Adsorption and decomposition of ammonia on W(100); XPS and UPS studies

The adsorption and decomposition of ammonia on a clean and c(2 × 2)-N ordered W(100) surface has been studied by photoemission spectroscopy (XPS and UPS). At 120 K molecularly adsorbed ammonia was identified by N(1s) core level emission at 400.9 eV and the valence emissions at 7.6 and 11.7 eV. By heating the sample stepwise the N(1s) core level shifted to lower binding energy. In the valence region, the corresponding spectral changes were obtained, where the dependence of the peak intensity on photon energy was observed. These observations were interpreted to demonstrate that adsorbed ammonia dissociates its hydrogen successively to form NH_x(a) and finally to atomic nitrogen. On the other hand, ammonia was molecularly adsorbed on a c(2 × 2)-N ordered surface even at temperatures as high as 300 K, although the spectra at 400 K or above were very similar to those under a steady state flow condition, where the tungsten surface was mostly covered by atomic nitrogen. At higher ammonia pressure up to about 100 Pa thicker nitride layers were formed at 700 K, which were characterized by the N(1s) core level at 397.3 eV and a broad emission around 6 eV in the valence level.     

Contractible edges in triangle-free graphs

An edge of a graph is calledk-contractible if the contraction of the edge results in ak-connected graph. Thomassen [5] proved that everyk-connected graph of girth at least four has ak-contractible edge. In this paper, we study the distribution ofk-contractible edges in triangle-free graphs and show the following: Whenk≧2, everyk-connected graph of girth at least four and ordern≧3k, hasn+(3/2)k ²-3k or morek-contractible edges.     

Effects of hydrostatic pressure on CdxHg1−xTe alloys

The hydrostatic pressure effects on Cd_xHg_1−xTe alloys have been investigated. In the relatively low pressure region, we find an anomaly in each Hall coefficient vs. pressure curve for the alloys (x<0.09), which is attributed to the inversion of the Γ₈/Γ₆ energy bands. In the relatively high pressure region, the electrical resistivities of the alloy crystals (x>0.09) show changes of three stages with increase of pressure. The crystal structures corresponding to the three stages are studied by an X-ray diffraction analysis. It is found that the crystal structure of the alloys exhibits transitions from the zinc-blende to the rock salt type and at last to the hexagonal cinnabar type.     

Quantitative study of F center in high-surface-area anatase titania nanoparticles prepared by MOCVD

The concentration of F center in high-surface-area anatase titania nanoparticles prepared by MOCVD are studied with electron paramagnetic resonance measurements at different time and different temperatures. The results show that the kinetics of F center is dominated by surface process rather than bulk process. With increasing temperature, two temperature dependent regions for the concentration of F center can be clearly identified. The starting temperature and activation energy of the surface process are determined to be 357.4 K and 0.085 eV, respectively. The reaction of F center with oxygen is of zero order and the concentration of F center can be well described by

     

Confirmation of the cation radicals of hexamethylethane and hexamethyldisilane by ESR and other spectroscopy

Unambiguous ESR spectra of cation radicals of hexamethylethane and hexamethyldisilane were obtained for γ-irradiated solutions in CCl₃F at 77 and 140 K. INDO calculations, optical measurements, and comparison with photoelectron spectra are consistent with the observed ESR spectra.     

A new synthetic route to 7-halo-1-cyclopropyl-6-fluoro-1,4-dihydro-4-oxoquinoline-3-carboxylic acid,an intermediate for the synthesis of quinolone antibacterial agents

Starting from m-fluorotoluene, 7-chloro-6-fluoro- and 6,7-difluoro-1-cyclopropyl-1,4-dihydro-4-oxoquino-line-3-carboxylic acids, 3 and 16 were synthesized. Compounds 3 and 16 are useful intermediates for the synthesis of a class of quinolone antibacterial agents. The synthetic route involves two processes; i) construction of the quinoline ring by an intramolecular cyclization accompanied by the elimination of a nitro group and ii) introduction of fluorine atom by replacement of a nitro group with potassium fluoride. 7-(3-Amino-1-pyrroli-dinyl)-1-cyclopropyl-6-fluoro-1,4-dihydro-4-oxoquinoline-3-carboxylic acid (18) was prepared from 3 or 16. The antibacterial activity of 18 compares favorably with that of ciprofloxacin (2) .     

Photocrosslinking of poly(2,3-epithiopropyl methacrylate) film with 254 nm irradiation

Poly(2,3-epithiopropyl methacrylate) (PETMA) has an absorption maximum at 258 nm (ε_max = 50) in dioxane which is due to episulfide groups. When irradiated at 254 nm under atmospheric pressure its film became insoluble. When kept at 70°C for several minutes a PETMA film became partially insoluble; however, the contribution of this thermal reaction to the photocrosslinking at room temperature was negligible. For photoreaction at 254 nm under nitrogen a low probability of main-chain scission was ascertained by the use of the Charlesby-Pinner equation. The IR spectrum of irradiated PETMA showed a decrease in episulfide groups and the formation of mercapto groups, which suggested that the photocrosslinking of PETMA results from free radicals formed by the cleavage of C? S bonds. Free radicals in the irradiated PETMA were detected by ESR spectroscopy and the assignment is discussed. In this photocrosslinking process oxygen was a retarder at the early stage but an accelerator at the later stage.     

A new mathematical formula to link near equilibrium relaxation kinetics and conformational selection steps in enzymatic reactions

A new mathematical formula was derived for near equilibrium relaxation processes of enzyme reactions including the conformational selection (CS) modes. CS is one of the most accepted molecular recognition mechanisms, in which protein conformers (CS conformers) are in an equilibrium with varying degrees of ligand binding affinity so that ligands select a particular conformer among them to bind. Using computer simulation techniques, our previous study (Egawa and Callender in Math Biosci 313: 61–70, 2019) predicted that the rate constant for the near equilibrium relaxation processes (k_NER) and the concentration-sum of substrate and product (C_L^t) of enzyme reactions uniquely related to the presence of CS steps in a manner that 1/k_NER versus C_L^t plot transformed from linear to quadratic as the elementary rate constants of inter-conversions among the CS conformers were becoming smaller relative to the rate constants at other steps of the enzymatic reaction system. Thus our previous work could provide a potential tool to detect the presence of CS steps in an enzyme reaction simply by assays using only trace amount of enzyme samples, although logical basis to have the quadratic deformation in the 1/k_NER versus C_L^t plot in response to the presence of CS steps could not be clarified. Employing mathematical approaches that were alternative to those in our previous study, this study succeeded in deriving a theoretical equation that fully explained why and how the CS modes caused the quadratic characters in the 1/k_NER versus C_L^t plot.