Thermally initiated radical reactions of K2S2O8: EPR spin trapping investigations

Abstract

The thermally initiated decomposition of K2S2O8 radical initiator coupled with EPR spin trapping represents a very powerful source of reactive free radicals which can be effectively used to initiate and to monitor various radical processes, as we were able to demonstrate in the characterization of the antioxidant properties of wine samples. The spin trapping agent α‐phenyl‐N‐tert‐butylnitrone (PBN) is a very effective trap, but with low selectivity. 5,5‐Dimethyl‐1‐pyrroline‐N‐oxide (DMPO), which has higher selectivity and a very pronounced dependence of adduct formation on the temperature (the formal activation energy of ·DMPO–OH formation is 81.1 kJ mol−1), offers more advantages. In pure aqueous solutions, formation of the ·DMPO–OH adduct dominates. In mixed water–ethanol solvents, carbon‐centred DMPO radical adducts are increasingly evident with higher CH3CH2OH ratios, and at very high ethanol ratios the formation of ·DMPO–OOR adduct prevails. Copyright © 2000 John Wiley & Sons, Ltd.

General Strategy for Enhancing Electrochemiluminescence of Semiconductor Nanocrystals by Hydrogen Peroxide and Potassium Persulfate as Dual Coreactants

“…As shown in Figure S1, the DMPO–OH signal had not been obviously enhanced while the concentration of H 2 O 2 was less than 10 mM, but was enhanced quickly when the amount of H 2 O 2 was more than 10 mM. Moreover, it is important to note that, in aqueous solutions, formation of the DMPO–OH adduct dominates, as the stability of DMPO–SO 4 is very limited (half-life only ∼30 s); thus, its spectrum intensity is not clearly evident, and the change of the SO 4 –DMPO signal based on different conditions can easily go undetected.…”

First Direct and Unequivocal Electron Spin Resonance Spin-Trapping Evidence for pH-Dependent Production of Hydroxyl Radicals from Sulfate Radicals

“…These results clearly demonstrate that • OH is indeed produced from UV-PDS under alkaline conditions, which was further confirmed by ESR simulation studies (Figure and Figures S12 and S13). However, for PBN and POBN, the ESR signals of these free radical adducts are weak and not easily distinguishable from each other, which may be attributed to the following facts: (i) SO 4 • – can react quickly with PBN and POBN to form their corresponding cation radicals, which are further hydrolyzed by water into PBN–OH and POBN–OH adducts; , (ii) the lifetimes of both PBN–OH and POBN–OH adducts are quite short (the half-lives of PBN–OH and POBN–OH are 38 and 10 s at pH = 7.4, respectively); , and (iii) for PBN and POBN, the ESR spectra of their corresponding SO 4 • – adducts and • OH adducts are very similar, exhibiting extremely close hyperfine splitting constants (Table S4). , Based on the findings, we suggest that DMPO and BMPO are more suitable than PBN and POBN for distinguishing • OH from SO 4 • – produced by the UV-PDS system.…”

Oxidation of Aromatic Aldehydes to Esters: A Sulfate Radical Redox System

“…In the case of aromatics substituted by formyl group under study, it is prone to liberate one proton in the subsequent step when no other reaction paths are available, with generating the corresponding acyl radicals. , Thus formed acyl radicals were expected to undergo a second one-electron oxidation by SRA with conversion to acyl cations in the light of Lin and Sen’s study on radical carboxylation of methane, therefore, making the model of alcoholysis of acyl cation probable (path 1, Scheme ). Independently, the radical cross-coupling (path 2, Scheme ) may also be possible since the oxygen centered radicals were once detected in the spin-trap experiments of reactions of peroxydisulfates with alkanols . The unresolved question for path 2 is the O–H group in alkanols is a poor hydrogen atom donor and H-abstraction generally occurs on the alkyl moiety rather than the O–H group (BDE: C H 3 OH, 96.06 ± 0.15 kcal/mol; CH 3 O H , 104.6 ± 0.7 kcal/mol). , The interpretation of formation and fate of alkoxyl radicals in the oxidation process warrant further study.…”