Proton-Coupled Electron Transfer in the Reaction of 3,4-Dihydroxyphenylpyruvic Acid with Reactive Species in Various Media

Abstract

The distinction of concerted proton-coupled electron transfer (CPCET) from sequential one as well as proton transfer-electron transfer (PT-ET) from electron transfer-proton transfer (ET-PT) in the O-H bond cleavage reactions in various media has always been a difficult task. In this work, the activation barrier of the CPCET mechanism, its rate constants, and reaction free energies related to ET-PT and PT-ET involving coreactive species were presented as good parameters to attempt the problem. DFT calculations were carried out studying the described pathways subsequent to the scavenging of• OH and OBr − by the 3,4-DHPPA in various media. The solvation was described in a hybrid manner using IEF-PCM model conjointly with a model that takes into account some solute-solvent interactions. As a result, we found that the scavenging of hydroxyl radical by 3,4-DHPPA is thermodynamically governed by a one-step hydrogen atom transfer (CPCET) from the acid to the radical in all media. In kinetic viewpoint, CPCET still dominates in the vacuum and in nonpolar solvents, but in polar solvents it could compete strongly with the ET-PT mechanism so that the latter could slightly dominate.

The Substitution Effect on Reaction Enthalpies of Antioxidant Mechanisms of Juglone and Its Derivatives in Gas and Solution Phase: DFT Study

“…In the whole, the substitution of electron withdrawing group leads to an increase of PDE values in this phase. The solvation J2 J3 J4 J5 J6 J7 J8 J9 J10 J11 J12 J13 J14 J15 J16 J17 J18 J1 causes an enormous decrease of PDEs in comparison to the gas phase values because of high solvation enthalpies [30]. The average decrease in PDEs reached approximately 874.5 kJ/mol.…”

“…In the same vein, the calculated gas free energies of a proton (H(H + )) and an electron (H(e − )), are respectively, −26.28 kJ/mol [27] and −3.72 kJ/mol [28]. Proton and electron solvation enthalpies and free energies were taken from the literature [29,30]. The hydrogen atom solvation reaction enthalpies and free energies were also taken from earlier researches [30].…”

“…Proton and electron solvation enthalpies and free energies were taken from the literature [29,30]. The hydrogen atom solvation reaction enthalpies and free energies were also taken from earlier researches [30].…”

Quantum Chemical Investigation on the Antioxidant Activity of Neutral and Anionic Forms of Juglone: Metal Chelation and Its Effect on Radical Scavenging Activity

“…On the other hand, the enthalpies of formation for the proton in gas phase and water are 6.197 kJ/mol [38] and −1022 kJ/mol [39], respectively. The values of the proton hydration free energies in these media are, respectively, −26.26 kJ/mol [38] and −1104.5 kJ/mol [40]. Table 4 are all positive, indicating that this mechanism is endothermic and nonfavorable at 298.15 K and 1 atm.…”

“…The gas phase enthalpy and free energy of electron employed here are 3.145 kJ/mol and −3.42 kJ/mol, respectively [38]. In water, the values −105 kJ/mol [39] and −156.8 kJ/mol [40] for the enthalpy and the free energy of electron, respectively, were used. On the other hand, the enthalpies of formation for the proton in gas phase and water are 6.197 kJ/mol [38] and −1022 kJ/mol [39], respectively.…”