Mono-Aryl/Alkylthio-Substituted (Hetero)acenes of Exceptional Thermal and Photochemical Stability by the Thio-Friedel–Crafts/Bradsher Cyclization Reaction

The highly substituted mono-aryl/alkylthio-(hetero)acenes prepared in this study have been found to be thermally more stable (T_decomp.=331–354 °C) than the known di-aryl/alkylthio-substituted acenes by an average of 25 °C. They are also much more photostable at 254 and 365 nm (in both argon and air) than the parent anthracene and other reported anthracenes. The most photostable aryl/alkylthio-anthracenes at 254 nm were found to be 60–70 (in air) and 130 (in argon) times more stable in solution than the unsubstituted anthracene, and much more stable than known EDG/EWG-substituted anthracenes (EDG=electron-donating group, EWG=electron-withdrawing group) with an extended aromatic core. Furthermore, the acenes showed significantly higher photostability at 365 nm in both air and argon. The anthracenes were obtained by the novel thio-Friedel–Crafts/Bradsher cyclization reaction of hitherto unknown [o-(1,3-dithian-2-yl)aryl](aryl)methyl thioethers. The developed approach provides a general access to mono-aryl/alkylthio-substituted (hetero)acene frameworks containing at least three fused (hetero)aromatic rings. The characteristic feature of this approach, which leads to highly substituted acenes, is that the substituents, unlike in other methods, may be introduced at an early stage of the synthesis. DFT and TD-DFT calculations confirmed the stabilizing role of the aryl/alkylthio substituent in the mono-aryl/alkylthio-substituted anthracenes, which are the most stable anthracenes prepared to date. Their high photostability is mainly due to the quenching of singlet oxygen by the acene and the quenching of the acene S₁ state by molecular oxygen.     

Influence of concentration on the activation energy for diffusion in polymer-solvent systems

The self-diffusion of benzene, toluene, and ethylbenzene in polystyrene have been analyzed using the Vrentas/Duda free-volume diffusion model. Diffusion coefficient predictions suggest an exponential concentration dependence of the activation energy required to overcome attractive forces, E. Without the use of any diffusion data approximating E as zero over the entire concentration range yields self-diffusion coefficient predictions which are in good agreement with experimental data. © 1992 John Wiley & Sons, Inc.     

Electrostatic potentials of molecules and prediction of the lewis acid-base properties of solvents,part I. General considerations

Summary Electrostatic potentials in the vicinity of isolated molecules of typical nonaqueous solvents were computed in the MNDO approximation. It was found that the calculated dipole moments are in good agreement with experimental values. However, it turned out that in many cases electrostatic interactions of ions with solvent molecules cannot be described in the frame of ion-dipole interactions. Thus, two computable parameters,UN andUP describing short-range electrostatic interactions of a solvent molecule with a cation or an anion of radiusR were proposed. The dependence ofUN andUP uponR is analyzed in the context of ion-solvent interactions.

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Elektrostatische Potentiale von Molekülen und Voraussage von Lewis-Säure-Base-Eigenschaften von Lösungsmitteln, 1. Mitt.: Allgemeine Grundlagen

Zusammenfassung Es wurden die elektrostatischen Potentiale in der Umgebung von isolierten Molekülen typischer nichtwäßriger Lösungsmittel in der MNDO-Näherung berechnet. Die berechneten Dipolmomente waren in guter Übereinstimmung mit den experimentellen Werten. Es wurde jedoch beobachtet, daß in vielen Fällen die elektrostatischen Wechselwirkungen von Ionen mit Lösungsmittelmolekülen nicht im Rahmen von Ionen-Dipol-Wechselwirkungen beschrieben werden können. Dafür wurden zwei berechenbare ParameterUN undUP zur Beschreibung von elektrostatischen Wechselwirkungen im Nahbereich eines Lösungsmittelmoleküls mit einem Kationen- oder AnionenradiusR vorgeschlagen. Die Abhängigkeit vonUN undUP vonR wurde im Zusammenhang von Ionen-Solvens-Wechselwirkungen untersucht.

     

Long time behaviour of a flow in infinite pipe conforming to slip boundary conditions

The paper analyses long time behaviour of solutions of the Navier–Stokes equations in a two‐dimensional pipe‐like domain. The system is studied with perfect slip boundary conditions with arbitrary inflow conditions at infinity. The main results show the existence of global in time solutions and of an attractor for the dynamical system generated by the model. The paper also establishes an upper bound for the Hausdorff dimension of the attractor. Copyright © 2005 John Wiley & Sons, Ltd.     

Asymptotic Completeness for Multiparticle Dispersive Charge Transfer Models

We prove the asymptotic completeness of wave operators associated with the scattering of a quantum particle in a field of classical particles in the dispersive case when the free hamiltonian has the formH₀=p(D) withpelliptic of degreem?1 and satisfying some convexity hypotheses.     

Structural transformations during the gas chromatography of carbamates

Summary There is no apparent paucity of examples illustrative of the structural changes observed during the gas chromatography of diverse carbamates and thiocarbamates. In many cases the resultat thermal degradation products observed are analogous in some measure to the three possible modes of decomposition reported in nonchromatographic thermal studies for N-phenyl carbamates [4, 35, 36], e.g., Dyer andWright [35] have shown that when R is a primary or secondary alkyl group, all three modes of decomposition can occur when the reaction is carried out in the absence of solvents. If the amine and isocyanate products are allowed to remain in the formation mixture, diphenyl urea is also formed, viz., As has been shown, the thermal and hydrolytic stabilities of carbamates exhibit a marked dependence upon the degree and type of N-substitution. Disubstituted carbamates are quite resistant to thermal decomposition (as well as hydrolysis). Monosubstituted carbamates readily undergo thermal decomposition at elevated temperatures to yield primarily the respective isocyanate while unsubstituted carbamates have been reported to decompose to allophanates, cyanuric acid, and alcohol [37]. The thermal decomposition of unsubstituted carbamates is accelerated appreciably in the presence of eventrace amounts of metal salts, giving rise to the formation of cyamelide. The degradative scheme for the diol dicarbamates (as typified by the medicinal carbamates, meprobamate and soma) is more difficult to interpret.Paquin [38] reported that the pyrolysis of the dicarbamate of 1,3-butylene glycol (I), yielded an oxazolidone (II) and a cyclic urea (III) as follows: The further degradation of compounds II and III under pyrolytic conditions remains a distinct possibility.

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Strukturumwandlungen w?hrend der Gas-Chromatographie von Carbamaten

Zusammenfassung Es besteht offenbar kein Mangel an Beispielen für Strukturumwandlungen w?hrend der Gas-Chromatographie verschiedener Carbamate und Thiocarbamate. In vielen F?llen entsprechen die beobachteten resultierenden thermischen Abbauprodukte in gewissem Ma?e den drei m?glichen Abbauformen, über die in nichtchromatographischen thermischen Untersuchungen von N-Phenylcarbamaten berichtet wird [4, 35, 36], z.B. Dyer Wright [35] haben gezeigt, da?, wenn R eine prim?re oder sekund?re Alkylgruppe ist, alle drei Abbauformen vorkommen k?nnen, wenn die Reaktion in Abwesenheit von L?sungsmitteln stattfindet. Wenn die Amin- und Isocyanat-Produkte in der Mischung belassen werden, bildet sich auch Diphenylharnstoff, n?mlich: Wie gezeigt wurde, besitzt die thermische und hydrolytische Stabilit?t von Carbamaten eine starke Abh?ngigkeit von Grad und Art der N-Substitution. Disubstituierte Carbamate sind ziemlich resistent gegen therm?sche Zersetzung (wie auch gegen Hydrolyse). Monosubstitutierte Carbamate lassen sich bei h?heren Temperaturen leicht thermisch zersetzen, wobei haupts?chlich das entsprechende Isocyanat entsteht, w?hrend sich unsubstituierte Carbamate nach [37] in Allophanate, Cyanurs?ure und Alkohol zersetzen. Der thermische Abbau unsubstituierter Carbamate wird durch Anwesenheit selbst geringer Spuren von Metallsalzen nennenswert beschleunigt, wobei Cyamelid entsteht. Das Abbauschema für die Diol-Dicarbamate (typische Beispiele: medizinische Carbamate, Meprobamat und Soma) ist schwieriger zu interpretieren.Paquin [38] berichtet, da? bei der Pyrolyse des Dicarbamats von 1,3-Butylenglycol (I) ein Oxazolidon (II) und ein cyclischer Harnstoff (III) in der folgenden Weise entstanden: Der weitere Abbau der Verbindungen II und III unter Pyrolysebedingungen ist nicht auszuschlie?en.

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Transformations de structure pendant la chromatographie en phase gazeuse de carbamates

Sommaire On ne manque pas d'exemples qui illustrent les transformations de structure observées pendant la chromatographie en phase gazeuse de divers carbamates et thiocarbamates. Souvent les produits de dégradation thermiques obtenus sont analogues, dans une certaine mesure, a ceux formés par décomposition thermique selon un des schémas probable signalé ci-après, procédés qui furent decrits pour des carbamates N-phényliques [4, 35, 36] dans des études de stabilité thermique non-chromatographiques, par exemple: Dyer etWright [35] ont démontré, si R est un groupe alkyle primaire ou secondaire, les trois modes de décomposition peuvent avoir lieu si la réaction se produit en l'absence de solvants. Si les amines et les isocyanates sont laissés dans le mélange, il y a aussi formation de diphénylurée, c'est-à-dire Il a été montré que la stabilité thermique et hydrolytique de carbamates dépend fortement du degré et type de la N-substitution. Des carbamates disubstitués sont assez résistants à la décomposition thermique (ainsi qu'à l'hydrolyse). Les carbamates monosubstitutés sont facilement sujets à la décomposition thermique à des températures élevées, produisant surtout l'isocyanate correspondant, tandis que, selon la littérature, des carbamates non-substitués sont décomposés en formant des allophanates, acide cyanurique, et alcool [37]. La décomposition thermique de carbamates non-substitués est accélérée sensiblement par la présence de sels métalliques, même à l'état de trace, provoquant la formation de cyamélide. Le schéma de dégradation des diol-dicarbamates (exemples typiques: carbamates médicinaux, méprobamate, soma) est plus difficile à interpréter.Paquin [38] a trouvé que la pyrolyse du dicarbamate de 1,3-butylène glycol (I) produit un oxazolidone (II) et une urée cyclique (III) d'après le schéma suivant: La dégradation ultérieure des composés II et III sous des conditions pyrolytiques n'est pas à exclure.

     

Polymerization of acrylonitrile by catalytic systems consisting of triphenylphosphine and a lewis acid

Polymerization of acrylonitrile in the presence of systems that consisted of triphenylphosphine (PPh₃) and a Lewis acid R_mMX_n (ZnCl₂, Me₃Al, Et₃Al, Et₂AlCl, EtAlCl₂, AlCl₃) was studied. The systems that contained Me₃Al and Et₃Al (i.e., Lewis acid of moderate acidity) were the most efficient catalysts. Conductometric measurements carried out in the polymerization systems showed the presence of ions. The presence of phosphonium cation in the polyacrylonitrile chain formed by the PPh₃–R_mMX_n catalytic systems was determined by IR, ¹H-NMR, and ³¹P-NMR spectroscopy. The average molecular weight measurements and kinetic chain lengths of polyacrylonitrile formed within the reaction time in the presence of PPh₃–Et₃Al showed that transfer reactions occur. According to the results obtained, the polymerization reaction of acrylonitrile by PPh₃–R_mMX_n involved a zwitterion formed by the attack of PPh₃ on acrylonitrile complexed by Lewis acid [Ph₃P^⊕? CH₂? C^?H? C?N → MR_mX_n] and the anion [CH₂?C^?? C?N] formed by the proton abstraction from the monomer.     

Examination of formamide,formamidic acid,amidine dimers,and the double proton transfer transition states involving these dimers

Structures and relative energies were obtained for the hydrogen bonded dimers of formamide and formamidic acid using the 3-21G basis set. A double proton transfer transition state is claimed to link these two dimers. While the structure of the transition state was intermediate between those of the two dimers, the energy was only 7.6 kJ/mol greater than the less stable formamidic acid dimer. The activation energy from the formamide dimer side of the reaction was found to be 125 kJ/mol of dimer. A similar transition state was found for the amidine dimer system. The activation energy for this model reaction was found to be 66.9 kJ/mol of dimer.     

Basicity of 4-aminopyrimidine and 2,4-diaminopyrimidine derivatives

Summary The basic dissociation constants for a series of 30 4-aminopyrimidine and 2,4-diaminopyrimidine derivatives were determined in 2% v/v aqueous methanol by a spectrophotometric method. The determinedpK _a values correlate well withHammet's constants.

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Basizität von 4-Aminopyrimidin- und 2,4-Diaminopyrimidinderivaten

Zusammenfassung Die basischen Dissoziationskonstanten einer Reihe von 30 4-Aminopyrimidin-and 2,4-Diaminopyrimidinderivaten wurden in 2% v/v wäßrigem Methanol durch spektroskopische Methoden bestimmt. Die ermitteltenpK _a-Werte korrelieren gut mit den entsprechendenHammett-Parametern .