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71.
Dipl.‐Chem. Alexander Rupp Dr. Nataliya Roznyatovskaya Dr. Harald Scherer Dr. Witali Beichel Petra Klose Carola Sturm Dr. Anke Hoffmann Dr. Jens Tübke Prof. Dr. Thorsten Koslowski Prof. Dr. Ingo Krossing 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(31):9794-9804
Several, partly new, ionic liquids (ILs) containing imidazolium and ammonium cations as well as the medium‐sized [NTf2]? (0.230 nm3; Tf=CF3SO3?) and the large [Al(hfip)4]? (0.581 nm3; hfip=OC(H)(CF3)2) anions were synthesized and characterized. Their temperature‐dependent viscosities and conductivities between 25 and 80 °C showed typical Vogel–Fulcher–Tammann (VFT) behavior. Ion‐specific self‐diffusion constants were measured at room temperature by pulsed‐gradient stimulated‐echo (PGSTE) NMR experiments. In general, self‐diffusion constants of both cations and anions in [Al(hfip)4]?‐based ILs were higher than in [NTf2]?‐based ILs. Ionicities were calculated from self‐diffusion constants and measured bulk conductivities, and showed that [Al(hfip)4]?‐based ILs yield higher ionicities than their [NTf2]? analogues, the former of which reach values of virtually 100 % in some cases.From these observations it was concluded that [Al(hfip)4]?‐based ILs come close to systems without any interactions, and this hypothesis is underlined with a Hirshfeld analysis. Additionally, a robust, modified Marcus theory quantitatively accounted for the differences between the two anions and yielded a minimum of the activation energy for ion movement at an anion diameter of slightly greater than 1 nm, which fits almost perfectly the size of [Al(hfip)4]?. Shallow Coulomb potential wells are responsible for the high mobility of ILs with such anions. 相似文献
72.
Marcileia Zanatta Dr. Anne‐Lise Girard Nathalia M. Simon Prof. Gunter Ebeling Prof. Hubert K. Stassen Prof. Paolo R. Livotto Prof. Francisco P. dos Santos Prof. Jairton Dupont 《Angewandte Chemie (International ed. in English)》2014,53(47):12817-12821
1‐n‐Butyl‐2,3‐dimethylimidazolium (BMMI) ionic liquids (ILs) associated with different anions undergo H/D exchange preferentially at 2‐Me group of the imidazolium in deuterated solvents. This process is mainly related to the existence of ion pairs rather than the anion basicity. The H/D exchange occurs in solvents (CDCl3 and MeCN for instance) in which intimate contact ion pairs are present and the anion possesses a labile H in its structure, such as hydrogen carbonate and prolinate. In D2O, separated ion pairs are formed and the H/D exchange does not occur. A plausible catalytic cycle is that the IL behaves as a neutral base in the course of all H/D exchange processes. NMR experiments, density functional calculations, and molecular dynamics simulations corroborate these hypotheses. 相似文献
73.
Formation of “Quasi” Contact or Solvent‐separated Ion Pairs in the Local Environment of Probe Molecules Dissolved in Ionic Liquids 下载免费PDF全文
Matthias Strauch Dr. Christian Roth Falk Kubatzki Prof. Dr. Ralf Ludwig 《Chemphyschem》2014,15(2):265-270
The change from “quasi” contact to “quasi” solvent‐separated ion‐pair configuration in the local environment of a probe molecule in ionic liquids depends on the varying interaction strength of the chosen anions. The ion speciation in these Coulomb fluids could be shown by combining infrared spectroscopy, density functional theory calculations, and natural bond orbital analysis using a low‐self‐clustering probe molecule. 相似文献
74.
Ion Pairing in Protic Ionic Liquids Probed by Far‐Infrared Spectroscopy: Effects of Solvent Polarity and Temperature 下载免费PDF全文
Dr. Koichi Fumino Verlaine Fossog Peter Stange Kai Wittler Wigbert Polet Prof. Dr. Rolf Hempelmann Prof. Dr. Ralf Ludwig 《Chemphyschem》2014,15(12):2604-2609
The cation–anion and cation–solvent interactions in solutions of the protic ionic liquid (PIL) [Et3NH][I] dissolved in solvents of different polarities are studied by means of far infrared vibrational (FIR) spectroscopy and density functional theory (DFT) calculations. The dissociation of contact ion pairs (CIPs) and the resulting formation of solvent‐separated ion pairs (SIPs) can be observed and analyzed as a function of solvent concentration, solvent polarity, and temperature. In apolar environments, the CIPs dominate for all solvent concentrations and temperatures. At high concentrations of polar solvents, SIPs are favored over CIPs. For these PIL/solvent mixtures, CIPs are reformed by increasing the temperature due to the reduced polarity of the solvent. Overall, this approach provides equilibrium constants, free energies, enthalpies, and entropies for ion‐pair formation in trialkylammonium‐containing PILs. These results have important implications for the understanding of solvation chemistry and the reactivity of ionic liquids. 相似文献
75.
Mechanism of Back Electron Transfer in an Intermolecular Photoinduced Electron Transfer Reaction: Solvent as a Charge Mediator 下载免费PDF全文
Sudhakar Narra Dr. Yoshifumi Nishimura Prof. Dr. Henryk A. Witek Prof. Dr. Shinsuke Shigeto 《Chemphyschem》2014,15(14):2945-2950
Back electron transfer (BET) is one of the important processes that govern the decay of generated ion pairs in intermolecular photoinduced electron transfer reactions. Unfortunately, a detailed mechanism of BET reactions remains largely unknown in spite of their importance for the development of molecular photovoltaic structures. Here, we examine the BET reaction of pyrene (Py) and 1,4‐dicyanobenzene (DCB) in acetonitrile (ACN) by using time‐resolved near‐ and mid‐IR spectroscopy. The Py dimer radical cation (Py2.+) and DCB radical anion (DCB.?) generated after photoexcitation of Py show asynchronous decay kinetics. To account for this observation, we propose a reaction mechanism that involves electron transfer from DCB.? to the solvent and charge recombination between the resulting ACN dimer anion and Py2.+. The unique role of ACN as a charge mediator revealed herein could have implications for strategies that retard charge recombination in dye‐sensitized solar cells. 相似文献
76.
Formation of Octameric Methylaluminoxanes by Hydrolysis of Trimethylaluminum and the Mechanisms of Catalyst Activation in Single‐Site α‐Olefin Polymerization Catalysis 下载免费PDF全文
Dr. Janne T. Hirvi Prof. Manfred Bochmann Dr. John R. Severn Prof. Mikko Linnolahti 《Chemphyschem》2014,15(13):2732-2742
Hydrolysis of trimethylaluminum (TMA) leads to the formation of methylaluminoxanes (MAO) of general formula (MeAlO)n(AlMe3)m. The thermodynamically favored pathway of MAO formation is followed up to n=8, showing the major impact of associated TMA on the structural characteristics of the MAOs. The MAOs bind up to five TMA molecules, thereby inducing transition from cages into rings and sheets. Zirconocene catalyst activation studies using model MAO co‐catalysts show the decisive role of the associated TMA in forming the catalytically active sites. Catalyst activation can take place either by Lewis‐acidic abstraction of an alkyl or halide ligand from the precatalyst or by reaction of the precatalyst with an MAO‐derived AlMe2+ cation. Thermodynamics suggest that activation through AlMe2+ transfer is the dominant mechanism because sites that are able to release AlMe2+ are more abundant than Lewis‐acidic sites. The model catalyst system is demonstrated to polymerize ethene. 相似文献
77.
Idan Avigdori Dr. Alla Pogoreltsev Dr. Alexander Kaushanski Dr. Natalia Fridman Prof. Dr. Mark Gandelman 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(52):23682-23685
N-heterocyclic nitrogen Lewis acids are a recent addition to the field of organic chemistry. Based on nitrenium cations, these acids where previously shown to generate Lewis adducts when combined with the appropriate Lewis bases. Herein, a triazinium-based Lewis acid was combined with tBu3P to generate a frustrated Lewis pair (FLP) capable of cleaving, for the first time, Si−H bonds in silanes. Whereas low yields were initially encountered owing to insufficient Lewis acidity, a new nitrenium-based Lewis acid was synthesized, and its superior Lewis acidity was experimentally and computationally confirmed. A FLP based on this acid cleaved the Si−H bond in PhSiH3, generating the triazane product in a quantitative yield. This unprecedented N−H triazane was fully characterized by multinuclear NMR techniques and single-crystal X-ray crystallography. A new class of compounds, N-H triazanes display the potential capacity to participate in hydride transfer reactions. 相似文献
78.
Shuvankar Naskar Rweetuparna Guha Jens Müller 《Angewandte Chemie (International ed. in English)》2020,59(4):1397-1406
The incorporation of metal ions into nucleic acids by means of metal‐mediated base pairs represents a promising and prominent strategy for the site‐specific decoration of these self‐assembling supramolecules with metal‐based functionality. Over the past 20 years, numerous nucleoside surrogates have been introduced in this respect, broadening the metal scope by providing perfectly tailored metal‐binding sites. More recently, artificial nucleosides derived from natural purine or pyrimidine bases have moved into the focus of AgI‐mediated base pairing, due to their expected compatibility with regular Watson–Crick base pairs. This minireview summarizes these advances in metal‐mediated base pairing but also includes further recent progress in the field. Moreover, it addresses other aspects of metal‐modified nucleic acids, highlighting an expansion of the concept to metal‐mediated base triples (in triple helices and three‐way junctions) and metal‐mediated base tetrads (in quadruplexes). For all types of metal‐modified nucleic acids, proposed or accomplished applications are briefly mentioned, too. 相似文献
79.
Alessandro Bismuto Gary S. Nichol Fernanda Duarte Michael J. Cowley Stephen P. Thomas 《Angewandte Chemie (International ed. in English)》2020,59(31):12731-12735
The reaction of a Lewis acidic borane with an alkyne is a key step in a diverse range of main group transformations. Alkyne 1,1‐carboboration, the Wrackmeyer reaction, is an archetypal transformation of this kind. 1,1‐Carboboration has been proposed to proceed through a zwitterionic intermediate. We report the isolation and spectroscopic, structural and computational characterization of the zwitterionic intermediates generated by reaction of B(C6F5)3 with alkynes. The stepwise reactivity of the zwitterion provides new mechanistic insight for 1,1‐carboboration and wider B(C6F5)3 catalysis. Making use of intramolecular stabilization by a ferrocene substituent, we have characterized the zwitterionic intermediate in the solid state and diverted reactivity towards alkyne cyclotrimerization. 相似文献
80.
Jaewoong Kim 《Linear and Multilinear Algebra》2017,65(6):1260-1275
In this paper, we study several properties for mono-weakly hyponormal 2-variable weighted shifts. First, we consider propagation phenomena for mono-weakly hyponormal (resp. mono-polynomially hyponormal) 2-variable weighted shifts. Next, we contemplate the mono-weak hyponormality under the Schur product. Finally, we study whether the mono-weak hyponormality is invariant under powers. 相似文献