全文获取类型
收费全文 | 1987篇 |
免费 | 75篇 |
国内免费 | 10篇 |
专业分类
化学 | 1380篇 |
晶体学 | 13篇 |
力学 | 25篇 |
数学 | 359篇 |
物理学 | 295篇 |
出版年
2023年 | 12篇 |
2020年 | 37篇 |
2019年 | 39篇 |
2018年 | 13篇 |
2017年 | 16篇 |
2016年 | 44篇 |
2015年 | 43篇 |
2014年 | 56篇 |
2013年 | 68篇 |
2012年 | 109篇 |
2011年 | 111篇 |
2010年 | 87篇 |
2009年 | 70篇 |
2008年 | 93篇 |
2007年 | 83篇 |
2006年 | 60篇 |
2005年 | 66篇 |
2004年 | 53篇 |
2003年 | 37篇 |
2002年 | 30篇 |
2001年 | 17篇 |
2000年 | 25篇 |
1999年 | 18篇 |
1997年 | 20篇 |
1996年 | 12篇 |
1995年 | 12篇 |
1994年 | 17篇 |
1993年 | 16篇 |
1992年 | 22篇 |
1991年 | 14篇 |
1988年 | 12篇 |
1987年 | 17篇 |
1985年 | 23篇 |
1984年 | 15篇 |
1983年 | 14篇 |
1981年 | 14篇 |
1980年 | 14篇 |
1978年 | 13篇 |
1956年 | 16篇 |
1934年 | 26篇 |
1933年 | 15篇 |
1932年 | 29篇 |
1931年 | 24篇 |
1930年 | 21篇 |
1929年 | 13篇 |
1928年 | 16篇 |
1926年 | 23篇 |
1923年 | 19篇 |
1922年 | 16篇 |
1910年 | 11篇 |
排序方式: 共有2072条查询结果,搜索用时 36 毫秒
1.
2.
Gary W. Breton Lauren A. Hahn Kenneth L. Martin 《Acta Crystallographica. Section C, Structural Chemistry》2019,75(9):1208-1212
Tetrahydrotetrazoles are five‐membered‐ring heterocycles containing four contiguous saturated nitrogen atoms. Very few examples of such compounds have been reported in the literature. Our previous attempt at the synthesis of a member of this class of compound suggested that the N—N bonds may be more labile than expected. This finding raised the question as to whether the structures of any of the previously reported tetrahydrotetrazoles had been properly assigned. We have reproduced the synthesis of a reported tetrahydrotetrazole, namely 1,2‐di‐tert‐butyl 3‐phenyl‐1H,2H,3H,10bH‐[1,2,3,4]tetrazolo[5,1‐a]isoquinoline‐1,2‐dicarboxylate, C25H30N4O4, and have now confidently confirmed its structure via X‐ray crystallography. However, while sufficiently stable in the crystal phase, we discovered that it remains very labile in solution (having a half‐life of only 15 min at 20 °C in CDCl3). A tentative reaction pathway for its dissociation based on 1H NMR spectral evidence is provided. 相似文献
3.
4.
Pieter Bouckenooge Oliver Bauer Kathrin Eckhard Herman Terryn 《Surface and interface analysis : SIA》2020,52(4):174-184
In this paper, the behaviour of the industrial applied Ti/Zr conversion coating (CC) pretreatment on rolled automotive aluminium samples (AA5182) is analysed. Due to its nanometre lateral and depth resolution, Auger electron spectroscopy (AES) is used to analyse the CC distribution at surface cathodic intermetallic particles. As a result of its high surface sensitivity, the AES technique is very susceptible to differences in the top contamination layer thickness. It is demonstrated that AES point measurements performed on aluminium model samples coated with 1.5 and 3 nm of Ti (oxide) layer cannot differentiate the two-layer thicknesses if a difference in the top contamination thickness is not taken into account. A data analysis methodology is introduced, based on the ratio of normalized peak areas (enrichment ratios), to eliminate the effect of the contamination layer thickness. The experimental validation of the methodology is performed on the model samples, demonstrating errors of 2% on the enrichment ratios on similar samples with different contamination layer thicknesses, while the conventional spectra quantification results in errors of 49%. The methodology is also theoretically substantiated within certain constraints. By the use of the AES methodology, an enriched Ti and Zr deposition is confirmed at the cathodic intermetallic particles at the surface of the industrial no-rinse CC sprayed automotive aluminium sheet samples. 相似文献
5.
Marcus Richter Sebastian Hahn Dr. Evgenia Dmitrieva Dr. Frank Rominger Dr. Alexey Popov Prof. Dr. Uwe H. F. Bunz Prof. Dr. Xinliang Feng Dr. Reinhard Berger 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(5):1345-1352
Polycyclic aromatic azomethine ylides (PAMYs) are powerful building blocks in the bottom-up synthesis of internally nitrogen-containing polycyclic aromatic hydrocarbons (N-PAHs) through 1,3-cycloaddition reactions. In this work, the cycloaddition reaction of PAMYs to asymmetric ortho-quinones is presented, which, in contrast to the addition to symmetric para-quinones, facilitates subsequent condensation reactions and allows the synthesis of three helical N-PAHs with ullazine-quinoxaline ( UQ - 1 – 3 ) backbones. UQ - 1 and UQ - 2 possess two helical centers; however, single-crystal X-ray analysis together with the computational modeling of UQ - 3 elucidate the formation of only the thermodynamically most stable geometry with four helical centers in a (P,P,M,M) configuration. For the series UQ - 1 – 3 , the number of redox steps is directly correlated with the number of ullazine or quinoxaline units incorporated into the targeted molecular backbones. A detailed investigation of the spectroscopic and magnetic properties of the radical cation and anion as well as the dication and dianion species by in situ EPR/UV/Vis-NIR spectroelectrochemistry is provided. The excellent optical and redox properties combined with helical geometries render them possibly applicable as chiral emitter or ambipolar charge transport material in organic electronics. 相似文献
6.
Bousquet, Lochet and Thomassé recently gave an elegant proof that for any integer , there is a least integer such that any tournament whose arcs are coloured with colours contains a subset of vertices of size with the property that any vertex not in admits a monochromatic path to some vertex of . In this note we provide a lower bound on the value . 相似文献
7.
David M. Koshy Dr. Shucheng Chen Dr. Dong Un Lee Dr. Michaela Burke Stevens Ahmed M. Abdellah Samuel M. Dull Gan Chen Dr. Dennis Nordlund Dr. Alessandro Gallo Dr. Christopher Hahn Dr. Drew C. Higgins Dr. Zhenan Bao Dr. Thomas F. Jaramillo 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(10):4072-4079
Ni,N-doped carbon catalysts have shown promising catalytic performance for CO2 electroreduction (CO2R) to CO; this activity has often been attributed to the presence of nitrogen-coordinated, single Ni atom active sites. However, experimentally confirming Ni−N bonding and correlating CO2 reduction (CO2R) activity to these species has remained a fundamental challenge. We synthesized polyacrylonitrile-derived Ni,N-doped carbon electrocatalysts (Ni-PACN) with a range of pyrolysis temperatures and Ni loadings and correlated their electrochemical activity with extensive physiochemical characterization to rigorously address the origin of activity in these materials. We found that the CO2R to CO partial current density increased with increased Ni content before plateauing at 2 wt % which suggests a dispersed Ni active site. These dispersed active sites were investigated by hard and soft X-ray spectroscopy, which revealed that pyrrolic nitrogen ligands selectively bind Ni atoms in a distorted square-planar geometry that strongly resembles the active sites of molecular metal–porphyrin catalysts. 相似文献
8.
9.
10.
Dr. Narayan Sinha Tristan T. Y. Tan Prof. Dr. Eduardo Peris Prof. Dr. F. Ekkehardt Hahn 《Angewandte Chemie (International ed. in English)》2017,56(26):7393-7397
Highly selective, narcissistic self-sorting has been observed in the one-pot synthesis of three organometallic molecular cylinders of type [M3{L-(NHC)3}2](PF6)3 (M=Ag+, Au+; L=1,3,5-benzene, triphenylamine, or 1,3,5-triphenylbenzene) from L-(NHC)3 and silver(I) or gold(I) ions. The molecular cylinders contain only one type of tris-NHC ligand with no crossover products detectable. Transmetalation of the tris-NHC ligands from Ag+ to Au+ in a one-pot reaction with retention of the supramolecular structures is also demonstrated. High-fidelity self-sorting was also observed in the one-pot reaction of benzene-bridged tris-NHC and tetrakis-NHC ligands with Ag2O. This study for the first time extends narcissistic self-sorting in metal–ligand interactions from Werner-type complexes to organometallic derivatives. 相似文献