全文获取类型
收费全文 | 2154篇 |
免费 | 130篇 |
国内免费 | 7篇 |
专业分类
化学 | 1457篇 |
晶体学 | 2篇 |
力学 | 54篇 |
数学 | 422篇 |
物理学 | 356篇 |
出版年
2023年 | 53篇 |
2022年 | 35篇 |
2021年 | 66篇 |
2020年 | 108篇 |
2019年 | 86篇 |
2018年 | 28篇 |
2017年 | 54篇 |
2016年 | 103篇 |
2015年 | 84篇 |
2014年 | 84篇 |
2013年 | 94篇 |
2012年 | 133篇 |
2011年 | 149篇 |
2010年 | 76篇 |
2009年 | 56篇 |
2008年 | 118篇 |
2007年 | 90篇 |
2006年 | 89篇 |
2005年 | 73篇 |
2004年 | 56篇 |
2003年 | 48篇 |
2002年 | 52篇 |
2001年 | 20篇 |
2000年 | 17篇 |
1999年 | 11篇 |
1998年 | 16篇 |
1997年 | 10篇 |
1996年 | 9篇 |
1995年 | 9篇 |
1992年 | 9篇 |
1989年 | 8篇 |
1986年 | 9篇 |
1985年 | 13篇 |
1984年 | 8篇 |
1983年 | 9篇 |
1982年 | 10篇 |
1981年 | 13篇 |
1980年 | 10篇 |
1979年 | 13篇 |
1977年 | 13篇 |
1976年 | 10篇 |
1975年 | 9篇 |
1970年 | 8篇 |
1968年 | 9篇 |
1967年 | 8篇 |
1934年 | 11篇 |
1933年 | 8篇 |
1930年 | 8篇 |
1905年 | 7篇 |
1881年 | 9篇 |
排序方式: 共有2291条查询结果,搜索用时 15 毫秒
111.
Metal‐Free Catalyst for the Chemoselective Methylation of Amines Using Carbon Dioxide as a Carbon Source 下载免费PDF全文
Dr. Shoubhik Das Felix D. Bobbink Prof. Dr. Gabor Laurenczy Prof. Dr. Paul J. Dyson 《Angewandte Chemie (International ed. in English)》2014,53(47):12876-12879
N‐methylation of amines is an important step in the synthesis of many pharmaceuticals and has been widely applied in the preparation of other key intermediates and chemicals. Therefore, the development of efficient methylation methods has attracted considerable attention. In this respect, carbon dioxide is an attractive C1 building block because it is an abundant, renewable, and nontoxic carbon source. Consequently, we developed a highly chemoselective, metal‐free catalytic system that operates under ambient conditions for the N‐methylation of amines. 相似文献
112.
Synthesis and EPR/UV/Vis‐NIR Spectroelectrochemical Investigation of a Persistent Phosphanyl Radical Dication 下载免费PDF全文
Kai Schwedtmann Stephen Schulz Felix Hennersdorf Prof. Dr. Thomas Strassner Dr. Evgenia Dmitrieva Prof. Dr. Jan J. Weigand 《Angewandte Chemie (International ed. in English)》2015,54(38):11054-11058
The reaction of the bis(imidazoliumyl)‐substituted PI cation [(2‐ImDipp)P(4‐ImDipp)]+ ( 10 +) (2‐Im=imidazolium‐2‐yl; 4‐Im=imidazolium‐4‐yl; Dipp=2,6‐di‐isopropylphenyl) with trifluoromethanesulfonic acid (HOTf) or methyl trifluoromethylsulfonate (MeOTf) yields the corresponding protonated [(2‐ImDipp)PH(4‐ImDipp)]2+ ( 11 2+) and methylated [(2‐ImDipp)PMe(4‐ImDipp)]2+ ( 12 2+) dications, respectively. EPR/UV/Vis‐NIR spectroelectrochemical investigation of the low‐coordinated PI cation 10 + predicted a stable and “bottleable” P‐centered radical dication [(2‐ImDipp)P(4‐ImDipp)]2+. ( 13 2+.). The reaction of 10 + with the nitrosyl salt NO[OTf] yields the persistent phosphanyl radical dication 13 2+. as triflate salt in crystalline form. Quantum chemical investigation revealed an exceptional high spin density at the P atom. 相似文献
113.
Felix Anyakudo Erwin Adams 《International journal of environmental analytical chemistry》2018,98(4):323-337
Fuel oils are mostly used in marine applications and in power plants. They are known to contain hazardous volatile organic compounds (VOCs) that are of health and environmental importance. Chlorinated compounds, phenolic compounds, styrenes, indene, dicyclopentadiene, dihydrodicyclopentadiene, cumene, benzene, toluene, ethylbenzene and xylenes are some of the VOCs that have found their way into fuel oil through various streams during bunkering operation. Chromatographic analysis of VOCs in the presence of complex matrices in fuel oil is one of the major challenges encountered when dealing with products of that nature. An analytical procedure using automated static headspace gas chromatography–mass spectrometry was developed for the analysis of these compounds in fuel oil. Styrene D8 and phenol D6 were used as internal standards for quantitation. Phenol D6 was used for the quantitation of phenolic compounds, while styrene D8 was used for the quantitation of other target analytes. The influence of headspace parameters on analyte response such as temperature, incubation time and sample amount were all investigated and optimised. Linear calibration curves were achieved for all components with determination coefficients R2 > 0.995. Repeatability, limit of detection, limit of quantitation and recovery were reported. The matrix effect in fuel oil was minimised by 1:1 dilution with mineral oil. This method was successfully applied to the analysis of commercial samples. 相似文献
114.
115.
116.
117.
Exciton‐Vibrational Couplings in Homo‐ and Heterodimer Stacks of Perylene Bisimide Dyes within Cyclophanes: Studies on Absorption Properties and Theoretical Analysis 下载免费PDF全文
David Bialas Christoph Brüning Dr. Felix Schlosser Dr. Benjamin Fimmel Johannes Thein Prof. Dr. Volker Engel Prof. Dr. Frank Würthner 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(42):15011-15018
The optical properties of a series of three cyclophanes comprising either identical or different perylene bisimide (PBI) chromophores were studied by UV/Vis absorption spectroscopy and their distinctive spectral features were analyzed. All the investigated cyclophanes show significantly different absorption features with respect to the corresponding constituent PBI monomers indicating strong coupling interactions between the PBI units within the cyclophanes. DFT calculations suggest a π‐stacked arrangement of the PBI units at close van der Waals distance in the cyclophanes with rotational displacement. Simulations of the absorption spectra based on time‐dependent quantum mechanics properly reproduced the experimental spectra, revealing exciton‐vibrational coupling between the chromophores both in homo‐ and heterodimer stacks. The PBI cyclophane comprising two different PBI chromophores represents the first example of a PBI heterodimer stack for which the exciton coupling has been investigated. The quantum dynamics analysis reveals that exciton coupling in heteroaggregates is indeed of similar strength as for homoaggregates. 相似文献
118.
We present a new method for resolving combinatorial ambiguities that arise in multi-particle decay chains at hadron colliders where the assignment of visible particles to the different decay chains has ambiguities. Our method, based on selection cuts favoring high transverse momentum and low invariant mass pairings, is shown to be significantly superior to the more traditional hemisphere method for a large class of decay chains, producing an increase in signal retention of up to a factor of 2. This new method can thus greatly reduce the combinatorial ambiguities of decay chain assignments. 相似文献
119.
120.
Anzenbacher P Tyson DS Jursíková K Castellano FN 《Journal of the American Chemical Society》2002,124(22):6232-6233
A new Ru(II) complex is described which serves as a luminescence lifetime-based sensor for fluoride and cyanide anions (KF = 640 000 mol-1, KCN = 430 000 mol-1). This chromophore displays observable changes in its UV-vis and steady-state luminescence spectra upon cyanide binding. Prior to cyanide addition, this complex exhibits a single-exponential lifetime (tau = 377 +/- 20 ns). With increasing cyanide concentrations, the intensity decays are composed of two exponentials: long tau (320-370 ns) and short tau (13-17 ns). The average lifetimes shorten as a function of cyanide concentration since the fractional intensity shifts from an initial dominant long lifetime component to the short lifetime component. This work represents the first example of a direct method for the luminescence lifetime-based sensing of anions. 相似文献