Role of boson-fermion statistics on the Raman spectra of Br2(X) in helium clusters

The role played by the bosonic or fermionic character of He atoms surrounding a Br2(X) molecule is analyzed through vibrotational Raman spectra simulations. Quantum chemistry-type calculations reveal the spin multiplicity to be chiefly responsible for the drastic difference observed by Grebenev et al. [Science 279, 2083 (1998)]] in the rotational structure of molecules embedded in helium droplets.     

Reaction of nitrogen dioxide with polystyrene films

The reaction of nitrogen dioxide with thin polystyrene films has been investigated at 35°C with different partial pressures of NO₂ (0.1, 2, 15, 30, and 60 cm Hg) and at several temperatures (25, 35, 45 and 55°C). The films were thin enough (ca. 20 μ) so that the reaction was independent of the diffusion of gas into the polymer. The experimental results can be represented by a chain mechanism. The whole degradation process is controlled by the diffusion of polymer radicals out of cages. This diffusion in turn, is affected by the decrease in viscosity or decrease in weight-average molecular weight as degradation proceeds. This leads to an acceleration of the degradation process. A straight-line relationship between the logarithm of the reciprocal weight-average molecular weight and the logarithm of a reaction–time function was found. The dependence on the rate was substantiated by degrading polymer fractions. The energy of activation for the process is small, in agreement with a diffusion process for chain scission. Nitro and nitrite groups are incorporated along the backbone of polystyrene during exposure. The number of these polar side groups appears to pass through a maximum with time, as is evidenced by aggregation of polymer molecules in benzene solution only during the middle stage of the degradation. The final stage of the process is slowed down by retarder being produced. This retarder can be removed by reprecipitation of exposed polymer films. Degradation in solution is similar to that of films. Isotactic polystyrene shows less irregularities in its degradation curve than the atactic polymer. This is, presumably, due to its more homogeneous morphology, large molecular weight, and broader molecular size distribution. The plot of the degree of degradation versus time for the isotactic polymer can be satisfactorily approximated by a straight line.     

Landolt-Reaktionen auf „Chlorgrundlage“ und ihre Anwendung zum Nachweis und zur Bestimmung katalytisch wirksamer Spurenelemente. II

Zusammenfassung Die Verfasser untersuchten das Chlorat-Chlorid-Zinn(II)-Landolt-System. Mit dessen Reaktion können 0,2g Vanadin/5 ml nachgewiesen werden. Das Landolt-System ist bei Anwendung der Simultankomparationsmethode zur Bestimmung von Vanadin in Mikrogrammengen geeignet; eine Genauigkeit von 1 bis 5% kann dabei erreicht werden. In diesem System wird auch durch Mo(VI), Fe(III), Ti(IV), W(VI), Se(IV) und Te(IV) ein katalytischer Effekt ausgeübt, hingegen wirken Os(VIII) und Cu(II) auf die katalysierte Reaktion stark behindernd.

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Summary The authors studied the chlorate-chloride-tin(II)-Landolt system. Its reaction revealed 0.2g vanadium in 5 ml. The Landolt system is suitable for the determination of vanadium in microgram amounts by the simultaneous comparison method; a precision of 1–5% can be attained. A catalytic effect is exerted in this system also by Mo(VI), Fe(III), Ti(IV), W(VI), Se(IV), and Te(IV). On the other hand, Os(VIII) and Cu(II) have a marked inhibiting effect on the catalyzed reaction.

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Résumé Les auteurs ont étudié la réaction deLandolt avec le système chlorate-chlorure-étain-II. Elle permet de déceler jusqu'à 0,2g de vanadium dans 5 ml. Elle peut être appliquée à la méthode de comparaison simultanée pour le dosage du vanadium en quantités de l'ordre du microgramme, en atteignant ainsi la précision de 1–5%. On trouve aussi dans cette réaction un effet catalytique du Mo-VI, Fe-III, Ti-IV, W-VI, Se-IV et Te-IV, tandis que Os-III et Cu-II gênent fortement la réaction catalysée.

     

Raman spectra of (He)N-Br2(X) clusters: the role of boson/fermion statistics in a quantum solvent

The aim of this paper is to elucidate the role played by the bosonic/fermionic character of N He atoms solvating a Br2(X) molecule. To this end, an adiabatic model in the molecular stretching coordinate is assumed and the ground energy levels of the complexes are searched by means of Hartree (or Hartree-Fock) Quantum Chemistry calculations for 4He (or 3He) solvent atoms. Simulations of vib-rotational Raman spectra point at the spin multiplicity as the main feature responsible for the drastic difference in the rotational structures of molecules embedded in boson or fermion helium drops as already observed by the experiments of Grebenev et al. [S. Grebenev, J. P. Toennies, and A. F. Vilesov, Science 279 (1998) 2083].     

Kinetcis and mechanism of HCN-evolution from the oxidation of polyurethanes

The oxidative degradation of aliphatic, aromatic, and secondary polyurethanes was investigated over a range of temperatures of 440–1000°C as a function of time and oxygen concentration. The degradation was studied with special emphasis on HCN-evolution; its rate constants and Arrhenius equations were determined. In some of the temperature ranges reactions were diffusion controlled. At relatively low temperatures the oxidative rate constant was a linear function of oxygen concentration, whereas at higher temperatures oxidation of the evolved HCN took place in passage through the hot zone of the furnace; in addition, at these higher temperatures thermal degradation of the polymers and thermal decomposition of HCN in the hot zone occurred simultaneously with oxidation to an appreciable extent. The kinetics and mechanism proposed and quantitatively evaluated account well for the experimental results.     

Polymer morphology and random chain scission

A theory has been developed for the kinetics of random chain scission of polymers consisting of amorphous and crystalline (spherulite) phases. The degrees of degradation have been derived for such scission due to a gas which in one case is able to penetrate spherulites where degradation is then diffusion-controlled and in another case cannot diffuse into spherulites. In the later case, special conditions are prevalent in the amorphous–crystalline interface. Main-chain links in crystalline fold regions are assumed to scission faster than all other main-chain links due to strain energy.     

Über die Struktur der künstlichen Harze

p-Nitrophenol reagiert mit Hexamethylentetramin zum 2,2-Dihydroxy-5,5-dinitro-dibenzylamin und nicht, wieJ. C. Duff undE. J. Bills annehmen, zum 2,2-Dihydroxy-5,5-dinitro-dibenzyl-N-hydroxymethyl-amin.     

13-atom Ni-Al alloy clusters: Structures and dynamics

Structural and dynamical properties of model 13-atom Ni_nAl_m alloy clusters derived from a many-body potential are presented and discussed. Characterization of the structures corresponding to a given stoichiometric composition (i.e., chosen number of Ni and Al atoms) is carried out in terms of isomeric (geometric) forms and different distributions of the two types of atoms between the sites of a chosen isomer. We use the term homotops (“the same topography or geometry”) to label the structural forms that differ only by these distributions. The number and the energy spectra of the homotops are sensitive functions of the stoichiometric composition and isomeric form. Similarly to homogeneous clusters, alloy clusters undergo a solid-to-liquidlike transition as their energy is increased. Individual stages in the transition, such as isomerizations involving only surface atoms, isomerizations involving all atoms, surface melting (in a system as small as 13 atoms), and complete melting are identified and characterized. The actual occurrence of some or all of these stages in the meltinglike transition of a given cluster depends on the character of the energy spectra of its homotops, i.e., ultimately, on its stoichiometric composition. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 62: 185–197, 1997