The effect of iodide on the electrochemical reduction of thick oxide films formed on platinum electrodes in sulfuric acid solution

The reduction of thick oxide films formed on Pt under severe anodic conditions was studied in the presence of adsorbed I^?. The Pt electrode covered with a thick oxide film does not adsorb I^?. However, when a superficial monolayer oxide on the thick oxide has been reduced, I^? is irreversibly adsorbed. Iodide adsorbed on its surface blocks the adsorption of hydrogen and retards markedly the cathodic reduction of the inner thick oxide remaining. It was found that the reduction rate of the inner oxide depends only on the coverage by hydrogen, which coexists with adsorbed I^?. These results support the proton-electron theory which has previously been proposed for the explanation of the characteristic reduction of the thick oxide films.     

Determination of carbon isotope ratios of methanol and acetaldehyde in air samples by gas chromatography-isotope ratio mass spectrometry combined with headspace solid-phase microextraction

Isotopic signatures of atmospheric methanol and acetaldehyde have the potential to improve our ability to quantitatively assess their importance in atmospheric chemistry. However, isotopic measurements of atmospheric methanol and acetaldehyde and their individual source and sink processes have been limited. In this study, we examined gas chromatography-isotope ratio mass spectrometry combined with headspace solid-phase microextraction to measure the carbon isotope ratios of methanol and acetaldehyde in air samples. The method enabled us to determine carbon isotope ratios with a precision (1 standard deviation) of±0.6 ‰ for 20 ml of air sample containing more than 3 ppm of methanol and±0.7 ‰ for 20 ml of air sample containing more than 2 ppm of acetaldehyde. Moreover, the applicability of this method to determine isotope ratios of methanol and acetaldehyde emitted from detached plant leaves was demonstrated.     

Highly Chemo‐, Enantio‐, and Regioselective Synthesis of α,α‐Disubstituted Furanones by Cu‐Catalyzed Conjugate Addition

A highly chemo‐, enantio‐, and regioselective synthesis of furanones bearing an α,α‐disubstituted quaternary stereogenic center is reported. The Cu‐catalyzed enantioselective conjugate addition of organoaluminum reagents to unsaturated ketoesters at room temperature and subsequent lactonization took place. Synthetic transformations of furanones represent facile approaches to various cyclic or acyclic compounds bearing a quaternary stereogenic center.     

Unusual crystallization behaviors of anatase nanocrystallites from a molecularly thin titania nanosheet and its stacked forms: increase in nucleation temperature and oriented growth

Crystallization behaviors of anatase nanocrystallites from an ultrathin two-dimensional reactant composed of exfoliated titania nanosheets have been studied by monitoring the heating process of their well-organized films, with which the film thickness can be controlled from a molecularly thin monolayer to a stacked multilayer structure with a stepwise increment of approximately 1 nm. The heated products were identified by means of total reflection fluorescence X-ray absorption near-edge structure analysis and in-plane X-ray diffraction measurements using a synchrotron radiation source. The films composed of five or more layers of stacked nanosheets were transformed into anatase at 400-500 degrees C, which is a normal crystallization temperature of anatase from bulk reactants. As the film became thinner by decreasing the number of nanosheet layers to five or less, the crystallization temperature was found to increase and finally reached 800 degrees C for the monolayer film. Interestingly, preferential growth of anatase along the c-axis was strongly promoted for these ultrathin films. These unusual behaviors may be understood in terms of crystallization from the two-dimensional system of scarcely distributed reactants. The titania nanosheet crystallite is much thinner than the unit cell dimensions of anatase, and therefore, extensive atomic diffusion is required for the transformation particularly for the ultrathin films with a critical number (2-3) of stacked nanosheet layers. There is some structural similarity between anatase and titania nanosheet, which may account for the oriented growth of anatase nanocrystallites.     

A novel and specific fluorescence reaction for uracil

Facile and specific methods to quantify a nucleobase in biological samples are of great importance for diagnosing disorders in nucleic acid metabolism. In the present study, a novel fluorogenic reaction specific for uracil has been developed. The reaction was carried out in an alkaline medium containing benzamidoxime and K₃[Fe(CN)₆] which were heated for 2.0 min. Under the optimum reaction conditions, strong fluorescence was produced only from uracil, not from other many biogenic compounds tested such as cytosine, thymine, adenine, guanine, nucleobases, nucleosides, nucleotides, amino acids, saccharides, creatine, creatinine and urea. The sensitivity of this method was compared with a known fluorogenic reaction using phenacylbromide which does not react with uracil but reacts with cytosine, adenine and their analogues. The proposed uracil-specific reaction showed approximately 400-fold higher sensitivity than the phenacylbromide reaction. The lower detection limit of uracil by the present method was 100 pmol mL⁻¹, and a good linearity of the calibration curve was obtained up to 100 nmol mL⁻¹ uracil. Due to its high sensitivity and specificity, the quantitative determination of uracil was possible by the proposed fluorimetric method.     

Observation of intermediate bands feeding the positive-parity yrast band in 155Gd

High-spin states of ¹⁵⁵Gd were populated by using the ¹⁵⁴Sm(α,3nγ)¹⁵⁵Gd reaction at E _α= 33 MeV. γ-γ coincidence, E _γ singles, excitation function, and the DCO ratios were measured. we have identified three intermediate bands with ΔI= 2 feeding the positive yrast band. The bands are interpreted as such candidate bands that are mixed with the negative-parity ground state band. This observation can provide a plausible explanation for unusually large population of the positive-parity yrast band observed in a recent Coulomb excitation. Received: 2 November 1999     

Kinetic and theoretical studies on the mechanism of alkaline hydrolysis of DNA

The reaction mechanism of alkaline hydrolysis of DNA has been investigated by kinetic analysis and density-functional-theory calculation. The rates of hydrolysis of thymidine 3'-monophosphate esters (including thymidylyl(3'-5')thymidine (Tp-OT)) monotonically decrease as the leaving groups get poorer. According to the theoretical calculation in which the solvent effects are incorporated, no intermediate is formed in the course of the reaction. In the alkaline hydrolysis of the activated Tp-OT analogues having good leaving groups, the 3',5'-cyclic monophosphate of thymidine is concurrently formed through the intramolecular attack by the 5'-alkoxide ion. In the hydrolysis of the native dinucleotide, however, this side reaction does not occur, since the transition state leading to the departure of its poor leaving group cannot be formed due to conformational restraint. These arguments are supported by the theoretical analysis on the hydrolysis of both dimethyl phosphate and its O(bridging)-->S substituted analogue.     

Rh-catalyzed intermolecular and enantioselective [4 + 2] cycloaddition of 1,3-dienes with dimethyl acetylenedicarboxylate

A Rh-BINAP complex catalyzed an intermolecular and enantioselective [4 + 2] cycloaddition of 1-monosubstituted, 1,1- or 1,2-disubstituted buta-1,3-dienes with dimethyl acetylenedicarboxylate to give chiral cyclohexa-1,4-dienes.