Mechanistic aspects of hydrogen abstraction for phenolic antioxidants. Electronic structure and topological electron density analysis

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    Abstract

    Density functional calculations using the B3LYP functional are used to provide insight into the hydrogen abstraction mechanism of phenolic antioxidants. The energy profiles for 13 ortho, meta, para and di-methyl substituted phenols with hydroperoxyl radical have been determined. An excellent correlation between the enthalpy (ΔH) and activation energy (ΔEa) was found, obeying the Evans-Polanyi rule. The effects of hydrogen bonding on ΔEa are also discussed. Electron donating groups at the ortho and para positions are able to lower the activation energy for hydrogen abstraction. The highly electron withdrawing fluoro substituent increases the activation energies relative to phenol at the meta position but not at the para position. The electron density is studied using the atoms in molecules (AIM) approach. Atomic and bond properties are extracted to describe the hydrogen atom abstraction mechanism. It is found that on going from reactants to transition state, the hydrogen atom experiences a loss in volume, electronic population and dipole moment. These features suggest that the phenol hydroperoxyl reactions proceed according to a proton coupled electron transfer (PCET) as opposed to a hydrogen atom transfer (HAT) mechanism. © The Owner Societies 2005.
    Original languageEnglish
    Pages (from-to)614-619
    Number of pages5
    JournalPhysical Chemistry Chemical Physics
    Volume7
    Issue number4
    DOIs
    Publication statusPublished - 21 Feb 2005

    Keywords

    • Dipole moment (AIM at. dipole moment as correlation parameter
    • mechanistic aspects of hydrogen abstraction for phenolic antioxidants including electronic structure and topol. electron d. anal.)
    • Molar volume (AIM at. vol. of at. hydrogen
    • Electron density (AIM topol. anal.
    • Abstraction reaction (hydrogen, proton coupled electron transfer mechanism
    • Abstraction reaction enthalpy (hydrogen
    • Activation energy
    • Evans-Polanyi equation
    • Potential energy surface
    • Transition state structure (mechanistic aspects of hydrogen abstraction for phenolic antioxidants including electronic structure and topol. electron d. anal.)
    • Phenols Role: CAT (Catalyst use), CPS (Chemical process), PEP (Physical, engineering or chemical process), PRP (Properties), RCT (Reactant), PROC (Process), USES (Uses), RACT (Reactant or reagent) (mechanistic aspects of hydrogen abstraction for phenoli
    • Substituent effects (on activation energies
    • Electron delocalization
    • Lone-pair electrons (oxygen lone pair delocalization as correlation parameter
    • Antioxidants (phenolic
    • Hydrogen bond (reactant and product complexes
    • Linear free energy relationship (using transition-state at. dipole moment as correlation parameter
    • hydrogen abstraction phenolic antioxidant mechanism calcn

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