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81.
Joachim Janssen Holger Rumpf Hartwig Modrow Ralf Rablbauer Georg Frommeyer Josef Hormes 《无机化学与普通化学杂志》2003,629(10):1701-1708
The surface oxidation of FeCr alloys with 18, 28, and 43 mass‐% Cr was investigated in situ using grazing‐incidence X‐ray absorption spectroscopy (GIXAS) at the chromium and iron K‐edges. Oxidation in air was monitored in situ in the temperature range from 290 K to 680 K. The standard GIXAS data analysis is extended for the treatment of a single layer model in order to estimate the chromium concentrations of the oxide layer and of the near‐interface substrate as well as the oxide layer thickness. XANES analysis shows transitions from b.c.c. Fe to corundum type Fe2O3 and from b.c.c. Cr to corundum type Cr2O3. The initial oxide layers are 1.1‐1.4 nm thick and contain 60‐90 mass‐% chromium, while the near‐interface substrate is depleted in Cr. During heating, iron oxide growth dominates up to 560‐600 K. Then the chromium oxide layer loses its passivation effect and Cr oxidation sets in. 相似文献
82.
83.
Matthias Westerhausen Mathias Krofta Stefan Schneiderbauer Holger Piotrowski 《无机化学与普通化学杂志》2005,631(8):1391-1396
Transmetallation of Tin(II) in [Sn(μ3‐PSitBu3)]4 by Barium – from Sn4P4 Heterocubane Structures to Heterobinuclear Cage Compounds with a Central BanSn4?nP4 Heterocubane Polyhedron (n = 1, 2 and 3) For the preparation of compounds of the type [BanSn4?n(PSitBu3)4] (n = 1 ( 2 ), 2 ( 3 ) and 3 ( 4 )) two synthetic routes are applicable: in the transmetallation reaction homometallic [Sn4(PSitBu3)4] ( 1 ) reacts with barium metal and in a deprotonation reaction (metallation) tri(tert‐butyl)silylphosphane reacts simultaneously with (thf)2Ba[N(SiMe3)2]2 and Sn[N(SiMe3)2]2. During the transmetallation reaction mixtures of the heterobimetallic cage compounds 2 to 4 are obtained, however, analytically pure compounds 2 and 3 are accessible by the metallation reaction. Compound 4 is formed as a minor product together with 3 . Due to the larger Ba‐P bond lengths compared to the Sn‐P values the substitution of tin by barium leads to strong distortions of the heterocubane moiety. With NMR‐spectroscopic experiments one could show that all the above mentioned compounds form BanSn4?nP4 heterocubane cage structures. 相似文献
84.
AES depth profiles on ceramic powders (untreated/hydrolyzed/oxidized/ (Al, Y)2O3 coated Si3N4, [BaO, SiO2] coated Al2O3) are feasible on thin, homogeneous layers or m sized agglomerations prepared on an Au foil. By means of the depth profiles one can qualitatively characterize the coating around the particles. Factor analysis of the depth profiles on the differently treated Si3N4 powders suggests the existence of an Si2N2O phase on the oxidized sample.Dedicated to Professor Günther Tölg on the occasion of his 60th birthday 相似文献
85.
To get informations on both the structure and dynamics of hydrogen chelates 1 of heteroaromatic systems, a great variety of quinazoline-2-acetonitrile chelates were synthesized (see 2–4 ). Similarly to the situation of the corresponding H-chelates in the pyrimidine-2-acetonitrile series, the investigation of these new derivatives 2–4 by NMR spectroscopic methods (DNMR, COSY, NOESY, ROESY, EXSY, HMQC, HMBC) confirms the presence of an equilibrium of the two possible H-chelate structures (two ‘rotamers’ I and II , i.e., (E)/(Z) isomers; see Scheme). The corresponding equilibria I ? II were determined by complete 1H-NMR signal assignment at low temperatures (after freezing the rotational processes). In addition, the tautomer equilibria A ? B (relative energies of the two minima of the nonsymmetrical double-well potential) for both ‘rotamers’ are ascertained by H,H and C,H couplings. The results are an important basis for the interpretation of both the UV/VIS absorptions and the dependence of fluorescence and fluorescence quantum yields on temperature. 相似文献
86.
87.
The bond dissociation energies for a series of silyl peroxides have been calculated at the G2 and CBS-Q levels of theory. A comparison is made with the O-O BDE of the corresponding dialkyl peroxides, and the effect of the O-O bond strength on the activation barrier for oxygen atom transfer is discussed. The O-O bond dissociation enthalpies (DeltaH(298)) for bis (trimethylsilyl) peroxide (1) and trimethylsilyl hydroperoxide (2) are 54.8 and 53.1 kcal/mol, respectively at the G2 (MP2) and CBS-Q levels of theory. The O-O bond dissociation energies computed at G2 and G2(MP2) levels for bis(tert-butyl) peroxide and tert-butyl hydroperoxide are 45.2 and 48.3 kcal/mol, respectively. The barrier height for 1,2-methyl migration from silicon to oxygen in trimethylsilyl hydroperoxide is 47.9 kcal/mol (MP4//MP2/6-31G). The activation energy for the oxidation of trimethylamine to its N-oxide by bis(trimethylsilyl) peroxide is 28.2 kcal/mol (B3LYP/6-311+G(3df,2p)// B3LYP/6-31G(d)). 1,2-Silicon bridging in the transition state for oxygen atom transfer to a nucleophilic amine results in a significant reduction in the barrier height. The barrier for the epoxidation of E-2-butene with bis(dimethyl(trifluoromethyl))silyl peroxide is 25.8 kcal/mol; a reduction of 7.5 kcal/mol relative to epoxidation with 1. The activation energy calculated for the epoxidation of E-2-butene with F(3)SiOOSiF(3) is reduced to only 2.2 kcal/mol reflecting the inductive effect of the electronegative fluorine atoms. 相似文献
88.
Max Winter Karl H. Schulte-Elte Alain Velluz Josef Limacher Wilhelm Pickenhagen Günther Ohloff 《Helvetica chimica acta》1979,62(1):131-134
Aroma Constituents of the Purple Passion Fruit. Two New Edulan Derivatives The isolation of (2R*, 4S*, 4aS*, 8aS*)-4,4a-epoxy-4,4a-dihydroedulan ( 1 ), and (2R*, 3S*, 8aS*)-3-hydroxyedulan ( 2 ), two new constituents of the purple passion-fruit (Passiflora edulis SIMS ), is reported. Racemic epoxide 1 was synthesized by oxidation of edulan 6 with peracidic acid, and racemate of alcohol 2 was obtained by reduction of ketone 7 , one of the chromic acid oxidation-products of edulan 6 . 相似文献
89.
A set of highly preorganized pyrazolate-bridged dimanganese complexes L(Mn)MnX have been prepared and structurally characterized. They can be described as hybrid organometallic/Werner-type systems that consist of a low-spin CpMn(I)(CO)2 subunit (Mn1) and a proximate tripodal tetradentate {N4} binding pocket accommodating a high-spin Mn(II) ion (Mn2), with Mn...Mn distances of approximately 4.3 A and different coligands bound to Mn2. Density functional theory (DFT) calculations (both the hybrid B3LYP and the pure BP86 functionals and the all-electron basis sets 6-311G and 6-311G*) confirm that the valence alpha and beta Kohn-Sham molecular orbitals (MOs) of these mixed-valent Mn(I)Mn(II) compounds have predominant Mn(3d) character and an almost perfectly localized nature: all five unpaired electrons are essentially localized at the Werner-type Mn2, whereas Mn1 possesses an effective closed-shell structure with the MOs of highest energy centered there. One-electron oxidation occurs in a clean process at approximately E(1/2) = -0.6 V (versus ferrocene/ferrocinium), giving the low-spin/high-spin Mn(II)Mn(II) species. UV/vis and IR spectroelectrochemistry as well as a detailed theoretical analysis reveal that the redox process takes place with strict site control at the organometallic subunit, while it does not significantly influence the spin and charge distribution on the Werner-type site. Positions and shifts of the nu(C[triple bond]O) absorptions are largely reproduced by the DFT calculations. These systems thus represent an exceptional example of the effect the unsymmetry of a dinucleating ligand scaffold has on the spin and charge distribution in homobimetallic complexes and might offer interesting prospects for the study of the cooperative effects of bimetallic arrays. 相似文献
90.
Keil O LeRiche T Deppe H Volmer DA 《Rapid communications in mass spectrometry : RCM》2002,16(8):814-820
This paper focuses on the technical aspects of chemical screening from 384-well plate nano-scale single-bead combinatorial libraries. The analytical technique utilized is a combination of capillary liquid chromatography with ultraviolet detection and matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. The HPLC/MALDI-MS hyphenation is achieved by means of a micro-fraction collector with a peak detection system that automatically collects the peaks onto the MALDI targets for subsequent characterization. Several experimental parameters such as type of 384-well plate, well-plate sealing foils, and a column-switching procedure were investigated using a small test library of nine components. Additionally, the influence of different MALDI matrices, different MALDI targets and sample-spotting techniques on the MALDI detection sensitivity as well as the ruggedness and sample throughput capacity of this technique were studied. Optimum results for the analytes investigated were obtained with 2,5-dihydroxybenzoic acid using on-line mixing of HPLC effluent and matrix solution. To demonstrate the potential of this capillary HPLC/MALDI-TOFMS method, its application to several single-bead libraries was investigated. The instrumental method allowed for the rapid identification and purity assessment of combinatorial libraries with detection limits down to the higher femtomole level using both UV detection and MALDI mass spectrometry. 相似文献