Analyse theoretique des lois cinetiques couramment utilisees en thermoanalyse pour l'etude des reactions solid-gaz. III. proposition d'une nouvelle methode d'approche en regime dynamique

Résumé Dans la première partie de ce travail, les auteurs mettent en évidence que la vitesse d'une réaction en régime non isotherme dépend d'un facteur supplémentaire: le régime de montée en température. Cette assertion est basée sur le fait que le taux de réactionx est une fonction de trois variables: températureT, tempst, et vitesse de chauffeT, variable qui n'avait pas été prise en considération jusqu'ici par les autres auteurs travaillant sur ce problème.Puis, se basant sur les deux hypothèses suivantes: a) l'équation de vitesse en régime isotherme est la forme limite de l'équation de vitesse en régime dynamique, b) la variation de température en régime dynamique affecte la fréquence d'apparition des germes de la nouvelle phase, une formulation théorique des loisg(x) est proposée pour le régime non-isotherme. Cette formulation est explicitée en fonction de l'écart, à une température donnée, entre les valeurs de Lnk _i (isotherme) et Lnk _d (dynamique) dans le diagramme Lnk = f(1/T).L'étude, par thermogravimétrie, de la décomposition du gypse, du sulfate de calcium semihydraté, et de l'oxalate de calcium monohydraté, réalisée en montée linéaire de température, permet de vérifier expérimentalement la formulation théorique proposée.Pour obtenir une bonne interprétation cinétique d'une réaction chimique en régime dynamique, la vitesse de chauffe ne doit pas excéder 20°/heure.

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In the first part of this paper, the authors point out that the rate of reaction with non-isothermal kinetics depends on a supplementary factor: the heating rate. This assertion is based on the fact that the degreex of reaction is a function of three variables:T (temperature),t (time) andT (heating rate); this was not taken into account by other authors concerned with the subject.Then from the two following hypotheses: a) the rate equation for isothermal conditions is the limit form of the rate equation for non-isothermal conditions, b) the temperature change under dynamic conditions affects the frequency of nucleus formation of the new phase, a theoretical formulation of non-isothermalg(x) laws is proposed. This formulation involves the value of the difference, at constant temperature, between Ink _i (isothermal) and In k_d (dynamic) in the Ink=f(1/T) diagram.The thermogravimetric study of the dehydration of gypsum, of calcium sulfate hemihydrate, and of calcium oxalate monohydrate, under non-isothermal conditions, allows experimental verification of the theoretical formulation proposed.For a good kinetic approach of a chemical reaction under dynamic conditions, the heating rate must not exceed 20°/hour.

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Zusammenfassung Im ersten Teil dieser Arbeit wird von den Autoren bewiesen, daß die Geschwindigkeit einer Reaktion unter nicht-isothermen Bedingungen von einem zusätzlichen Faktor, den Bedingungen des Temperaturenstiegs, abhängt. Diese Behauptung beruht auf der Tatsache, daß der Reaktionsverlauf eine Funktion dreier Variablen ist: der TemperaturT, der Zeitt und der AufheizungsgeschwindigkeitT, wobei letztere Variable bisher von den auf diesem Gebiet arbeitenden Autoren außer Acht gelassen worden ist.Nachfolgend wird an Hand der zwei Voraussetzungen, daß a) die Geschwindigkeitsgleichung unter isothermen Bedingungen die Grenzform der Geschwindigkeitsgleichung unter dynamischen Bedingungen ist und b) die Temperaturänderung unter dynamischen Bedingungen sich auf die Erscheinungsfrequenz der Keime der neuen Phase auswirkt, eine theoretische Formulierung der Gesetzeg(x) für nicht-isotherme Bedingungen vorgeschlagen. Diese Formulierung wird als Funktion der Abweichung ausgedrückt, welche bei einer gegebenen Temperatur zwischen den Werten von Lnk _i (isotherm) und Lnk _d (dynamisch) im Diagramm Lnk=f(1/T) besteht.Die bei linearem Temperaturanstieg thermogravimetrisch durchgeführte Untersuchung der Zersetzung von Gips, von Calciumsulfat Semihydrat und von Calciumoxalat Monohydrat ermöglicht die vorgeschlagene theoretische Formulierung experimentell zu bestätigen.Um unter dynamischen Bedingungen eine gute kinetische Interpretation einer chemischen Reaktion zu erhalten, darf die Aufheizungsgeschwindigkeit den Wert von 20° pro Stunde nicht überschreiten.

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, : . , : (),t () ' ( ), , . : ) ) t(x) . Lnk _j () Lnk _d ( ) Lnk=f(1/T) . , , . , 20° .

     

Oxytropis alkaloids II. Structure of oxytriphine

The new alkaloid oxytriphine, the parent of natural 2-oxazoline derivatives, has been isolated from the epigeal part ofOxytropis trichophysa; and its structure has been established as (5S)-(+)-2,5-diphenyl-2-oxazoline. (-)-N-Benzoyl-2-phenyl-2-hydroxyethylamine and benzoic acid have been detected in this plant for the first time. A hypothesis of the possible biogenetic interrelationship of the compounds isolated is put forward.Institute of the Chemistry of Plant Substances. Uzbekistan Academy of Sciences, Tashkent. Translated from Khimiya Prirodnykh Soedinenii. No. 6, pp. 873–876, November–December, 1993.     

Development of an amperometric biosensor for the determination of phenolic compounds in reversed micelles

The possibilities of amperometric enzyme electrodes in reversed micellar systems for the determination of phenol, 4-chloro-3-methylphenol and 2,4-dimethylphenol are illustrated. The used enzymatic reaction consisted of the oxidation of the phenolic compounds by oxygen, catalysed by tyrosinase. The reduction of the liberated quinones was amperometrically detected. The concentration of the components of the reversed micelles, as well as the potential applied to the tyrosinase electrode have been optimized. The stability of the enzyme electrode with time was also evaluated. The effect of the analyte solubility in water upon the analytical performance of the electrode was explored. Advantages of amperometric biosensors in reversed micelles are shown with respect to aqueous media and organic phase enzyme electrodes.     

Calcium-selective electrodes based on calcium tetra(4-chlorophenyl)borate complexes of macrocyclic polyether diamides

The preparation of macrocyclic 7,19-dibenzyl-7,19-diaza-1,4,10,13,16-pentaoxa-cycloheneicosane-6, 20-diones substituted in positions 2 and 3 with methyl groups, and their properties in PVC membranes as calcium sensors are described. Complexes of these polyether diamides (PEDA) with calcium and tetra(4-chlorophenyl)borate (TClPB) ions having the composition 2PEDA·Ca·2TClPB were prepared. Calcium electrodes based on these complexes have selectivity coefficients for calcium over barium up to 10³, and over alkali metals up to 3 × 10⁴.     

Synthesis of beta-lactams by Ag+-induced ring expansion of 1-hydroxycyclopropylamines: a theoretical analysis

A theoretical analysis of the silver-induced ring expansion of N-chloro-N-methyl-1-hydroxycyclopropylamine to form N-methyl-2-azetidinone, and of the Cl(-) elimination from this substrate without Ag(+) assistance, was performed using the B3LYP method and the 6-31+G(d) basis set for C, N, O, H, and Cl atoms and the relativistic effective core pseudopotential LANL2DZ complemented with one set of f polarization functions (zeta(f) = 0.473) for the Ag atom. The partial Ag(+)-assisted extrusion of Cl(-) at the rate-determining transition state provokes an important change in the nodal properties of the frontier molecular orbitals of the H(3)CClNCOHAg(+) fragment, thus making very stabilizing HOMO-LUMO interactions between this fragment and the C(2)H(4) moiety possible. This interaction leads to the ring opening and release of most of the strain energy, giving rise to a low energy barrier for the process. Also, by assisting the Cl(-) extrusion, Ag(+) avoids the elimination of the hydroxyl hydrogen atom, which would provoke the fragmentation of the system instead of the formation of the beta-lactam.     

Eu(III)-cyclen-phen conjugate as a luminescent copper sensor: the formation of mixed polymetallic macrocyclic complexes in water

The cationic cyclen based Eu(III)-phen conjugated 1.Eu was synthesised as a chemosensor for Cu(II), where the recognition in water at pH 7.4 gave rise to quenching of the Eu(III) luminescence and the formation of tetranuclear polymetallic Cu(II)-Eu(III) macrocyclic complexes in solution where Cu(II) was bound by three 1.Eu conjugates.     

Topographical properties of polymer films deposited in capillaries for electrophoretic separations of large organic molecules

Topography and thickness of hydrophilic polymer coatings of fused-silica capillaries for capillary electrophoresis (CE) were investigated using atomic force microscopy (AFM), scanning electron microscopy (SEM), and profilometry. Three hydrogels, poly(2-hydroxyethyl methacrylate) [poly(HEMA)], poly(diethylene glycol monomethacrylate) [poly(DEGMA)], and poly(triethylene glycol monomethacrylate) [poly(TEGMA)], were deposited using two procedures, either by simple physical sorption of the polymers, or by derivatization of the capillary wall surface with glycidyl methacrylate (EPMA) followed by polymerization of the appropriate monomers. The performance of the modified capillaries was tested under CE conditions (decrease in the electroosmotic flow, EOF dependence on pH, separation of milk and standard proteins). It has been found that the most important property of the polymer coating is its thickness, whereas its topography and the degree of its hydrophobicity are less significant. Film deposition by physical adsorption is preferable to polymerization on the derivatized surface.     

Photoinduced electron transfer at liquid|liquid interfaces: dynamics of the heterogeneous photoreduction of quinones by self-assembled porphyrin ion pairs

The initial stages of the heterogeneous photoreduction of quinone species by self-assembled porphyrin ion pairs at the water|1,2-dichloroethane (DCE) interface have been studied by ultrafast time-resolved spectroscopy and dynamic photoelectrochemical measurements. Photoexcitation of the water-soluble ion pair formed by zinc meso-tetrakis(p-sulfonatophenyl)porphyrin (ZnTPPS(4)(-)) and zinc meso-tetrakis(N-methylpyridyl)porphyrin (ZnTMPyP(4+)) leads to a charge-separated state of the form ZnTPPS(3)(-)-ZnTMPyP(3+) within 40 ps. This charge-separated state is involved in the heterogeneous electron injection to acceptors in the organic phase in the microsecond time scale. The heterogeneous electron transfer manifests itself as photocurrent responses under potentiostatic conditions. In the case of electron acceptors such as 1,4-benzoquinone (BQ), 2,6-dichloro-1,4-benzoquinone (DCBQ), and tetrachloro-1,4-benzoquinone (TCBQ), the photocurrent responses exhibit a strong decay due to back electron transfer to the oxidized porphyrin ion pair. Interfacial protonation of the radical semiquinone also contributes to the photocurrent relaxation in the millisecond time scale. The photocurrent responses are modeled by a series of linear elementary steps, allowing estimations of the flux of heterogeneous electron injection to the acceptor species. The rate of electron transfer was studied as a function of the thermodynamic driving force, confirming that the activation energy is controlled by the solvent reorganization energy. This analysis also suggests that the effective redox potential of BQ at the liquid|liquid boundary is shifted by 0.6 V toward positive potentials with respect to the value in bulk DCE. The change of the redox potential of BQ is associated with the formation of hydrogen bonds at the liquid|liquid boundary. The relevance of this approach toward modeling the initial processes in natural photosynthetic reaction centers is briefly discussed.     

An alternative method for pucker determination in carbohydrates from residual dipolar couplings: a solution NMR study of the fructofuranosyl ring of sucrose

A simple solution NMR method is presented for pucker determination of five-membered rings using only residual dipolar couplings obtained in a single liquid crystalline medium, DMPC/DHPC bicelles (DMPC = dimyristoylphosphatidylcholine; DHPC = dihexanoylphosphatidylcholine). The method was applied to determine the pucker of the fructofuranosyl ring of sucrose. The results indicate a fructofuranosyl pucker phase in the 20 degrees - 70 degrees range. The pucker phases are in agreement with those from previous NMR and optical spectroscopic studies and, importantly, do not rely on empirically parametrized Karplus curves. Furthermore, the analysis implies more than one stable pucker phase and rapid ring interconversion in this range. The present results suggest that using residual dipolar couplings alone can reveal multiple conformations present in solution. Hence, when a sufficient number of residual dipolar coupling constants is measured, the outcome is a robust, reliable, and independent route for determining carbohydrate and nucleic acid structure by NMR spectroscopy.