全文获取类型
收费全文 | 1565篇 |
免费 | 48篇 |
国内免费 | 12篇 |
专业分类
化学 | 990篇 |
晶体学 | 2篇 |
力学 | 32篇 |
数学 | 181篇 |
物理学 | 420篇 |
出版年
2021年 | 12篇 |
2020年 | 13篇 |
2019年 | 17篇 |
2018年 | 18篇 |
2017年 | 11篇 |
2016年 | 31篇 |
2015年 | 20篇 |
2014年 | 35篇 |
2013年 | 67篇 |
2012年 | 65篇 |
2011年 | 99篇 |
2010年 | 50篇 |
2009年 | 28篇 |
2008年 | 94篇 |
2007年 | 96篇 |
2006年 | 81篇 |
2005年 | 92篇 |
2004年 | 84篇 |
2003年 | 57篇 |
2002年 | 60篇 |
2001年 | 30篇 |
2000年 | 31篇 |
1999年 | 13篇 |
1998年 | 16篇 |
1997年 | 19篇 |
1996年 | 18篇 |
1995年 | 18篇 |
1994年 | 18篇 |
1993年 | 22篇 |
1992年 | 19篇 |
1991年 | 17篇 |
1990年 | 18篇 |
1989年 | 21篇 |
1988年 | 11篇 |
1987年 | 11篇 |
1986年 | 16篇 |
1985年 | 26篇 |
1984年 | 11篇 |
1983年 | 9篇 |
1982年 | 18篇 |
1981年 | 20篇 |
1980年 | 17篇 |
1979年 | 16篇 |
1978年 | 14篇 |
1977年 | 21篇 |
1976年 | 21篇 |
1975年 | 17篇 |
1974年 | 12篇 |
1973年 | 15篇 |
1972年 | 10篇 |
排序方式: 共有1625条查询结果,搜索用时 31 毫秒
1.
Giuseppe Spedalotto Dr. Robert Gericke Marta Lovisari Dr. Erik R. Farquhar Dr. Brendan Twamley Dr. Aidan R. McDonald 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(51):11983-11990
Hydroxide-bridged high-valent oxidants have been implicated as the active oxidants in methane monooxygenases and other oxidases that employ bimetallic clusters in their active site. To understand the properties of such species, bis-μ-hydroxo-NiII2 complex ( 1 ) supported by a new dicarboxamidate ligand (N,N′-bis(2,6-dimethyl-phenyl)-2,2-dimethylmalonamide) was prepared. Complex 1 contained a diamond core made up of two NiII ions and two bridging hydroxide ligands. Titration of the 1 e− oxidant (NH4)2[CeIV(NO3)6] with 1 at −45 °C showed the formation of the high-valent species 2 and 3 , containing NiIINiIII and NiIII2 diamond cores, respectively, maintaining the bis-μ-hydroxide core. Both complexes were characterised using electron paramagnetic resonance, X-ray absorption, and electronic absorption spectroscopies. Density functional theory computations supported the spectroscopic assignments. Oxidation reactivity studies showed that bis-μ-hydroxide-NiIII2 3 was capable of oxidizing substrates at −45 °C at rates greater than that of the most reactive bis-μ-oxo-NiIII complexes reported to date. 相似文献
2.
Investigation of N‐Heterocyclic Carbene‐Supported Group 12 Triflates as Pre‐catalysts for Hydrosilylation/Borylation 下载免费PDF全文
Matthew M. D. Roy Dr. Michael J. Ferguson Dr. Robert McDonald Prof. Dr. Eric Rivard 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(50):18236-18246
N‐Heterocyclic carbene (NHC) complexes of Cd and Hg triflates (OTf) were prepared and their attempted conversion into rare cadmium and mercury hydrides was explored. In contrast to zinc, which forms stable [ZnH]+ complexes with NHCs, the heavier Cd and Hg congeners could not be formed; the increased instability of Cd‐H and Hg‐H units was rationalized with the aid of computations. It was also discovered that the dimeric adduct [IPr?Cd(μ‐OTf)2]2 (IPr=[(HCNDipp)2C:]; Dipp=2,6‐iPr2C6H3) is an active precatalyst for the hydrosilylation and hydroborylation of hindered aldehydes and ketones. The related zinc congener was inactive as a catalyst highlighting a distinct advantage of using heavy Group 12 metals to promote catalytic hydrosilylation/borylation. 相似文献
3.
Dr. Luis M. Mori‐Quiroz Kirk W. Shimkin Dr. Sina Rezazadeh Ryan A. Kozlowski Prof. Dr. Donald A. Watson 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(44):15654-15658
The oxidative copper‐catalyzed cross‐coupling of functionalized alkyl boronic esters with primary amides is reported. Through the identification of appropriate diketimine ligands, conditions for efficient coupling of both primary and secondary alkyl boronic esters with diverse primary amides, including acetamide, have been developed. 相似文献
4.
A series of carbon-coated, nitrogen-doped titanium dioxide photocatalysts was produced and characterized. N-doped TiO2 powder samples were prepared using a sol-gel method and subsequently used for making doped-TiO2 thin films on glass substrates. Carbon layers were coated on the films by a thermal decomposition method using catechol. Diffuse reflectance spectra and Mott-Schottky analyses of the samples proved that nitrogen doping and carbon coating can slightly lower the band gap of TiO2, broaden its absorption to visible light and enhance its n-type character. According to photocatalytic tests against model contaminants, carbon-coated nitrogen-doped TiO2 films have better performance than simple TiO2 on the degradation of Rhodamine B dye molecules, but are poorly effective for degrading 4-chlorophenol molecules. Several possible explanations are proposed for this result, supported by scavenging experiments. This reveals the importance of a broad substrate scope when assessing new photocatalytic materials for water treatment, something which is often overlooked in many literature studies. 相似文献
5.
Hui Li Jian Liu Juan Zhao Huiting He Dr. Dabo Jiang Prof. Steven Robert Kirk Prof. Qiong Xu Dr. Xianxiang Liu Prof. Dulin Yin 《ChemistryOpen》2021,10(4):477-485
A series of tetraimidazolium salts with different anions was prepared and applied in the isomerization of β-pinene oxide. After examining the activity of different catalysts, a remarkable enhancement of the selectivity of perillyl alcohol (47 %) was obtained over [PEimi][HNO3]4 under mild reaction conditions and using DMSO as the solvent. Furthermore, noncovalent interactions between solvent molecules and the catalyst were found by FT-IR spectroscopy and confirmed by computational chemistry. The homogeneous catalyst showed excellent stability and was reused up to six times without significant loss. 相似文献
6.
Guanidinoamidized linear polyethyleneimine for gene delivery 总被引:1,自引:0,他引:1
Guanidine was introduced to low molecular weight linear polyethyleneimine(LPEI) via amide groups, to explore the effect of both guanidine degree and pendant chain length on its transfection behavior. The resulting guanidinoamidized LPEIs(GLPEIs) could dramatically reduce LPEI's toxicity, enhance its DNA-packaging capability, cellular uptake and therefore transfection efficiency. These polyplexes were taken up very efficiently via caveolae-mediated endocytosis and their transfection efficiencies in ovarian cancer cells were significantly improved compared to native LPEI10 k polyplexes. Among these GLPEIs, LPEI-C3-G100 showed higher DNA affinity even than LPEI25 k and the highest transfection efficiency, probably due to the optimization of polymer chain flexibility. Of notice, LPEI-C3-G100 polyplexes could more effectively accumulate into cytoplasm than LPEI25 k, although the transfection efficiency of LPEI-C3-G100 polyplexes was not superior to that of LPEI25 k polyplexes, which would be probably attributed to the more efficient release of LPEI25 k polyplexes than LPEI-C3-G100 polyplexes in the cytoplasm. 相似文献
7.
Cover Picture: Thermal and Photolytic Transformation of NHC–B,N‐Heterocycles: Controlled Generation of Blue Fluorescent 1,3‐Azaborinine Derivatives and 1H‐Imidazo[1,2‐a]indoles by External Stimuli (Chem. Eur. J. 40/2015) 下载免费PDF全文
8.
Dr. Saurabh S. Chitnis Prof. Neil Burford Prof. Jan J. Weigand Dr. Robert McDonald 《Angewandte Chemie (International ed. in English)》2015,54(27):7828-7832
Reactions of triarylphosphines with fluoroantimony(III) triflates give phosphine antimony(III) complexes, which undergo spontaneous reductive elimination of fluorophosphonium cations. The resulting phosphine antimony(I) complexes catenate to give the first examples of cationic antimony bicyclic compounds, [(R3P)4Sb6]4+, featuring a bicyclo[3.1.0]hexastibine framework stabilized by four phosphine ligands. The unprecedented 14‐electron redox process illustrates the generality of the reductive catenation method. 相似文献
9.
Chao Zhang Zengtian Cheng Zaihui Fu Yachun Liu Xianfeng Yi Anmin Zheng Steven Robert Kirk Dulin Yin 《Cellulose (London, England)》2017,24(1):95-106
This article first discloses that the fluorine anion-containing ionic liquids-functionalized biochar sulfonic acids (BCSA-IL-F1–3s), which were simply synthesized by an ionic exchange of 1-trimethoxysilylpropyl-3-methylimidazolium chloride (IL-Cl) grafted on the BCSA with CF3SO3H (HF1), HBF4 (HF2), HPF6 (HF3), respectively, can efficiently catalyze cellulose hydrolysis into reducing sugars (RSs) and 5-hydroxymethyl furfural (HMF) in water under microwave irradiation. This process provides a very high catalysis efficiency (turnover numbers, 4.03–4.89) at mild temperature (80 °C) for 3 h, but also possesses an excellent repeatability. More outstandingly, they can achieve much higher HMF yields (12.70–27.94%) compared to the IL-Cl-functionalized BCSA catalyst (HMF yields are lower than 0.1%) under the same reaction conditions. This is likely because the introduction of IL-F1–3s groups can significantly improve the accessibility, acidity and thermal stability of BCSA’s SO3H sites, as supported by evidence from a solid 31P NMR spectrum and thermogravimetric analysis. It is proposed that the good selectivity for HMF perhaps originates from a co-catalysis action of the IL-F1–3s and SO3H groups on BCSA-IL-F1–3s in the further conversion of RSs to HMF. 相似文献
10.
dm Tlgyesi Tams Farkas Mria Blint Thomas J. McDonald Virender K. Sharma 《Molecules (Basel, Switzerland)》2021,26(4)
Alternaria toxins are emerging mycotoxins whose regulation and standardization are in progress by the European Commission and the European Committee for Standardization. This paper describes a dilute and shoot approach to determine five Alternaria toxins in selected food samples using liquid chromatography–tandem mass spectrometry (LC-MS/MS). The strategy involves sample extraction with acidified aqueous methanol, followed by a solvent change accomplished via sample evaporation and reconstitution. The quantification is based on isotope dilution, applying all corresponding isotopically labeled internal standards to compensate possible matrix effects of the analysis. The main advantages of the present method over other existing methods includes simple and effective sample preparation, as well as detection with high sensitivity. The five-fold sample dilution can decrease matrix effects, which were evaluated with both external and internal standard methods. The results demonstrated a limit of quantification lower than 1.0 µg/kg for all five analytes for the first time. The newly presented method showed acceptable accuracy (52.7–111%) when analyzing naturally contaminated and spiked standard samples at the described levels. The method was validated for tomato-based and flour samples (wheat, rye, and maize). The absolute recovery ranged from 66.7% to 91.6% (RSD < 10%). The developed method could be an alternative approach for those laboratories that exclude sample cleanup and pre-concentration of state-of-the-art instruments with enhanced sensitivity. 相似文献