Quantum chemical modeling of CO oxidation by the active site of molybdenum CO dehydrogenase

The catalytic mechanism of molybdenum containing CO dehydrogenase has been studied using hybrid DFT methods with quite large chemical models. The recent high-resolution X-ray structure, showing the surprising presence of copper linked to molybdenum, was used as a starting point. A pathway was initially found with a low barrier for C-O bond formation and CO2 release. However, this pathway did not include the formation of any S-CO2 species, which had been suggested by experiments with an n-butylisocyanide inhibitor. When these SCO2 structures were studied they were found to lead to deep minima, making CO2 release much more difficult. A large effort was spent, including investigations of other spin states, varying the number of protons and electrons, adding water, etc., until a plausible pathway for S-C bond cleavage was found. In this pathway a water molecule is inserted in between molybdenum and the SCO2 group. Full catalytic cycles, including electron and proton transfers, are constructed both with and without S-C bond formation. When these pathways are extended to two full catalytic cycles it can be understood why the formation of the S-C bond actually makes catalysis faster, even though the individual step of CO2 release becomes much more difficult. These results agree well with experimental findings.     

Mechanism of the thermal decomposition of hydrophosphites of lithium,sodium and potassium

The thermal decomposition of some M ₂ ^I HPO₃ (M-Li, Na, K) phosphites under nitrogen atmosphere was investigated. A stepwise mechanism of thermal decomposition has been proposed.

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Zusammenfassung Es wurde die thermische Zersetzung von Phosphiten M ₂ ^I HPO₃ (M-Li, Na, K) in Stickstoffatmosphere untersucht. Die Schritte des Mechanismus der thermischen Zersetzung wurden vorgeschlagen.

     

铁氰化锰修饰玻碳电极的制备及其电化学性能

A thin film of manganese hexacyanoferrate （MnHCF） was electrochemically formed on a glassy carbon （GC） electrode to prepare a chemically modified electrode （CME）. The mechanism of film formation of MnHCF and its growth process were investigated in detail by cyclic voltammetry. The results show that the stoichiometric composition of MnHCF is Mn^ⅢFe^Ⅲ（CN）6, an analogue of prussian yellow. There exist three clear-cut stages in the whole modification process and the last stage is indispensable to the fabrication of homogenized, stable MnHCF film and must last for an appropriate time. The surface morphology of MnHCF/GC electrode was characterized by scanning electron microscopy （SEM）, which further verified the effective deposition of MnHCF film on GC. The kinetic constants of MnHCF/GC electrode process were also evaluated. The resulting MnHCF film modified electrode presented good stability and high electrocatalytic activity toward the oxidation of H2O2, indicating that MnHCF film possesses function of catalase and can be expected for analytical purposes.     

铑配合物催化甲醇羰基化反应的性能和机理

本文合成了２－丁酸甲酯吡啶与四羰基二氯二铑形成的螯合形配合物，并用Ｘ射线光电子能谱（ＸＰＳ），红外光谱（ＩＲ）及核磁共振（ＮＭＲ）研究确定了其配位结构特征，通过甲醇羰基化制备乙酸的反应揭示了其结构特征对催化剂活性及稳定性的影响，并提出了可能的反应机理。     

Voltammetric behaviour of bromhexine and its determination in pharmaceuticals

A complete electrochemical study and a novel electroanalytical procedure for bromhexine quantitation are described. Bromhexine in methanol/0.1 mol L⁻¹ Britton–Robinson buffer solution (2.5/97.5) shows an anodic response on glassy carbon electrode between pH 2 and 7.5. By DPV and CV, both peak potential and current peak values were pH-dependent in all the pH range studied. A break at pH 5.5 in E_P versus pH plot revealing a protonation–deprotonation (pK_a) equilibrium of bromhexine was observed. Spectrophotometrically, an apparent pK_a value of 4.3 was also determined.

An electrodic mechanism involving the oxidation of bromhexine via two-electrons and two-protons was proposed. Controlled potential electrolysis followed by HPLC–UV and GC–MS permitted the identification of three oxidation products: N-methylcyclohexanamine, 2-amino-3,5-dibromobenzaldehyde and 2,4,8,10-tetrabromo dibenzo[b,f][1,5] diazocine.

DPV at pH 2 was selected as optimal pH for analytical purposes. Repeatability, reproducibility and selectivity parameters were adequate to quantify bromhexine in pharmaceutical forms. The recovery was 94.50 ± 2.03% and the detection and quantitation limits were 1.4 × 10⁻⁵ and 1.6 × 10⁻⁵ mol L⁻¹, respectively. Furthermore, the DPV method was applied successfully to individual tablet assay in order to verify the uniformity content of bromhexine. No special treatment of sample were required due to excipients do not interfered with the analytical signal. Finally the method was not time-consuming and less expensive than the HPLC one.     

CH bond activation in the presence or absence of oxygen or nitrous oxide

The reaction of C₂H₆with lattice oxygen, O^2- (in the absence of gaseous oxygen), or “adsorbedℍ oxygen (in the presence of gaseous oxygen) over NiMoO₄ catalysts has been performed and compared to C₃H₈ activation. The results obtained indicate that adsorbed oxygen exhibits a higher reactivity to C₂H₆, while lattice oxygen is more reactive relative to C₃H₈. Kinetic studies of these two reactions in presence of molecular oxygen have indeed shown that the ethane oxidative dehydrogenation (ODH) is dependent on the oxygen partial pressure, whilst on the contrary propane ODH is not. In order to confirm the presence of “adsorbed” oxygen for ethane activation, ODH tests have been performed with N₂O. On increasing temperature, the O^- adsorbed species enhances the mild oxidation of ethane. The activation energy of ethane consumption E_C2H6, relative to propane (E_C3H8 = 133 kJ/mol) is 145 kJ/mol. A possible mechanism is proposed for the oxidative dehydrogenation of ethane. This revised version was published online in August 2006 with corrections to the Cover Date.     

半化学法制备Pb(Fe2/3W1/3)O3陶瓷粉体的反应机理

铅系弛豫铁电陶瓷钨铁酸铅Pb(Fe2/3W1/3)O3(PFW)是一种重要的介电材料,具有较大的介电常数(8000)和较低的烧结温度(小于900℃),适用于制备低烧高介的多层陶瓷电容器犤1～4犦。在传统氧化物法合成PFW的过程中,易生成恶化介电性能的钨酸铅(PbWO4或Pb2WO5)或焦绿石相(Pb2FeWO6)等其他相犤3,4犦。尽管通过加入过量5%的Fe2O3可以消除这些其他相,但因含较多的变价铁离子(Fe3+和Fe2+)而产生介电老化的缺点犤5犦。虽然二次合成法被广泛用于制备铅系弛豫铁电陶瓷犤6犦,但对制备PFW陶瓷的效果并不明显,仍有少量的钨酸铅PbWO4存在,并且预烧…     

Effect of the Structure of the Acyl Group on the Kinetics of Exchange between O-Nucleophiles

We have studied the rate of 15 reactions of acyl transfer from O-acyl salts of 4-dimethylaminostyryl-4-pyridine N-oxide to 4-morpholinopyridine and 4-dimethylaminopyridine N-oxides in acetonitrile solutions. Analysis of the results based on the Shaik – Pross approach and the Marcus equation shows that if the structure of the acyl group is varied, then the reactivity is determined by such parameters as the resonance interaction in the transition state (B) or the internal barrier (G ₀) of the reaction.     

Aktivierung von CO2 an Übergangsmetallzentren: Zum Ablauf der CO2-Reduktion an Nickel(0)-Fragmenten

Activation of CO₂ at Transition Metal Centres: The Route of the CO₂ Reduction at Nikel(0) Moieties A competing reaction in the catalytic cyclooligomerization of hex-3-yne and CO₂ at the (TMED)Ni(0)-fragment (TMED = N,N,N′,N′-tetramethylethylendiamine) is the formation of carbon monoxide and (TMED)Ni(CO₃). So it is possible to explain the generation of II (TMED)Ni(diethylmalicacidanhydride) and III (a nickel trimer with two (TMED)Ni(CO₃) units). Both complexes are characterized by X-ray analysis. The reduction of CO₂ to CO most likely proceeds via an intermediate in which two molecules of carbon dioxide are coupled head-to-tail to form a metallacycle. An ab initio scf geometry optimization supports the existence of such an intermediate.     

有机质谱学法研究气相S_N2离子－分子反应进展

总结了近年来应用质谱方法研究气相SN2离子－分子反应的进展，主要包括气 相SN2离子－分子反应的机理，动力学和热力学的各种因素。与凝固相中反应对比 ，阐明了影响反应机理、方向与程度的决定性因素，及溶剂化效应对凝聚相中反应 的影响方式。还介绍了一些气相中特有的SN2离子－分子反应和研究气相反应特殊 的质谱学研究方法与技术。