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91.
Jens Reinbold Martin Bertau Torsten Voss Dieter Hunkler Lothar Knothe Horst Prinzbach Dmytro Neschchadin Georg Gescheidt Bernhard Mayer Hans‐Dieter Martin Jürgen Heinze G.K.Surya Prakash GeorgeA. Olah 《Helvetica chimica acta》2001,84(6):1518-1560
Secodecahedradiene 2a , featuring very proximate, perfectly syn‐periplanar and significantly pyramidalized C=C bonds, was synthesized as testing object for in‐plane(σ)‐homoconjugational electron delocalization, starting from the available pagodane 15b . The response of 2a (and in part its diester 2b ) – in π,π‐distance (average 3.08 Å), olefinic pyramidalization (average 26.9°), and π,π‐split (PE, 1.15 eV) intermediate between disecododecahedradiene 1a and 1,16‐dodecahedradiene 3a – to selected 4π‐reagents, electrophiles, and radicals was explored experimentally and by calculations. Intriguing multistep reaction sequences attest to the ease of competing stabilization pathways for the 3C/2e in‐plane homoconjugated cationic intermediates. PE, CV, and ESR measurements and calculations (DFT) characterize the radical cation generated from 2a as in‐plane homoconjugated 4C/3e‐species 2a .+, persistent in a Freon matrix, but only very shortly existent in solution (CIDNP). Consequently, NMR control of the two‐electron oxidation in SbF5/SO2ClF did not disclose the σ‐bis‐homoaromatic dication 4C/2e (see 2a 2 + ), but a bis‐allylic dication 75 as persistent species. In support of 2a 2 + as intermediate, evidence is presented for very limited kinetic protection offered by the secododecahedral framework to through H‐cage σ‐homoconjugated cations. 相似文献
92.
C.R. Hoffman T. Baumann D. Bazin J. Brown G. Christian D.H. Denby P.A. DeYoung J.E. Finck N. Frank J. Hinnefeld S. Mosby W.A. Peters W.F. Rogers A. Schiller A. Spyrou M.J. Scott S.L. Tabor M. Thoennessen P. Voss 《Physics letters. [Part B]》2009
The decay energy spectrum for neutron unbound states in 24O (Z=8, N=16) has been observed for the first time. The resonance energy of the lowest lying state, interpreted as the 2+ level, has been observed at a decay energy above 600 keV. The resulting excitation energy of the 2+ level above 4.7 MeV, supplies strong evidence that 24O is a doubly magic nucleus. The data is also consistent with the presence of a second excited state around 5.33 MeV which can be interpreted as the 1+ level. 相似文献
93.
Wahl F Weiler A Landenberger P Sackers E Voss T Haas A Lieb M Hunkler D Wörth J Knothe L Prinzbach H 《Chemistry (Weinheim an der Bergstrasse, Germany)》2006,12(24):6255-6267
“One‐pot” substitution of the twenty hydrogen atoms in pentagonal dodecahedrane (C20H20) by OH, F, Cl, and Br atoms is explored. Electrophilic insertion of oxygen atoms with DMDO and TFMDO as oxidizing reagents ended, far off the desired C20(OH)20, in complex polyol mixtures (up to C20H10(OH)10 decols, a trace of C20H(OH)19?). Perfluorination was successful in a NaF matrix but (nearly pure) C20F20 could be secured only in very low yield. “Brute‐force” photochlorination (heat, light, pressure, time) provided a mixture of hydrogen‐free, barely soluble C20Cl16 dienes in high yield and C20Cl20 as a trace component. Upon electron‐impact ionization of the C20Cl16 material sequential loss of the chlorine atoms was the major fragmentation pathway furnishing, however, only minor amounts of chlorine‐free C20+ ions. “Brute‐force” photobrominations delivered an extremely complex mixture of polybromides with C20HBr13 trienes as the highest masses. The MS spectra exhibited exclusive loss of the Br substituents ending in rather intense singly, doubly, and triply charged C20H4–0+(2+)(3+) ions. The insoluble ~C20HBr13 fraction (C20Br14 trienes as highest masses) obtained along a modified bromination protocol, ultimately allowed the neat mass selection of C20? ions. The C20Cl16 dienes and C20H0–3Br14–12 tri‐/tetraenes, in spite of their very high olefinic pyramidalization, proved resistant to oxygen and dimerization (polymerization) but added CH2N2 smoothly. Dehalogenation of the respective cycloaddition products through electron‐impact ionization resulted in C22–24H4–8+(2+) ions possibly constituting bis‐/tris‐/tetrakis‐methano‐C20 fullerenes or partly hydrogenated C22, C23, and C24 cages. 相似文献
94.
Rumpel M Haefner M Schoder T Pruss C Voss A Osten W Ahmed MA Graf T 《Optics letters》2012,37(10):1763-1765
We report on the generation of beams with azimuthal polarization using resonant grating waveguide structures (GWSs) inside an Yb:YAG thin-disk laser (TDL) oscillator. Two different GWS concepts were used to select the polarization of the emitted beam. The first uses the resonant reflection principle, and the second is based on the leaky-mode approach already reported in our previous work. Up to 93 W and 103 W of output power were extracted from a TDL with an optical efficiency, η(oo), of 36.2% and 40.1% using the first and the second approaches, respectively. In both cases, a pure azimuthal polarization and a beam quality factor, M2, of about 2.2 were measured. The design, fabrication, and different experimental results, as well as the laser performances for both GWSs, are discussed in the present Letter. 相似文献
95.
This paper presents a new approach to computing an approximate solution of Tikhonov-regularized large-scale ill-posed least-squares problems with a general regularization matrix. The iterative method applies a sequence of projections onto generalized Krylov subspaces. A suitable value of the regularization parameter is determined by the discrepancy principle. 相似文献
96.
Heinrich Voss 《PAMM》2007,7(1):1021001-1021002
The Jacobi–Davidson method is known to converge at least quadratically if the correction equation is solved exactly, and it is common experience that the fast convergence is maintained if the correction equation is solved only approximately. Here we derive the Jacobi–Davidson method in a way that explains this robust behavior. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
97.
Konrad Voss 《Results in Mathematics》2001,40(1-4):310-320
Necessary and sufficient conditions are studied for the existence of surfaces in space forms with prescribed shape operator S. We consider three cases: that of the non-existence of a solution, the case of a unique solution and that of more than one solution. Examples are given for local and global S-deformations and S-rigidity. 相似文献
98.
Heinrich Voss 《PAMM》2005,5(1):783-784
We consider the problem to compute relevant energy states and corresponding wave functions of a three dimensional semiconductor quantum dot. Let Ωq ⊂ ℝ3 be a domain occupied by the quantum dot, which is embedded in a matrix Ωm of different material. A typical example is an InAs pyramidal quantum dot embedded in a cuboid GaAs matrix. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
99.
Moritz Voss Dr. Sumire Honda Malca Prof. Dr. Rebecca Buller 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(33):7336-7345
Freestanding Fe/α-ketoglutarate-dependent halogenases are oxidoreductases that catalyze the installation of halogen atoms into unactivated sp3-hybridized carbon centers with high stereo- and regioselectivity. Since their discovery in 2014, a small number of indole alkaloid and amino acid halogenases have been identified and characterized. First enzyme engineering examples suggest that the accessible substrate range of these enzymes may be expanded through the use of rational enzyme design and directed evolution. Structural investigations of non-heme iron halogenases acting on freestanding as well as tethered substrates are beginning to inform about the principles of the underlying halogenation mechanism. 相似文献
100.