- Bimolecular hydrogen abstraction from phenols by aromatic ketone triplets
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Absolute rate constants for hydrogen abstraction from 4-methylphenol (para-cresol) by the lowest triplet states of 24 aromatic ketones have been determined in acetonitrile solution at 23°C, and the results combined with previously reported data for roughly a dozen other compounds under identical conditions. The ketones studied include various ring-substituted benzophenones and acetophenones, α,α,α-trifluoroacetophenone and its 4-methoxy analog, 2-benzoylthiophene, 2-acetonaphthone, and various other polycyclic aromatic ketones such as fluorenone, xanthone and thioxanthone, and encompass n,π*, π,π*(CT) and arenoid π,π* lowest triplets with (triplet) reduction potentials (Ered*) varying from about -10 to -38 kcal mol-1. The 4-methylphenoxyl radical is observed as the product of triplet quenching in almost every case, along with the corresponding hemipinacol radical in most instances. Hammett plots for the acetophenones and benzophenones are quite different, but plots of log log kQ vs Ered* reveal a common behavior for most of the compounds studied. The results are consistent with reaction via two mechanisms: a simple electron-transfer mechanism, which applies to the n,π* triplet ketones and those π,π* triplets that possess particularly low reduction potentials, and a coupled electron-/proton-transfer mechanism involving the intermediacy of a hydrogen-bonded exciplex, which applies to the π,π* ketone triplets. Ketones with lowest charge-transfer π,π* states exhibit rate constants that vary only slightly with triplet reduction potential over the full range investigated; this is due to the compensating effect of substituents on triplet state basicity and reduction potential, which both play a role in quenching by the hydrogen-bonded exciplex mechanism. Ketones with arenoid π,π* states exhibit the fall-off in rate constant that is typical of photoinduced electron transfer reactions, but it occurs at a much higher potential than would be normally expected due to the effects of hydrogen-bonding on the rate of electron-transfer within the exciplex.
- Lathioor, Edward C.,Leigh, William J.
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p. 291 - 300
(2008/02/05)
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- Comparison of reactions of radical cations of 1-phenylalkanols produced by photoionization and by one-electron oxidation in aqueous solution
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Benzyl alcohols in aqueous solution react with photo- and radiation-chemically produced .OH and SO4.- radicals with diffusion-controlled rates to yield OH-adducts and benzyl alcohol radical cations, respectively. The former can be converted to the radical cations by H+-induced (heterolytic) dehydroxylation, whereas the latter decay by a) electrophilic reaction with water (= reverse of the dehydroxylation reaction) giving rise to Cnucleus-OH-adducts and by b) side chain C-H deprotonation yielding α-hydroxybenzyl-type radicals. If, however, the radical cation is produced by biphotonic ionization of the benzyl alcohol, the pattern of Cnucleus-OH bond formation and side chain C-H bond breakage is different from that in the reaction with SO4.-. It is concluded that, at least in this reaction, it is not the free, solvated radical cation that reacts with water but the ion pair [radical cation-SO42-].
- Steenken, Steen,Ramaraj, Ramasamy
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p. 1613 - 1619
(2007/10/03)
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- The reactivity of ketyl and alkyl radicals in reactions with carbonyl compounds
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A parabolic model of bimolecular radical reactions was used for analysis of the hydrogen transfer reactions of ketyl radicals: >C+OH + R1COR2 → >C=O + R1R2C+OH. The parameters describing the reactivity of the reagents were calculated from the experimental data. The parameters that characterize the reactions of ketyl and alkyl radicals as hydrogen donors with olefins and with carbonyl compounds were obtained: >C+OH + R1CH=CH2 → >C=O + R1C+ HCH3; >R1CH=CH2 + R2C+HCH2R3 → R2C+HCH3 + R2CH=CHR3. These parameters were used to calculate the activation energies of these transformations. The kinetic parameters of reactions of hydrogen abstraction by free radicals and molecules (aldehydes, ketones, and quinones) from the C-H and O-H bonds were compared.
- Denisov
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p. 2110 - 2116
(2007/10/03)
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- Diffusion of electrically neutral radicals and anion radicals created by photochemical reactions
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Diffusion processes of the intermediate radicals created by the photochemical reactions of ketones in alcoholic solvents are investigated by using the transient grating (TG) method. The electrically neutral radicals and the anion radicals of acetophenone, benzaldehyde, xanthone, benzophenone and benzil were created selectively by controlling the concentration of sodium hydroxide (NaOH) in alcoholic solvents. The translational diffusion constants (D) of the anion radicals, the neutral radicals, and the parent stable molecules can be successfully measured under the same conditions by this method. It is found that both the neutral and anion radicals diffuse slower than the parent molecules. Values of D of the anion radicals, the neutral radicals and the parent molecules are compared in detail in wide ranges of solvent viscosities, solute sizes and temperatures. Under any conditions, D values of the charged radicals are similar to those of the neutral radicals. A possible origin of such a similarity is discussed in term of the intermolecular charge polarizabilities of the radicals.
- Okamoto, Koichi,Hirota, Noboru,Terazima, Masahide
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p. 185 - 194
(2007/10/03)
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- Reactivity of HO2. Radicals in Alcohols
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Photoinitiated liquid phase oxidation of 1-phenyl ethanol and benzyl alcohol have been investigated in the temperature range of 50-70 deg C using the intermittent illumination (rotating sector) technique.The rate of oxidation was followed by gas chromatography while the rate of initiation was determined by the inhibitor method.Results were explained by assuming HO2. radicals to be the main chain carriers, supported by comparison of the results with literature data. - The termination rate constant of HO2. was calculated from the average lifetime of radicals in both alcohols: 2kt=9.91*1012 exp(-29.2 kJ*mol-1/RT) in 1-phenyl ethanol and 2kt=8.98*1012 exp(-31.3 kJ*mol-1/RT) in benzyl alcohol. (in l*mol-1*s-1, the uncertainties given by the covariance matrix, see in the text.) - The rate constants for hydrogen abstraction by HO2. from the substrate molecules have also been determined in the case of both alcohols: kp=4.61*108 exp(-52.0 kJ*mol-1/RT) in 1-phenyl ethanol, kp=1.04*109 exp(-54.3 kJ*mol-1/RT) in benzyl alcohol, (in l*mol-1*s-1, uncertainties see in the text.) Keywords: Chemical Kinetics/ Elementary Reactions / Photochemistry / Radicals
- Keszler, Agnes,Irinyi, Gyoergy,Heberger, Karoly,Gal, Dezso
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p. 175 - 179
(2007/10/02)
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- Photochemical Production of Peroxy Radicals and Their Interaction with Aromatic Alcohols
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The reaction between 1-phenylethyl peroxy radicals and 1-phenyl ethanol, benzyl alcohol and cumyl alcohol has been studied at T = 55 deg C.The rate coefficients of the hydrogen abstraction reactions for the three alcohols are 18.7 +/- 2.7, 17.8 +/- 1.9 and 4.8 +/- 0.5 M-1s-1, respectively.In the case of 1-phenyl ethanol experiments were carried out at T = 25 and 40 deg C as well, resulting in rate coefficients of 7.3 +/- 0.7 and 10.5 +/- 0.9 M-1s-1, while the energy of activation - taking into consideration also the data of Hajdu et al. - was 27.7 +/- 0.5 kJ mol-1. - Keywords: Chemical Kinetics / Elementary Reactions / Radicals
- Paal-Lukacs, Julia,Gal, Dezsoe,Butovskaya, Galina,Agabekov, V. E.
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p. 609 - 612
(2007/10/02)
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- Triplet Quenching by tert-Butyl Hydroperoxide
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Laser flash photolysis studies show that tert-butyl hydroperoxide is an excellent quencher.For example, the rate constants for benzophenone and phenanthrene in benzene at 300 K are 1.8 x 108 and 2.3 x 107 M-1 s-1/sup
- Stewart, Laura C.,Carlsson, D. J.,Wiles, D. M.,Scainano, J. C.
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p. 3605 - 3609
(2007/10/02)
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