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991.
Internal waves from a body accelerating in a thermocline 总被引:1,自引:0,他引:1
Many papers study the steady wave system around bodies moving in thermoclines but little attention has been given to unsteady wave systems. This paper concentrates on the unsteady wave systems around accelerating bodies in thermoclines. The wave shapes are calculated using a theory derived from a dispersion relation based on an exp-tanh density profile. All modes of oscillation can be determined and it is shown that for the lowest mode both oblique and transverse waves occur whereas for the higher modes the presence of transverse waves depends on the background conditions and on the speed of the body. Cauchy-Poisson impulsive start waves are included. The theoretical wave shapes compare quite well with those calculated using finite-difference formulations of the full Navier-Stokes equations when a body accelerates from rest.It is also shown how the dispersion relation =N sin together with the WKB approximation can produce the same plan-view wave forms as those obtained using the thermocline wave dispersion relation given by [17, 30]. 相似文献
992.
Ammonia Activation by a Nickel NCN‐Pincer Complex featuring a Non‐Innocent N‐Heterocyclic Carbene: Ammine and Amido Complexes in Equilibrium
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Rudy M. Brown Dr. Javier Borau Garcia Juuso Valjus Christopher J. Roberts Dr. Heikki M. Tuononen Dr. Masood Parvez Prof. Roland Roesler 《Angewandte Chemie (International ed. in English)》2015,54(21):6274-6277
A Ni0‐NCN pincer complex featuring a six‐membered N‐heterocyclic carbene (NHC) central platform and amidine pendant arms was synthesized by deprotonation of its NiII precursor. It retained chloride in the square‐planar coordination sphere of nickel and was expected to be highly susceptible to oxidative addition reactions. The Ni0 complex rapidly activated ammonia at room temperature, in a ligand‐assisted process where the carbene carbon atom played the unprecedented role of proton acceptor. For the first time, the coordinated (ammine) and activated (amido) species were observed together in solution, in a solvent‐dependent equilibrium. A structural analysis of the Ni complexes provided insight into the highly unusual, non‐innocent behavior of the NHC ligand. 相似文献
993.
Prof. Dr. Helmut Schwarz 《Angewandte Chemie (International ed. in English)》2015,54(35):10090-10100
Gas‐phase investigations of judiciously doped oxide clusters permit to address fundamental challenges related to, for example, the low‐temperature oxidation of CO or the selective conversion of hydrocarbons. Modifying the size and composition of a free cluster in a controlled way enables the modification of local charge effects and of spin states, and spectroscopic studies in combination with computational work help to identify the active site of a catalyst and to unravel mechanistic details. Also, the interplay of the support material with the reactive part of a composite catalyst cluster can be addressed. Examples will be presented demonstrating how and why the gas‐phase reactivities of heteronuclear clusters, in comparison with their homonuclear counterparts, toward small, generally rather inert molecules can be increased, decreased, or not significantly affected. 相似文献
994.
Cooperative Bond Activation and Catalytic Reduction of Carbon Dioxide at a Group 13 Metal Center
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Joseph A. B. Abdalla Dr. Ian M. Riddlestone Rémi Tirfoin Prof. Simon Aldridge 《Angewandte Chemie (International ed. in English)》2015,54(17):5098-5102
A single‐component ambiphilic system capable of the cooperative activation of protic, hydridic and apolar H? X bonds across a Group 13 metal/activated β‐diketiminato (Nacnac) ligand framework is reported. The hydride complex derived from the activation of H2 is shown to be a competent catalyst for the highly selective reduction of CO2 to a methanol derivative. To our knowledge, this process represents the first example of a reduction process of this type catalyzed by a molecular gallium complex. 相似文献
995.
Dr. Thomas Lunkenbein Dr. Frank Girgsdies Anna Wernbacher Dr. Johannes Noack Dr. Gudrun Auffermann Akira Yasuhara Achim Klein‐Hoffmann Prof. Wataru Ueda Dr. Maik Eichelbaum Dr. Annette Trunschke Prof. Robert Schlögl Dr. Marc G. Willinger 《Angewandte Chemie (International ed. in English)》2015,54(23):6828-6831
Complex Mo,V‐based mixed oxides that crystallize in the orthorhombic M1‐type structure are promising candidates for the selective oxidation of small alkanes. The oxygen sublattice of such a complex oxide has been studied by annular bright field scanning transmission electron microscopy. The recorded micrographs directly display the local distortion in the metal oxygen octahedra. From the degree of distortion we are able to draw conclusions on the distribution of oxidation states in the cation columns at different sites. The results are supported by X‐ray diffraction and electron paramagnetic resonance measurements that provide integral details about the crystal structure and spin coupling, respectively. 相似文献
996.
Prof. Dr. Holger Braunschweig Theresa Dellermann Dr. William C. Ewing Thomas Kramer Christoph Schneider Stefan Ullrich 《Angewandte Chemie (International ed. in English)》2015,54(35):10271-10275
The syntheses of sulfur‐ and selenium‐bridged cyclic compounds containing boron stabilized by N‐heterocyclic carbenes (NHCs) have been achieved by the reductive insertion of elemental chalcogens into boron–boron multiple bonds. The three pairs of bonding electrons between the boron atoms in the triply bonded diboryne enabled six‐electron reduction reactions, resulting in the formation of [2.2.1]‐bicyclic systems wherein bridgehead boron atoms are spanned by three chalcogen bridges. A similar reaction using a diborene (boron–boron double bond) resulted in the reductive transfer of both pairs of bonding electrons to three sulfur atoms, yielding a NHC‐stabilized trisulfidodiborolane. The demonstration of these six‐ and four‐electron reductions lends support to the presence of three and two pairs of bonding electrons between the boron atoms of the diboryne and diborene, respectively, a fact that may be useful in future discussions on bond order. 相似文献
997.
Six‐Coordinate Group 13 Complexes: The Role of d Orbitals and Electron‐Rich Multi‐Center Bonding
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Maarten G. Goesten Dr. Célia Fonseca Guerra Prof. Dr. Freek Kapteijn Prof. Dr. Jorge Gascon Prof. Dr. F. Matthias Bickelhaupt 《Angewandte Chemie (International ed. in English)》2015,54(41):12034-12038
Bonding in six‐coordinate complexes based on Group 13 elements (B, Al, Ga, In, Tl) is usually considered to be identical to that in transition‐metal analogues. We herein demonstrate through sophisticated electronic‐structure analyses that the bonding in these Group 13 element complexes is fundamentally different and better characterized as electron‐rich hypervalent bonding with essentially no role for the d orbitals. This characteristic is carried through to the molecular properties of the complex. 相似文献
998.
Acetyl Methyl Torsion in N‐Ethylacetamide: A Challenge for Microwave Spectroscopy and Quantum Chemistry
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Raphaela Kannengießer Marcel J. Lach Prof. Dr. Wolfgang Stahl Dr. Ha Vinh Lam Nguyen 《Chemphyschem》2015,16(9):1906-1911
The gas‐phase structures and parameters describing acetyl methyl torsion of N‐ethylacetamide are determined with high accuracy, using a combination of molecular beam Fourier‐transform microwave spectroscopy and quantum chemical calculations. Conformational studies at the MP2 level of theory yield four minima on the energy surface. The most energetically favorable conformer, which possesses C1 symmetry, is assigned. Due to the torsional barrier of 73.4782(1) cm?1 of the acetyl methyl group, fine splitting up to 4.9 GHz is found in the spectrum. The conformational structure is not only confirmed by the rotational constants, but also by the orientation of the internal rotor. The 14N quadrupole hyperfine splittings are analyzed and the deduced coupling constants are compared with the calculated values. 相似文献
999.
Atomic sp, sp2, and sp3 hybrid orbitals were introduced by Linus Pauling to explain the nature of the chemical bond. Quantum dynamics simulations show that they can be sculpted by means of a selective series of coherent laser pulses, starting from the 1s orbital of the hydrogen atom. Laser hybridization generates atoms with state‐selective electric dipoles, opening up new possibilities for the study of chemical reaction dynamics and heterogeneous catalysis. 相似文献
1000.
Simon Wübbolt Prof. Dr. Martin Oestreich 《Angewandte Chemie (International ed. in English)》2015,54(52):15876-15879
A C? H silylation of pyridines that seemingly proceeds through electrophilic aromatic substitution (SEAr) is reported. Reactions of 2‐ and 3‐substituted pyridines with hydrosilanes in the presence of a catalyst that splits the Si? H bond into a hydride and a silicon electrophile yield the corresponding 5‐silylated pyridines. This formal silylation of an aromatic C? H bond is the result of a three‐step sequence, consisting of a pyridine hydrosilylation, a dehydrogenative C? H silylation of the intermediate enamine, and a 1,4‐dihydropyridine retro‐hydrosilylation. The key intermediates were detected by 1H NMR spectroscopy and prepared through the individual steps. This complex interplay of electrophilic silylation, hydride transfer, and proton abstraction is promoted by a single catalyst. 相似文献