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901.
902.
The syntheses of a variety of fluorine-containing adamantane derivatives are described. The 13C NMR spectra of those compounds with different configurations are interpreted in terms of through-bond and through-space substituent interactions. 相似文献
903.
The 13C NMR spectra of four series of di- and tricyclic 1,4-dithiepane spiranes are presented. The cis-fused tricyclic derivatives 4a - 4d exhibit two coales-cences in their 100.6 MHz spectra. These are discussed and assigned to conformational processes, namely a conversion of the molecules into their mirror images and a restricted pseudorotation of the dithiepane ring, respectively. 相似文献
904.
905.
Helmut Alt Kurt Mehlhorn Hubert Wagener Emo Welzl 《Discrete and Computational Geometry》1988,3(1):237-256
We consider the problem of computing geometric transformations (rotation, translation, reflexion) that map a point setA exactly or approximately into a point setB. We derive efficient algorithms for various cases (Euclidean or maximum metric, translation or rotation, or general congruence).Part of this research was supported by the DFG under Grants Me 620/6-1 and Al 253/1-1. 相似文献
906.
907.
Helmut Pottmann 《Results in Mathematics》1986,9(1-2):131-159
908.
J.Rayford Nix Arnold J. Sierk Helmut Hofmann Franz Scheuter Dominique Vautherin 《Nuclear Physics A》1984,424(2):239-261
By use of both analytical and numerical techniques, we study the relaxation of time-dependent solutions of the Fokker-Planck equation for an inverted oscillator to Kramers' stationary solution. This is done by integrating over all time the time-dependent solutions for given initial conditions at the saddle point to obtain stationary solutions, whose densities and higher velocity moments are compared as functions of the coordinate with the corresponding quantities calculated from Kramers' stationary solution. For large values of the coordinate an a symptotic expansion of the density is obtained, but for general values of the coordinate and for higher velocity moments the time integration must be done numerically. With increasing dissipation the relaxation to Kramers' stationary solution occurs at successively smaller values of the coordinate. By use of Kramers' stationary solution, we derive analytical expressions as functions of nuclear temperature and dissipation strength for several quantities of interest in fission dynamics, including the mean time from the saddle point to scission, the mean fission-fragment kinetic energy at the scission point and the contribution to the variance in the fission-fragment kinetic energy resulting from fluctuations in the fission degree of freedom. We apply these results to some examples that have been studied experimentally, including the mean saddle-to-scission time for the heavy-ion-induced fission of the compound nucleus 168Yb and the mean fission-fragment kinetic energy at scission and the contribution to its variance for the α-particle-induced fission of the compound nucleus 213At. 相似文献
909.
910.
η3-Allylnickel alkoxides {η3-C3H5NiOR}2 (R = Me, Et, i-Pr, Ph, SiPh3) may be activated by gaseous boron trifluoride (BF3) to give active catalysts for the dimerization of propene in homogeneous phase. In CH2Cl2 at ?20 °C catalytic turnover numbers of 5000 mol propene(mol Ni)?1h?1 were measured. The nature of the OR group influences both the catalytic activity and the oligomerization product distribution. The ratio of methylpentenes to dimethylbutenes in the dimer fraction may be controlled by the presence of additional phosphine ligands at the nickel atom. The nickel alkoxide precursor was heterogenized on alumina to give {Al2O3}–O–Ni–(η3-C3H5). Subsequent activation using gaseous BF3 generates a powerful heterogeneous olefin dimerization catalyst which converts 50 × 103 mol propene (mol Ni)?1 at ?10° to ?5°C in a batchwise process and 143 × 103 mol propene (mol Ni)?1 continuously to give 75% dimers and 25% higher oligomers. The solvent-free treatment of oxide supports, e.g. alumina or silica, with gaseous BF3 produces strong ‘solid acids’. The activated hydroxyl groups on the support surface serve as effective anchor sites for organometallic complexes to form heterogenous catalysts. By reaction of Ni(cod)2 with {Al2O3}O(BF3)H or {SiO2}O(BF3)H, η1, η2-cyclo-octenylnickel–O fragments may be fixed to the surface. In the absence of halogenated solvents, the resulting catalysts, e.g. {SiO2}O–(BF3)–Ni–(η1, η2-C8H13), dimerize propene continuously at +5°C at the rate of 800 × 103 mol liquid propene (mol Ni)?1. 相似文献