3225-29-4Relevant articles and documents
Electron-transfer studies of a peroxide dianion
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.
supporting information, p. 5384 - 5391 (2014/06/09)
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.
Investigation of the oxidation of hydroquinone at the liquid/liquid interface
Lu, Xiao Quan,Dong, De Fang,Liu, Xiu Hui,Yao, Dong Na,Wang, Wen Ting,Xu, Yu Mei
experimental part, p. 225 - 228 (2010/12/24)
The oxidation of hydroquinone (QH2) was investigated for the first time at liquid/liquid (L/L) interface by scanning electrochemical microscopy (SECM). In this study, electron transfer (ET) from QH2 in aqueous to ferrocene (Fc) in nitrobenzene (NB) was probed. The apparent heterogeneous rate constants for ET reactions were obtained by fitting the experimental approach curves to the theoretical values. The results showed that the rate constants for oxidation reaction of QH2 were sensitive to the changes of the driving force, which increased as the driving force increased. In addition, factors that would affect ET of QH2 were studied. Experimental results indicated ion situation around QH2 molecule could change the magnitude of the rate constants because the capability of oxidation of QH2 would be affected by them.
Quenching of triplet-excited flavins by flavonoids. Structural assessment of antioxidative activity
Huvaere, Kevin,Olsen, Karsten,Skibsted, Leif H.
experimental part, p. 7283 - 7293 (2010/01/16)
(Figure Presented) The mechanism of flavin-mediated photooxidation of flavonoids was investigated for aqueous solutions. Interaction of triplet-excited flavin mononucleotide with phenols, as determined by laser flash photolysis, occurred at nearly diffusion-controlled rates (k～1.6x10 9 Lmol-1 s-1 for phenol at pH 7, 293 K), but protection of the phenolic function by methylation inhibited reaction. Still, electron transfer was proposed as the dominating mechanism due to the lack of primary kinetic hydrogen/ deuterium isotope effect and the low activation enthalpy (-1) for photooxidation. Activation entropy worked compensating in a series of phenolic derivatives, supporting a common oxidation mechanism. Anortho-hydroxymethoxy pattern was equally reactive (k～2.3x109Lmol-1 s-1 for guaiacol at pH 7) as compounds with ortho-dihydroxy substitution (k～2.4x109 L mol-1 s-1 for catechol at pH 7), which are generally referred to as good antioxidants. This refutes the common belief that stabilization of incipient phenoxyl radicals through intramolecular hydrogen bonding is the driving force behind the reducing activity of catechol-like compounds. Instead, such bonding improves ionization characteristics of the substrates, hence the differences in reactivity with (photo)oxidation of isolated phenols. Despite the similar reactivity, radicals from ortho-dihydroxy compounds are detected in high steady-state concentrations by electron paramagnetic resonance (EPR) spectroscopy, while those resulting from oxidation of ortho-hydroxymethoxy (or isolated phenolic) patterns were too reactive to be observed. The ability to deprotonate and form the corresponding radical anions at neutral pH was proposed as the decisive factor for stabilization and, consequently, for antioxidative action. Thus, substituting other ionizable functions for the ortho- or para-hydroxyl in phenolic compounds resulted in stable radical anion formation, as demonstrated for para-hydroxybenzoic acid, in contrast to its methyl ester. 2009 American Chemical Society.
