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301.
A new approach to 2-amino-6-(methoxycarbonyl)amino-4-(1,2,3,6-tetrahydro-1-pyridyl)pyrimidine 1-oxide ( 3 ) is described. Methyl [1-ethoxy-2-(ethoxycarbonyl)-ethylidene]carbamate ( 5 ) reacted with guanidine to the pyrimidinecarbamate 6 , which was successively transformed into methyl 2-amino-6-(p-tolyslulfonyl)oxy-4-pyrimidinecarbamate ( 8 ). Oxidation of 8 led to the corresponding pyrimidine N-oxide 9 , a useful starting material to 3 . 相似文献
302.
Karl Jug Hans-Peter Schluff Hans Kupka Rüdiger Iffert 《Journal of computational chemistry》1988,9(8):803-809
Small Sin and Aln clusters (n = 3–10) were studied with the semiempirical molecular orbital method (MO) method SINDO1. For each n, various structures were optimized to determine the most stable structure. To obtain good qualitative agreement with available ab initio calculations d orbitals had to be omitted from the basis set. Both silicon and aluminum tend to build three-dimensional structures rather than two- or one-dimensional structures, except for n = 3 or 4. The structure growth was studied by approaching various sites of stable structures with one or more atoms. It was found that silicon and aluminum exhibit different structure growth, and consequently, different most-stable structures. Ionization potentials, HOMO -LUMO energy differences, binding energies per atom, and average atomic valencies are presented. 相似文献
303.
Sven?UhlenbruckEmail author Frank?Tietz Vincent?Haanappel Doris?Sebold Hans-Peter?Buchkremer Detlev?St?ver 《Journal of Solid State Electrochemistry》2004,8(11):923-927
Silver (Ag) at 0.1–2.0 wt% was incorporated into cathodes for solid oxide fuel cells as a catalyst for oxygen reduction. A novel processing route for Ag incorporation ensuring a very homogeneous Ag ion distribution is presented. From the results of X-ray powder diffraction it can be concluded that the La0.65Sr0.3MnO3– perovskite phase is already formed at 900 °C. The solubility of Ag in the crystal lattice in this type of perovskite was below 1 wt%. The electrochemical tests of these materials show that there is only a slight catalytic effect of Ag. Scanning electron microscopy reveals a low mechanical contact of the cathode grains to the electrolyte due to the low cathode sintering temperature that was chosen. 相似文献
304.
Jan Kuliga Rodrigo Cezar de Campos Ferreirra Rajan Adhikari Stephen Massicot Dr. Michael Lepper Helen Hölzel Prof. Dr. Norbert Jux Dr. Hubertus Marbach Prof. Dr. Abner de Siervo Prof. Dr. Hans-Peter Steinrück 《Chemphyschem》2021,22(4):396-403
We study the interaction and metalation reaction of a free base 5,10,15,20-terakis(4-cyanophenyl)porphyrin (2HTCNPP) with post-deposited Zn atoms and the targeted reaction product Zn-5,10,15,20-terakis(4-cyanophenyl)porphyrin (ZnTCNPP) on a Ag(111) surface. The investigations are performed with scanning tunneling microscopy at room temperature after Zn deposition and subsequent heating. The goal is to obtain further insights in the metalation reaction and the influence of the cyanogroups on this reaction. The interaction of 2HTCNPP with post-deposited Zn leads to the formation of three different 2D ordered island types that coexist on the surface. All contain a new species with a bright appearance, which increases with the amount of post-deposited Zn. We attribute this to metastable SAT (“sitting atop”) complexes formed by Zn and the macrocycle, that is, an intermediate in the metalation reaction to ZnTCNPP, which occurs upon heating to 500 K. Interestingly, the activation barrier for the successive reaction of the SAT complex to the metalated ZnTCNPP species can also be overcome by a voltage pulse applied to the STM tip. 相似文献
305.
Neyman KM Lim KH Chen ZX Moskaleva LV Bayer A Reindl A Borgmann D Denecke R Steinrück HP Rösch N 《Physical chemistry chemical physics : PCCP》2007,9(27):3470-3482
We review systematic experimental and theoretical efforts that explored formation, structure and reactivity of PdZn catalysts for methanol steam reforming, a material recently proposed to be superior to the industrially used Cu based catalysts. Experimentally, ordered surface alloys with a Pd : Zn ratio of approximately 1 : 1 were prepared by deposition of thin Zn layers on a Pd(111) surface and characterized by photoelectron spectroscopy and low-energy electron diffraction. The valence band spectrum of the PdZn alloy resembles closely the spectrum of Cu(111), in good agreement with the calculated density of states for a PdZn alloy of 1 : 1 stoichiometry. Among the issues studied with the help of density functional calculations are surface structure and stability of PdZn alloys and effects of Zn segregation in them, and the nature of the most likely water-related surface species present under the conditions of methanol steam reforming. Furthermore, a series of elementary reactions starting with the decomposition of methoxide, CH(3)O, along both C-H and C-O bond scission channels, on various surfaces of the 1 : 1 PdZn alloy [planar (111), (100) and stepped (221)] were quantified in detail thermodynamically and kinetically in comparison with the corresponding reactions on the surfaces Pd(111) and Cu(111). The overall surface reactivity of PdZn alloy was found to be similar to that of metallic Cu. Reactive methanol adsorption was also investigated by in situ X-ray photoelectron spectroscopy for pressures between 3 x 10(-8) and 0.3 mbar. 相似文献
306.
307.
We compute the asymptotic distribution of the sample covariance matrix for independent and identically distributed random vectors with regularly varying tails. If the tails of the random vectors are sufficiently heavy so that the fourth moments do not exist, then the sample covariance matrix is asymptotically operator stable as a random element of the vector space of symmetric matrices. 相似文献
308.
Let be an i.i.d. sequence of -valued random vectors belonging to the generalized domain of semistable attraction of some nonnormal law. Assume further
that is a sequence of positive integer valued random variables such that for some
for some discrete positive random variable D, where we do not assume that and are independent. Let . Then various laws of the iterated logarithm for the norm of as well as the radial projection onto a unit vector θ are presented. 相似文献
309.
310.
Manfred Pinnow Hans-Peter Fink Carola Fanter Jürgen Kunze 《Macromolecular Symposia》2008,262(1):129-139
Highly porous cellulose was formed by gelation of cellulose carbamate solutions in caustic soda. Two methods for gel preparation were optimized for the formation of beads and bulky materials – the chemical precipitation from dilute sulfuric acid and the thermal gelation by annealing at elevated temperatures. Various methods were used for characterizing of the pores of low density materials: scanning electron microscopy, small angle X-ray scattering, mercury intrusion and nitrogen sorption. These methods were optimized and used for characterizing the complete pore system from micro to macro pores. The effects of different preparation (cellulose carbamate concentration in caustic soda) and processing (precipitation, drying and pyrolysis) on the pore structure were studied by the set of complementary methods. Aerocell samples with a minimum density of 0.06 g/cm3 were prepared from cellulose carbamate. They are characterized by a broad pore size distribution ranging from 0.5 nm to 1 mm, specific internal surfaces of up to 660 m2/g and total pore volumes of up to 18 cm3/g. 相似文献