- Time-Resolved Resonance Raman Observation of Tetrafluoro-p-benzosemiquinone Anion Radical
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Time-resolved resonance Raman spectroscopy has been used to examine tetrafluoro-p-benzosemiquinone radical anion produced in the pulse radiolytic oxidation of tetraflourohydroquinone in aqueous solution.This radical is much more reactive than p-benzosemiquinone and is observed to decay on the millisecond time scale in both Raman and pulse radiolytic experiments.For the Raman experiments excitation was on the red edge of the moderately strong absorption band of this radical at 430 nm.Two resonance-enhanced Raman bands are exhibited at 1556 and 1677 cm-1 and are assigned to the in-phase CO and symmetrical CC stretch vibrations.These frequencies are considerably higher than the corresponding values of 1435 and 1620 cm-1 observed in this radical's protonated counterpart.The relatively large increase in the CO stretch frequency, in particular, indicates that fluorination induces a substantial increase in the quinoid character of this radical.
- Tripathi, G. N. R.,Schuler, Robert H.
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- Pulse Radiolysis Study of One-Electron Reduction Reaction of Fluoranil in Aqueous Solution
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Hydrated electron reacts with fluoranil (tetrafluoro-1,4-benzoquinone) with a rate constant of 2.5 * 1010M-1 s-1 to yield fluoranil anion.The kinetics of the reaction was monitored at 300 and at 700 nm for the anion and e
- Shoute, Lian C. T.,Mittal, Jai P.
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- Electron-transfer studies of a peroxide dianion
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A peroxide dianion (O22-) can be isolated within the cavity of hexacarboxamide cryptand, [(O2)∪mBDCA-5t-H 6]2-, stabilized by hydrogen bonding but otherwise free of proton or metal-ion association. This feature has allowed the electron-transfer (ET) kinetics of isolated peroxide to be examined chemically and electrochemically. The ET of [(O2)∪mBDCA-5t-H6] 2- with a series of seven quinones, with reduction potentials spanning 1 V, has been examined by stopped-flow spectroscopy. The kinetics of the homogeneous ET reaction has been correlated to heterogeneous ET kinetics as measured electrochemically to provide a unified description of ET between the Butler-Volmer and Marcus models. The chemical and electrochemical oxidation kinetics together indicate that the oxidative ET of O22- occurs by an outer-sphere mechanism that exhibits significant nonadiabatic character, suggesting that the highest occupied molecular orbital of O 22- within the cryptand is sterically shielded from the oxidizing species. An understanding of the ET chemistry of a free peroxide dianion will be useful in studies of metal-air batteries and the use of [(O 2)∪mBDCA-5t-H6]2- as a chemical reagent.
- Ullman, Andrew M.,Sun, Xianru,Graham, Daniel J.,Lopez, Nazario,Nava, Matthew,De Las Cuevas, Rebecca,Mueller, Peter,Rybak-Akimova, Elena V.,Cummins, Christopher C.,Nocera, Daniel G.
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supporting information
p. 5384 - 5391
(2014/06/09)
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- In Situ Electrochemical EPR Studies of Charge Transfer across the Liquid/Liquid Interface
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The in situ measurement of EPR spectra of radical ions generated at the polarized liquid/liquid interface is described in relation to the 7,7,8,8-tetracyanoquinodimethane (TCNQ), 2,3,5,6-tetrachloro-p-benzoquinone (TCBQ), and 2,3,5,6-tetrafluoro-p-benzoquinone (TFBQ) radical anions and the tetrathiafulvalene (TTF) radical cation. TCNQ and TTF were chosen as model compounds with which to quantify the performance of a novel liquid/ liquid electrochemical EPR cell. The anion radical of TCNQ and the cation radical of TTF in 1,2-dichloroethane (DCE) were produced at the water interface by electron transfer from/to the aqueous-phase ferricyanide/ferrocyanide redox couple by applying a potential difference between the two phases with a four-electrode potentiostat. The EPR signal intensity (at constant magnetic field) of the resultant organic radicals was monitored during potential step experiments which indicated that the EPR data could be modeled in terms of diffusional transport. TCBQ and TFBQ were chosen as compounds to model the electron transfer behavior of biologically important quinones at the oil/water interface. The EPR and voltammetric data obtained from TCBQ/TCBQ-· and TFBQ/ TFBQ-· indicated unambiguously that the two semiquinones are stable at the DCE/water interface and do not undergo immediate protonation.
- Webster, Richard D.,Dryfe, Robert A. W.,Coles, Barry A.,Compton, Richard G.
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p. 792 - 800
(2007/10/03)
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- Oxidation mechanism of NAD dimer model compounds
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The oxidation of a dimeric N-benzyldihydronicotinamide with various oxidants such as quinones, triphenyl carbenium ions and a triplet exited tris(bipyridine) ruthenium(II) complex occurs via initial outer-sphere electron transfer followed by fast C-C bond cleavage and second electron transfer. The kinetic studies allow the determination of the oxidation potential of this compound.
- Patz, Matthias,Kuwahara, Yoshihiro,Suenobu, Tomoyoshi,Fukuzumi, Shunichi
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p. 567 - 568
(2007/10/03)
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- Entropy Changes and Electron Affinities from Gas-Phase Electron-Transfer Equilibria: A(-) + B = A + B(-)
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By measuring the electron-transfer equilibria 1, A(-) + B = A + B(-), at 150 deg C with a pulsed electron high-pressure mass spectrometer we determined the ΔGo1 values involving 12 new compounds.Measurements of the temperature dependence of K1 for 21 reactions involving some of the new compounds and many compounds whose ΔGo1 had been determined previously led, via van't Hoff plots, to ΔHo1 and ΔSo1 values.These were interconnecting such that ΔHo and ΔSo continuous scales (ladders) were obtained.These were anchored to SO2 whose electron affinity is accurately known.Available geometries and vibrational frequencies for SO2 and SO2(-) permit the evaluation of So(SO2(-)) - So(SO2).Through the ΔSo scale the So(B(-)) - So(B) for the other compounds B could be obtained also.Certain regularities in the So(B(-)) - So(B) data permitted entropy estimates to be made also for compounds for which no van't Hoff plots were made.In this manner a table of ΔHo, ΔSo, and ΔGo data for the electron capture e + B = B(-) was obtained, which contains some 50 compounds B.Most of the compounds are substituted benzenes, quinones, conjugated acid anhydrides, and perfluorinated organics.
- Chowdhury, Swapan,Heinis, Thomas,Grimsrud, Eric P.,Kebarle, Paul
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p. 2747 - 2752
(2007/10/02)
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- Electron affinity of SF6 and perfluoromethylcyclohexane. The unusual kinetics of electron transfer reactions A- + B = A + B-, where A = SF6 or perfluorinated cyclo-alkanes
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Measurements of the equilibria ( 1 ): A- + B = A + B- with a pulsed electron high pressure mass spectrometer lead to ΔG, ΔH, and ΔS.Equilibria involving SF6 as A and perfluoromethylcyclohexane C7F14 provide electron affinities E.A.(SF6) =1.05 +/- 0.1 eV and E.A.(C7F14) =1.06 +/- 0.15 eV.The kinetics of reactions (1) involving the above two compounds were studied.The rate constants kj for SF6- + B = SF6 + B- were found to increase with exothermicity of the reaction.The temperature dependence for k f was determined.B compounds leading to high exothermicities were associated with kj at the collision limit and essentially no temperature dependence.B of progressively lower electron affinity led to kf below the collision limit and negative temperature dependence, while B with the lowest E.A. but still leading to exothermic reaction produced lowest kf with positive temperature dependence.A model is provided explaining the above behavior.The known large change of geometry between SF6- and SF6 introduces an internal barrier in the reaction coordinate for the reactions ( 1 ) involving SF6.
- Grimsrud, E. P.,Chowdhury, S.,Kebarle, P.
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p. 1059 - 1068
(2007/10/02)
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