Non-isothermal crystallization in amorphous selenium films

The usual determination of kinetic parameters of crystallization of amorphous products is based on isothermal measurements. In general the crystallization of amorphous selenium thin films is studied by non-isothermal experiments (DTA). The adaptation to non-isothermal crystallization of the Avrami transformation rate equation allows us to determine different types of crystallization. These different regions enable one to determine the variations of the growth rate and of the nucleation rate versus temperature. The influence of the wavelength of illumination during the crystallization time on these parameters is also investigated.

---

Zusammenfassung Die übliche Bestimmung kinetischer Parameter der Kristallisation amorpher Produkte basiert auf isothermen Messungen. Die Kristallisation von amorphem Selen in dünnen Schichten wird im allgemeinen in nicht-isothermen Experimenten (DTA) untersucht. Die Adaptation der Transformationsgeschwindigkeitsgleichung nach Avrami an die nicht-isotherme Kristallisation ermöglicht die Bestimmung verschiedener Kristallisationtypen. Dadurch wird es möglich, die Veränderungen der Wachstumsgeschwindigkeit und der Geschwindigkeit der Nukleation in Abhängigkeit von der Temperatur zu bestimmen. Der Einfluß der Wellenlänge bei der Bestrahlung während der Kristallisation auf diese Parameter wird ebenfalls untersucht.

---

. () . , , . , .

     

1,3-Dipolar cycloaddition reactions of nitrones with vinylacetic olefins in the racemic series and synthesis of δ-lactams via isoxazolidines

A study of the cycloaddition behavior of a series of esters and nitriles α-chloro- and α-hydroxyvinyl-acetic dipolarophiles with C-aryl-N-alkylnitrones has been carried out. Regiospecific cycloadditions are observed; the reactions lead to a mixture of 5-substituted isoxazolidines either erythro or threo, wherever the nitrone is involved. We report the synthesis of some δ-lactams in which isoxazolidines are used as latent synthons.     

MECHANISM OF PHOTOREDUCTION OF THIAZINE DYES BY EDTA STUDIED BY FLASH PHOTOLYSIS—III CONSEQUENCES OF A NEWLY FOUND pKT OF THIONINE ON THE MECHANISM IN BASIC SOLUTIONS

Abstract— Evidence for an acid-base equilibrium between the neutral and monocationic forms of the triplet state of thionine, ³ T and ³ TH ⁺ (p K _T# 8–9) has been found. This equilibrium, which has no counterpart in the methylene blue system can explain the different behaviours of these two similar dyes concerning their photoreduction by ethylene diamine tetracetic acid (EDTA) in basic solutions. With reasonable assumptions about the reactivity of the various species present in solution, it is now possible to calculate over the whole pH range the quantum yield of semireduced dye formation φs, which, according to the previously proposed mechanism, must be proportional to the overall photoreduction quantum yield φR. The very good fit between the calculated curve φs = f (pH) and the experimental one φR = f (pH) for both dyes strongly supports the proposed mechanism.     

Twenty-species and 15-species chemical kinetic mechanisms for cyclohexane using the local self-similarity tabulation method

The 1081 species cyclohexane-oxidation elementary reaction mechanism of Silke et al. (DOI: 10.1021/jp067592d ) is reduced in the number of species by a factor using the local self-similarity tabulation (LS2T) method. Reduced-species mechanisms of both 20 (R20) and 15 (R15) species are created in the high-pressure combustion regime typical of diesel engines. To evaluate the performance of R20 and R15 against the elementary kinetics, simulations are performed for cyclohexane/air mixtures at initial temperatures of 1150, 900, 750, and 680 K and constant pressures of 20 and 40 bar for a variety of equivalence ratios (, 1.0, and 2 for 1150 and 900 K; for 750 K; for 680 K). Very good agreement between R20 and R15 with the elementary kinetics mechanism is demonstrated at 1150 and 900 K for which the self-similarity is very well obeyed; however, only fair agreement is obtained at 750 and 680 K, a fact which is traced to the less faithful adherence to the self-similarity due to the one order of magnitude increase in ignition time over the range 750-680 K. These results are found to be quasi-independent of the tabulation grid. Future work is proposed to improve the reduction in the cold-ignition, high-pressure regime.     

Lubricated squeezing flow of thin slabs of wheat flour dough: comparison of results at constant plate speed and constant extension rates

Lubricated squeezing flow experiments on wheat flour dough have, until now, mostly been performed in constant plate speed mode (CPS), i.e. at a permanently increasing extension rate. We have compared the results obtained under the CPS and constant extension rate (CER) modes using one of the very few commercial rheometers that allow operation in the CER mode. In both cases, and at any constant biaxial strain, a power law could be fitted to the stress versus extension rate data, the “consistency index” (K) increasing continuously with the strain and the “flow behaviour index” (n) being constant only up to a low strain value (≈0.25) and then decreasing. When compared to the CER mode, the CPS mode produced higher K and n values. For wheat flour doughs, an increase in K with extension may be associated to a strain-hardening phenomenon but the roles of viscoelasticity and lubricant thinning are discussed.     

Analysis of the importance of the metallo-beta-lactamase active site loop in substrate binding and catalysis

The role of the mobile loop comprising residues 60-66 in metallo-beta-lactamases has been studied by site-directed mutagenesis, determination of kinetic parameters for six substrates and two inhibitors, pre-steady-state characterization of the interaction with chromogenic nitrocefin, and molecular modeling. The W64A mutation was performed in IMP-1 and BcII (after replacement of the BcII 60-66 peptide by that of IMP-1) and always resulted in increased K(i) and K(m) and decreased k(cat)/K(m) values, an effect reinforced by complete deletion of the loop. k(cat) values were, by contrast, much more diversely affected, indicating that the loop does not systematically favor the best relative positioning of substrate and enzyme catalytic groups. The hydrophobic nature of the ligand is also crucial to strong interactions with the loop, since imipenem was almost insensitive to loop modifications.     

Catalysis of nucleophilic aromatic substitutions in the 2,6,8-trisubstituted purines and application in the synthesis of combinatorial libraries

The following paper summarizes our work on compound libraries of 2,6,8-trisubstituted purines. This synthesis route on a polystyrene support begins with 2,6-dichloro purine making extensive use of catalysis. During the synthesis the polymer bound purines were brominated selectively on C8. The substitution reaction of C6-Cl by amines was found to be acid catalyzed. The substitution of C2-Cl by amines and aryls, as well as the substitution of a C8-Br by aryls, alkenyl and alkynyl groups can be catalyzed by transition metals. Under some bromination conditions novel selective oxidative transformations of 2-amino groups in 2,6-diamino purines have been found.     

A method of parameter differentiation applied to flow in porous annuli

A general noniterative method of solution of nonlinear boundary value problems involving a parameter is presented. Through a parametric differentiation of the original system, the boundary value problem is converted into an initial value problem with the parameter as the independent variable and differentiation with respect to the original variable is completely eliminated. A step by step integration with respect to the parameter of the newly obtained initial value system yields a large family of solutions which may be of practical interest when the parameter chosen is of physical significance.The method is applied to study the effect of uniform wall suction and injection on the laminar flow in an annulus. The integration is carried out with respect to the transpiration Reynolds number and the deviation from the Poiseuille flow can be seen at each step of integration.

---

Résumé Une méthode générale non itérative est développée pour résoudre des problèmes aux limites non linéaires contenant un paramètre. La dérivation du système différentiel par rapport au paramètre transforme le problème en un problème aux valeurs initiales, où la variable indépendante est le paramètre et où la dérivation par rapport à la variable d'origine est complètement éliminée. L'intégration du nouveau système par rapport au paramètre est menée pas à pas et conduit à une famille de solutions qui peuvent être d'intérêt pratique si le paramètre a une signification physique.La méthode est appliquée à l'étude de l'écoulement laminaire d'un fluide dans un tube annulaire à parois uniformément perméables. La variable d'intégration est le nombre de Reynolds de transpiration et la déviation par rapport à l'écoulement de Poiseuille peut être déterminée à chaque pas d'intégration.

     

Fuzzy Boolean algebras

The purpose of this paper is to generalize the following situation: from the concrete structure $\text{B}$, we define the notion of Boolean algebras; the Stone representation theorem allows us to replace the algebraic study of Boolean algebras by a topological one. Let E be a non-empty set, and J a non-empty ordered set. Note $\text{B}$ the set of all fuzzy subsets of (E,J). We shall introduce the concept of fuzzy Boolean algebra and find a representation theorem. But it will be difficult to speak of the dual fuzzy topological space of a fuzzy Boolean algebra as we shall see further, except in certain particular cases.