A Unified Perspective on the Hydrogen Atom Transfer and Proton-Coupled Electron Transfer Mechanisms in Terms of Topographic Features of the Ground and Excited Potential Energy Surfaces As Exemplified by the Reaction between Phenol and Radicals

Abstract

The relation between the hydrogen atom transfer (HAT) and proton-coupled electron transfer (PCET) mechanisms is discussed and is illustrated by multiconfigurational electronic structure calculations on the ArOH + R(*) --> ArO(*) + RH reactions. The key topographic features of the Born-Oppenheimer potential energy surfaces that determine the predominant reaction mechanism are the conical intersection seam of the two lowest states and reaction saddle points located on the shoulders of this seam. The saddle point corresponds to a crossing of two interacting valence bond states corresponding to the reactant and product bonding patterns, and the conical intersection corresponds to the noninteracting intersection of the same two diabatic states. The locations of mechanistically relevant conical intersection structures and relevant saddle point structures are presented for the reactions between phenol and the N- and O-centered radicals, (*)NH2 and (*)OOCH3. Points on the conical intersection of the ground doublet D0 and first excited doublet D1 states are found to be in close geometric and energetic proximity to the reaction saddle points. In such systems, either the HAT mechanism or both the HAT mechanism and the proton-coupled electron transfer (PCET) mechanism can take place, depending on the relative energetic accessibility of the reaction saddle points and the D0/D1 conical intersection seams. The discussion shows how the two mechanisms are related and how they blend into each other along intermediate reaction paths. The recognition that the saddle point governing the HAT mechanism is on the shoulder of the conical intersection governing the PCET mechanism is used to provide a unified view of the competition between the two mechanisms (and the blending of the two mechanisms) in terms of the prominent and connected features of the potential energy surface, namely the saddle point and the conical intersection. The character of the dual mechanism may be understood in terms of the dominant valence bond configurations of the intersecting states, which are zero-order approximations to the diabatic states.

Proton‐coupled electron transfer of the phenoxyl/phenol couple: Effect of Hartree‐Fock exchange on transition structures

“…[1][2][3] There are two mechanisms proposed for those reactions, i.e., hydrogen atom transfer (HAT) and proton-coupled electron transfer (PCET) [4,5] reactions. In the HAT reaction, the hydrogen transfer occurs between the same sets of σ orbitals associated with breaking and forming bonds, which is typical of hydrocarbon combustion reactions.…”

Metal-organic charge transfer can produce biradical states and is mediated by conical intersections

“…The effect of a CI on a thermally activated reaction depends on the energetic accessibility of the dividing surface and of the seam of intersection. Examples of thermally activated reactions where the CI seam is (or can be) accessed under usual conditions involve chemiluminescence, energy transfer, reactions involving metal atoms (29,32), and hydrogen atom abstractions from phenolic antioxidants (69). The CIs in systems considered here are particularly interesting because of their relevance in nonreactive processes, such as attachment of metal atoms to aromatic systems, which is of utmost importance in organometallic chemistry and materials chemistry.…”

Synthesis, Characterization and Antioxidant Studies of Quinazolin Derivatives

“…In another study, leopoldini et al found that the addition of OH radical to caffeic acid is slightly favored with respect to the hydrogen atom transfer, while the single electron transfer is unlikely, since thermodynamically, it is unfavorable 9 . Mayer et al and Oksana et al reported that the reactivity of phenols toward free radicals, such as CH3 OO• and PhO, involved a PCET mechanism 10,11 . In this study, we describe the synthesis of quinozoline derivatives (Scheme 1) and we describe their characterization through spectral data (FT-IR, H-NMR) and elemental analysis (CHN) inadition to discuss the in vitro antioxidant activities of the compounds (3-6).…”