Theoretical study of the electronic spectra of oxidized and reduced states of lumiflavin and its derivative

Time-dependent density functional theory has been applied to investigate the electronic absorption spectrum of oxidized and reduced lumiflavin and its derivative, 8-NH(2)-lumiflavin. The calculations allow the authors to explain the origin of the difference in spectral features between oxidized and reduced states of lumiflavin. For the reduced lumiflavin, a reasonable assignment of the experimental spectrum has been made for the first time. Furthermore, the results obtained reveal that the NH(2) group plays a critical role in shaping the spectral features of 8-NH(2)-lumiflavin, and offer a reasonable explanation for the spectral changes upon substituting the NH(2) group for the CH(3) group of lumiflavin.     

Genesis of catalysts for methanol synthesis

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Excited states and electronic spectra of extended tetraazaporphyrins

Electronic excited states, electronic absorption, and magnetic circular dichroism (MCD) spectra of free-base tetraazaporphyrin (TAP), phthalocyanine (Pc), naphthalocyanine (Nc), and anthracocyanine (Ac) were studied by quantum chemical calculations using the symmetry-adapted cluster-configuration interaction (SAC-CI) method. Not only optically allowed states including the Q- and B-bands but also optically forbidden states were calculated for transitions whose excitation energies were lower than 4.5 eV. The present SAC-CI calculations consistently assigned the absorption and MCD peaks as optically allowed π→π(?) excitations, although these calculations using double-zeta basis limit quantitative agreement and discussion. For Nc and Ac, excited states beyond the four-orbital model appeared in the low-energy region. The low-energy shifts of the Q-bands with the extension of molecular size were explained by the orbital energies. The splitting of the Q-bands decreases with extension of the molecular size. This feature was reproduced by the SAC-CI calculations but the configuration interaction with single excitations and time-dependent density functional theory calculations failed to reproduce this trend. Electron correlation in the excited states is important in reproducing this splitting of the Q-bands and in describing the energy difference between the B(2u) and B(3u) states of free-base porphyrins.     

Variations in electronic state of silver catalysts for methanol oxidation

Variations in the electronic state of silver catalysts for methanol oxidation under the effect of the reaction medium have been studied, using diffuse reflectance spectroscopy. The nature of active centers-Ag⁺ cations, Ag _n ⁺ complexes and clusters of various sizes for bulk and supported catalysts are shown to be identical.

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. - Ag⁺, Ag _n ⁺ .

     

Formation of methane in methanol synthesis on zinc-chromium catalysts

The rate of methane formation on zinc-chromium catalysts is described by zero-order rate law, If the catalyst is activated by copper, the rate of methane formation increases due to a decrease in the activation energy of the reaction. Presumably the formation of methane and methanol takes place on different active centers.

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Energies of electronic states of promoter ions in hydrodesulfurization catalysts

Poly-vinylpyrrolidone-anchored (PVP) supported platinum catalyst (PVP-Pt) and rare earth metals (Sm³⁺, Pr³⁺, Ce³⁺, Nd³⁺and La³⁺) promoted PVP-Pt catalysts were prepared and used in the hydrogenation of o- and m-chloronitrobenzene (CNB) in ethanol at 303 K and atmospheric pressure. It was found that rare earth metals improved the selectivity of chloronitrobenzene to chloroaniline (CAN), while the hydrogenation rate of CNB decreased. In the presence of NaOH, PVP-Pr-Pt and PVP-La-Pt catalysts exhibit the highest selectivity in the hydrogenation of CNB. At the same time different support materials of Pt catalyst (such as polyethylene glycol (PEG), ethyl cellulose (EC)) possessed great influence on catalytic activity and selectivity of p-CAN.This revised version was published online in December 2005 with corrections to the Cover Date.     

Copper-supported catalysts in methanol synthesis and water gas shift reaction

The comparative studies of physicochemical properties of Cu-support catalysts (support = ZnO, Al₂O₃, Cr₂O₃, ZnAl₂O₄, FeAlO₃, or CrAl₃O₆) and their catalytic activity in methanol synthesis and water gas shift reaction were the main goal of this work. The promotion effect of copper addition in both reaction was proved. The formation of spinel type structure CuCr₂O₄, ZnAl₂O₄ and binary oxide CrAl₃O₆, FeAlO₃ during calcination process was confirmed by XRD technique. Results showed that 20% Cu/FeAlO₃ had the best performance in water gas shift reaction. The best selective and active catalyst in methanol synthesis was 20% Cu/ZnAl₂O₄.     

Active state of copper in catalysts for low-temperature methanol synthesis

Copper-containing catalysts have been investigated in low-temperature methanol synthesis. It is shown that the active component of the catalyst is the copper ion and not the metallic copper as is supposed previously.

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The nature of the electronic states and photoelectron spectra of oxyanion crystals

The densities of states, atomic charges, and partial components were calculated by the B3LYP method for lithium, sodium, and potassium nitrites, nitrates, chlorates, perchlorates, sulfites, and sulfates using a localized basis of atomic orbitals and CRYSTAL06 software. The calculated densities of states N(E) are in good agreement with the experimental photoelectron spectrum (UPS). The crystallographically nonequivalent metal and oxygen atoms are in different charged states, which leads to a splitting of the N(E) bands.     

Electron spectroscopic studies of copper in catalysts for methanol synthesis

The state of Cu²⁺ ions in Cu–Zn–Al catalysts for methanol synthesis has been studied by electron spectroscopy (5000–50000 cm^–1). It has been established that low-temperature ZnO stabilizes copper in a distorted octahedral coordination as cluster structures resembling (but not similar to) CuO. The presence of Al promotes the formation of these clusters and inhibits the formation of CuO.

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(5000–50000 ^–1) Cu²⁺ Cu–Zn–Al . , ZnO , ( ) CuO. , CuO.

     

One-pot synthesis of oxidized and reduced heptaphyrins with unusual TFA binding

Synthesis and reversible redox interconversion between core-modified [32]heptaphyrin (1.1.1.1.1.0.0) and [30]heptaphyrin (1.1.1.1.1.0.0) are reported.     

Activity of tungstate catalysts in the synthesis of metylmercaptane from methanol and hydrogen sulfide

Catalysis obtained by impregnation of Al₂O₃ with solutions of hepta-, dodeca-, meta- and normal alkaline metal tungstates showed approximately the same activity. Most selective towards methylmercaptane were catalysts with an alkaline metal to tungsten atomic ratio equal to 21.

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, Al₂O₃ -, -, -, . , : 21.

     

Peculiarities of the vibratioanl spectra of molecules adsorbed on the surface of heterogeneous catalysts: theoretical analysis

The vibrational spectra of adsorption complexes resulting from the adsorption of ammonia on the surface of heterogeneous catalysts are interpreted by computation of frequencies and forms of normal vibrations. The characteristics of normal vibrations are computed from the state of separate fragments of adsorption complexes. It is shown that an interaction of internal coordinates, associated with the adsorption bond, with internal coordinates of the adsorbed molecule leads to a decrease of characteristics of vibrations sensitive to the surface area. A correlation is established between the characteristics of such vibrations and the changes in their frequencies in comparison with the free molecule.     

负载型金属催化剂上合成气制甲醇反应体系的耐硫性能

研究了共沉淀法制备的系列金属负载型催化剂合成气制甲醇反应性能,重点考察了催化剂上合成甲醇反应体系的耐硫性能。结果表明,Cu/ZnO催化剂显示了较好的甲醇合成反应性能,但该反应在含硫气氛下迅速失活;Pd/CeO₂催化剂体现了良好的甲醇合成反应性能和该反应体系的高耐硫性能。结合多种物理化学表征手段分析得出, Cu/ZnO催化剂在含硫气氛下因活性组分形成金属硫化物而失活;Pd/CeO₂催化剂中的载体CeO₂可优先与硫作用而保护金属活性组分,进而保持了Pd/CeO₂反应体系的高抗硫性能。     

Free-electron model for electronic spectra of benzene

Using a three dimensional free-electron model for the π-electron system the energies of the various singly and doubly excited configurations of benzene have been calculated. It has been found that the energies of the various singly excited configurations are in good agreement with the values obtained by Craig; whereas the energies of the doubly excited configurations are higher than those obtained by Craig. Therefore the configurational mixing for the doubly excited configurations is not necessary in this kind of model.     

Monitoring in situ catalytically active states of Ru catalysts for different methanol oxidation pathways

One of the prerequisites for the detailed understanding of heterogeneous catalysis is the identification of the dynamic response of the catalyst surface under variable reaction conditions. The present study of methanol oxidation on different model Ru pre-catalysts, performed approaching the realistic catalytic reaction conditions, provides direct evidence of the significant effect of reactants' chemical potentials and temperature on the catalyst surface composition and the corresponding catalytic activity and selectivity. The experiments were carried out for three regimes of oxygen potentials in the 10(-1) mbar pressure range, combining in situ analysis of the catalyst surface by synchrotron-based photoelectron core level spectroscopy with simultaneous monitoring of the products released in the gas phase by mass spectroscopy. Metallic Ru with adsorbed oxygen and transient 'surface oxide', RuO(x), with varying x have been identified as the catalytically active states under specific reaction conditions, favouring partial or full oxidation pathways. It has been shown that the composition of catalytically active steady states, exhibiting different activity and selectivity, evolves under the reaction conditions, independent of the crystallographic orientation and the initial pre-catalyst chemical state, metallic Ru or RuO(2).