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  • 1
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 1 (1995), S. 423-429 
    ISSN: 0947-6539
    Keywords: kinetics ; peroxyl radicals ; pulse radiolysis ; radicals ; superoxide radicals ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Hydroxyl radicals were generated radiolytically and reacted with a number of benzene derivatives (PhH). In the presence of oxygen, the hydroxycyclohexadienyl radicals thus formed were converted into their corresponding peroxyl radicals. Pulse radiolysis has shown the oxygen addition (forward reaction, f) to be reversible (reverse reaction, r) [Eq. (1)]. The peroxyl radicals can eliminate HO2· to yield phenols, alongside some ring-fragmentation products (product-forming reaction, p). The rate constants for the forward and reverse reactions (kf and kr) and the corresponding stability constants K (= kf/kr) were determined for the hydroxy-cyclohexadienyl radicals derived from anisole, toluene, fluorobenzene, benzene, chlorobenzene, benzyl chloride, benzoate ion, phenylalanine, and terephthalate ion. The constants kf lie between 8 × 108 (anisole) and 1.6 × 107 dm3 mol-1 s-1 (terephthalate ion), and kr between 7.5 × 104 (toluene) and 3.4 × 103 s-1 (terephthalate ion). The stability constants lie between 2.6 × 104 (benzene) and 3.3 × 103 dm3 mol-1 (phenylalanine). The rate constants for the product-forming reactions kp are between 5.5 × 103 (anisole) and 3.4 × 102 s-1 (benzoate). For the peroxyl radical derived from phenylalanine, a bond dissociation energy of 5.5 kcal mol-1 has been derived. A number of hydroxy-cyclohexadienyl radicals (e.g., those derived from benzoic acid, ethylbenzoate, benzonitrile, and nitrobenzene) react too slowly to allow the equilibrium constant to be determined by means of pulse radiolysis. These reactions have rate constants kf in the order of 5 × 106 dm3 mol-1 s-1, except for nitrobenzene where the reaction is too slow for measurement. The rate constants kr are below 500 s-1, and the product-forming reaction is too slow to be detected by pulse radiolysis. γ-Radiolysis of N2O/O2(4:1)-saturated aqueous solutions of benzonitrile gave dimeric compounds (e.g., dicyanobiphenyls) in low yield, alongside the three isomeric phenols; this again proves the low reactivity of its hydroxycyclohexadienyl radical toward oxygen.
    Additional Material: 6 Ill.
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  • 2
    ISSN: 0009-2940
    Keywords: Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Die Radiolyse von Trimethylamin in N2O/O2 (4:1)-gesättigter basischer Lösung führt zur Bildung von Dimethylamin, Formaldehyd und Wasserstoffperoxid. Die Anwendung der Pulsradiolyse in Verbindung mit schneller Leitfähigkeitsmessung und Spektralphotometrie ermöglicht die Beobachtung der chemischen Prozesse, die nach dem Angriff des OH-Radikals am Trimethylamin ablaufen. Das (Dimethylamino)methyl-Radikal reagiert rasch mit Sauerstoff (k=3.5×109 M-1 s-1), wobei sich Dimethylimonium und Superoxid bildet, möglicherweise über ein kurzlebiges (t1/2〈10-6 s) Peroxylradikal. Das Dimethylimonium-Ion lagert OH- an und bildet (Hydroxymethyl)dimethylamin (k=2.8×108 M-1 s-1). Dieses hydrolysiert in Dimethylamin und Formaldehydhydrat (kobs=4.0 ± 0.6 s-1. Es wird gezeigt, daß ein Reaktionsmechanismus, der diese Elementarschritte enthält, in guter übereinstimmung mit den experimentellen Daten (pH von 9 bis 11.3) steht.
    Notes: The radiolysis of trimethylamine in N2O/O2 (4:1 v/v) saturated basic solutions eventually gives rise to dimethylamine, formaldehyde, and hydrogen peroxide. Pulse radiolysis using conductivity and optical detection permits the observation of the chemical transformations following the generation of the (dimethylamino)methyl radical from trimethylamine upon attack by the hydroxyl radical. The (dimethylamino)methyl radical reacts rapidly with oxygen (k = 3.5×109 M-1 s-1) to give O2-1 and dimethylimonium, possibly via a short-lived (t1/2 -6s) peroxyl radical. The dimethylimonium ion adds OH- to form (hydroxymethyl)dimethylamine (k=2.8×108 M-1 s-1). This is hydrolyzed to dimethylamine and formaldehyde hydrate (kobs=4.0 ± 0.6 s-1). A mechanism incorporating these reactions is shown to be in good accordance with the experimental data in the pH range from 9 to 11.3.
    Additional Material: 4 Ill.
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  • 3
    ISSN: 0044-8249
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Additional Material: 1 Ill.
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  • 4
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Zeitschrift für die chemische Industrie 103 (1991), S. 1255-1279 
    ISSN: 0044-8249
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Immer wenn es zur Bildung von Radikalen kommt, sei es thermisch, durch UV- und ionisierende Strahlen oder in Redoxreaktionen, werden sie in Gegenwart von Sauerstoff rasch in die entsprechenden Peroxyl-Radikale überführt. Nicht nur in aquatischen Systemen wie den Flüssen, Seen und Meeren, sondern auch in der lebenden Zelle sowie in der Atmosphäre in beträchtlichem Ausmaß (in Wassertröpfchen) laufen Peroxyl-Radikalreaktionen in wäßriger Umgebung ab. Die Peroxyl-Radikalchemie in diesem Medium unterscheidet sich häufig sehr von der in der Gasphase oder in organischen Lösungsmitteln. Trotz der großen Bedeutung dieser Reaktionen auch in der Medizin, beispielsweise bei der Strahlentherapie des Krebses und der Ischämie, liegen vergleichsweise wenige Untersuchungen über Peroxyl-Radikalreaktionen in wäßrigem Medium vor. Für derartige Untersuchungen liefern uns jedoch strahlenchemische Techniken, z. B. die Pulsradiolyse, die besten Handwerkszeuge. So überrascht es nicht, daß der überwiegende Teil der Kenntnisse auf diesem Gebiet mit strahlenchemischen Methoden gewonnen wurde. Die Strahlenchemie des Wassers kann so gelenkt werden, daß im wesentlichen ·OH-Radikale entstehen (Ausbeute ca. 90%), die mit Substraten zu Substrat-Radikalen und in Gegenwart von Sauerstoff zu den entsprechenden Substratperoxyl-Radikalen reagieren. Auch kann man die experimentellen Bedingungen so wählen, daß ausschließlich HO2·/O2·⊖-Radikale gebildet werden, die dann mit Substraten zur Reaktion gebracht werden können. Dies erlaubt somit auch einen Zugang zu diesen in biologischen Systemen so wichtigen Intermediaten. Durch eine detaillierte Produktanalyse und kinetische Untersuchungen mit Pulsradiolyse wurde die Chemie einer Reihe von Peroxyl-Radikalen aufgeklärt, so daß jetzt ausreichend Material vorliegt, um die Vielfalt der mit strahlenchemischen Methoden untersuchten Peroxyl-Radikalreaktionen zusammenfassend darstellen zu können. Nach einer etwas generelleren Übersicht über die physikalischen Eigenschaften der Peroxyl-Radikale und ihre unimolekularen und bimolekularen Reaktionen wird auf ausgewählte Vertreter verschiedener Verbindungsklassen eingegangen. Wegen der großen biologischen Bedeutung der radikalinduzierten DNA-Schädigung wird auch dieses Gebiet kurz gestreift.
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  • 5
    ISSN: 0570-0833
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Additional Material: 1 Ill.
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  • 6
    ISSN: 0570-0833
    Keywords: Reaction mechanisms ; Free radicals ; Peroxyl radicals ; Radical reactions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Whenever free radicals are formed, independent of whether this occurs thermally, is induced by UV or ionizing irradiation, or takes place in redox reactions, they are converted rapidly into the corresponding peroxyl radicals in the presence of oxygen. Peroxyl radical reactions in aqueous environments are observed not only in aquatic systems (e.g., rivers, lakes and oceans) but also in the living cell and to a considerable degree even in the atmosphere (in water droplets). The peroxyl radical chemistry occurring in this medium is often very different from that observed in the gas phase or in organic solvents. In spite of the great importance of these reactions in medicine (for example in anti-cancer irradiation therapy and ischaemia) there have been comparatively few studies of peroxyl reactions in aqueous media. Radiation-chemical techniques such as pulse radiolysis offer the best means for carrying out such studies, so that it is not surprising that the majority of the information available in this area has been obtained with the help of radiation-chemical methods. The radiation chemistry of water can be con trolled in such a manner that the main products are ·OH radicals (90 % yield), which react with substrate molecules to give substrate radicals and in the presence of oxygen to give substrate peroxyl radicals. The experimental conditions can also be varied in such a way that HO2·/O2·⊖ radicals can be formed in 100 % yield and caused to react with substrates. We therefore have a simple access to these intermediates, which are extremely important in biological systems. A detailed product analysis, supported by kinetic studies carried out with the help of pulse radiolysis, has been used to clarify the chemistry of a series of peroxyl radicals, so that sufficient material is now available to justify a review of the variety of the peroxyl radical reactions studied by means of radiation-chemical methods. A more general survey of the physical properties of the peroxyl radicals and their unimolecular and bimolecular reactions will be followed by a discussion of selected examples of various classes of substance. Because of the great biological importance of radical-induced DNA damage this area will also be treated briefly.
    Additional Material: 6 Ill.
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