Monitoring of the Heterogroup Twisting Dynamics in Phenol Type Molecules via Different Characteristic Free-Electron-Transfer Products

Abstract

Phenol radical cations as well as phenoxyl radicals were observed as direct products of free electron transfer from phenol type solute molecules to solvent parent radical cations generated by ionizing irradiation. It is shown that the finding of the two species in comparable amounts can be explained by a nuclear-structure dependent solute cation dissociation behavior: Quantum-chemical calculations indicate that for phenol as solute primarily its conformers with perpendicular C−OH axis orientation to the aromatic ring tend to prompt deprotonation after ionization. A quite similar behavior could be predicted also for the heteroanalogous thiophenols and selenophenols. Quite generally considerable changes in the electron distribution of the ground-state molecules with the twisting angle of the −OH, −SH and −SeH groups could be calculated, with the greatest differences between “parallel” and “perpendicular” conformations. On the assumption that the fast electron-transfer projects the equilibrium solute conformer distribution onto the solute cation conformer one it is demonstrated that the experimental findings are compatible with a simple solute-cation internal relaxation model. By applying the quantum-chemically calculated conformer interconversion barrier heights it can be understood that radical fraction among the direct products increases if phenols are substituted by thiophenols or those by selenophenols, as observed in experiment.