全文获取类型
收费全文 | 345篇 |
免费 | 15篇 |
国内免费 | 1篇 |
专业分类
化学 | 235篇 |
力学 | 1篇 |
数学 | 53篇 |
物理学 | 72篇 |
出版年
2023年 | 7篇 |
2022年 | 8篇 |
2021年 | 11篇 |
2020年 | 14篇 |
2019年 | 4篇 |
2018年 | 2篇 |
2017年 | 6篇 |
2016年 | 12篇 |
2015年 | 17篇 |
2014年 | 9篇 |
2013年 | 20篇 |
2012年 | 20篇 |
2011年 | 17篇 |
2010年 | 10篇 |
2009年 | 12篇 |
2008年 | 13篇 |
2007年 | 22篇 |
2006年 | 18篇 |
2005年 | 15篇 |
2004年 | 10篇 |
2003年 | 11篇 |
2002年 | 6篇 |
2001年 | 3篇 |
2000年 | 2篇 |
1999年 | 3篇 |
1997年 | 2篇 |
1996年 | 3篇 |
1994年 | 5篇 |
1993年 | 2篇 |
1987年 | 5篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1981年 | 3篇 |
1979年 | 2篇 |
1976年 | 3篇 |
1973年 | 2篇 |
1972年 | 2篇 |
1935年 | 2篇 |
1931年 | 4篇 |
1930年 | 3篇 |
1928年 | 4篇 |
1926年 | 2篇 |
1915年 | 2篇 |
1907年 | 2篇 |
1904年 | 2篇 |
1889年 | 3篇 |
1888年 | 2篇 |
1873年 | 1篇 |
排序方式: 共有361条查询结果,搜索用时 31 毫秒
111.
Generation of Dicoordinate Boron(I) Units by Fragmentation of a Tetra‐Boron(I) Molecular Square
下载免费PDF全文
![点击此处可从《Angewandte Chemie (International ed. in English)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Dr. Merle Arrowsmith Dominic Auerhammer Dr. Rüdiger Bertermann Prof. Dr. Holger Braunschweig Prof. Dr. Gerhard Bringmann Dr. Mehmet Ali Celik Dr. Rian D. Dewhurst Prof. Dr. Maik Finze Dr. Matthias Grüne Michael Hailmann Thomas Hertle Dr. Ivo Krummenacher 《Angewandte Chemie (International ed. in English)》2016,55(46):14464-14468
Reduction of carbene‐borane adduct [(cAAC)BBr2(CN)] (cAAC=1‐(2,6‐diisopropylphenyl)‐3,3,5,5‐tetramethylpyrrolidin‐2‐ylidene) cleanly yielded the tetra(cyanoborylene) species [(cAAC)B(CN)]4 presenting a 12‐membered (BCN)4 ring. The analysis of the Kohn–Sham molecular orbitals showed significant borylene character of the BI atoms. [(cAAC)B(CN)]4 was found to reduce two equivalents of AgCN per boron center to yield [(cAAC)B(CN)3] and fragmented into two‐coordinate boron(I) units upon reaction with IMeMe (1,3,4,5‐tetramethylimidazol‐2‐ylidene) to yield the corresponding tricoordinate mixed cAAC‐NHC cyanoborylene. The analogous cAAC‐phosphine cyanoborylene was obtained by reduction of [(cAAC)BBr2(CN)] in the presence of excess phosphine. 相似文献
112.
The quantum dynamics of beta-hydrogen elimination in cationic transition metal complexes [CpML(C(2)H(4))H](+) (M = Co, Rh; L = PH(3), PF(3), PMe(3), P(OMe)(3)) is studied theoretically. The underlying potential energy profiles are obtained at the DFT level of theory; the nuclear motion is computed by the method of wave packet propagation (i.e., by integrating the time-dependent Schrodinger equation). For the Co as well as the Rh complexes, agostic intermediates relevant to the insertion/elimination process exist. Complexes containing each ligand are studied for both transition metals from an electronic structure point of view and, in addition, the ligand influence on the dynamics is compared. The results allow us to draw conclusions concerning the influence of the energetics as well as the different masses on vibrational periods and lifetimes of these complexes due to beta-elimination. 相似文献
113.
Ludwig Zapf Sven Peters Prof. Dr. Udo Radius Prof. Dr. Maik Finze 《Angewandte Chemie (International ed. in English)》2023,62(17):e202300056
First examples of anionic cyclic (alkyl)(amino)carbenes (Ani-cAACs) that contain borane substituents have been synthesized. The nature of the borane substituents allows a modulation of the σ-donor or π-acceptor abilities compared to their neutral analogues. A B(CN)3-substituted Ani-cAAC has been generated and used in situ. The corresponding C2F5BF2-Ani-cAAC 6 was obtained in high yield on a multigram scale. First reactions of these novel ligands with elemental selenium and chloro(triphenylphosphine)gold(I) led to the anionic selenium adducts 7 and 8 and the Ani-cAAC gold complex 9 . The properties of these compounds and data derived from theoretical calculations provide an insight into the electronic and steric properties of these novel anionic cAACs. Especially the ease of synthesis and the combination of properties such as negative charge, large buried volume, and good σ-donor and π-acceptor ability renders Ani-cAACs unique and promising new building blocks. 相似文献
114.
115.
Glaser T Heidemeier M Strautmann JB Bögge H Stammler A Krickemeyer E Huenerbein R Grimme S Bothe E Bill E 《Chemistry (Weinheim an der Bergstrasse, Germany)》2007,13(33):9191-9206
A series of trinuclear Cu(II) complexes with the tris(tetradentate) triplesalen ligands H(6)talen, H(6)talen(tBu(2) ), and H(6)talen(NO(2) ), namely [(talen)Cu(II) (3)] (1), [(talen(tBu(2) ))Cu(II) (3)] (2), and [(talen(NO(2) ))Cu(II) (3)] (3), were synthesized and their molecular and electronic structures determined. These triplesalen ligands provide three salen-like coordination environments bridged in a meta-phenylene arrangement by a phloroglucinol backbone. The structure of [(talen)Cu(II) (3)] (1) was communicated recently. The structure of the tert-butyl derivative [(talen(tBu(2) ))Cu(II) (3)] (2) was established in three different solvates. The molecular structures of these trinuclear complexes show notable differences, the most important of which is the degree of ligand folding around the central Cu(II)-phenolate bonds. This folding is symmetric with regard to the central phloroglucinol backbone in two structures, where it gives rise to bowl-shaped overall geometries. For one solvate two trinuclear triplesalen complexes form a supramolecular disk-like arrangement, hosting two dichloromethane molecules like two pearls in an oyster. The FTIR spectra of these complexes indicate the higher effective nuclear charge of Cu(II) in comparison to the trinuclear Ni(II) complexes by the lower C--O and higher C=N stretching frequencies. The UV/Vis/NIR spectra of 1-3 reflect the stronger ligand folding in the tert-butyl complex 2 by an intense phenolate-to-Cu(II) LMCT. This absorption is absent in 1 and is obscured by the nitro chromophore in 3. The more planar molecular structures cause orthogonality of the Cu(II) d(x(2)-y(2) ) orbital and the phenolate O p(z) orbital, which leads to small LMCT dipole strengths. Whereas 1 and 3 exhibit only irreversible oxidations, 2 exhibits a reversible one-electron oxidation at +0.26 V, a reversible two-electron oxidation at +0.59 V, and a reversible one-electron oxidation at +0.81 V versus Fc(+)/Fc. The one-electron oxidized form 2(+) is strongly stabilized with respect to reference mononuclear salen-like Cu complexes. Chemical one-electron oxidation of 2 to 2(+) allows the determination of its UV/Vis/NIR spectrum, which indicates a ligand-centered oxidation that can be assigned to the central phloroglucinol unit by analogy with the trinuclear Ni triplesalen series. Delocalization of this oxidation over three Cu(II)-phenolate subunits causes the observed energetic stabilization of 2(+). Temperature-dependent magnetic susceptibility measurements reveal ferromagnetic couplings for all three trinuclear Cu(II) triplesalen complexes. The trend of the coupling constants can be rationalized by two opposing effects: 1) electron-withdrawing terminal substituents stabilize the central Cu(II)-phenolate bond, which results in a stronger coupling, and 2) ligand folding around the central Cu(II)-phenolate bond opens a bonding pathway between the magnetic Cu(II) d(x(2)-y(2) ) orbital and the phenolate O p(z) orbital, which results in a stronger coupling. Density functional calculations indicate that both spin-polarization and spin-delocalization are operative and that slight geometric variations alter their relative magnitudes. 相似文献
116.
In the present study a headspace solid-phase dynamic extraction method coupled to gas chromatography–mass spectrometry (HS-SPDE-GC/MS)
for the trace determination of volatile halogenated hydrocarbons and benzene from groundwater samples was developed and evaluated.
As target compounds, benzene as well as 11 chlorinated and brominated hydrocarbons (vinyl chloride, dichloromethane, cis-1,2-dichloroethylene, trans-1,2-dichloroethylene, carbon tetrachloride, chloroform, trichloroethylene, tetrachloroethylene, bromoform) of environmental
and toxicological concern were included in this study. The analytes were extracted using a SPDE needle device, coated with
a poly(dimethylsiloxane) with 10% embedded activated carbon phase (50-μm film thickness and 56-mm film length) and were analyzed
by GC/MS in full-scan mode. Parameters that affect the extraction yield such as extraction and desorption temperature, salting-out,
extraction and desorption flow rate, extraction volume and desorption volume, the number of extraction cycles, and the pre-desorption
time have been evaluated and optimized. The linearity of the HS-SPDE-GC/MS method was established over several orders of magnitude.
Method detection limits (MDLs) for the compounds investigated ranged between 12 ng/L for cis-dichloroethylene and trans-dichloroethylene and 870 ng/L for vinyl chloride. The method was thoroughly validated, and the precision at two concentration
levels (0.1 mg/L and a concentration 5 times above the MDL) was between 3.1 and 16% for the analytes investigated. SPDE provides
high sensitivity, short sample preparation and extraction times and a high sample throughput because of full automation. Finally,
the applicability to real environmental samples is shown exemplarily for various groundwater samples from a former waste-oil
recycling facility. Groundwater from the site showed a complex contamination with chlorinated volatile organic compounds and
aromatic hydrocarbons.
Figure SPDE Principle 相似文献
117.
Dr. Michael Drisch Lisa A. Bischoff Dr. Jan A. P. Sprenger Dr. Philipp T. Hennig Raphael Wirthensohn Dr. Johannes Landmann Dr. Szymon Z. Konieczka Dr. Michael Hailmann Dr. Nikolai V. Ignat'ev Prof. Dr. Maik Finze 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(50):11625-11633
Different types of high-yield, easily scalable syntheses for cyano(fluoro)borates Kt[BFn(CN)4−n] (n=0–2) (Kt=cation), which are versatile building blocks for materials applications and chemical synthesis, have been developed. Tetrafluoroborates react with trimethylsilyl cyanide in the presence of metal-free Brønsted or Lewis acid catalysts under unprecedentedly mild conditions to give tricyanofluoroborates or tetracyanoborates. Analogously, pentafluoroethyltrifluoroborates are converted into pentafluoroethyltricyanoborates. Boron trifluoride etherate, alkali metal salts, and trimethylsilyl cyanide selectively yield dicyanodifluoroborates or tricyanofluoroborates. Fluorination of cyanohydridoborates is the third reaction type that includes direct fluorination with, for example, elemental fluorine, stepwise halogenation/fluorination reactions, and electrochemical fluorination (ECF) according to the Simons process. In addition, fluorination of [BH(CN)2{OC(O)Et}]− to result in [BF(CN)2{OC(O)Et}]− is described. 相似文献
118.
The fluorination of K[B(CN)(4)] with ClF is studied by millimolar test reactions in aHF and CH(2)Cl(2) solution and by subsequent identification of intermediates such as B-CF═NCl, B-CF(2)-NCl(2), and B-CF(3) species as well as NCl(3) by (19)F, (11)B NMR, and Raman spectroscopy, respectively. At first one cyano group of K[B(CN)(4)] is converted fast into a CF(3) group, and with increasing fluorination the reaction becomes slower and several intermediates could be observed. On the basis of these results, a synthesis was developed for K[B(CF(3))(4)] on a 0.2 molar scale by treatment of K[B(CN)(4)] diluted in aHF with ClF. The course of the reactions was followed by (i) monitoring the vapor pressure inside the reactor, (ii) observing the heat dissipation during ClF uptake, and (iii) measuring the volume of the released nitrogen gas. Since the fluorination of the last cyano group proceeds very slowly, the selective synthesis of K[(CF(3))(3)BCN] on a 0.2 molar scale is possible, as well. The analysis of the mechanisms, thermodynamics, and kinetics of the fluorination reactions is supported by density functional theory (DFT) calculations. 相似文献
119.
The coordination of acetonitrile and water to the Hg atom in [Hg(closo-1-CB(11)F(11))(2)](2-) (1) reveals the Lewis acidity of the Hg(II) center, which is unprecedented, since 1 is a dianion. Both coordination compounds were characterized by single-crystal X-ray diffraction, vibrational spectroscopy, and differential scanning calorimetry (DSC). In contrast, the Hg atom in [PhHg(closo-1-CB(11)F(11))](-) (2) does not coordinate to CH(3)CN and H(2)O, although it has only a single negative charge. 相似文献
120.