Zeros of the Whittaker function associated to Coulomb waves

The zeros in the complex z plane of the Whittaker function W_c/z,µ(z),closely related to spherical waves in the quantum-mechanicalCoulomb problem, are investigated for varying real values ofthe parameters c and µ     

Synthesis and Diels-Alder Reactions of a New Kind of Chiral Dienophiles: Cyclic Vinyl-p-tolylsulfilimines

A new kind of chiral dienophiles, cyclic vinyl-p-tolylsulfilimines (2a and 2b), were obtained from the corresponding (Z)-sulfinylacrylonitriles with HBF(4) and methanol. The asymmetric Diels-Alder reaction of optically pure 2a with cyclopentadiene under mild thermal or catalyzed conditions afforded only the endo-4a adduct with complete endo and pi-facial selectivities. The ability of the sulfilimine moiety to enhance the dienophilic reactivity of the double bond is similar to that of the sulfinyl group.     

Excited-state interactions in diastereomeric flurbiprofen-thymine dyads

Excited-state interactions between (S)- or (R)-flurbiprofen ((S)- or (R)-FBP) and thymidine (dThd) covalently linked in dyads 1 or 2 have been investigated. In both dyads, the only emitting species is (1)FBP*, but with a lower fluorescence quantum yield (?(F)) and a shorter fluorescence lifetime (τ(F)) than when free in solution. These results indicate that dynamic quenching occurs either by electron transfer or via exciplex formation, with FBP as the charge-donating species. In acetonitrile, both mechanisms are favored, while in dioxane exciplex formation is predominating. Isomer 1 displays lower values of ?(F) and τ(F) than its analogue 2, indicating that the relative spatial arrangement of the chromophores plays a significant role. The triplet quantum yields (?(T)) of 1 and 2 are significantly higher than the expectations based solely on (1)FBP*-dThd intersystem crossing quantum yields (?(ISC)), with ?(T) (1) > ?(T) (2). This can be explained in terms of intramolecular charge recombination at the radical ion pairs and/or the exciplexes, which would be again dependent on geometrical factors. The triplet lifetimes (τ(T)) of (3)FBP*-dThd and free (3)FBP* are similar, indicating the lack of excited-state interactions at this stage. The FBP-dThd dyads could, in principle, constitute appropriate model systems for the elucidation of the excited-state interactions in noncovalent DNA-ligand complexes.     

Mesomechanics of plastic deformation and fracture of partially crystalline polyethylene

From the standpoint of physical mesomechanics, we have investigated plastic deformation mechanisms and the mechanical properties of partially crystalline polyethylene. We show that from the very beginning, plastic deformation occurs at the mesoscopic level. Fracture is preceded by fragmentation of the material. The observed stages of the process of plastic deformation of polyethylene are qualitatively similar to the stages of this process for metallic materials. The effect of electron bombardment on the mechanical properties of polyethylene is explained by the size reduction in the mesoscopic substructure formed on plastic deformation. Tomsk Polytechnical University. Zhilin University, People’s Republic of China. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 48–53, January, 1997.     

Structure électronique et stabilité thermique d'heterocycles appartenant à la serie de l'imidazole et du thiazole

Résumé La structure électronique de l'imidazole, du benzimidazole, du thiazole et du benzothiazole a été calculée par une méthode de champ self-consistant simplifiée tenant compte des électrons et . L'énergie d'arrachement d'un atome d'hydrogène fixé sur le noyau hétérocyclique a été évaluée de faÇon approximative, en vue d'interpréter la variation de stabilité observée dans les expériences de pyrolyse.

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The electronic structure of imidazole, benzimidazole, thiazole and benzothiazole has been calculated by a simplified LCAO-SCF method taking into account and electrons. Breacking energy for an hydrogen atom bounded to the heterocyclic ring is estimated by an approximated way and used for explaining changes of stability observed in pyrolysis experiments.

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Zusammenfassung Die Elektronenstruktur von Imidazol, Benzimidazol, Thiazol und Benzthiazol wurde mittels eines vereinfachten LCAO-SCF-Verfahrens unter Einschlu\ von - und -Elektronen berechnet. Dann wurde die Trennungsenergie für ein am Heteroring gebundenes H-Atom abgeschÄtzt und zur Interpretation der relativen StabilitÄt, wie sie bei Pyrolysen beobachtet wird, verwendet.