- Predicting organic hydrogen atom transfer rate constants using the Marcus cross relation
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Chemical reactions that involve net hydrogen atom transfer (HAT) are ubiquitous in chemistry and biology, from the action of antioxidants to industrial and metalloenzyme catalysis. This report develops and validates a procedure to predict rate constants for HAT reactions of oxyl radicals (RO ?) in various media. Our procedure uses the Marcus cross relation (CR) and includes adjustments for solvent hydrogen-bonding effects on both the kinetics and thermodynamics of the reactions. Kinetic solvent effects (KSEs) are included by using Ingold's model, and thermodynamic solvent effects are accounted for by using an empirical model developed by Abraham. These adjustments areshown to be critical to the success of our combined model, referred to as the CR/KSE model. As an initial test of the CR/KSE model we measured self-exchange and cross rate constants in different solvents for reactions of the 2,4,6-tri-tert-butylphenoxyl radical and the hydroxylamine 2,2′-6,6′-tetramethylpiperidin-1-ol. Excellent agreement is observed between the calculated and directly determined cross rate constants. We then extend the model to over 30 known HAT reactions of oxyl radicals with OH or CH bonds, including biologically relevant reactions of ascorbate, peroxyl radicals, and α-tocopherol. The CR/KSE model shows remarkable predictive power, predicting rate constants to within a factor of 5 for almost all of the surveyed HAT reactions.
- Warren, Jeffrey J.,Mayer, James M.
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scheme or table
p. 5282 - 5287
(2010/09/10)
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- Direct Comparison of the reactivity of model complexes for compounds 0, I, and II in oxygenation, hydrogen-abstraction, and hydride-transfer processes
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The iron(III) meso-tetramesitylporphyrin complex is a good biomimetic to study the catalytic reactions of cytochrome P450. All of the three most discussed reactive intermediates concerning P450 catalysis (namely, Cpd 0, Cpd I, and Cpd II) can be selective
- Fertinger, Christoph,Hessenaue-Ilicheva, Natalya,Franke, Alicja,Van Eldik, Rudi
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supporting information; scheme or table
p. 13435 - 13440
(2010/06/11)
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- Generation of diarylcarbenium ion poolsviaelectrochemical C-H bond dissociation
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The "cation pools" of diarylcarbenium ions have been generated by the low-temperature electrochemical oxidation of diphenylmethane derivatives. In addition to diphenylmethanes having various substituents, 9,10-dihydroanthracene, dibenzosuberane, and xanth
- Okajima, Masayuki,Soga, Kazuya,Watanabe, Takashi,Terao, Kimitada,Nokami, Toshiki,Suga, Seiji,Yoshida, Jun-Ichi
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experimental part
p. 594 - 599
(2009/11/30)
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- C-H activation by a mononuclear manganese(III) hydroxide complex: Synthesis and characterization of a manganese-lipoxygenase mimic?
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Lipoxygenases are mononuclear non-heme metalloenzymes that regio- and stereospecifically convert 1,4-pentadiene subunit-containing fatty acids into alkyl peroxides. The rate-determining step is generally accepted to be hydrogen atom abstraction from the p
- Goldsmith, Christian R.,Cole, Adam P.,Stack, T. Daniel P.
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p. 9904 - 9912
(2007/10/03)
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- Comparison of pKR values of fluorenyl and anthracenyl cations
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A value of pKR = -5.1 for the anthracenonium ion in 50:50 (v/v) aqueous trifluorethanol is reported based on a ratio of measured rate constants kH for acid-catalysed dehydration of the 9,10-hydrate of anthracene and kH2O for the reverse hydrolysis of the carbocation (KR = kH2O/kH. Comparison with pKR = -15.9 for the fluorenyl cation indicates that the latter ion is less stable by more than 10 log units (15 kcal mol-1). This difference is (a) considerably larger than that between the benzhydryl (pKR = -11.7) and fluorenyl cations (ΔpK = 4.2), which has been considered too small to indicate antiaromatic character for the fluorenyl cation, and (b) comparable to that between pKas for the ionization (in DMSO) of fluorene (22.6) and diphenylmethane (32.2), which has been interpreted as implying aromatic character for the fluorenyl anion. It is shown that a difference in stability of anthracene hydrate and 9-hydroxyfluorene makes only a minor contribution to the difference in pKR values and that the fluorenyl cation is destabilized by ca 10 kcal mol-1. A smaller difference in pKas for protonation of fluorenimine (5.85) and benzhydrylimine (7.0) is consistent with the expected moderating effect of an electron-donating substituent on relative carbocation stabilities. Evidence from calculations relating to the antiaromaticity of the fluorenyl cation is reviewed in the light of these measurements. An additional comparison between equilibrium constants for the ionization of aralkylazides (Kaz) and alcohols (KR) reveals the influence of differences in geminal σ-bond interactions for the hydroxy and azido groups in their respective reactants. Copyright
- Courtney, Michael C.,MacCormack, Aoife C.,More O'Ferrall, Rory A.
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p. 529 - 539
(2007/10/03)
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- Hydrocarbon oxidation by bis-μ-oxo manganese dimers: Electron transfer, hydride transfer, and hydrogen atom transfer mechanisms
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Described here are oxidations of alkylaromatic compounds by dimanganese μ-xo and μ-hydroxo dimers [(phen)2MnIV (μ-O)2MnIV(phen)2]4+ ([Mn2(O)2]4+), [(phen)2MnIV (μ-O)2MnIII(phen)2]3+ ([Mn2(O)2]3+), and [(phen)2MnIII (μ-O)(μ-OH)MnIII(phen)2]3+ ([Mn2(O)(OH)]3+). Dihydroanthracene, xanthene, and fluorene are oxidized by [Mn2(O)2]3+ to give anthracene, bixanthenyl, and bifluorenyl, respectively. The manganese product is the bis(hydroxide) dimer, [(phen)2MnIII (μ-OH)2Mn(phen)2]3+ ([Mn2(OH)2]3+). Global analysis of the UV/vis spectral kinetic data shows a consecutive reaction with buildup and decay of [Mn2(O)(OH)]3+ as an intermediate. The kinetics and products indicate a mechanism of hydrogen atom transfers from the substrates to oxo groups of [Mn2(O)2]3+ and [Mn2(O)(OH)]3+. [Mn2(O)2]4+ is a much stronger oxidant, converting toluene to tolyl-phenylmethanes and naphthalene to binaphthyl. Kinetic and mechanistic data indicate a mechanism of initial preequilibrium electron transfer for p-methoxytoluene and naphthalenes because, for instance, the reactions are inhibited by addition of [Mn2(O)2]3+. The oxidation of toluene by [Mn2(O)2]4+, however, is not inhibited by [Mn2(O)2]3+. Oxidation of a mixture of C6H5CH3 and C6H5CD3 shows a kinetic isotope effect of 4.3 ± 0.8, consistent with C-H bond cleavage in the rate-determining step. The data indicate a mechanism of initial hydride transfer from toluene to [Mn2(O)2]4+. Thus, oxidations by manganese oxo dimers occur by three different mechanisms: hydrogen atom transfer, electron transfer, and hydride transfer. The thermodynamics of e-, H?, and H- transfers have been determined from redox potential and pKa measurements. For a particular oxidant and a particular substrate, the choice of mechanism is influenced both by the thermochemistry and by the intrinsic barriers. Rate constants for hydrogen atom abstraction by [Mn2(O)2]3+ and [Mn2(O)(OH)]3+ are consistent with their 79 and 75 kcal mol-1 affinities for H?. In the oxidation of p-methoxytoluene by [Mn2(O)2]4+, hydride transfer is thermochemically 24 kcal mol-1 more facile than electron transfer; yet the latter mechanism is preferred. Thus, electron transfer has a substantially smaller intrinsic barrier than does hydride transfer in this system.
- Larsen, Anna S.,Wang, Kun,Lockwood, Mark A.,Rice, Gordon L.,Won, Tae-Jin,Lovell, Scott,Sadilek, Martin,Turecek, Frantisek,Mayer, James M.
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p. 10112 - 10123
(2007/10/03)
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- Rate Constants for Termination and TEMPO Trapping of Some Resonance Stabilized Hydroaromatic Radicals in the Liquid Phase
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The rate constants for the termination reaction (2k1) of some resonance stabilized carbon centered radicals (SR.) derived from hydroaromatics (Sr. + SR. -> P) have been determined at 294 +/- 2 K by laser flash photolysis with UV-vis detection.The radicals were generated by hydrogen atom abstraction by t-BuO-radicals from the corresponding hydrocarbon (SRH + t-BuO. -> SR. + t-BuOH. k4).The extinction coefficients (e) of the SR., essential to calculate 2k1, were obtained using a relative kinetic technique.The change in 2k1 for the radicals derived from 1,4-cyclohexadiene, fluorine, 9,10-dihydroanthracene, diphenylmethane, tetralin, indan, indene, and phenol appeared to be modest; a range of 2k1 = 2-10 x 1E9 M-1 s-1 in mixtures of benzene and di-tert-butyl peroxide was observed.Most of the rate constants are near the diffusion controlled limit.In contrast, quenching the radicals with a persistent radical, 2,2,5,5-tetramethylpiperidin-1-oxyl (TEMPO), resulted in a larger variation of -1 s-1.The strength of the N-O bond formed in the latter process may have an important contribution to the observed rate constant.
- Arends, I.W.C.E.,Mulder, P.,Clark, K.B.,Wayner, D.D.M.
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p. 8182 - 8189
(2007/10/02)
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- Bimolecular Formation of Radicals by H-Transfer, 7. - Bimolecular Formation of Radicals via H-Transfer with Catalysis by 7H-Benzanthracene
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The transfer hydrogenation of α-methylstyrene (2) by 9,10-dihydroanthracene (1) (DHA) with 7H-benzanthracene (8) as a catalyst proceeds already at 210-260 deg C.Isotopic labeling, ESR spectroscopy, radical clock experiments, solvents effects, and thermochemical calculations support the postulated stepwise radical mechanism induced by bimolecular formation of radicals. - Key Words: Molecule-induced radical formation, kinetics of / Catalysis / ESR spectroscopy / Radical clock / Isokinetic relationship
- Gerst, Matthias,Morgenthaler, Jens,Ruechardt, Christoph
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p. 691 - 696
(2007/10/02)
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- Unifying the Solution Thermochemistry of Molecules, Radicals, and Ions
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A general cycle was developed that defines the thermodynamics for all of the homolytic and heterolytic cleavage reactions of a hydrocarbon, R-R', in solution.Only seven experimental parameters were needed in order to define the energetics for all 11 of the possible cleavage reactions of R-R'.These parameters were the oxidation and reduction potentials of R-R', R(.), and R'(.) and the homolytic, R-R', bond energy.The utility of this approach was demonstrated for the case where R was an arylmethyl group and R' was hydrogen.The oxidation and reduction potentials of thearylmethyl radicals were measured by modulation voltammetry in acetonitrile, and the homolytic C-H bond energies of the corresponding hydrocarbons were measured by photoacoustic calorimetry.The cycle was also extended to a case where R-R' was a radical rather than a closed-shell molecule.
- Griller, D.,Simoes, J. A. Martinho,Mulder, P.,Sim, B. A.,Wayner, D. D. M.
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p. 7872 - 7876
(2007/10/02)
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- Mobility in Single Crystals Studied by Optical Nuclear Polarization
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By optical excitation of molecular triplet states along with selective population of their spin sublevels nuclear spin polarization can be generated up to a level far above the Boltzmann equilibrium.This effect known as Optical Nuclear Polarization (ONP) has been applied for the investigation of dynamical processes in solid state matrices in the following three ways: (i) The efficiency of the ONP process and its dependency on an external magnetic field is affected by the fluctuation of local fields which is correlated to molecular motions. (ii) By pulsed irradiation of light and radio-frequencies one can achieve a temporal resolution of the polarization process; in this way the time evolution of the spin order of the molecular states involved in the optical excitation cycle can be mapped out. (iii) The high nuclear polarization makes time resolved NMR measurements on solid state systems feasible which cannot be studied by conventional techniques because of unsufficient sensitivity, e.g. guest molecules in low concentrations or isotopes with small natural abundance.The three methods, which cover together a time range between 1E-12 and 1E3 seconds are illustrated by experiments on various organic crystals. - Crystals / Photochemistry / Spectroscopy, Nuclear Magnetic Resonance
- Allgeier, J.,Buntkowsky, G.,Hentrich, S.,Nack, M.,Vieth, H.-M.
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p. 1281 - 1285
(2007/10/02)
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- Kinetic Isotope Effects in Hydrogen Atom Transfer Reactions between Benzylic Carbons
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The kinetic deuterium isotope effect for transfer of hydrogen from tetralin, dihydroanthracene, fluorene, diphenylmethane, triphenylmethane, and acenaphthene to the benzyl radical was measured at 170 deg C.The range of values for the effect was from 6.5 to 8.0.Activation energy parameters were obtained for five of the hydrogen donors.The apparent difference between activation energies for deuterium or hydrogen transfer was 2 kcal/mol for triphenylmethane, diphenylmethane, and tetralin.Applications of several tests indicate that a tunnel effect plays a significant role in these hydrogen transfers.
- Bockrath, Bradley C.,Bittner, Edward W.,Marecic, Thomas C.
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- The Source and the Resolution of the Controversy Regarding the Interpretation of Linear Sweep Voltammetry Data for the Protonation of Anthracene Anion Radical by Phenol
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The results of a number of linear sweep voltammetry (LSV) studies have appeared to be contradictory and have resulted in controversy.It is shown that there are no inconsistencies in the published data when all data are treated in the same manner.The apparent inconsistencies noted earlier are artefacts in the nature of the data treatment.Two important points which have not previously been recognised contribute to the confusion surrounding the interpretation of the data.These are i) the reaction of anthracene anion radical with phenol does not fulfill the requirements for a purely kinetic LSV process under the experimental conditions used and ii) the stoichiometry of the reaction is 2A-/. + 4PhOH --> products.The latter results from the fact that the 2PhO- formed in the reaction remove 2PhOH by virtue of the large association constant between these species.Point ii) is important in the LSV data interpretation in that the assumption of pseudo first order in A-/. is not valid over the entire range of used in the experimental work.Theoretical calculations were carried out taking points i) and ii) into account and excellent agreement with experimental data was found for the simple ECEh mechanism, the essential feature of which is rate determining irreversible protonation.
- Nielsen, Merete Folmer,Hammerich, Ole,Parker, Vernon D.
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p. 101 - 118
(2007/10/02)
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- Magnetic Field Effect on the Hydrogen Abstraction Reaction of Xanthone in Sodium Dodecyl Sulfate Micellar Solution
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The magnetic field effect on the hydrogen abstraction of xanthone from xanthene, 9,10-dihydroanthracene, and sodium dodecyl sulfate (SDS) surfactant has been studied in SDS micellar solution by steady-state (/= 260 mT) and laser flash photolysis (/= 80 mT).In the steady-state photolysis, the relative quantum yield of the disappearance of xanthone decreases in the magnetic field.Transient absorption intensities of 9-xanthenyl and 9,10-dihydroanthracene-9-yl radicals in the laser flash photolysis show remarkable magnetic field dependence, when xanthene and 9,10-dihydroanthracene are used as the respective hydrogen donors.All the results are interpreted in terms of the radical-pair model.
- Tanimoto, Yoshifumi,Takashima, Masanobu,Itoh, Michiya
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p. 6053 - 6056
(2007/10/02)
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- OPTICAL STUDIES OF HYDRONAPHTHYL RADICALS EMBEDDED IN DIHYDRONAPHTHALENE CRYSTAL.
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Radiation-induced radicals in dihydronaphthalene were investigated at 4. 2 K by absorption, fluorescence and fluorescence excitation studies. It was shown that the radicals in dihydronaphthalene are hydronaphtyl radicals. The advantges of using the dihydro compounds in the identification of cyclohexadienyl type radicals is discussed.
- Nakayama,Sheng
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p. 199 - 206
(2007/10/02)
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- Generation of Arenium Ions by a Self-Protonation Reaction in an Aprotic Molten Salt Medium
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We have examined the reaction behavior of a group of polycyclic aromatic hydrocarbons in the aprotic liquid SbCl3-10 mol percent AlCl3 from 100 to 130 deg C by 1H NMR and by quench and separation techniques.For anthracene, pyrene, 9,10-dimethylanthracene, 9,10-diphenylanthracene, and naphthacene, we have observed a novel arene self-protonation reaction for which the proton source is the condensation-dehydrogenation of a portion of the arene combined with arene oxidation by SbCl3.Naphthalene and phenanthrene, however, do not undergo this reaction.Evidence is presented which indicates that the self-protonation reaction proceeds through the oxidation of the arene to its radical cation by SbCl3, and that the function of AlCl3 is to enhance the oxidizing power of the Sb3+/Sb0 couple.
- Buchanan, A. C.,Dworkin, A. S.,Smith, G. P.
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p. 5262 - 5265
(2007/10/02)
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- CHARGE TRANSFER TRANSITIONS FROM CYCLOHEXADIENYL-TYPE RADICALS TO HOST MOLECULES IN ANTHRACENE SINGLE CRYSTALS.
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The optical absorption spectra of 1-, 2- and 9-dibenzo-cyclohexadienyl radical in anthracene crystals were obtained and analyzed. It is shown that the absorption lines consist of vibrational progression and charge-transfer lines for 2- and minus 9DBCR. Each line belonging to the former has the same polarization as the most intense 0-0 vibrational line and the latter show polarization character depending on the direction of charge transfer and the transition energy was found to have a coulombic dependence on the distance of charge transfer. It is pointed out that only charge transfer lines were observed for 1-DBCR. The splitting of lines was observed and ascribed either to the resonance transfer interaction between two neighboring molecules situated at an equal distance from the radical or to the site symmetry. It is suggested that the intensity of the charge-transfer lines are borrowed from the intra-radical transition. The result that the charge transfer lines are more intense than the intra-radical lines for 1-DBCR was discussed.
- Shirakawa,Nakagawa,Itoh
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p. 211 - 226,214,219
(2007/10/12)
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