni(0)/TDA‐1; and (2) nucleophilic attack of carbanions to Si? Cl bonds of a second monomer or oligomer to extend the polymer chain. The investigation reveals that the R spacer has a considerable impact on the polymerizability of the corresponding monomer. Such interfacial polymerization resembles a template polymerization, leading to unique microstructures that were preserved even after converted to silicon carbide ceramics at high temperatures. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7677–7689, 2008 相似文献
150.
Nguyen HP Ortiz IP Temiyasathit C Kim SB Schug KA 《Rapid communications in mass spectrometry : RCM》2008,22(14):2220-2226
Crude oil fingerprints were obtained from four crude oils by laser desorption/ionization mass spectrometry (LDI-MS) using a silver nitrate cationization reagent. Replicate analyses produced spectral data with a large number of features for each sample (>11,000 m/z values) which were statistically analyzed to extract useful information for their differentiation. Individual characteristic features from the data set were identified by a false discovery rate based feature selection procedure based on the analysis of variance models. The selected features were, in turn, evaluated using classification models. A substantially reduced set of 23 features was obtained through this procedure. One oil sample containing a high ratio of saturated/aromatic hydrocarbon content was easily distinguished from the others using this reduced set. The other three samples were more difficult to distinguish by LDI-MS using a silver cationization reagent; however, a minimal number of significant features were still identified for this purpose. Focus is placed on presenting this multivariate statistical method as a rapid and simple analytical procedure for classifying and distinguishing complex mixtures. 相似文献