- ESR Spin-Trapping Study of the Radicals Produced in NOx/Olefin Reactions. A Mechanism for the Production of the Apparently Long-Lived Radicals in Gas-Phase Cigarette Smoke.
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Gas-phase cigarette smoke contains high concentrations of both oxygen- and carbon-centered free radicals.We have detected these radicals using several variations of the electron spin resonance (ESR) spin-trapping technique, including the use of spin traps in the solid state, to show that the radicals are trapped directly from the gas phase.These gas-phase radicals can still be trapped from gas-phase smoke that is more than 5 min old, a result that is clearly inconsistent with the highly reactive nature of oxygen- and carbon-centered radicals.To rationalize this apparent paradox, we hypothesize that free radicals are continuously produced and destroyed in cigarette smoke and exist in a steady state.We suggest that one mechanism by which radicals can be formed involves the slow oxidation of the relatively unreactive nitric oxide (which acts as a "radical reservoir") to the much more reactive nitrogen dioxide.Nitrogen dioxide can then react with a number of the species that are present in smoke to produce the radicals that we detect.As a model, we have studied the reactions of NO/air mixtures with unsaturated hydrocarbons.Isoprene is one of the most abundant species in smoke and is known to be very reactive toward NO2; therefore we have studied the nature of the radicals that can be spin trapped from gaseous mixtures of NO, isoprene, and air.We find that the NO/air/isoprene model system gives essentially the same types of radicals (oxygen and carbon centered) as does cigarette smoke.We have also studied the gas-phase reactions of NO2 with several small olefins and 1,3-butadiene and find evidence for peroxyl radical intermediates.In solution, NO2 reacts with isoprene much faster than it does with the spin-trap phenyl-tert-butylnitrone (PBN).We find that NO2 oxidizes PBN to benzoyl tert-butyl nitroxide and propose a mechanism for this reaction.
- Pryor, William A.,Tamura, Masamitsu,Church, Daniel F.
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- Oxidative cleavage of CH3 and CF3 radicals from BOXAM nickel complexes
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Oxidation of the nickel complexes [(BOXAM)Ni(R)] (HBOXAM = bis((4-isopropyl-4,5-dihydrooxazol-2-yl)phenyl)amine) with R = CF3, CH3, Cl leads to the corresponding radical cationic complexes, which rapidly undergo homolytic Ni-R bond s
- Klein, Axel,Vicic, David A.,Biewer, Christian,Kieltsch, Iris,Stirnat, Kathrin,Hamacher, Claudia
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p. 5334 - 5341
(2012/11/13)
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- Titanium tetra-tert-butoxide-tert-butyl hydroperoxide oxidizing system: Physicochemical and chemical aspects
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The reaction of titanium tetra-tert-butoxide with tert-butyl hydroperoxide (1: 2) (C6H6, 20 C) involves the steps of formation of the titanium-containing peroxide (t-BuO)3TiOOBu-t and peroxytrioxide (t-BuO)3TiOOOBu-t. The latter decomposes with the release of oxygen, often in the singlet form, and also homolytically with cleavage of both peroxy bonds. The corresponding alkoxy and peroxy radicals were identified by ESR using spin traps. The title system oxidizes organic substrates under mild conditions. Depending on the substrate structure, the active oxidant species can be titanium-containing peroxide, peroxytrioxide, and oxygen generated by the system.
- Stepovik,Gulenova,Martynova,Mar'Yasin,Cherkasov
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p. 266 - 276
(2008/09/20)
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- Alkoxyamines of stable aromatic nitroxides: N-O vs. C-O bond homolysis
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A series of stable 2,2-disubstituted 3-(phenylimino)indol-1-oxyls, the alkoxyamines 3, were prepared, characterized, and tested as possible candidates in controlled radical polymerization (CRP). The sturctures of 3d and 10 were additionally solved by X-ra
- Gigmes, Didier,Gaudel-Siri, Anouk,Marque, Sylvain R. A.,Bertin, Denis,Tordo, Paul,Astolfi, Paola,Greci, Lucedio,Rizzoli, Corrado
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p. 2312 - 2326
(2007/10/03)
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- Fluoro spin adducts and their modes of formation
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The reactions of two fluorinating reagents, XeF2 and N-fluorodibenzenesulfonamide [(PhSO2)2N-F], with several spin traps have been investigated. In dichloromethane, the strong oxidant XeF2 cleanly gives fluoro spin adducts with N-tert-butyl-α-phenylnitrone (PBN) or 5,5-dimethyl-1-pyrroline 1-oxide (DMPO) according to a mechanism mediated by the radical cation of the spin trap. In both cases, further fluorination takes place with replacement of the a hydrogen by fluorine. The much weaker oxidant (PhSO2)2N-F reacts with PBN or DMPO in dichloromethane giving both the fluoro adduct and an adduct formally derived from an N-centred radical, assigned the structure of PhSO2N(F)-PBN? or (PhSO2)2N-DMPO?, respectively. This type of reaction proceeds by a version of the Forrester-Hepburn mechanism, in which an acid HA, in this case HF, initially adds to the nitrone function to give a hydroxylamine derivative which is oxidized by (PhSO2)N-F giving the fluoro spin adduct, a proton and the highly labile radical anion (PhSO2)2N-F?-. By decomposition of the latter to PhSO2(F)N- and PhSO2?, conditions are set up for propagation of the reaction by a new molecule of HA [now PhSO2(F)NH] and thus formation of the PhSO2(F)N spin adduct.
- Eberson, Lennart,Persson, Ola
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p. 893 - 898
(2007/10/03)
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- Mass spectrometry and electron paramagnetic resonance study of free radicals spontaneously formed in nitrone-peracid reactions
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Reactions of spin traps (C-phenyl N-tert-butyl nitrone (PBN) and 5,5-dimethyl-2-phenyl-1-pyrroline N-oxide (2-Ph-DMPO)) with peracids have been investigated by both mass spectrometry (MS) and electron paramagnetic resonance (EPR). The peracids m-chloroperbenzoic acid, perbenzoic acid, and perpropionic acid, which can be considered models of biological peracids produced during lipid peroxidation, were found to react with spin traps to spontaneously produce significant amount of aminoxyl radicals. The radical products, as well as the nonradical products were detected and their structures identified by EPR and/or MS. Mechanisms for the formation of these products are proposed.
- Sang, Hong,Janzen, Edward G.,Lewis, Brian H.
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p. 2358 - 2363
(2007/10/03)
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- Spin trapping of fluoroalkyl radicals
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The formation of fluoroalkyl radicals in liquid chlorofluorocarbons (CF2Cl-CFCl2, CFCl3, CFCl2-CH3, CHF2-CF2-CF2Cl, CF3-CFH-CF3 and CCl4) has been investigated by means of the spin trapping technique and ESR spectroscopy.It is shown that the primary fluoroalkyl radicals which were generated in these compounds by ionising radiation are preferably formed by bond cleavage at carbon atoms attached to one fluorine atom.Besides the liberation of chlorine atoms in chlorine-containing compounds, in CHF2-CF2-CF2Cl and CF3-CFH-CF3 fluorine atoms were also detected.By comparison of radicals formed from γ-irradiated CF2Cl-CFCl2 (CFC-113) and from CF2Cl-CFClI or CF2I-CFCl2 by zinc reduction, it was demonstrated that in CFC-113 both .CF2-CFCl2 and .CFCl-CF2Cl radicals are formed, whereas in iodine-containing homologues the formation of one species is favoured. - Keywords: Spin trapping; Fluoroalkyl radicals; ESR spectroscopy; γ-Irradiation; Zinc reduction; Phenyl-tert-butylnitrone
- Gille, Lars,Stoesser, Reinhard
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p. 191 - 194
(2007/10/02)
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- Mechanistic Studies by Electron Paramagnetic Resonance Spectroscopy on the Formation of 2-(N-Chloroimino)-5,5-dimethylpyrrolidine-1-oxyl Radical from 5,5-Dimethyl-1-pyrroline 1-Oxide and Hypochlorite-Treated Ammonia
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Mixtures of ammonium ion (NH4(1+)) or ammonia (NH3), hypochlorous acid (HOCl), and 5,5-dimethyl-1-pyrroline-1-oxide (DMPO) gave the radical 2-(N-chloroimino)-5,5-dimethylpyrrolidine-1-oxyl (1).The first step in the formation of 1 was the reaction of HOCl and NH4(1+) to provide ammonia chloramines: monochloramine (NH2Cl), dichloramine (NHCl2), and trichloramine (NCl3).Chloramine composition and the formation of 1 were dependent on pH, the ratio of NH4(1+) to HOCl, and, at acidic pH, on the concentration of chloride in the medium.Conditions were chosen to isolate the individual chloramines in solution for further study.NCl3 and DMPO gave 1; however, NHCl2 and NH2Cl required additional oxidants such as HOCl or PbO2 to produce 1.Studies with 15N-labeled chloramines confirmed that NHCl2 and NH2Cl reacted with DMPO to form N-chloro intermediates that yielded 1 upon subsequent oxidation.Light had no effect on the formation of 1, and UV irradiation did not enhance the EPR signal intensity but caused rapid decay, indicating that radical intermediates of ammonia chloramines were not involved.The mechanism of formation of 1 appeared to involve temporary attachment of chloramine Cl to the nitroxide oxygen of DMPO which activated its β-carbon for nucleophilic addition of the chloramine N.Subsequent N-chlorination and/or dehydrochlorination, depending on the reactive chloramine, would then provide 1.However, nucleophilic addition of H2O to the activated β-carbon of DMPO was competitive because 5,5-dimethyl-2-hydroxypyrrolidine-1-oxyl (DMPO-OH) or 5,5-dimethyl-2-pyrrolidone-1-oxyl (DMPOX) radicals were sometimes observed as minor products along with 1.Analogous chloroimine radicals were not obtained from the reaction of ammonia chloramines with 3,3,4,4-tetramethyl-1-pyrroline 1-oxide (M4PO) and N-tert-butyl-α-phenylnitrone (PBN), although their 2-oxo nitroxyl derivatives and hydroxyl adducts were formed as radical products suggesting that nucleophilic addition of H2O was dominant with these nitroxides.
- Bandara, B. M. Ratnayake,Hinojosa, Oscar,Bernofsky, Carl
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p. 1642 - 1654
(2007/10/02)
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- Spontaneous Free-Radical Formation in Reactions of m-Chloroperbenzoic Acid with C-Phenyl-N-tert-butylnitrone (PBN) and 3- or 4-Substituted PBN's
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A molecular reaction between m-chloroperbenzoic acid and C-phenyl-N-tert-butylnitrone (PBN) produces significant amounts of aminoxyl radicals assigned to the m-chlorobenzoyloxyl adduct of PBN and benzoyl-tert-butylaminoxyl.
- Janzen, Edward G.,Lin, Chiou-Rong,Hinton, Randall D.
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p. 1633 - 1635
(2007/10/02)
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- The Mechanistic Diversity of the Thermal and Photochemical Decomposition of Bis(phenylphosphonoyl)Peroxides: Concerted Polar, Homolytic and Electron-Transfer Processes. On the Reactivity of (Phenylphosponoyl)oxyl Radicals
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The thermal and photochemical decomposition of the first bis(phenylphosphonoyl)peroxides, dioxybis (5), and dioxybis (6) has been studied in various solvents by 1H-, 13C-, and 31P-NMR spectroscopy, laser flash photolysis (LFP), and ESR spin-trapping experiments.Kinetic studies reveal at 20 deg C a ca. 270 times slower thermal decay for 5 than for 6, which primarily results from a lower A factor rather than differences in the activation energies.The thermal decay of 5 occurs predominantly by a novel, presumably concerted polar rearrangement with formation of a thermally unstable, mixed phosphonoyl-phosphoryl anhydride.Photolysis of 5 induces homolytical cleavage of the peroxy bond with release of oxyl radicals 7.Radical 7 is characterized by a broad, transient UV/Vis absorption spectrum in the 400 to >700 nm range (λmax ca. 580 nm), as has been demonstrated by 248-nm LFP of 5 in acetonitrile solution.The short lifetime of this absorption indicates an extremely high reactivity (in hydrogen abstraction and addition) of this electrophilic radical.The thermal and photochemical decomposition of peroxide 6 leads to a virtually identical product distribution, suggesting O-O bond cleavage to be the major initial reaction under both conditions.LFP at 248 and 308 nm of a solution of 6 in acetonitrile initially produces a weak, broad absorption at ca. 500 nm and stronger bands at 280 and 400 nm.The highly transient 500-nm absorption is assigned to the oxyl radical 8, the other bands are attributed to the phosphonoyloxy-substituted benzene radical cation 8Z.The formation of this species can be explained in terms of electron transfer in the first-formed oxyl radical 8 and/or the intact peroxide 6, followed by cleavage of the peroxy bond.The decay of 8Z is accompanied by the build-up of the absorption spectrum of a 1,4-dioxy-substituted biphenyl radical cation.Key Words: Oxyl radicals / Phosphonoyl peroxides / Laser flash photolysis /ESR-Spin trapping / Electron transfer
- Korth, Hans-Gert,Lommes, Petra
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p. 2419 - 2438
(2007/10/02)
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- The Reaction of Nitrogen Dioxide with N-Benzylidene-t-butylamine N-Oxide. Formation of a White Solid Adduct and Its Properties
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The reaction of N-benzylidene-t-butylamine N-oxide (PBN) with NO2 gave white diamagnetic solid C11H16N2O4 in addition to greenish blue free radical species.The white solid was shown to decompose quantitatively into the starting PBN and nitric acid when it was dissolved in water.
- Sonokawa, Koji,Machida, Hidenori,Sakakibara, Kazuhisa,Hirota, Minoru
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p. 207 - 210
(2007/10/02)
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- Direct Spectroscopic Detection of Sulfonyloxy Radicals and First Measurements of Their Absolute Reactivities
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Two sulfonyloxyl radicals, CH3S(=O)2O., 2a, and 3-CF3C6H4S(=O)2O., 2b, have been generated by 308-nm laser flash photolysis (LFP) of their parent symmetrical peroxides in CH3CN solution, in which they have lifetimes of 7-20 μs.Both radicals exhibit a broad, structureless absorption similar to that known for SO4.- with λmax ca. 450 nm.This absorption can be bleached for 2a but not for 2b by firing a second laser at 480 nm, presumably reflecting a photoinduced cleavage of the H3C-SO3. bond.Radicals 2a and 2b react with the acetonitrile solvent by abstraction of a hydrogen atom, kH ca. 1.6*105 M-1 s-1, kH/kD ca. 2.0.Bimolecular rate constants for attack of these radicals on cyclohexane (viz., 1.9*108 and 6.5*108 M-1 s-1 for 2a and 2b, respectively) and chloroform (viz. ca., 3*105 M-1 s-1 for both) demonstrate that they are more reactive than almost all other oxygen-centered radicals.Product studies demonstrate that both the photodecomposition and the thermal decomposition of the parent peroxides yield the corresponding sulfonyloxy radicals, a result that contrasts with that we have previously obtained for the decomposition of 2, which yields radicals on photolysis but few if any radicals on thermolysis.Semiempirical AM1/PM3-UHF calculations on 2a are also reported.
- Korth, Hans-Gert,Neville, Anthony G.,Lusztyk, Janusz
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p. 8835 - 8839
(2007/10/02)
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- ESR Spin-Trapping Study of the Thermal Decomposition of N-Pivaloxyacetanilides in Benzene
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Spin-trapping results favor a mechanism involving homolytic cleavage of the N-O bond for the thermal decomposition in benzene of the N-pivaloxyacetanilides, which are models for carcinogenic metabolites of aromatic amides.
- Novak, Michael,Brodeur, Brenda A.
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p. 1142 - 1144
(2007/10/02)
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- Spin Trapping of Peroxy Radicals by Phenyl-N-(tert-butyl)nitrone and Methyl-N-durylnitrone
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The spin trapping of tert-butylperoxy and tetralylperoxy radicals by phenyl-N-(tert-butyl)nitrone (PBN) and methyl-N-durylnitrone (MDN) has been investigated.The peroxy radicals were generated by alkoxy radical induced decomposition of hydroperoxides, hydrogen atom abstraction from hydrocarbon in the presence of oxygen, and decomposition of hydroperoxydes by cobaltous ion and lead tetraacetate.The spin adducts of peroxy radicals were observed by ESR and their hyperfine splitting constants were detrmined.The spin adducts of peroxy radicals by MDN could be clearly distinguished from that of alkoxy radical.The nitrone spin adducts of oxygen radicals were found to be reasonably stable at room temperature in the dark, but they decayed readily in the ordinary laboratory light.
- Niki, Etsuo,Yokoi, Seiichi,Tsuchiya, Jyunichi,Kamiya, Yoshio
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p. 1498 - 1503
(2007/10/02)
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- Acyl Nitroxides. Part 4. Estimation of OH Bond Dissociation Energies for N-t-Butylhydroxamic Acids
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E.s.r. spectroscopy has been used to determine the equilibrium constants for the hydrogen exchange reactions between N-t-butylhydroxamic acids and the stable piperidine N-oxyl (1); since the O-H bond strength in (1)-H is known, this gives an estimate of the O-H bond strengths in the hydroxamic acids.These are found to be stronger than those in dialkyl nitroxides and increase with increasing electron demand in the acyl group.
- Jenkins, Terence C.,Perkins, M. John
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p. 717 - 720
(2007/10/02)
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- Dibenzoyl Peroxide Induced Photodecarboxylation of Amino Acids and Peptides. A Spin-Trapping Study
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In a new photochemical reaction the radicals produced from several amino acids and peptides by UV irradiation at 313 nm in dimethyl sulfoxide solutions in the presence of dibenzoyl peroxide were characterized by spin trapping using 2-methyl-2-nitrosopropane.The most predominant reactions were the decarboxylation of the amino acids and of the carboxyl-terminal residue in peptides.An unusual behavior was exhibited by the valine moiety which consistently yielded H-atom abstraction radicals.No radicals derived from dimethyl sulfoxide could be detected under our reaction conditions.
- Rosenthal, Ionel,Mossoba, Magdi M.,Riesz, Peter
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p. 2398 - 2403
(2007/10/02)
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- Aminyloxide (Nitroxide), XXXIV. Amidinyl-N-oxide und -N,N'-dioxide als Sekundaerradikale bei der oxidativen Kupplung von Aminen und Hydroxylaminen mit Nitronen
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Aminyl oxides 3, formed by oxidation of a mixture of nitrones 1 and primary amines 2, are converted into the secondary radicals 4 with increasing reaction time.In addition to dehydrogenation of 3 elimination of R2H is observed, starting with nitrones 1B - E.Radicals 10 and subsequently 4Ae arise from 1A and N-tert-butylhydroxylamine (15), but 17 is detected, too; from the reaction mixture the lead complex 14 can be isolated.Oxidative coupling of 2 with cyclic nitrones 20 and 28, respectively, affords aminyloxides 23 and 29 via their primary radicals.Further oxidation gives 24 and 30, respectively.From this reaction the lead complexes 25 can be isolated.
- Aurich, Hans Guenter,Duggal, Suresh K.,Hoehlein, Peter,Klingelhoefer, Hans-Georg,Weiss, Wolfram
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p. 2440 - 2449
(2007/10/02)
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