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1.
Robin F. Weitkamp Beate Neumann Hans‐Georg Stammler Berthold Hoge 《Angewandte Chemie (International ed. in English)》2020,59(14):5494-5499
We report on the first examples of isolated silanol–silanolate anions, obtained by utilizing weakly coordinating phosphazenium counterions. The silanolate anions were synthesized from the recently published phosphazenium hydroxide hydrate salt with siloxanes. The silanol–silanolate anions are postulated intermediates in the hydroxide‐mediated polymerization of aryl and alkyl siloxanes. The silanolate anions are strong nucleophiles because of the weakly coordinating character of the phosphazenium cation, which is perceptible in their activity in polysiloxane depolymerization. 相似文献
2.
3.
Guido Pintacuda Andrei Kaikkonen Gottfried Otting 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2004,171(2):387-243
Paramagnetic metal ions with fast-relaxing electronic spin and anisotropic susceptibility tensor provide a rich source of structural information that can be derived from pseudo-contact shifts, residual dipolar couplings, dipole-dipole Curie spin cross-correlation, and paramagnetic relaxation enhancements. The present study draws attention to a cross-correlation effect between nuclear relaxation due to anisotropic chemical shielding (CSA) and due to the anisotropic dipolar shielding (DSA) caused by the electronic Curie spin. This CSA x DSA cross-correlation contribution seems to have been overlooked in previous interpretations of paramagnetic relaxation enhancements. It is shown to be sufficiently large to compromise the 1/r6 distance dependence usually assumed. The effect cannot experimentally be separated from auto-correlated DSA relaxation. It can increase or decrease the observed paramagnetic relaxation enhancement. Under certain conditions, the effect can dominate the entire paramagnetic relaxation, resulting in nuclear resonances narrower than in the absence of the paramagnetic center. CSAxDSA cross-correlation becomes important when paramagnetic relaxation is predominantly due to the Curie rather than the Solomon mechanism. Therefore the effect is most pronounced for relaxation by metal ions with large magnetic susceptibility and fast-relaxing electron spin. It most strongly affects paramagnetic enhancements of transverse relaxation in macromolecules and of longitudinal relaxation in small molecules. 相似文献
4.
Gottfried Mayer Vitali Vogel Bas G. G. Lohmeijer Jean‐Franois Gohy Jacomina A. Van Den Broek Winfried Haase Ulrich S. Schubert Dieter Schubert 《Journal of polymer science. Part A, Polymer chemistry》2004,42(17):4458-4465
Micelles prepared from amphiphilic block copolymers in which a poly(styrene) segment is connected to a poly(ethylene oxide) block via a bis‐(2,2′:6′,2″‐terpyridine‐ruthenium) complex have been intensely studied. In most cases, the micelle populations were found to be strongly heterogeneous in size because of massive micelle/micelle aggregation. In the study reported in this article we tried to improve the homogeneity of the micelle population. The variant preparation procedure developed, which is described here, was used to prepare two “protomer”‐type micelles: PS20‐[Ru]‐PEO70 and PS20‐[Ru]‐PEO375. The dropwise addition of water to a solution of the compounds in dimethylformamide was replaced by the controlled addition of water by a syringe pump. The resulting micelles were characterized by sedimentation velocity and sedimentation equilibrium analyses in an analytical ultracentrifuge and by transmission electron microscopy of negatively stained samples. Sedimentation analysis showed virtually unimodal size distributions, in contrast to the findings on micelles prepared previously. PS20‐[Ru]‐PEO70 micelles were found to have an average molar mass of 318,000 g/mol (corresponding to 53 protomers per micelle, which is distinctly less than after micelle preparation by the standard method) and an average hydrodynamic diameter (dh) of 18 nm. For PS20‐[Ru]‐PEO375 micelles, the corresponding values were M = 603,000 g/mol (31 protomers per micelle) and dh = 34 nm. The latter particles were found to be identical to the “equilibrium” micelles prepared in pure water. Both micelle types had a very narrow molar mass distribution but a much broader distribution of s values and thus of hydrodynamic diameters. This indicates a conformational heterogeneity that is stable on the time scale of sedimentation velocity analysis. The findings from electron microscopy were in disagreement with those from the sedimentation analysis both in average micelle diameter and in the width of the distributions, apparently because of imperfections in the staining procedure. The preparation procedure described also may be useful in micelle formation from other types of protomers. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4458–4465, 2004 相似文献
5.
Christo I. Ivanov Nikolai Tyutyulkov Gottfried Olbrich H. Barentzen Oscar E. Polansky 《Theoretical chemistry accounts》1988,73(1):27-42
Utilizing an extended Hubbard-type Hamiltonian which incorporates both nearest-neighbour Coulomb repulsion and exchange interactions, we have studied the energy dispersion of the lowest elementary excitation from the ferromagnetically aligned state of quasi one-dimensional alternant hydrocarbon networks. It was found that the main effect of the long range Coulomb interaction may be thought of as a renormalization (screening) of the on-site Hubbard integral. This implies an enhancement of the kinetic exchange term and impairs the stability of the ferromagnetic state towards single spin inversions. However, for physically relevant values of the parameters entering the model Hamiltonian, the collective spin excitation represents a magnon, whose energy band lies above the reference value pertaining to the magnetically saturated configuration.Dedicated to Prof. Dr. Adolf Neckel on the occasion of his 60th birthday 相似文献
6.
Jörn Müller Helmuth Menig Gottfried Huttner Albin Frank 《Journal of organometallic chemistry》1980,185(2):251-263
The complexes [Ir(COD)(η5-C7H9)] and [Ir(COD)(η5-C8H11)] are obtained by the isoprophyl Grignard synthesis of [Ir(COD)Cl]2 (COD = η4-1,5-cyclooctadiene) in the presence of cycloheptatriene, and cyclooctatriene, respectively. The later reaction yields [IrH(COD)(δ4-1,3,6-C8H10)] as a by-product which, in contrast to other [IrH(η4-cyclodiene)2] complexes, does not show H-addition-elimination equilibria. Reduction of [Ir(1,3-C7H10)2Cl] with C2H5OH/Na2CO3 yields [Ir(η4-1,3-C7H10)](η5-C7H9)] which was characterized by X-ray analysis. [Ir(COD)Cl]2 reacts with Na2C8H8, and after hydrolysis unstable [Ir(COD)(η5-C8H9)] is formed which by protonation with HPF6 is converted into the [Ir(COD)(η6-1,3,5-C8H10)]+ cation. All these compounds are fluxional in solution. 相似文献
7.
Reaction of the vicinal diols of steroids1, 5, 7, 10, 13, and16 with TPP/DEAD yields both regio-and stereospecifically the oxosteroids2, 6, 8, 11, 14, and15 by displacement of an axial hydrogen and extrusion ofTPPO besides the cholest-4-en-6-ols9 and12 and the cyclic carbonate3. 16, 17-androstandiol16 gives only the cyclic carbonate17. The different structures of the carbohydrates withcis-diol arrangement19 and21 lead exclusively to cyclic carbonates20 and22 in moderate yields. Treatment of1 with TPP/DEAD/HN3 affords 3-azido-2-hydroxycholestane4 in addition to the above mentioned2. 相似文献
8.
Reinhard Haase Dorit S. Meinhold Berthold Thomas Edwin Weber Gerd Rheinwald 《Structural chemistry》2002,13(5-6):471-477
The crystal structures of inclusion compounds of 3,3-bis(9-hydroxy-9-fluorenyl)-2,2-binaphthyl host (1) and its chloro (2) or bromo (3) derivatives substituted in 2,7-positions of the fluorene units with acetone guests (1A–3A) were determined by X-ray studies as well as by 1H-CRAMPS solid-state NMR. Using this NMR technique allows identification of differently bound guest molecules due to their different chemical shifts caused by the influence of the ring current effects of the host aryl units. 相似文献
9.
Filip Fratev Gottfried Olbrich Oskar E. Polansky 《Monatshefte für Chemie / Chemical Monthly》1979,110(3):505-515
UsingGleason's theorem the pars orbital concept is developed for excited states of conjugated systems described by CI-methods. A significance index is introduced to evaluate different patterns of fragmentation. The usefulness of this approach is illustrated for butadiene, biphenyl, naphthalene, and azulene.
11. Mitt.:H. Karpf, O. E. Polansky undM. Zander, Tetrahedron Lett.1978, 2069. 相似文献
10.
The Metropolis Monte Carlo method has been added to the program FANTOM for energy refinement of polypeptides and proteins using a Newton–Raphson minimizer in torsion angle space. With this extension, different strategies for global minimization of the semiempirical energy function ECEPP/2 by various temperature schedules and restriction of conformational space were tested for locating local minimum conformations with low energy of the pentapeptide Met-enkephalin. In total, 1881 conformations below ?10 kcal/mol were found. These conformations could be represented by 77 nonidentical conformations which were analysed for their pattern of hydrogen bonds, types of tight turn, pairwise root-mean-square-deviation (rmsd), Zimmermann codes and side chain conformations. All low energy conformations below ?10.4 kcal/mol show strong similarity to the global minimum conformation in the backbone structure. 相似文献