Enhanced low-temperature CO oxidation on a stepped platinum surface for oxygen pressures above 10(-5) Torr

The rate of CO oxidation has been characterized on the stepped Pt(411) surface for oxygen pressures up to 0.002 Torr, over the 100-1000 K temperature range. CO oxidation was characterized using both temperature-programmed reaction spectroscopy (TPRS) and in situ soft X-ray fluorescence yield near-edge spectroscopy (FYNES). New understanding of the important role surface defects play in accelerating CO oxidation for oxygen pressure above 10(-5) Torr is presented in this paper for the first time. For saturated monolayers of CO, the oxidation rate increases and the activation energy decreases significantly for oxygen pressures above 10(-5) Torr. This enhanced CO oxidation rate is caused by a change in the rate-limiting step to a surface reaction limited process above 10(-5) Torr oxygen from a CO desorption limited process at lower oxygen pressure. For example, in oxygen pressures above 0.002 Torr, CO(2) formation begins at 275 K even for the CO saturated monolayer, which is well below the 350 K onset temperature for CO desorption. Isothermal kinetic measurements in flowing oxygen for this stepped surface indicate that activation energies and preexponential factors depend strongly on oxygen pressure, a factor that has not previously been considered critical for CO oxidation on platinum. As oxygen pressure is increased from 10(-6) to 0.002 Torr, the oxidation activation energies for the saturated CO monolayer decrease from 24.1 to 13.5 kcal/mol for reaction over the 0.95-0.90 ML CO coverage range. This dramatic decrease in activation energy is associated with a simple increase in oxygen pressure from 10(-5) to 10(-3) Torr. Activation energies as low as 7.8 kcal/mol were observed for oxidation of an initially saturated CO layer reacting over the 0.4-0.25 ML coverage range in oxygen pressure of 0.002 Torr. These dramatic changes in reaction mechanism with oxygen pressure for stepped surfaces are consistent with mechanistic models involving transient low activation energy dissociation sites for oxygen associated with step sites. Taken together these experimental results clearly indicate that surface defects play a key role in increasing the sensitivity of CO oxidation to oxygen pressure.     

Design, synthesis, and biochemical evaluation of 1,5,6,7-tetrahydro-6,7-dioxo-9-D-ribitylaminolumazines bearing alkyl phosphate substituents as inhibitors of lumazine synthase and riboflavin synthase

The last two steps in the biosynthesis of riboflavin, an essential metabolite that is involved in electron transport, are catalyzed by lumazine synthase and riboflavin synthase. To obtain structural probes and inhibitors of these two enzymes, two ribityllumazinediones bearing alkyl phosphate substituents were synthesized. The synthesis involved the generation of the ribityl side chain, the phosphate side chain, and the lumazine system in protected form, followed by the simultaneous removal of three different types of protecting groups. The products were designed as intermediate analogue inhibitors of lumazine synthase that would bind to its phosphate-binding site as well as its lumazine binding site. Both compounds were found to be effective inhibitors of Bacillus subtilislumazine synthase as well as Escherichia coli riboflavin synthase. Molecular modeling of the binding of one of the two compounds provided a structural explanation for how these compounds are able to effectively inhibit both enzymes. In phosphate-free buffer, the phosphate moieties of the inhibitors were found to contribute positively to their binding to Mycobacterium tuberculosis lumazine synthase, resulting in very potent inhibitors with Ki values in the low nanomolar range. The additional carbonyl in the dioxolumazine system versus the purinetrione system was found to make a positive contribution to its binding to E. coli riboflavin synthase.     

Synthesis and structural characterization of novel neutral hexacoordinate silicon(IV) complexes with SiO2N4 skeletons containing cyanato-N or thiocyanato-N ligands

A series of novel hexacoordinate silicon(IV) complexes with an SiO2N4 skeleton (compounds (OC-6-12)-3, (OC-6-12)-4, (OC-6-12)-5, (OC-6-12)-6, and (OC-6-2'2)-7) were synthesized, starting from Si(NCO)4 or Si(NCS)4. These compounds contain (i) two bidentate O,N-chelate ligands (or one tetradentate O,N,N,O-chelate ligand) derived from 4-aminopent-3-en-2-ones of the formula type Me-C(NRH)=CH-C(O)-Me (R = organyl) and (ii) two monodentate cyanato-N or thiocyanato-N ligands. Formally, the bidentate singly negatively charged O,N-chelate ligands (tetradentate 2-fold negatively charged O,N,N,O-chelate ligand) behave as ligands of the imino-enolato type. In addition, the adduct trans-8 was synthesized by reaction of Si(NCS)4 with 2 molar equiv of Me-C(Ni-PrH)=CH-C(O)-Me. This hexacoordinate silicon(IV) complex contains (i) four monodentate thiocyanato-N ligands and (ii) two neutral monodentate ligands of the iminio-enolato type. All compounds synthesized were structurally characterized by single-crystal X-ray diffraction and solid-state and solution NMR spectroscopy. To get more information about the stereochemistry of these compounds, the experimental investigations were complemented by computational studies.     

A mixed adiabatic—diabatic representation for the vibronic coupling problem of polyatomic molecules

A model consisting of two electronic states with different symmetries and an arbitrary number of vibrational modes which couple these states is considered. After a series of canonical transformations the hamiltonian is brought into a form suited to study effects of anharmonic and vibronic couplings. Divergences typical for non-adiabatic coupling terms are avoided.     

2-Hydroxyindoxyles: Preparation nouvelle et generale,proprietes et leur role lors de l'oxydation perphtalique des indoles

Oxidation with monoperphthalic acid of 2-substituted indoles led to corresponding 2-hydroxyindoxyles of which five were isolated (4c, 4d, 4f, 4g, 4h). Two more were characterized in solution by spectroscopic evidence and/or the preparation of crystalline derivatives (4a, 4c), and one showing characteristic behaviour on thin layer chromatography (4b). Longer reaction times with excess oxidant gave benzoxazones (8c, 8d, 8f, 8g). Reactions at the indole α-carbon led to bridged compounds (141, 14m).     

One- electron transfer properties and herbicidal activity of diquaternary salts of 2,4-di-(4-pyridyl)-1,3,5-triazines

Diquaternary salts of 2,4-di-(4-pyridyl)-1,3,5-triazines have been prepared. The salts can be regarded as reversible one electron transfer systems with redox potentials (E₀) of about ?0·44 V in the pH range 7·0–8·5. 2,4-Bis-(4-methyl-4-pyridinio)-1,3,5-triazinediium dihalides are effective post-emergent herbicides at application rates of 4–6 kg/hectare.     

Photoelectron spectroscopy of f-element organometallic compounds II. Tricyclopentadienyl derivatives of uranium(IV) and thorium(IV)

The photoelectron spectra of some cyclopentadienyI derivatives of uranium(IV) and thorium(IV), namely (C₅H₅)₃ ThCl, (C₅H₅)₃ UCl, (C₅ H₄ CH₃)₃ ThCl, (C₅ H₄ CH₃)₃ UCl, (C₅ H₄ CH₃)₃ UBr and (C₅ H₄ CH₃ )₃ UBH₄, are reported the metal 5f ionization has been detected in all the uranium derivative spectra and a simple molecular orbital scheme qualitatively accounts for the number and position of the observed low energy bands.     

Absolute Rate Constants for the Addition of Benzyl (PhĊH2) and Cumyl (PhĊMe2) Radicals to Alkenes in Solution

Absolute rate constants and their temperature dependence were determined by time-resolved electron spin resonance for the addition of the radicals Ph?H₂ and Ph?Me₂ to a variety of alkenes in toluene solution. To vinyl monomers CH₂=CXY, Ph?H₂ adds at the unsubstituted C-atom with rate constants ranging from 14 M ^?1S ^?1 (ethoxyethene) to 6.7 · 10³ M ^?1S ^?1 (4-vinylpyridine) at 296 K, and the frequency factors are in the narrow range of log (A/M ^?1S ^?1) = 8.6 ± 0.3, whereas the activation energy varies with the substituents from ca. 51 kJ/mol to ca. 26 kJ/mol. The rate constants and the activation energies increase both with increasing exothermicity of the reaction and with increasing electron affinity of the alkenes and are mainly controlled by the reaction enthalpy, but are markedly influenced also by nucleophilic polar effects for electron-deficient substrates. For 1,2-disubstituted and trisubstituted alkenes, the rate constants are affected by additional steric substituent effects. To acrylate and styrenes, Ph?Me₂ adds with rate constants similar to those of Ph?H₂, and the reactivity is controlled by the same factors. A comparison with relative-rate data shows that reaction enthalpy and polar effects also dominate the copolymerization behavior of the styrene propagation radical.     

In Process Citation

The electrochemical reduction of NO(2)ClO(4) solutions at a platinum electrode in sulpholane has been studied. The results suggest the following scheme for the reduction of NO(+)(2) : 2NO(+)(2) + 2e(-) right harpoon over left harpoon 2NO(2) right harpoon over left harpoon N(2)O(4). A fast dimerization of nitrogen dioxide, which is favoured by the high donor number of sulpholane, follows the charge-transfer step. The NO(+)(2) ion is much more oxidative than the NO(+) ion in sulpholane, whereas the oxidizing power of the two species is nearly the same in nitromethane or acetonitrile solution. This result can be used to explain the role of water present at trace level.